經過源碼學習ListView（大章）
從源碼開始學習ListView
前言
這個是我人生中寫的地一篇博客，若是有寫的很差，思惟混亂的地方請請請請輕噴。並謝謝指出不對的地方，我好修改。畢竟博客寫出來更多的是爲了記錄學習。
依舊現行感謝一下幾個博客對個人指導以及借鑑。若是有看的不明白的能夠移步去了解，排名不分先後。
Yogurt_QaQ ：ListView 源碼研究 帶你從源碼中讚歎谷歌大吊們的代碼藝術
康志：Android ListView工做原理徹底解析，帶你從源碼的角度完全理解
孫羣：源碼解析ListView中的RecycleBin機制
foxleezh：Android系統源碼分析--如何下載和閱讀Android源碼android
1、ListView的組成
一、ListView的繼承關係
ListView-->AbsListView-->AdapterView-->ViewGroup-->View
由上可知，ListView繼承於AbsListView繼承AdapterView，再日後就是View的繼承，後面暫時不說。等我慢慢看完，學習完在作總結。AdapterView能夠知道，google的工程師把ListView分紅了兩部分，ListView和Adapter。
二、淺談Adapter
google爲了讓listview適應更多的需求，承載更多的數據類型。因此使用了Adpater去承載數據，起到一個橋樑的做用，固然adapter還有一個重要的方法getview()；這個後面會詳細提到spring
2、ListView的源碼研究
一、RcycleBin機制
這個機制是Listview處理成大數據量時不會OOM的重要部分，先看源碼，以下。數組
/**
* The RecycleBin facilitates reuse of views across layouts. The RecycleBin has two levels of
* storage: ActiveViews and ScrapViews. ActiveViews are those views which were onscreen at the
* start of a layout. By construction, they are displaying current information. At the end of
* layout, all views in ActiveViews are demoted to ScrapViews. ScrapViews are old views that
* could potentially be used by the adapter to avoid allocating views unnecessarily.
*
* @see android.widget.AbsListView#setRecyclerListener(android.widget.AbsListView.RecyclerListener)
* @see android.widget.AbsListView.RecyclerListener
*/
class RecycleBin {
private RecyclerListener mRecyclerListener;
/**
* The position of the first view stored in mActiveViews.
*/
private int mFirstActivePosition;
/**
* Views that were on screen at the start of layout. This array is populated at the start of
* layout, and at the end of layout all view in mActiveViews are moved to mScrapViews.
* Views in mActiveViews represent a contiguous range of Views, with position of the first
* view store in mFirstActivePosition.
*/
private View[] mActiveViews = new View[0];
/**
* Unsorted views that can be used by the adapter as a convert view.
*/
private ArrayList<View>[] mScrapViews;
private int mViewTypeCount;
private ArrayList<View> mCurrentScrap;
private ArrayList<View> mSkippedScrap;
private SparseArray<View> mTransientStateViews;
private LongSparseArray<View> mTransientStateViewsById;
public void setViewTypeCount(int viewTypeCount) {
if (viewTypeCount < 1) {
throw new IllegalArgumentException("Can't have a viewTypeCount < 1");
}
//noinspection unchecked
ArrayList<View>[] scrapViews = new ArrayList[viewTypeCount];
for (int i = 0; i < viewTypeCount; i++) {
scrapViews[i] = new ArrayList<View>();
}
mViewTypeCount = viewTypeCount;
mCurrentScrap = scrapViews[0];
mScrapViews = scrapViews;
}
public void markChildrenDirty() {
if (mViewTypeCount == 1) {
final ArrayList<View> scrap = mCurrentScrap;
final int scrapCount = scrap.size();
for (int i = 0; i < scrapCount; i++) {
scrap.get(i).forceLayout();
}
} else {
final int typeCount = mViewTypeCount;
for (int i = 0; i < typeCount; i++) {
final ArrayList<View> scrap = mScrapViews[i];
final int scrapCount = scrap.size();
for (int j = 0; j < scrapCount; j++) {
scrap.get(j).forceLayout();
}
}
}
if (mTransientStateViews != null) {
final int count = mTransientStateViews.size();
for (int i = 0; i < count; i++) {
mTransientStateViews.valueAt(i).forceLayout();
}
}
if (mTransientStateViewsById != null) {
final int count = mTransientStateViewsById.size();
for (int i = 0; i < count; i++) {
mTransientStateViewsById.valueAt(i).forceLayout();
}
}
}
public boolean shouldRecycleViewType(int viewType) {
return viewType >= 0;
}
/**
* Clears the scrap heap.
*/
void clear() {
if (mViewTypeCount == 1) {
final ArrayList<View> scrap = mCurrentScrap;
clearScrap(scrap);
} else {
final int typeCount = mViewTypeCount;
for (int i = 0; i < typeCount; i++) {
final ArrayList<View> scrap = mScrapViews[i];
clearScrap(scrap);
}
}
clearTransientStateViews();
}
/**
* Fill ActiveViews with all of the children of the AbsListView.
*
* @param childCount The minimum number of views mActiveViews should hold
* @param firstActivePosition The position of the first view that will be stored in
* mActiveViews
*/
void fillActiveViews(int childCount, int firstActivePosition) {
if (mActiveViews.length < childCount) {
mActiveViews = new View[childCount];
}
mFirstActivePosition = firstActivePosition;
//noinspection MismatchedReadAndWriteOfArray
final View[] activeViews = mActiveViews;
for (int i = 0; i < childCount; i++) {
View child = getChildAt(i);
AbsListView.LayoutParams lp = (AbsListView.LayoutParams) child.getLayoutParams();
// Don't put header or footer views into the scrap heap if (lp != null && lp.viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { // Note: We do place AdapterView.ITEM_VIEW_TYPE_IGNORE in active views. // However, we will NOT place them into scrap views. activeViews[i] = child; // Remember the position so that setupChild() doesn't reset state.
lp.scrappedFromPosition = firstActivePosition + i;
}
}
}
/**
* Get the view corresponding to the specified position. The view will be removed from
* mActiveViews if it is found.
*
* @param position The position to look up in mActiveViews
* @return The view if it is found, null otherwise
*/
View getActiveView(int position) {
int index = position - mFirstActivePosition;
final View[] activeViews = mActiveViews;
if (index >=0 && index < activeViews.length) {
final View match = activeViews[index];
activeViews[index] = null;
return match;
}
return null;
}
View getTransientStateView(int position) {
if (mAdapter != null && mAdapterHasStableIds && mTransientStateViewsById != null) {
long id = mAdapter.getItemId(position);
View result = mTransientStateViewsById.get(id);
mTransientStateViewsById.remove(id);
return result;
}
if (mTransientStateViews != null) {
final int index = mTransientStateViews.indexOfKey(position);
if (index >= 0) {
View result = mTransientStateViews.valueAt(index);
mTransientStateViews.removeAt(index);
return result;
}
}
return null;
}
/**
* Dumps and fully detaches any currently saved views with transient
* state.
*/
void clearTransientStateViews() {
final SparseArray<View> viewsByPos = mTransientStateViews;
if (viewsByPos != null) {
final int N = viewsByPos.size();
for (int i = 0; i < N; i++) {
removeDetachedView(viewsByPos.valueAt(i), false);
}
viewsByPos.clear();
}
final LongSparseArray<View> viewsById = mTransientStateViewsById;
if (viewsById != null) {
final int N = viewsById.size();
for (int i = 0; i < N; i++) {
removeDetachedView(viewsById.valueAt(i), false);
}
viewsById.clear();
}
}
/**
* @return A view from the ScrapViews collection. These are unordered.
*/
View getScrapView(int position) {
final int whichScrap = mAdapter.getItemViewType(position);
if (whichScrap < 0) {
return null;
}
if (mViewTypeCount == 1) {
return retrieveFromScrap(mCurrentScrap, position);
} else if (whichScrap < mScrapViews.length) {
return retrieveFromScrap(mScrapViews[whichScrap], position);
}
return null;
}
/**
* Puts a view into the list of scrap views.
* <p>
* If the list data hasn't changed or the adapter has stable IDs, views * with transient state will be preserved for later retrieval. * * @param scrap The view to add * @param position The view's position within its parent
*/
void addScrapView(View scrap, int position) {
final AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();
if (lp == null) {
// Can't recycle, but we don't know anything about the view.
// Ignore it completely.
return;
}
lp.scrappedFromPosition = position;
// Remove but don't scrap header or footer views, or views that // should otherwise not be recycled. final int viewType = lp.viewType; if (!shouldRecycleViewType(viewType)) { // Can't recycle. If it's not a header or footer, which have // special handling and should be ignored, then skip the scrap // heap and we'll fully detach the view later.
if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
getSkippedScrap().add(scrap);
}
return;
}
scrap.dispatchStartTemporaryDetach();
// The the accessibility state of the view may change while temporary
// detached and we do not allow detached views to fire accessibility
// events. So we are announcing that the subtree changed giving a chance
// to clients holding on to a view in this subtree to refresh it.
notifyViewAccessibilityStateChangedIfNeeded(
AccessibilityEvent.CONTENT_CHANGE_TYPE_SUBTREE);
// Don't scrap views that have transient state. final boolean scrapHasTransientState = scrap.hasTransientState(); if (scrapHasTransientState) { if (mAdapter != null && mAdapterHasStableIds) { // If the adapter has stable IDs, we can reuse the view for // the same data. if (mTransientStateViewsById == null) { mTransientStateViewsById = new LongSparseArray<>(); } mTransientStateViewsById.put(lp.itemId, scrap); } else if (!mDataChanged) { // If the data hasn't changed, we can reuse the views at
// their old positions.
if (mTransientStateViews == null) {
mTransientStateViews = new SparseArray<>();
}
mTransientStateViews.put(position, scrap);
} else {
// Otherwise, we'll have to remove the view and start over. clearScrapForRebind(scrap); getSkippedScrap().add(scrap); } } else { clearScrapForRebind(scrap); if (mViewTypeCount == 1) { mCurrentScrap.add(scrap); } else { mScrapViews[viewType].add(scrap); } if (mRecyclerListener != null) { mRecyclerListener.onMovedToScrapHeap(scrap); } } } private ArrayList<View> getSkippedScrap() { if (mSkippedScrap == null) { mSkippedScrap = new ArrayList<>(); } return mSkippedScrap; } /** * Finish the removal of any views that skipped the scrap heap. */ void removeSkippedScrap() { if (mSkippedScrap == null) { return; } final int count = mSkippedScrap.size(); for (int i = 0; i < count; i++) { removeDetachedView(mSkippedScrap.get(i), false); } mSkippedScrap.clear(); } /** * Move all views remaining in mActiveViews to mScrapViews. */ void scrapActiveViews() { final View[] activeViews = mActiveViews; final boolean hasListener = mRecyclerListener != null; final boolean multipleScraps = mViewTypeCount > 1; ArrayList<View> scrapViews = mCurrentScrap; final int count = activeViews.length; for (int i = count - 1; i >= 0; i--) { final View victim = activeViews[i]; if (victim != null) { final AbsListView.LayoutParams lp = (AbsListView.LayoutParams) victim.getLayoutParams(); final int whichScrap = lp.viewType; activeViews[i] = null; if (victim.hasTransientState()) { // Store views with transient state for later use. victim.dispatchStartTemporaryDetach(); if (mAdapter != null && mAdapterHasStableIds) { if (mTransientStateViewsById == null) { mTransientStateViewsById = new LongSparseArray<View>(); } long id = mAdapter.getItemId(mFirstActivePosition + i); mTransientStateViewsById.put(id, victim); } else if (!mDataChanged) { if (mTransientStateViews == null) { mTransientStateViews = new SparseArray<View>(); } mTransientStateViews.put(mFirstActivePosition + i, victim); } else if (whichScrap != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { // The data has changed, we can't keep this view.
removeDetachedView(victim, false);
}
} else if (!shouldRecycleViewType(whichScrap)) {
// Discard non-recyclable views except headers/footers.
if (whichScrap != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
removeDetachedView(victim, false);
}
} else {
// Store everything else on the appropriate scrap heap.
if (multipleScraps) {
scrapViews = mScrapViews[whichScrap];
}
lp.scrappedFromPosition = mFirstActivePosition + i;
removeDetachedView(victim, false);
scrapViews.add(victim);
if (hasListener) {
mRecyclerListener.onMovedToScrapHeap(victim);
}
}
}
}
pruneScrapViews();
}
/**
* At the end of a layout pass, all temp detached views should either be re-attached or
* completely detached. This method ensures that any remaining view in the scrap list is
* fully detached.
*/
void fullyDetachScrapViews() {
final int viewTypeCount = mViewTypeCount;
final ArrayList<View>[] scrapViews = mScrapViews;
for (int i = 0; i < viewTypeCount; ++i) {
final ArrayList<View> scrapPile = scrapViews[i];
for (int j = scrapPile.size() - 1; j >= 0; j--) {
final View view = scrapPile.get(j);
if (view.isTemporarilyDetached()) {
removeDetachedView(view, false);
}
}
}
}
/**
* Makes sure that the size of mScrapViews does not exceed the size of
* mActiveViews, which can happen if an adapter does not recycle its
* views. Removes cached transient state views that no longer have
* transient state.
*/
private void pruneScrapViews() {
final int maxViews = mActiveViews.length;
final int viewTypeCount = mViewTypeCount;
final ArrayList<View>[] scrapViews = mScrapViews;
for (int i = 0; i < viewTypeCount; ++i) {
final ArrayList<View> scrapPile = scrapViews[i];
int size = scrapPile.size();
while (size > maxViews) {
scrapPile.remove(--size);
}
}
final SparseArray<View> transViewsByPos = mTransientStateViews;
if (transViewsByPos != null) {
for (int i = 0; i < transViewsByPos.size(); i++) {
final View v = transViewsByPos.valueAt(i);
if (!v.hasTransientState()) {
removeDetachedView(v, false);
transViewsByPos.removeAt(i);
i--;
}
}
}
final LongSparseArray<View> transViewsById = mTransientStateViewsById;
if (transViewsById != null) {
for (int i = 0; i < transViewsById.size(); i++) {
final View v = transViewsById.valueAt(i);
if (!v.hasTransientState()) {
removeDetachedView(v, false);
transViewsById.removeAt(i);
i--;
}
}
}
}
/**
* Puts all views in the scrap heap into the supplied list.
*/
void reclaimScrapViews(List<View> views) {
if (mViewTypeCount == 1) {
views.addAll(mCurrentScrap);
} else {
final int viewTypeCount = mViewTypeCount;
final ArrayList<View>[] scrapViews = mScrapViews;
for (int i = 0; i < viewTypeCount; ++i) {
final ArrayList<View> scrapPile = scrapViews[i];
views.addAll(scrapPile);
}
}
}
/**
* Updates the cache color hint of all known views.
*
* @param color The new cache color hint.
*/
void setCacheColorHint(int color) {
if (mViewTypeCount == 1) {
final ArrayList<View> scrap = mCurrentScrap;
final int scrapCount = scrap.size();
for (int i = 0; i < scrapCount; i++) {
scrap.get(i).setDrawingCacheBackgroundColor(color);
}
} else {
final int typeCount = mViewTypeCount;
for (int i = 0; i < typeCount; i++) {
final ArrayList<View> scrap = mScrapViews[i];
final int scrapCount = scrap.size();
for (int j = 0; j < scrapCount; j++) {
scrap.get(j).setDrawingCacheBackgroundColor(color);
}
}
}
// Just in case this is called during a layout pass
final View[] activeViews = mActiveViews;
final int count = activeViews.length;
for (int i = 0; i < count; ++i) {
final View victim = activeViews[i];
if (victim != null) {
victim.setDrawingCacheBackgroundColor(color);
}
}
}
private View retrieveFromScrap(ArrayList<View> scrapViews, int position) {
final int size = scrapViews.size();
if (size > 0) {
// See if we still have a view for this position or ID.
// Traverse backwards to find the most recently used scrap view
for (int i = size - 1; i >= 0; i--) {
final View view = scrapViews.get(i);
final AbsListView.LayoutParams params =
(AbsListView.LayoutParams) view.getLayoutParams();
if (mAdapterHasStableIds) {
final long id = mAdapter.getItemId(position);
if (id == params.itemId) {
return scrapViews.remove(i);
}
} else if (params.scrappedFromPosition == position) {
final View scrap = scrapViews.remove(i);
clearScrapForRebind(scrap);
return scrap;
}
}
final View scrap = scrapViews.remove(size - 1);
clearScrapForRebind(scrap);
return scrap;
} else {
return null;
}
}
private void clearScrap(final ArrayList<View> scrap) {
final int scrapCount = scrap.size();
for (int j = 0; j < scrapCount; j++) {
removeDetachedView(scrap.remove(scrapCount - 1 - j), false);
}
}
private void clearScrapForRebind(View view) {
view.clearAccessibilityFocus();
view.setAccessibilityDelegate(null);
}
private void removeDetachedView(View child, boolean animate) {
child.setAccessibilityDelegate(null);
AbsListView.this.removeDetachedView(child, animate);
}
}複製代碼
源碼很少只有300多行，我們先看開頭，google對這個的註釋。緩存
The RecycleBin facilitates reuse of views across layouts. The RecycleBin has two levels of
* storage: ActiveViews and ScrapViews. ActiveViews are those views which were onscreen at the
* start of a layout. By construction, they are displaying current information. At the end of
* layout, all views in ActiveViews are demoted to The RecycleBin facilitates reuse of views across layouts. The RecycleBin has two levels of
* storage: ActiveViews and ScrapViews. ActiveViews are those views which were onscreen at the
* start of a layout. By construction, they are displaying current information. At the end of
* layout, all views in ActiveViews are demoted to ScrapViews. ScrapViews are old views that
* could potentially be used by the adapter to avoid allocating views unnecessarily.. ScrapViews are old views that
* could potentially be used by the adapter to avoid allocating views unnecessarily.複製代碼
可知，這個內部類中最關鍵的是ActiveViews和ScrapViews。更據這兩個關鍵詞咱們去找對應類，以下：bash
fillActiveViews()app
* Fill ActiveViews with all of the children of the AbsListView. * * @param childCount The minimum number of views mActiveViews should hold * @param firstActivePosition The position of the first view that will be stored in * mActiveViews */ void fillActiveViews(int childCount, int firstActivePosition) { if (mActiveViews.length < childCount) { mActiveViews = new View[childCount]; } mFirstActivePosition = firstActivePosition; //noinspection MismatchedReadAndWriteOfArray final View[] activeViews = mActiveViews; for (int i = 0; i < childCount; i++) { View child = getChildAt(i); AbsListView.LayoutParams lp = (AbsListView.LayoutParams) child.getLayoutParams(); // Don't put header or footer views into the scrap heap if (lp != null && lp.viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { // Note: We do place AdapterView.ITEM_VIEW_TYPE_IGNORE in active views. // However, we will NOT place them into scrap views. activeViews[i] = child; // Remember the position so that setupChild() doesn't reset state. lp.scrappedFromPosition = firstActivePosition + i; } } }複製代碼老樣子，先從註釋看起，這裏仍是要感嘆一下本身平時，寫代碼要多加註釋！！言歸正傳，註釋上說，把AbsListView中的指定元素放到ActiveViews數組中。而後看傳入的兩個參數，childCount，firstActivePosition。@param中都有寫，一個是傳入須要儲存view的數量，另外一個是ListView中第一個可見view的position。調用這個函數主要是將view添加進ActiveViews數組。less
getActiveView()async
* Get the view corresponding to the specified position. The view will be removed from * mActiveViews if it is found. * * @param position The position to look up in mActiveViews * @return The view if it is found, null otherwise */ View getActiveView(int position) { int index = position - mFirstActivePosition; final View[] activeViews = mActiveViews; if (index >=0 && index < activeViews.length) { final View match = activeViews[index]; activeViews[index] = null; return match; } return null; }複製代碼調用這個函數，其實註釋裏面說的很是清楚了，獲取一個position下的View而後它就從mActiveViews數組中移除了。意思是什麼呢，一個view只能用一個，用完之後就被廢棄了。那麼新的view怎麼加入，廢棄的view怎麼再利用呢？接着往下看。ide
- addScrapView()
Puts a view into the list of scrap views.函數
* <p> * If the list data hasn't changed or the adapter has stable IDs, views * with transient state will be preserved for later retrieval. * * @param scrap The view to add * @param position The view's position within its parent */ void addScrapView(View scrap, int position) { final AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams(); if (lp == null) { // Can't recycle, but we don't know anything about the view. // Ignore it completely. return; } lp.scrappedFromPosition = position; // Remove but don't scrap header or footer views, or views that // should otherwise not be recycled. final int viewType = lp.viewType; if (!shouldRecycleViewType(viewType)) { // Can't recycle. If it's not a header or footer, which have // special handling and should be ignored, then skip the scrap // heap and we'll fully detach the view later. if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { getSkippedScrap().add(scrap); } return; } scrap.dispatchStartTemporaryDetach(); // The the accessibility state of the view may change while temporary // detached and we do not allow detached views to fire accessibility // events. So we are announcing that the subtree changed giving a chance // to clients holding on to a view in this subtree to refresh it. notifyViewAccessibilityStateChangedIfNeeded( AccessibilityEvent.CONTENT_CHANGE_TYPE_SUBTREE); // Don't scrap views that have transient state. final boolean scrapHasTransientState = scrap.hasTransientState(); if (scrapHasTransientState) { if (mAdapter != null && mAdapterHasStableIds) { // If the adapter has stable IDs, we can reuse the view for // the same data. if (mTransientStateViewsById == null) { mTransientStateViewsById = new LongSparseArray<>(); } mTransientStateViewsById.put(lp.itemId, scrap); } else if (!mDataChanged) { // If the data hasn't changed, we can reuse the views at // their old positions. if (mTransientStateViews == null) { mTransientStateViews = new SparseArray<>(); } mTransientStateViews.put(position, scrap); } else { // Otherwise, we'll have to remove the view and start over. clearScrapForRebind(scrap); getSkippedScrap().add(scrap); } } else { clearScrapForRebind(scrap); if (mViewTypeCount == 1) { mCurrentScrap.add(scrap); } else { mScrapViews[viewType].add(scrap); } if (mRecyclerListener != null) { mRecyclerListener.onMovedToScrapHeap(scrap); } } }複製代碼這個函數，更據他的代碼量就知道他是recyclebin中很關鍵的函數，意思很簡單，調用這個函數之後將view放進scrapviews數組。兩個傳入的變量，一個是廢棄的view，一個是view的位置。
getScrapView()
* @return A view from the ScrapViews collection. These are unordered. */ View getScrapView(int position) { final int whichScrap = mAdapter.getItemViewType(position); if (whichScrap < 0) { return null; } if (mViewTypeCount == 1) { return retrieveFromScrap(mCurrentScrap, position); } else if (whichScrap < mScrapViews.length) { return retrieveFromScrap(mScrapViews[whichScrap], position); } return null; }複製代碼這個函數就很好理解了，就是將mScrapViews中最後一個取出來。
上面這四個函數超級重要，必定要牢記3、ListView源碼流程
- 第一次layout
首先，ListView也是ViewGoup也是View，因此說ListView也是基本的三步繪製流程。onmeasure()測量View的大小,onlayout()確認View的佈局,ondraw()繪製View到界面上。onmeasure()在ListView中沒有什麼特殊的。可是在onlayout中體現出不同了，咱們從onlayout的源碼一層層看。
看源碼能夠發現ListView中並無onlayout方法，那麼咱們去父類AbsListView中去看，果真咱們發現了onlayout。
/**
* Subclasses should NOT override this method but
* {@link #layoutChildren()} instead.
*/
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
super.onLayout(changed, l, t, r, b);
mInLayout = true;
final int childCount = getChildCount();
if (changed) {
for (int i = 0; i < childCount; i++) {
getChildAt(i).forceLayout();
}
mRecycler.markChildrenDirty();
}
layoutChildren();
mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR;
// TODO: Move somewhere sane. This doesn't belong in onLayout(). if (mFastScroll != null) { mFastScroll.onItemCountChanged(getChildCount(), mItemCount); } mInLayout = false; }複製代碼
方法代碼量不多，主要是一個判斷，若是View發生改變，那麼就從新繪製全部的view。而後就是layoutChildren()方法，這個從名字上能夠看出，是繪製子view的方法，咱們跟進去看。
* Subclasses must override this method to layout their children.
*/
protected void layoutChildren() {
}複製代碼
發現並無寫什麼，固然這句話是廢話，由於這是父類，不會涉及到具體的view繪製。因此咱們回到ListView中去找這個方法。
@Override
protected void layoutChildren() {
final boolean blockLayoutRequests = mBlockLayoutRequests;
if (blockLayoutRequests) {
return;
}
mBlockLayoutRequests = true;
try {
super.layoutChildren();
invalidate();
if (mAdapter == null) {
resetList();
invokeOnItemScrollListener();
return;
}
final int childrenTop = mListPadding.top;
final int childrenBottom = mBottom - mTop - mListPadding.bottom;
final int childCount = getChildCount();
int index = 0;
int delta = 0;
View sel;
View oldSel = null;
View oldFirst = null;
View newSel = null;
// Remember stuff we will need down below
switch (mLayoutMode) {
case LAYOUT_SET_SELECTION:
index = mNextSelectedPosition - mFirstPosition;
if (index >= 0 && index < childCount) {
newSel = getChildAt(index);
}
break;
case LAYOUT_FORCE_TOP:
case LAYOUT_FORCE_BOTTOM:
case LAYOUT_SPECIFIC:
case LAYOUT_SYNC:
break;
case LAYOUT_MOVE_SELECTION:
default:
// Remember the previously selected view
index = mSelectedPosition - mFirstPosition;
if (index >= 0 && index < childCount) {
oldSel = getChildAt(index);
}
// Remember the previous first child
oldFirst = getChildAt(0);
if (mNextSelectedPosition >= 0) {
delta = mNextSelectedPosition - mSelectedPosition;
}
// Caution: newSel might be null
newSel = getChildAt(index + delta);
}
boolean dataChanged = mDataChanged;
if (dataChanged) {
handleDataChanged();
}
// Handle the empty set by removing all views that are visible
// and calling it a day
if (mItemCount == 0) {
resetList();
invokeOnItemScrollListener();
return;
} else if (mItemCount != mAdapter.getCount()) {
throw new IllegalStateException("The content of the adapter has changed but "
+ "ListView did not receive a notification. Make sure the content of "
+ "your adapter is not modified from a background thread, but only from "
+ "the UI thread. Make sure your adapter calls notifyDataSetChanged() "
+ "when its content changes. [in ListView(" + getId() + ", " + getClass()
+ ") with Adapter(" + mAdapter.getClass() + ")]");
}
setSelectedPositionInt(mNextSelectedPosition);
AccessibilityNodeInfo accessibilityFocusLayoutRestoreNode = null;
View accessibilityFocusLayoutRestoreView = null;
int accessibilityFocusPosition = INVALID_POSITION;
// Remember which child, if any, had accessibility focus. This must
// occur before recycling any views, since that will clear
// accessibility focus.
final ViewRootImpl viewRootImpl = getViewRootImpl();
if (viewRootImpl != null) {
final View focusHost = viewRootImpl.getAccessibilityFocusedHost();
if (focusHost != null) {
final View focusChild = getAccessibilityFocusedChild(focusHost);
if (focusChild != null) {
if (!dataChanged || isDirectChildHeaderOrFooter(focusChild)
|| (focusChild.hasTransientState() && mAdapterHasStableIds)) {
// The views won't be changing, so try to maintain // focus on the current host and virtual view. accessibilityFocusLayoutRestoreView = focusHost; accessibilityFocusLayoutRestoreNode = viewRootImpl .getAccessibilityFocusedVirtualView(); } // If all else fails, maintain focus at the same // position. accessibilityFocusPosition = getPositionForView(focusChild); } } } View focusLayoutRestoreDirectChild = null; View focusLayoutRestoreView = null; // Take focus back to us temporarily to avoid the eventual call to // clear focus when removing the focused child below from messing // things up when ViewAncestor assigns focus back to someone else. final View focusedChild = getFocusedChild(); if (focusedChild != null) { // TODO: in some cases focusedChild.getParent() == null // We can remember the focused view to restore after re-layout // if the data hasn't changed, or if the focused position is a
// header or footer.
if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)
|| focusedChild.hasTransientState() || mAdapterHasStableIds) {
focusLayoutRestoreDirectChild = focusedChild;
// Remember the specific view that had focus.
focusLayoutRestoreView = findFocus();
if (focusLayoutRestoreView != null) {
// Tell it we are going to mess with it.
focusLayoutRestoreView.dispatchStartTemporaryDetach();
}
}
requestFocus();
}
// Pull all children into the RecycleBin.
// These views will be reused if possible
final int firstPosition = mFirstPosition;
final RecycleBin recycleBin = mRecycler;
if (dataChanged) {
for (int i = 0; i < childCount; i++) {
recycleBin.addScrapView(getChildAt(i), firstPosition+i);
}
} else {
recycleBin.fillActiveViews(childCount, firstPosition);
}
// Clear out old views
detachAllViewsFromParent();
recycleBin.removeSkippedScrap();
switch (mLayoutMode) {
case LAYOUT_SET_SELECTION:
if (newSel != null) {
sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
} else {
sel = fillFromMiddle(childrenTop, childrenBottom);
}
break;
case LAYOUT_SYNC:
sel = fillSpecific(mSyncPosition, mSpecificTop);
break;
case LAYOUT_FORCE_BOTTOM:
sel = fillUp(mItemCount - 1, childrenBottom);
adjustViewsUpOrDown();
break;
case LAYOUT_FORCE_TOP:
mFirstPosition = 0;
sel = fillFromTop(childrenTop);
adjustViewsUpOrDown();
break;
case LAYOUT_SPECIFIC:
final int selectedPosition = reconcileSelectedPosition();
sel = fillSpecific(selectedPosition, mSpecificTop);
/**
* When ListView is resized, FocusSelector requests an async selection for the
* previously focused item to make sure it is still visible. If the item is not
* selectable, it won't regain focus so instead we call FocusSelector * to directly request focus on the view after it is visible. */ if (sel == null && mFocusSelector != null) { final Runnable focusRunnable = mFocusSelector .setupFocusIfValid(selectedPosition); if (focusRunnable != null) { post(focusRunnable); } } break; case LAYOUT_MOVE_SELECTION: sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom); break; default: if (childCount == 0) { if (!mStackFromBottom) { final int position = lookForSelectablePosition(0, true); setSelectedPositionInt(position); sel = fillFromTop(childrenTop); } else { final int position = lookForSelectablePosition(mItemCount - 1, false); setSelectedPositionInt(position); sel = fillUp(mItemCount - 1, childrenBottom); } } else { if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) { sel = fillSpecific(mSelectedPosition, oldSel == null ? childrenTop : oldSel.getTop()); } else if (mFirstPosition < mItemCount) { sel = fillSpecific(mFirstPosition, oldFirst == null ? childrenTop : oldFirst.getTop()); } else { sel = fillSpecific(0, childrenTop); } } break; } // Flush any cached views that did not get reused above recycleBin.scrapActiveViews(); // remove any header/footer that has been temp detached and not re-attached removeUnusedFixedViews(mHeaderViewInfos); removeUnusedFixedViews(mFooterViewInfos); if (sel != null) { // The current selected item should get focus if items are // focusable. if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) { final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild && focusLayoutRestoreView != null && focusLayoutRestoreView.requestFocus()) || sel.requestFocus(); if (!focusWasTaken) { // Selected item didn't take focus, but we still want to
// make sure something else outside of the selected view
// has focus.
final View focused = getFocusedChild();
if (focused != null) {
focused.clearFocus();
}
positionSelector(INVALID_POSITION, sel);
} else {
sel.setSelected(false);
mSelectorRect.setEmpty();
}
} else {
positionSelector(INVALID_POSITION, sel);
}
mSelectedTop = sel.getTop();
} else {
final boolean inTouchMode = mTouchMode == TOUCH_MODE_TAP
|| mTouchMode == TOUCH_MODE_DONE_WAITING;
if (inTouchMode) {
// If the user's finger is down, select the motion position. final View child = getChildAt(mMotionPosition - mFirstPosition); if (child != null) { positionSelector(mMotionPosition, child); } } else if (mSelectorPosition != INVALID_POSITION) { // If we had previously positioned the selector somewhere, // put it back there. It might not match up with the data, // but it's transitioning out so it's not a big deal. final View child = getChildAt(mSelectorPosition - mFirstPosition); if (child != null) { positionSelector(mSelectorPosition, child); } } else { // Otherwise, clear selection. mSelectedTop = 0; mSelectorRect.setEmpty(); } // Even if there is not selected position, we may need to // restore focus (i.e. something focusable in touch mode). if (hasFocus() && focusLayoutRestoreView != null) { focusLayoutRestoreView.requestFocus(); } } // Attempt to restore accessibility focus, if necessary. if (viewRootImpl != null) { final View newAccessibilityFocusedView = viewRootImpl.getAccessibilityFocusedHost(); if (newAccessibilityFocusedView == null) { if (accessibilityFocusLayoutRestoreView != null && accessibilityFocusLayoutRestoreView.isAttachedToWindow()) { final AccessibilityNodeProvider provider = accessibilityFocusLayoutRestoreView.getAccessibilityNodeProvider(); if (accessibilityFocusLayoutRestoreNode != null && provider != null) { final int virtualViewId = AccessibilityNodeInfo.getVirtualDescendantId( accessibilityFocusLayoutRestoreNode.getSourceNodeId()); provider.performAction(virtualViewId, AccessibilityNodeInfo.ACTION_ACCESSIBILITY_FOCUS, null); } else { accessibilityFocusLayoutRestoreView.requestAccessibilityFocus(); } } else if (accessibilityFocusPosition != INVALID_POSITION) { // Bound the position within the visible children. final int position = MathUtils.constrain( accessibilityFocusPosition - mFirstPosition, 0, getChildCount() - 1); final View restoreView = getChildAt(position); if (restoreView != null) { restoreView.requestAccessibilityFocus(); } } } } // Tell focus view we are done mucking with it, if it is still in // our view hierarchy. if (focusLayoutRestoreView != null && focusLayoutRestoreView.getWindowToken() != null) { focusLayoutRestoreView.dispatchFinishTemporaryDetach(); } mLayoutMode = LAYOUT_NORMAL; mDataChanged = false; if (mPositionScrollAfterLayout != null) { post(mPositionScrollAfterLayout); mPositionScrollAfterLayout = null; } mNeedSync = false; setNextSelectedPositionInt(mSelectedPosition); updateScrollIndicators(); if (mItemCount > 0) { checkSelectionChanged(); } invokeOnItemScrollListener(); } finally { if (mFocusSelector != null) { mFocusSelector.onLayoutComplete(); } if (!blockLayoutRequests) { mBlockLayoutRequests = false; } } }複製代碼
源碼超級多，咱們走大流程分析吧。首先先看invalidate();這個是在合適的時候出發ondarw()方法，就是在Adapter中數據有變化的時候調用，final int childCount = getChildCount();第一次ListView繪製的時候，是沒有子view的，因此確定是0。
再日後就是 final RecycleBin recycleBin = mRecycler;
if (dataChanged) {
for (int i = 0; i < childCount; i++) {
recycleBin.addScrapView(getChildAt(i), firstPosition+i);
}
} else {
recycleBin.fillActiveViews(childCount, firstPosition);
}複製代碼
這裏執行了一個判斷若是dataChanged，那麼會執行addScrapView把view所有廢棄。若是沒有chang那麼執行fillActiveViews，把view添加進數組。這兩個函數在recyclebin中有說過，忘了回看。可是如今尚未填充數據，因此子view中沒有東西，因此這行代碼暫時不起效果。
關於mLayoutMode這個值默認都是LAYOUT_NORMAL，因此會進入default。
if (childCount == 0) {
if (!mStackFromBottom) {
final int position = lookForSelectablePosition(0, true);
setSelectedPositionInt(position);
sel = fillFromTop(childrenTop);
} else {
final int position = lookForSelectablePosition(mItemCount - 1, false);
setSelectedPositionInt(position);
sel = fillUp(mItemCount - 1, childrenBottom);
}
} else {
if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
sel = fillSpecific(mSelectedPosition,
oldSel == null ? childrenTop : oldSel.getTop());
} else if (mFirstPosition < mItemCount) {
sel = fillSpecific(mFirstPosition,
oldFirst == null ? childrenTop : oldFirst.getTop());
} else {
sel = fillSpecific(0, childrenTop);
}
}複製代碼
首先判斷了childCount是否等於0，而後驗證了mSelectedPosition是否合規。默認佈局就是從上向下依次填充view，因此是fillTOP，而後FillFromtop的return是filldown()。
* Fills the list from top to bottom, starting with mFirstPosition
*
* @param nextTop The location where the top of the first item should be
* drawn
*
* @return The view that is currently selected
*/
private View fillFromTop(int nextTop) {
mFirstPosition = Math.min(mFirstPosition, mSelectedPosition);
mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);
if (mFirstPosition < 0) {
mFirstPosition = 0;
}
return fillDown(mFirstPosition, nextTop);
}複製代碼
可知fillDown纔是繪製的真正方法，咱們進入方法內部。
* Fills the list from pos down to the end of the list view.
*
* @param pos The first position to put in the list
*
* @param nextTop The location where the top of the item associated with pos
* should be drawn
*
* @return The view that is currently selected, if it happens to be in the
* range that we draw.
*/
private View fillDown(int pos, int nextTop) {
View selectedView = null;
int end = (mBottom - mTop);
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
end -= mListPadding.bottom;
}
while (nextTop < end && pos < mItemCount) {
// is this the selected item?
boolean selected = pos == mSelectedPosition;
View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);
nextTop = child.getBottom() + mDividerHeight;
if (selected) {
selectedView = child;
}
pos++;
}
setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1);
return selectedView;
}複製代碼
能夠看到，這裏使用了一個while循環來執行重複邏輯，一開始nextTop的值是第一個子元素頂部距離整個ListView頂部的像素值，pos則是剛剛傳入的mFirstPosition的值，而end是ListView底部減去頂部所得的像素值，mItemCount則是Adapter中的元素數量。所以一開始的狀況下nextTop一定是小於end值的，而且pos也是小於mItemCount值的。那麼每執行一次while循環，pos的值都會加1，而且nextTop也會增長，當nextTop大於等於end時，也就是子元素已經超出當前屏幕了，或者pos大於等於mItemCount時，也就是全部Adapter中的元素都被遍歷結束了，就會跳出while循環。
跳出循環中執行了makeAndAddView()方法，進入這個方法看一下。
* Obtains the view and adds it to our list of children. The view can be
* made fresh, converted from an unused view, or used as is if it was in
* the recycle bin.
*
* @param position logical position in the list
* @param y top or bottom edge of the view to add
* @param flow {@code true} to align top edge to y, {@code false} to align
* bottom edge to y
* @param childrenLeft left edge where children should be positioned
* @param selected {@code true} if the position is selected, {@code false}
* otherwise
* @return the view that was added
*/
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
boolean selected) {
if (!mDataChanged) {
// Try to use an existing view for this position.
final View activeView = mRecycler.getActiveView(position);
if (activeView != null) {
// Found it. We're reusing an existing child, so it just needs // to be positioned like a scrap view. setupChild(activeView, position, y, flow, childrenLeft, selected, true); return activeView; } } // Make a new view for this position, or convert an unused view if // possible. final View child = obtainView(position, mIsScrap); // This needs to be positioned and measured. setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]); return child; }複製代碼
首先看mDataChanged，前面getchildview是0，因此mDataChanged爲FALSE，而後從ActiveView數組中拿出來一個view。可是數組爲空，因此確定view爲空，因此跳出if，執行obtainView。
View obtainView(int position, boolean[] isScrap) {
isScrap[0] = false;
View scrapView;
scrapView = mRecycler.getScrapView(position);
View child;
if (scrapView != null) {
child = mAdapter.getView(position, scrapView, this);
if (child != scrapView) {
mRecycler.addScrapView(scrapView);
if (mCacheColorHint != 0) {
child.setDrawingCacheBackgroundColor(mCacheColorHint);
}
} else {
isScrap[0] = true;
dispatchFinishTemporaryDetach(child);
}
} else {
child = mAdapter.getView(position, null, this);
if (mCacheColorHint != 0) {
child.setDrawingCacheBackgroundColor(mCacheColorHint);
}
}
return child;
}複製代碼
能夠看得出來，這裏確定會返回一個child，一個view。這個view怎麼來的呢，先看源碼。首先嚐試從scrapView數組中獲取一個view，可是咱們是第一次繪製listview因此確定失敗了。而後從else看，child = mAdapter.getView(position, null, this);從mAdapter中獲取一個view。就是前文提到的mAdapter中的getview了。這個時候ListView和adapter聯繫上了。
如今咱們去adapter中看getview方法。
* Get a View that displays the data at the specified position in the data set. You can either
* create a View manually or inflate it from an XML layout file. When the View is inflated, the
* parent View (GridView, ListView...) will apply default layout parameters unless you use
* {@link android.view.LayoutInflater#inflate(int, android.view.ViewGroup, boolean)}
* to specify a root view and to prevent attachment to the root.
*
* @param position The position of the item within the adapter's data set of the item whose view * we want. * @param convertView The old view to reuse, if possible. Note: You should check that this view * is non-null and of an appropriate type before using. If it is not possible to convert * this view to display the correct data, this method can create a new view. * Heterogeneous lists can specify their number of view types, so that this View is * always of the right type (see {@link #getViewTypeCount()} and * {@link #getItemViewType(int)}). * @param parent The parent that this view will eventually be attached to * @return A View corresponding to the data at the specified position. */ View getView(int position, View convertView, ViewGroup parent);複製代碼
經過註釋咱們能夠看到getview在GridView, ListView...等地方都會用到，在LayoutInflater#inflate中實現,那麼咱們看getview()的三個參數，position view viewgroup。這裏咱們第一次執行getview，傳入的參數爲geview(position,null,this),主要說view爲null的狀況，咱們會調LayoutInflater.inflate()。getview返回，固然obtainView也返回了一個child。這就是爲何第一次加載頁面卡的緣由，每個view都須要經過LayoutInflater.inflate()調用出來。
而後obtainView返回了一個view，makeAndAddView的child那麼也有view了，而後執行setupChild()方法。源碼上面都有，可仔細看一下。若是不方便我爭取作一個思惟導圖。
* Adds a view as a child and make sure it is measured (if necessary) and
* positioned properly.
*
* @param child the view to add
* @param position the position of this child
* @param y the y position relative to which this view will be positioned
* @param flowDown {@code true} to align top edge to y, {@code false} to
* align bottom edge to y
* @param childrenLeft left edge where children should be positioned
* @param selected {@code true} if the position is selected, {@code false}
* otherwise
* @param isAttachedToWindow {@code true} if the view is already attached
* to the window, e.g. whether it was reused, or
* {@code false} otherwise
*/
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
boolean selected, boolean isAttachedToWindow) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem");
final boolean isSelected = selected && shouldShowSelector();
final boolean updateChildSelected = isSelected != child.isSelected();
final int mode = mTouchMode;
final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL
&& mMotionPosition == position;
final boolean updateChildPressed = isPressed != child.isPressed();
final boolean needToMeasure = !isAttachedToWindow || updateChildSelected
|| child.isLayoutRequested();
// Respect layout params that are already in the view. Otherwise make
// some up...
AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
if (p == null) {
p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
}
p.viewType = mAdapter.getItemViewType(position);
p.isEnabled = mAdapter.isEnabled(position);
// Set up view state before attaching the view, since we may need to
// rely on the jumpDrawablesToCurrentState() call that occurs as part
// of view attachment.
if (updateChildSelected) {
child.setSelected(isSelected);
}
if (updateChildPressed) {
child.setPressed(isPressed);
}
if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) {
if (child instanceof Checkable) {
((Checkable) child).setChecked(mCheckStates.get(position));
} else if (getContext().getApplicationInfo().targetSdkVersion
>= android.os.Build.VERSION_CODES.HONEYCOMB) {
child.setActivated(mCheckStates.get(position));
}
}
if ((isAttachedToWindow && !p.forceAdd) || (p.recycledHeaderFooter
&& p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
attachViewToParent(child, flowDown ? -1 : 0, p);
// If the view was previously attached for a different position,
// then manually jump the drawables.
if (isAttachedToWindow
&& (((AbsListView.LayoutParams) child.getLayoutParams()).scrappedFromPosition)
!= position) {
child.jumpDrawablesToCurrentState();
}
} else {
p.forceAdd = false;
if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
p.recycledHeaderFooter = true;
}
addViewInLayout(child, flowDown ? -1 : 0, p, true);
// add view in layout will reset the RTL properties. We have to re-resolve them
child.resolveRtlPropertiesIfNeeded();
}
if (needToMeasure) {
final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
mListPadding.left + mListPadding.right, p.width);
final int lpHeight = p.height;
final int childHeightSpec;
if (lpHeight > 0) {
childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
} else {
childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(),
MeasureSpec.UNSPECIFIED);
}
child.measure(childWidthSpec, childHeightSpec);
} else {
cleanupLayoutState(child);
}
final int w = child.getMeasuredWidth();
final int h = child.getMeasuredHeight();
final int childTop = flowDown ? y : y - h;
if (needToMeasure) {
final int childRight = childrenLeft + w;
final int childBottom = childTop + h;
child.layout(childrenLeft, childTop, childRight, childBottom);
} else {
child.offsetLeftAndRight(childrenLeft - child.getLeft());
child.offsetTopAndBottom(childTop - child.getTop());
}
if (mCachingStarted && !child.isDrawingCacheEnabled()) {
child.setDrawingCacheEnabled(true);
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}複製代碼
老樣子，先看註釋。做爲一個子view添加確保這個view已經被測量過以及位置正確。代碼比較多，咱們看關鍵的。既然要添加view，因此咱們看咱們傳進去的子view，何時被調用。attachViewToParent()和addViewInLayout()。這兩個viewgroupo的方法有調用，好咱們具體來看一下。主要是一個判斷Header和Footer，還記得前面的代碼註釋麼？第一次layout不添加Header和Footer，因此咱們先執行addViewInLayout()，把view添加到listliew中，fillDown()，更據while的魂環咱們可知，不管多少數據量，只會填充第一屏的數據。到此爲止，第一次layout執行結束。
- 第二、3次onlayout
爲何要把第二次和第三次放在一塊兒呢，這個我卻是沒試過，可是前人的經驗和結果告訴我，view中都會經歷至少兩次onMeasure()和兩次onLayout()的過程。對於其餘的view還行，可是對於listview這個可就是個問題了，listview若是經歷了兩次onMeasure()和onLayout()，這樣的話就會有重複數據。
那麼問題來了，如何保證不添加劇複數據的呢。在第一次layout中的layoutchild()，第19行，geichild()，由於添加了子view，因此不爲0，這樣在第90行，第一次layout的時候由於當時沒有view，代碼不起做用。（忘了能夠回看第一次layout）如今有了子view，因此就會被添加到mActiveViews數組中，而後關鍵來了！官方也爲此添加了一份註釋。
Clear out old views detachAllViewsFromParent();
他就是反覆屢次onMeasure()和onLayout不會有重複數據的緣由。由於，咱們在清除前以及作了一次view的緩存因此，並不會有屢次加載浪費資源的狀況出現。而後繼續走141行if (childCount == 0) （沒找到怎麼加行數=。=尷尬），
由於有子view了，因此childCount不等於0，而後走else，就不是filldown了，而是fillSpecific()，咱們進入這個方法。
/**
* Put a specific item at a specific location on the screen and then build
* up and down from there.
*
* @param position The reference view to use as the starting point
* @param top Pixel offset from the top of this view to the top of the
* reference view.
*
* @return The selected view, or null if the selected view is outside the
* visible area.
*/
private View fillSpecific(int position, int top) {
boolean tempIsSelected = position == mSelectedPosition;
View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected);
// Possibly changed again in fillUp if we add rows above this one.
mFirstPosition = position;
View above;
View below;
final int dividerHeight = mDividerHeight;
if (!mStackFromBottom) {
above = fillUp(position - 1, temp.getTop() - dividerHeight);
// This will correct for the top of the first view not touching the top of the list
adjustViewsUpOrDown();
below = fillDown(position + 1, temp.getBottom() + dividerHeight);
int childCount = getChildCount();
if (childCount > 0) {
correctTooHigh(childCount);
}
} else {
below = fillDown(position + 1, temp.getBottom() + dividerHeight);
// This will correct for the bottom of the last view not touching the bottom of the list
adjustViewsUpOrDown();
above = fillUp(position - 1, temp.getTop() - dividerHeight);
int childCount = getChildCount();
if (childCount > 0) {
correctTooLow(childCount);
}
}
if (tempIsSelected) {
return temp;
} else if (above != null) {
return above;
} else {
return below;
}
}複製代碼
老規矩，咱們先看一下官方對他的註解：添加一個特殊的item在屏幕中特殊的位置，而後在從他的上或者下添item。而後咱們一層層看，看到了個老朋友makeAndAddView();這個確定要進去看一下走什麼邏輯了。
/**
* Obtains the view and adds it to our list of children. The view can be
* made fresh, converted from an unused view, or used as is if it was in
* the recycle bin.
*
* @param position logical position in the list
* @param y top or bottom edge of the view to add
* @param flow {@code true} to align top edge to y, {@code false} to align
* bottom edge to y
* @param childrenLeft left edge where children should be positioned
* @param selected {@code true} if the position is selected, {@code false}
* otherwise
* @return the view that was added
*/
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
boolean selected) {
if (!mDataChanged) {
// Try to use an existing view for this position.
final View activeView = mRecycler.getActiveView(position);
if (activeView != null) {
// Found it. We're reusing an existing child, so it just needs // to be positioned like a scrap view. setupChild(activeView, position, y, flow, childrenLeft, selected, true); return activeView; } } // Make a new view for this position, or convert an unused view if // possible. final View child = obtainView(position, mIsScrap); // This needs to be positioned and measured. setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]); return child; }複製代碼
又是在19行，仍是同樣的方法，獲取getActiveView(),這下確定沒問題了，進入if執行setupChild(),而後就直接return了。進去setupChild()，注意，這裏的setupchild()，最後一個參數是true。
/**
* Adds a view as a child and make sure it is measured (if necessary) and
* positioned properly.
*
* @param child the view to add
* @param position the position of this child
* @param y the y position relative to which this view will be positioned
* @param flowDown {@code true} to align top edge to y, {@code false} to
* align bottom edge to y
* @param childrenLeft left edge where children should be positioned
* @param selected {@code true} if the position is selected, {@code false}
* otherwise
* @param isAttachedToWindow {@code true} if the view is already attached
* to the window, e.g. whether it was reused, or
* {@code false} otherwise
*/
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
boolean selected, boolean isAttachedToWindow) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem");
final boolean isSelected = selected && shouldShowSelector();
final boolean updateChildSelected = isSelected != child.isSelected();
final int mode = mTouchMode;
final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL
&& mMotionPosition == position;
final boolean updateChildPressed = isPressed != child.isPressed();
final boolean needToMeasure = !isAttachedToWindow || updateChildSelected
|| child.isLayoutRequested();
// Respect layout params that are already in the view. Otherwise make
// some up...
AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
if (p == null) {
p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
}
p.viewType = mAdapter.getItemViewType(position);
p.isEnabled = mAdapter.isEnabled(position);
// Set up view state before attaching the view, since we may need to
// rely on the jumpDrawablesToCurrentState() call that occurs as part
// of view attachment.
if (updateChildSelected) {
child.setSelected(isSelected);
}
if (updateChildPressed) {
child.setPressed(isPressed);
}
if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) {
if (child instanceof Checkable) {
((Checkable) child).setChecked(mCheckStates.get(position));
} else if (getContext().getApplicationInfo().targetSdkVersion
>= android.os.Build.VERSION_CODES.HONEYCOMB) {
child.setActivated(mCheckStates.get(position));
}
}
if ((isAttachedToWindow && !p.forceAdd) || (p.recycledHeaderFooter
&& p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
attachViewToParent(child, flowDown ? -1 : 0, p);
// If the view was previously attached for a different position,
// then manually jump the drawables.
if (isAttachedToWindow
&& (((AbsListView.LayoutParams) child.getLayoutParams()).scrappedFromPosition)
!= position) {
child.jumpDrawablesToCurrentState();
}
} else {
p.forceAdd = false;
if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
p.recycledHeaderFooter = true;
}
addViewInLayout(child, flowDown ? -1 : 0, p, true);
// add view in layout will reset the RTL properties. We have to re-resolve them
child.resolveRtlPropertiesIfNeeded();
}
if (needToMeasure) {
final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
mListPadding.left + mListPadding.right, p.width);
final int lpHeight = p.height;
final int childHeightSpec;
if (lpHeight > 0) {
childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
} else {
childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(),
MeasureSpec.UNSPECIFIED);
}
child.measure(childWidthSpec, childHeightSpec);
} else {
cleanupLayoutState(child);
}
final int w = child.getMeasuredWidth();
final int h = child.getMeasuredHeight();
final int childTop = flowDown ? y : y - h;
if (needToMeasure) {
final int childRight = childrenLeft + w;
final int childBottom = childTop + h;
child.layout(childrenLeft, childTop, childRight, childBottom);
} else {
child.offsetLeftAndRight(childrenLeft - child.getLeft());
child.offsetTopAndBottom(childTop - child.getTop());
}
if (mCachingStarted && !child.isDrawingCacheEnabled()) {
child.setDrawingCacheEnabled(true);
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}複製代碼
最後一個boolean值是isAttachedToWindow，不同的地方我們打高亮看，看到了if，咱們進去，由於是TRUE，因此執行attachViewToParent();由於，咱們第二次layout的view是被清除掉的，狀態爲death的view，因此咱們使用attachViewToParent()方法，爲何第一次layout使用addViewInLayout呢？
是由於viewgroup中建立view使用addViewInLayout，重用也就是狀態爲death的view用attachViewToParent()方法。再深層次就不細究了。等之後有時間再說吧。。。。。這裏兩次onlayout，才能夠說Listview真正的展示出來了。
4、滑動LIsetView
由於我是作機頂盒TV的因此我想探究一下用按鍵事件ListView的數據是怎麼變化的。這塊都是本身找的源碼，有不對請指正！
1、首先肯定監聽器，按鍵事件是onkey和onfocus。首先在AbsListView中查看。
@Override
protected void onFocusChanged(boolean gainFocus, int direction, Rect previouslyFocusedRect) {
super.onFocusChanged(gainFocus, direction, previouslyFocusedRect);
if (gainFocus && mSelectedPosition < 0 && !isInTouchMode()) {
if (!isAttachedToWindow() && mAdapter != null) {
// Data may have changed while we were detached and it's valid // to change focus while detached. Refresh so we don't die.
mDataChanged = true;
mOldItemCount = mItemCount;
mItemCount = mAdapter.getCount();
}
resurrectSelection();
}
}
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
return false;
}
@Override
public boolean onKeyUp(int keyCode, KeyEvent event) {
if (KeyEvent.isConfirmKey(keyCode)) {
if (!isEnabled()) {
return true;
}
if (isClickable() && isPressed() &&
mSelectedPosition >= 0 && mAdapter != null &&
mSelectedPosition < mAdapter.getCount()) {
final View view = getChildAt(mSelectedPosition - mFirstPosition);
if (view != null) {
performItemClick(view, mSelectedPosition, mSelectedRowId);
view.setPressed(false);
}
setPressed(false);
return true;
}
}
return super.onKeyUp(keyCode, event);
}複製代碼
然而並無什麼發現，而後去ListView中尋找。
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
return commonKey(keyCode, 1, event);
}
@Override
public boolean onKeyMultiple(int keyCode, int repeatCount, KeyEvent event) {
return commonKey(keyCode, repeatCount, event);
}
@Override
public boolean onKeyUp(int keyCode, KeyEvent event) {
return commonKey(keyCode, 1, event);
}
private boolean commonKey(int keyCode, int count, KeyEvent event) {
if (mAdapter == null || !isAttachedToWindow()) {
return false;
}
if (mDataChanged) {
layoutChildren();
}
boolean handled = false;
int action = event.getAction();
if (KeyEvent.isConfirmKey(keyCode)
&& event.hasNoModifiers() && action != KeyEvent.ACTION_UP) {
handled = resurrectSelectionIfNeeded();
if (!handled && event.getRepeatCount() == 0 && getChildCount() > 0) {
keyPressed();
handled = true;
}
}
if (!handled && action != KeyEvent.ACTION_UP) {
switch (keyCode) {
case KeyEvent.KEYCODE_DPAD_UP:
if (event.hasNoModifiers()) {
handled = resurrectSelectionIfNeeded();
if (!handled) {
while (count-- > 0) {
if (arrowScroll(FOCUS_UP)) {
handled = true;
} else {
break;
}
}
}
} else if (event.hasModifiers(KeyEvent.META_ALT_ON)) {
handled = resurrectSelectionIfNeeded() || fullScroll(FOCUS_UP);
}
break;
case KeyEvent.KEYCODE_DPAD_DOWN:
if (event.hasNoModifiers()) {
handled = resurrectSelectionIfNeeded();
if (!handled) {
while (count-- > 0) {
if (arrowScroll(FOCUS_DOWN)) {
handled = true;
} else {
break;
}
}
}
} else if (event.hasModifiers(KeyEvent.META_ALT_ON)) {
handled = resurrectSelectionIfNeeded() || fullScroll(FOCUS_DOWN);
}
break;
case KeyEvent.KEYCODE_DPAD_LEFT:
if (event.hasNoModifiers()) {
handled = handleHorizontalFocusWithinListItem(View.FOCUS_LEFT);
}
break;
case KeyEvent.KEYCODE_DPAD_RIGHT:
if (event.hasNoModifiers()) {
handled = handleHorizontalFocusWithinListItem(View.FOCUS_RIGHT);
}
break;
case KeyEvent.KEYCODE_PAGE_UP:
if (event.hasNoModifiers()) {
handled = resurrectSelectionIfNeeded() || pageScroll(FOCUS_UP);
} else if (event.hasModifiers(KeyEvent.META_ALT_ON)) {
handled = resurrectSelectionIfNeeded() || fullScroll(FOCUS_UP);
}
break;
case KeyEvent.KEYCODE_PAGE_DOWN:
if (event.hasNoModifiers()) {
handled = resurrectSelectionIfNeeded() || pageScroll(FOCUS_DOWN);
} else if (event.hasModifiers(KeyEvent.META_ALT_ON)) {
handled = resurrectSelectionIfNeeded() || fullScroll(FOCUS_DOWN);
}
break;
case KeyEvent.KEYCODE_MOVE_HOME:
if (event.hasNoModifiers()) {
handled = resurrectSelectionIfNeeded() || fullScroll(FOCUS_UP);
}
break;
case KeyEvent.KEYCODE_MOVE_END:
if (event.hasNoModifiers()) {
handled = resurrectSelectionIfNeeded() || fullScroll(FOCUS_DOWN);
}
break;
case KeyEvent.KEYCODE_TAB:
// This creates an asymmetry in TAB navigation order. At some
// point in the future we may decide that it's preferable to // force the list selection to the top or bottom when receiving // TAB focus from another widget, but for now this is adequate. if (event.hasNoModifiers()) { handled = resurrectSelectionIfNeeded() || arrowScroll(FOCUS_DOWN); } else if (event.hasModifiers(KeyEvent.META_SHIFT_ON)) { handled = resurrectSelectionIfNeeded() || arrowScroll(FOCUS_UP); } break; } } if (handled) { return true; } if (sendToTextFilter(keyCode, count, event)) { return true; } switch (action) { case KeyEvent.ACTION_DOWN: return super.onKeyDown(keyCode, event); case KeyEvent.ACTION_UP: return super.onKeyUp(keyCode, event); case KeyEvent.ACTION_MULTIPLE: return super.onKeyMultiple(keyCode, count, event); default: // shouldn't happen
return false;
}
}複製代碼
在ListView中onkey事件被分紅了3種事件：onKeyDown、onKeyMultiple、onKeyUp。都是調用commonKey()方法，結果沒有什麼發現。咱們再看onfocus()事件。
@Override
protected void onFocusChanged(boolean gainFocus, int direction, Rect previouslyFocusedRect) {
super.onFocusChanged(gainFocus, direction, previouslyFocusedRect);
final ListAdapter adapter = mAdapter;
int closetChildIndex = -1;
int closestChildTop = 0;
if (adapter != null && gainFocus && previouslyFocusedRect != null) {
previouslyFocusedRect.offset(mScrollX, mScrollY);
// Don't cache the result of getChildCount or mFirstPosition here, // it could change in layoutChildren. if (adapter.getCount() < getChildCount() + mFirstPosition) { mLayoutMode = LAYOUT_NORMAL; layoutChildren(); } // figure out which item should be selected based on previously // focused rect Rect otherRect = mTempRect; int minDistance = Integer.MAX_VALUE; final int childCount = getChildCount(); final int firstPosition = mFirstPosition; for (int i = 0; i < childCount; i++) { // only consider selectable views if (!adapter.isEnabled(firstPosition + i)) { continue; } View other = getChildAt(i); other.getDrawingRect(otherRect); offsetDescendantRectToMyCoords(other, otherRect); int distance = getDistance(previouslyFocusedRect, otherRect, direction); if (distance < minDistance) { minDistance = distance; closetChildIndex = i; closestChildTop = other.getTop(); } } } if (closetChildIndex >= 0) { setSelectionFromTop(closetChildIndex + mFirstPosition, closestChildTop); } else { requestLayout(); } }複製代碼
代碼量不多，古話說得好，有備註就有但願，結果然被我發現了。
// Don't cache the result of getChildCount or mFirstPosition here, // it could change in layoutChildren. if (adapter.getCount() < getChildCount() + mFirstPosition) { mLayoutMode = LAYOUT_NORMAL; layoutChildren(); }複製代碼
備註是不要緩存這裏得到子節點個數或mfirstposition結果，它能夠改變layoutchildren。
這裏的if判斷就是若是adapter.getCount()小於getChildCount() + mFirstPosition。
意思就是在最後一個item變成倒數第二個item了，這個時候就要新增一個item，而且回收。咱們繼續往裏面看，前面也有說mLayoutMode的默認方式是LAYOUT_NORMAL，而後進入 layoutChildren()方法。
又回到熟悉的方法了。老樣子前面的就不說了，不過要變得是datachange了，因此咱們要把最後一個item前面的全部item加入scrpview數組，而後直接去關鍵點，fillSpecific()方法中makeAndAddView()，返回一個view就是新增的item，而後往下走。在第二次layout沒有往下走是由於咱們新增的是第一個item，因此後面的無所謂，可是如今不同了。
if (!mStackFromBottom) {
above = fillUp(position - 1, temp.getTop() - dividerHeight);
// This will correct for the top of the first view not touching the top of the list
adjustViewsUpOrDown();
below = fillDown(position + 1, temp.getBottom() + dividerHeight);
int childCount = getChildCount();
if (childCount > 0) {
correctTooHigh(childCount);
}
} else {
below = fillDown(position + 1, temp.getBottom() + dividerHeight);
// This will correct for the bottom of the last view not touching the bottom of the list
adjustViewsUpOrDown();
above = fillUp(position - 1, temp.getTop() - dividerHeight);
int childCount = getChildCount();
if (childCount > 0) {
correctTooLow(childCount);
}
}複製代碼
之前都是走上面，如今是走else，先filldown，加載下一個數據，而後關鍵（註釋）來了adjustViewsUpOrDown()，這將更正最後一個視圖的底部，而不是在列表的底部。意思是將屏幕外的第一個item加載到視圖底部，而後再用fillup()加載如今最後一個item上面的其他item。
fillup()和filldown()都是經過循壞去對listview進行加載，可是涉及到加載就會使用makeAndAddView()方法。此次咱們再看一下流程，具體能夠參考第一次layout。到了obtainView()這個方法的時候就不同了。咱們看一下源碼。
/**
* Gets a view and have it show the data associated with the specified
* position. This is called when we have already discovered that the view
* is not available for reuse in the recycle bin. The only choices left are
* converting an old view or making a new one.
*
* @param position the position to display
* @param outMetadata an array of at least 1 boolean where the first entry
* will be set {@code true} if the view is currently
* attached to the window, {@code false} otherwise (e.g.
* newly-inflated or remained scrap for multiple layout
* passes)
*
* @return A view displaying the data associated with the specified position
*/
View obtainView(int position, boolean[] outMetadata) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");
outMetadata[0] = false;
// Check whether we have a transient state view. Attempt to re-bind the
// data and discard the view if we fail.
final View transientView = mRecycler.getTransientStateView(position);
if (transientView != null) {
final LayoutParams params = (LayoutParams) transientView.getLayoutParams();
// If the view type hasn't changed, attempt to re-bind the data. if (params.viewType == mAdapter.getItemViewType(position)) { final View updatedView = mAdapter.getView(position, transientView, this); // If we failed to re-bind the data, scrap the obtained view. if (updatedView != transientView) { setItemViewLayoutParams(updatedView, position); mRecycler.addScrapView(updatedView, position); } } outMetadata[0] = true; // Finish the temporary detach started in addScrapView(). transientView.dispatchFinishTemporaryDetach(); return transientView; } final View scrapView = mRecycler.getScrapView(position); final View child = mAdapter.getView(position, scrapView, this); if (scrapView != null) { if (child != scrapView) { // Failed to re-bind the data, return scrap to the heap. mRecycler.addScrapView(scrapView, position); } else if (child.isTemporarilyDetached()) { outMetadata[0] = true; // Finish the temporary detach started in addScrapView(). child.dispatchFinishTemporaryDetach(); } } if (mCacheColorHint != 0) { child.setDrawingCacheBackgroundColor(mCacheColorHint); } if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) { child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES); } setItemViewLayoutParams(child, position); if (AccessibilityManager.getInstance(mContext).isEnabled()) { if (mAccessibilityDelegate == null) { mAccessibilityDelegate = new ListItemAccessibilityDelegate(); } if (child.getAccessibilityDelegate() == null) { child.setAccessibilityDelegate(mAccessibilityDelegate); } } Trace.traceEnd(Trace.TRACE_TAG_VIEW); return child; }複製代碼
scrapView，這裏是第一次layout沒有調到的地方，此次scrapView數組裏面是有值了。咱們一旦有新的數據須要顯示，就會從scrapView數組中取出。因此關於LIstview，無論有多少條數據，永遠只有那麼幾個，這個就是OOM不會出現的緣由。
這個是圖片總結，從康志大大的博客上取來的，偷了點懶
5、總結
這篇博客從準備到學習花了有快1個星期的事件，都是下班時間一點點看，一點點學，才畢業沒多久不想讓本身荒廢。後續我還會學習其餘的源碼，還有一些實踐app但願與你們分享。在這個知識共享的年代一塊兒進步學習！
不是打廣告，是我下一步的學習目標。
HenCoder Android 開發進階: 自定義 View隨便看了看，真的受益不淺。
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