推薦閱讀 java
（一）RecycleView 初探回收複用，onCreateView和onBindView調用關係ide
（二）Android RecycleView實現吸附小標題的Demo（附源碼）佈局
（三）RecycleView 自定義下拉刷新，上拉加載監聽學習
（四）RecycleView 滑動到置頂、Adapter局部刷新ui
（五）RecycleView 動態設置改變列表顯示的高度this
前言
RecycleView 是一個可回收複用的列表控件，也是使用較廣泛的。在使用時也會結合業務功能需求作出一些改變。好比兩個Recycleview之間有交互，又或者嵌套滑動處理，又或者高度動態設置。本篇正是關於如何動態改變列表的高度。spa
先看效果圖：.net
1、RecycleView測量原理
RecyclerView.onMeasure() 方法源碼，測量順序以下：code
protected void onMeasure(int widthSpec, int heightSpec) {
if (mLayout == null) {
defaultOnMeasure(widthSpec, heightSpec);
return;
}
// 一、是否進入 自動測量自身尺寸
if (mLayout.mAutoMeasure) {
final int widthMode = MeasureSpec.getMode(widthSpec);
final int heightMode = MeasureSpec.getMode(heightSpec);
final boolean skipMeasure = widthMode == MeasureSpec.EXACTLY
&& heightMode == MeasureSpec.EXACTLY;
mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
if (skipMeasure || mAdapter == null) {
return;
}
if (mState.mLayoutStep == State.STEP_START) {
dispatchLayoutStep1();
}
mLayout.setMeasureSpecs(widthSpec, heightSpec);
mState.mIsMeasuring = true;
dispatchLayoutStep2();
// 關鍵：經過測量孩子view寬高來肯定自身尺寸
mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
if (mLayout.shouldMeasureTwice()) {
mLayout.setMeasureSpecs(
MeasureSpec.makeMeasureSpec(getMeasuredWidth(), MeasureSpec.EXACTLY),
MeasureSpec.makeMeasureSpec(getMeasuredHeight(), MeasureSpec.EXACTLY));
mState.mIsMeasuring = true;
dispatchLayoutStep2();.
mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
}
} else {
//二、若是是固定大小，執行會和上面效果同樣
if (mHasFixedSize) {
mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
return;
}
// 定製測量
if (mAdapterUpdateDuringMeasure) {
eatRequestLayout();
processAdapterUpdatesAndSetAnimationFlags();
if (mState.mRunPredictiveAnimations) {
mState.mInPreLayout = true;
} else {
// 使用剩餘的更新來提供與佈局傳遞一致的狀態。
mAdapterHelper.consumeUpdatesInOnePass();
mState.mInPreLayout = false;
}
mAdapterUpdateDuringMeasure = false;
resumeRequestLayout(false);
}
if (mAdapter != null) {
mState.mItemCount = mAdapter.getItemCount();
} else {
mState.mItemCount = 0;
}
eatRequestLayout();
mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
resumeRequestLayout(false);
mState.mInPreLayout = false; // 清除
}
}
有兩個判斷比較顯眼：mAutoMeasure、mHasFixedSize。這倆都會讓RecycleView自動測量所有孩子的高度，從而能肯定自身尺寸MeasuredDimension大小。對象
2、實現方案1：經過重寫onMeasure
經過重寫 LayoutManage的onMeasure()方法，獲取到RecycleView的一個item的viewholder對象實例，若是這個item實例對象存在，就進行測量item的大小，拿到確切的高度Height值後，就能夠動態設置Recycleview顯示多少個item的高度了。
須要注意，item的佈局最好提早設定固定的高度，不然獲取爲0。
記得要設置mAutoMeasure、mHasFixedSize值爲false，不設置可能會報錯。
LinearLayoutManager mLayoutManager = new LinearLayoutManager(this){
@Override
public void onMeasure(RecyclerView.Recycler recycler, RecyclerView.State state, int widthSpec, int heightSpec) {
View view = recycler.getViewForPosition(0);
if (view != null) {
measureChild(view, widthSpec, heightSpec);
int measuredHeight = view.getMeasuredHeight();
//int measuredWidth = View.MeasureSpec.getSize(widthSpec);
int showHeight = measuredHeight * state.getItemCount();
if(state.getItemCount() >= 5){
showHeight = measuredHeight * 5;
}
setMeasuredDimension(widthSpec, showHeight);
}
}
};
mLayoutManager.setAutoMeasureEnabled(false);
mRecyclerview.setHasFixedSize(false);
mRecyclerview.setLayoutManager(mLayoutManager);
3、實現方案2：經過修改LayoutParams（推薦）
經過adapter傳入不一樣的viewType拿到ViewHolder對象，對這個ViewHolder進行測量，而後獲得測量後的高度值。最後，就能夠根據item調整設置列表的佈局參數的高度。
須要注意，在NestedScrollView嵌套RecycleView時，在RecycleView徹底展現時(即按itemCount總數)，RecycleView仍然會有上下可滑動的小空間，雖然只是一點點，也是會影響用戶體驗。所以，須要在徹底展開時，將它設置禁止滑動。
boolean isOpen ; //記錄展開、收起狀態
private boolean setFitHeight(RecyclerView recyclerView){
RecyclerView.Adapter adapter = recyclerView.getAdapter();
int itemCount = adapter.getItemCount();
int measuredHeight = 0;
if (itemCount >0){
RecyclerView.ViewHolder holder = adapter.createViewHolder(recyclerView, adapter
.getItemViewType(0));//經過viewType類型返回ViewHolder
adapter.onBindViewHolder(holder, 0);
holder.itemView.measure(
View.MeasureSpec.makeMeasureSpec(recyclerView.getWidth(), View.MeasureSpec.EXACTLY),
View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED));
holder.itemView.layout(0, 0, holder.itemView.getMeasuredWidth(), holder.itemView.getMeasuredHeight());
holder.itemView.setDrawingCacheEnabled(true);
holder.itemView.buildDrawingCache();
measuredHeight = holder.itemView.getMeasuredHeight();
}
if (isOpen){
LinearLayout.LayoutParams layoutParams = new LinearLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
measuredHeight * 3);
recyclerView.setLayoutParams(layoutParams);
recyclerView.setNestedScrollingEnabled(true);//容許滑動
return isOpen = false;
}else{
LinearLayout.LayoutParams layoutParams = new LinearLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
measuredHeight * itemCount);
recyclerView.setLayoutParams(layoutParams);
recyclerView.setNestedScrollingEnabled(false);//禁止滑動
return isOpen = true;
}
}
4、總結
上面兩種實現方式，都離開View的測量，所以建議你們多深刻學習自定義View流程mesure\layout\draw源碼。
第一種方案代碼簡單，使用方便，但擴展性和靈活性不強。適用於該頁面靜態顯示高度，不動態改變。
第二種方案更值得推薦。由於咱們的RecycleView的item會有不一樣風格大小的時候，它能夠經過viewType獲得每一種item高度，從而設置固定高度。另外，RecycleView的佈局參數LayoutParams的值改變即響應。
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