淺析NestedScrolling嵌套滑動機制之CoordinatorLayout.Behavior
嵌套系列導航
- 1.淺析NestedScrolling嵌套滑動機制之基礎篇
- 2.淺析NestedScrolling嵌套滑動機制之實踐篇-仿寫餓了麼商家詳情頁
- 3.淺析NestedScrolling嵌套滑動機制之CoordinatorLayout.Behavior
- 4.淺析NestedScrolling嵌套滑動機制之實踐篇-自定義Behavior實現小米音樂歌手詳情
本文已在公衆號鴻洋原創發佈。未經許可，不得以任何形式轉載！java
概述
在前面《淺析NestedScrolling嵌套滑動機制之基礎篇》裏的常見效果提到Behavior也是走NestedScrolling機制來實現各類神奇的滑動效果，它伴隨CoordinatorLayout在Revision 24.1.0的android.support.v4兼容包被引入，和CoordinatorLayout結合實現各個控件聯動，能夠攔截代理CoordinatorLayout的測量、佈局、WindowInsets、觸摸事件、嵌套滑動。node
Behavior簡介
Behavior是做用於 CoordinatorLayout的直接子View 的交互行爲插件。一個Behavior 實現了用戶的一個或者多個交互行爲，它們可能包括拖拽、滑動、快滑或者其餘一些手勢。android
/** * 泛型<V>是Behavior關聯的View */
public static abstract class Behavior<V extends View> {
/** * 默認構造方法,用於註解的方式建立或者在代碼中建立 */
public Behavior() {}
/** * 用於xml解析layout_Behavior屬性的構造方法,若是須要Behavior支持在xml中使用,則必須有此構造方法 */
public Behavior(Context context, AttributeSet attrs) {}
/** * 在LayoutParams實例化後調用,或者在調用了LayoutParams.setBehavior(behavior)時調用. */
public void onAttachedToLayoutParams(@NonNull CoordinatorLayout.LayoutParams params) {}
/** * 同上面onAttachedToLayoutParams相反 * 當LayoutParams移除Behavior時調用,例如調用了LayoutParams.setBehavior(null). * View被從View Tree中移除時不會調用此方法. */
public void onDetachedFromLayoutParams() {}
/** * 在CoordinatorLayout分發給子View前攔截Touch事件 */
public boolean onInterceptTouchEvent(CoordinatorLayout parent, V child, MotionEvent ev) {
return false;
}
/** * 在CoordinatorLayout分發給子View前消費Touch事件 */
public boolean onTouchEvent(CoordinatorLayout parent, V child, MotionEvent ev) {
return false;
}
/** * 阻斷此Behavior所關聯View下層的View的交互 */
public boolean blocksInteractionBelow(CoordinatorLayout parent, V child) {
return getScrimOpacity(parent, child) > 0.f;
}
/** * 當blocksInteractionBelow返回爲true時，CoordinatorLayout將會在View的上層繪製 * 一個屏蔽的getScrimColor()顏色來顯示沒法進行交互的區域 */
@ColorInt
public int getScrimColor(CoordinatorLayout parent, V child) {
return Color.BLACK;
}
/** * getScrimColor()繪製顏色的透明度 */
@FloatRange(from = 0, to = 1)
public float getScrimOpacity(CoordinatorLayout parent, V child){
return 0.f;
}
/** * 關聯的View和感興趣的View進行依賴 */
public boolean layoutDependsOn(CoordinatorLayout parent, V child, View dependency) {
return false;
}
/** * 依賴View的位置、大小改變時回調 */
public boolean onDependentViewChanged(CoordinatorLayout parent, V child, View dependency) {
return false;
}
/** * 依賴View從佈局移除時回調 */
public void onDependentViewRemoved(CoordinatorLayout parent, V child, View dependency) {}
/** * 代理CoordinatorLayout子View的測量，注意這個子View是關聯了當前Behavior, * 返回true表示使用Behavior的*onMeasureChild()來測量參數裏child的這個子View， * 返回false則使用*CoordinatorLayout的默認測量子View的方法。 */
public boolean onMeasureChild(CoordinatorLayout parent, V child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) {
return false;
}
/** * 代理CoordinatorLayout子View的佈局 * 返回true表示使用Behavior的onLayoutChild()來佈局子View * 返回false則使用CoordinatorLayout的默認測量子View的方法。 */
public boolean onLayoutChild(CoordinatorLayout parent, V child, int layoutDirection) {
return false;
}
/** *代理消費CoordinatorLayout的WindowInsets */
@NonNull
public WindowInsetsCompat onApplyWindowInsets(CoordinatorLayout coordinatorLayout, V child, WindowInsetsCompat insets) {
return insets;
}
//如下是NestedScrolling相關方法//
@Deprecated
public boolean onStartNestedScroll(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View directTargetChild, @NonNull View target, @ScrollAxis int axes) {
return false;
}
public boolean onStartNestedScroll(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View directTargetChild, @NonNull View target, @ScrollAxis int axes, @NestedScrollType int type) {
if (type == ViewCompat.TYPE_TOUCH) {
return onStartNestedScroll(coordinatorLayout, child, directTargetChild,
target, axes);
}
return false;
}
@Deprecated
public void onNestedScrollAccepted(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View directTargetChild, @NonNull View target, @ScrollAxis int axes) {
}
public void onNestedScrollAccepted(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View directTargetChild, @NonNull View target, @ScrollAxis int axes, @NestedScrollType int type) {
if (type == ViewCompat.TYPE_TOUCH) {
onNestedScrollAccepted(coordinatorLayout, child, directTargetChild,
target, axes);
}
}
@Deprecated
public void onStopNestedScroll(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View target) {
}
public void onStopNestedScroll(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View target, @NestedScrollType int type) {
if (type == ViewCompat.TYPE_TOUCH) {
onStopNestedScroll(coordinatorLayout, child, target);
}
}
@Deprecated
public void onNestedScroll(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed) {
}
public void onNestedScroll(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed, @NestedScrollType int type) {
if (type == ViewCompat.TYPE_TOUCH) {
onNestedScroll(coordinatorLayout, child, target, dxConsumed, dyConsumed,
dxUnconsumed, dyUnconsumed);
}
}
@Deprecated
public void onNestedPreScroll(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View target, int dx, int dy, @NonNull int[] consumed) {
}
public void onNestedPreScroll(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View target, int dx, int dy, @NonNull int[] consumed, @NestedScrollType int type) {
if (type == ViewCompat.TYPE_TOUCH) {
onNestedPreScroll(coordinatorLayout, child, target, dx, dy, consumed);
}
}
public boolean onNestedFling(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View target, float velocityX, float velocityY, boolean consumed) {
return false;
}
public boolean onNestedPreFling(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View target, float velocityX, float velocityY) {
return false;
}
//省略部分很是用方法
...
}
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View設置Behavior
xml佈局文件設置
<!-- 佈局文件 -->
<android.support.design.widget.CoordinatorLayout>
<android.support.v4.widget.NestedScrollView app:layout_behavior="@string/appbar_scrolling_view_behavior"/>
</android.support.design.widget.CoordinatorLayout>
<!-- values.xml -->
<string name="appbar_scrolling_view_behavior" translatable="false">
android.support.design.widget.AppBarLayout$ScrollingViewBehavior
</string>
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在佈局文件對CoordinatorLayout的直接子View添加app:layout_behavio屬性，屬性是Behavior類全限包名，你能夠把值放在values文件裏，也能夠直接寫在佈局文件裏。在CoordinatorLayout的parseBehavior()調用Behavior兩個參數的構造方法建立。app
代碼動態設置
AppBarLayout.ScrollingViewBehavior behavior = new AppBarLayout.ScrollingViewBehavior();
CoordinatorLayout.LayoutParams params =(CoordinatorLayout.LayoutParams) view.getLayoutParams();
params.setBehavior(behavior);
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註解方式
@CoordinatorLayout.DefaultBehavior(AppBarLayout.Behavior.class)
public class AppBarLayout extends LinearLayout {}
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注意若是同時使用註解和xml佈局文件爲同一個view設置Behavior，生效的是註解方式的Behavior,若在自定義Behavior使用此方式須要一個無參的構造函數，由於CoordinatorLayout在getResolvedLayoutParams()解析時調用反射Behavior的無參構造函數建立，而這種註解方式在support27.1.0版本打上了@Deprecated過期標籤。less
接口實現返回
View實現CoordinatorLayout.AttachedBehavior接口並複寫getBehavior()返回Behavior。在CoordinatorLayout在getResolvedLayoutParams()解析時調用getBehavior()獲取Behavior，而後調用CoordinatorLayout.LayoutParams.setBehavior()傳入。ide
public class MyLayout extends LinearLayout implements CoordinatorLayout.AttachedBehavior{
@NonNull
@Override
Behavior getBehavior(){
return new AppBarLayout.ScrollingViewBehavior()
};
}
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Behavior中的代理
代理CoordinatorLayout子View的測量
Behavior的onMeasureChild()能夠代理CoordinatorLayout子View的測量，注意這個子View是關聯了當前Behavior，它的返回值爲Boolean類型，返回true表示使用Behavior的onMeasureChild()來測量參數裏child的這個子View，返回false則使用CoordinatorLayout的默認測量子View的方法。函數
//CoordinatorLayout.Behavior
public boolean onMeasureChild(CoordinatorLayout parent, V child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) {
return false;
}
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在CoordinatorLayout的onMeasure()裏能夠看出Behavior中的代理子View的測量：佈局
//CoordinatorLayout
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
...
final int childCount = mDependencySortedChildren.size();
for (int i = 0; i < childCount; i++) {
final View child = mDependencySortedChildren.get(i);
...
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
...
//Behavior判空檢測是否能夠代理measure
final Behavior b = lp.getBehavior();
if (b == null || !b.onMeasureChild(this, child, childWidthMeasureSpec, keylineWidthUsed,
childHeightMeasureSpec, 0)) {
onMeasureChild(child, childWidthMeasureSpec, keylineWidthUsed,
childHeightMeasureSpec, 0);
}
...
}
}
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代理CoordinatorLayout子View的佈局
和上面相似，Behavior的onLayoutChild()能夠代理CoordinatorLayout子View的佈局，它的返回值爲Boolean類型，返回true表示使用Behavior的onLayoutChild()來佈局子View，返回false則使用CoordinatorLayout的默認測量子View的方法。post
//CoordinatorLayout.Behavior
public boolean onLayoutChild(CoordinatorLayout parent, V child, int layoutDirection) {
return false;
}
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在CoordinatorLayout的onLayout()裏能夠看出Behavior中的代理子View的佈局：動畫
//CoordinatorLayout
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
...
final int childCount = mDependencySortedChildren.size();
for (int i = 0; i < childCount; i++) {
final View child = mDependencySortedChildren.get(i);
...
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final Behavior behavior = lp.getBehavior();
//Behavior判空檢測是否能夠代理layout
if (behavior == null || !behavior.onLayoutChild(this, child, layoutDirection)) {
onLayoutChild(child, layoutDirection);
}
}
}
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代理CoordinatorLayout的WindowInsets
Behavior的onApplyWindowInsets()能夠代理消費CoordinatorLayout的WindowInsets。
//CoordinatorLayout.Behavior
public WindowInsetsCompat onApplyWindowInsets(CoordinatorLayout coordinatorLayout, V child, WindowInsetsCompat insets) {
return insets;
}
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在CoordinatorLayout的onLayout()裏能夠看出Behavior中的消費CoordinatorLayout的WindowInsets： setFitsSystemWindows()->setupForInsets()->setWindowInsets()->dispatchApplyWindowInsetsToBehaviors()
//CoordinatorLayout
private WindowInsetsCompat dispatchApplyWindowInsetsToBehaviors(WindowInsetsCompat insets) {
...
for (int i = 0, z = getChildCount(); i < z; i++) {
final View child = getChildAt(i);
if (ViewCompat.getFitsSystemWindows(child)) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final Behavior b = lp.getBehavior();
if (b != null) {
// If the view has a behavior, let it try first
insets = b.onApplyWindowInsets(this, child, insets);
if (insets.isConsumed()) {
// If it consumed the insets, break
break;
}
}
}
}
return insets;
}
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代理CoordinatorLayout的Touch事件
Behavior的onInterceptTouchEvent()、onTouchEvent()能夠在CoordinatorLayout分發給子View前被攔截消費，若Behavior攔截了來自CoordinatorLayout的Touch事件，CoordinatorLayout的各個子View天然就接受不到Touch事件，Behavior的blocksInteractionBelow()表示是否阻斷此Behavior所關聯View下層的View的交互，則這個方法能影響Touch事件的攔截，若blocksInteractionBelow()爲true時，getScrimOpacity()返回值大於0，CoordinatorLayout將會在View的上層繪製一個屏蔽的getScrimColor()顏色來顯示沒法進行交互的區域：
//CoordinatorLayout.Behavior
public boolean onInterceptTouchEvent(CoordinatorLayout parent, V child, MotionEvent ev) {
return false;
}
public boolean onTouchEvent(CoordinatorLayout parent, V child, MotionEvent ev) {
return false;
}
public boolean blocksInteractionBelow(CoordinatorLayout parent, V child) {
return getScrimOpacity(parent, child) > 0.f;
}
public float getScrimOpacity(CoordinatorLayout parent, V child) {
return 0.f;
}
public int getScrimColor(CoordinatorLayout parent, V child) {
return Color.BLACK;
}
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接下來看看CoordinatorLayout的onInterceptTouchEvent()、onTouchEvent()如何被Behavior代理：
//CoordinatorLayout
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
...
final boolean intercepted = performIntercept(ev, TYPE_ON_INTERCEPT);
...
return intercepted;
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
boolean handled = false;
boolean cancelSuper = false;
MotionEvent cancelEvent = null;
...
if (mBehaviorTouchView != null || (cancelSuper = performIntercept(ev, TYPE_ON_TOUCH))) {
// Safe since performIntercept guarantees that
// mBehaviorTouchView != null if it returns true
final LayoutParams lp = (LayoutParams) mBehaviorTouchView.getLayoutParams();
final Behavior b = lp.getBehavior();
//Behavior不爲空，事件分發給Behavior
if (b != null) {
handled = b.onTouchEvent(this, mBehaviorTouchView, ev);
}
}
// Keep the super implementation correct(走CoordinatorLayout默認方法)
if (mBehaviorTouchView == null) {
handled |= super.onTouchEvent(ev);
} else if (cancelSuper) {
if (cancelEvent == null) {
final long now = SystemClock.uptimeMillis();
cancelEvent = MotionEvent.obtain(now, now,
MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0);
}
super.onTouchEvent(cancelEvent);
}
...
return handled;
}
private boolean performIntercept(MotionEvent ev, final int type) {
boolean intercepted = false;
//記錄是否Behavior的blocksInteractionBelow()返回true，根據這個標
//識來給剩餘遍歷的Behavior分發個CANCEL的MotionEvent
boolean newBlock = false;
MotionEvent cancelEvent = null;
final int action = ev.getActionMasked();
//根據View的層級由高到低排序，儲放在臨時的容器
final List<View> topmostChildList = mTempList1;
getTopSortedChildren(topmostChildList);
//(先遍歷最外層View的Behavior的Touch事件代理)
// Let topmost child views inspect first
final int childCount = topmostChildList.size();
for (int i = 0; i < childCount; i++) {
final View child = topmostChildList.get(i);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final Behavior b = lp.getBehavior();
//若Touch事件已經被前面遍歷的Behavior攔截或者newBlock爲true表示前面遍歷的Behavior已阻斷交互、且action不是DOWN時
//那麼後面剩餘遍歷的Behavior分發個CANCEL的MotionEvent
if ((intercepted || newBlock) && action != MotionEvent.ACTION_DOWN) {
// Cancel all behaviors beneath the one that intercepted.
// If the event is "down" then we don't have anything to cancel yet.
if (b != null) {
if (cancelEvent == null) {
final long now = SystemClock.uptimeMillis();
cancelEvent = MotionEvent.obtain(now, now,
MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0);
}
switch (type) {
case TYPE_ON_INTERCEPT:
b.onInterceptTouchEvent(this, child, cancelEvent);
break;
case TYPE_ON_TOUCH:
b.onTouchEvent(this, child, cancelEvent);
break;
}
}
continue;
}
//沒有攔截Touch事件，Behavior不爲空，事件分發給Behavior
if (!intercepted && b != null) {
switch (type) {
case TYPE_ON_INTERCEPT:
intercepted = b.onInterceptTouchEvent(this, child, ev);
break;
case TYPE_ON_TOUCH:
intercepted = b.onTouchEvent(this, child, ev);
break;
}
//若是Behavior攔截了Touch事件，標記其關聯的View
if (intercepted) {
mBehaviorTouchView = child;
}
}
// Don't keep going if we're not allowing interaction below this.
// Setting newBlock will make sure we cancel the rest of the behaviors.
final boolean wasBlocking = lp.didBlockInteraction();
final boolean isBlocking = lp.isBlockingInteractionBelow(this, child);
newBlock = isBlocking && !wasBlocking;
if (isBlocking && !newBlock) {
//這裏要考慮onInterceptTouchEvent()進入performIntercept()Behavior阻斷過，
//再到onTouchEvent()進入performIntercept()就沒必要再遍歷
// Stop here since we don't have anything more to cancel - we already did
// when the behavior first started blocking things below this point.
break;
}
}
topmostChildList.clear();
return intercepted;
}
//CoordinatorLayout.LayoutParams
/** * Behavior是否以前已經阻斷過此Behavior所關聯View下層的View的交互 */
boolean didBlockInteraction() {
if (mBehavior == null) {
mDidBlockInteraction = false;
}
return mDidBlockInteraction;
}
/** * Behavior已經阻斷過此Behavior所關聯View下層的View的交互返回true， * 不然返回調用Behavior的blocksInteractionBelow並記錄已阻斷過 */
boolean isBlockingInteractionBelow(CoordinatorLayout parent, View child) {
if (mDidBlockInteraction) {
return true;
}
return mDidBlockInteraction |= mBehavior != null
? mBehavior.blocksInteractionBelow(parent, child)
: false;
}
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CoordinatorLayout的onInterceptTouchEvent()執行攔截主要邏輯在performIntercept()裏：
- 1.首先根據子View的層級由高到低排序後按順序遍歷子View的Behavior；
- 2.在遍歷中先判斷Touch事件已經被前面遍歷的Behavior攔截或者阻斷、且不是DOWN事件，若符合這些條件則給剩餘遍歷的Behavior分發個CANCEL的MotionEvent；
- 3.而後將根據參數type調用Behavior對應的事件攔截、消費的方法，若是Behavior攔截了Touch事件則以變量mBehaviorTouchView記錄其關聯的View；
- 4.接着調用CoordinatorLayout.LayoutParams的兩個判斷阻斷交互方法用變量newBlock記錄Behavior的阻斷交互。
CoordinatorLayout的onTouchEvent()邏輯以下：
- 1.先判斷以前在onInterceptTouchEvent()是否有記錄mBehaviorTouchView，如有則直接調用Behavior的onTouchEvent()；若無則調用performIntercept()且返回值賦值變量cancelSuper；
- 2.若cancelSuper爲true說明已有Behavior調用onTouchEvent()消費Touch事件了並記錄mBehaviorTouchView,而後經過mBehaviorTouchView的LayoutParam 再次調用Behavior的onTouchEvent()（ps:雖然根據源碼註釋說在這調用performIntercept()返回true是爲了確保mBehaviorTouchView不爲空，但按邏輯理解Behavior的onTouchEvent()被執行2次）；
- 3.接着若是沒有Behavior作出攔截,則會調用父類的onTouchEvent()，若是沒則判讀前面的變量cancelSuper是否爲true，若true則爲了防止以前已經給父類傳了事件給父類的onTouchEvent傳一個cancel事件。
這裏小結一下：若是重寫Behavior的onInterceptTouchEvent()、onTouchEvent()應當很是注意其邏輯在 CoordinatorLayout中onInterceptTouchEvent()、onTouchEvent()的合理性，由於在Behavior代理觸摸事件的處理顯得有點複雜並且繁瑣,並且會有大量的非正常的cancel事件出現。
代理CoordinatorLayout的嵌套滑動
CoordinatorLayout實現了NestedScrollingParent2接口並也覆寫兼容NestedScrollingParent，但它自己並無處理嵌套滑動而是所有給Behavior代理，Behavior代理嵌套滑動是經過NestedScrollingParent二、NestedScrollingParent對應的方法多了兩個參數：一個是CoordinatorLayout，一個是Behavior關聯的View。由於涉及到方法比較多，這裏不宜展開，關於嵌套滑動能夠參考我以前寫的的《淺析NestedScrolling嵌套滑動機制之基礎篇》
//CoordinatorLayout.Behavior
public void onNestedPreScroll(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View target, int dx, int dy, @NonNull int[] consumed, @NestedScrollType int type) {
if (type == ViewCompat.TYPE_TOUCH) {
onNestedPreScroll(coordinatorLayout, child, target, dx, dy, consumed);
}
}
@Deprecated
public void onNestedPreScroll(@NonNull CoordinatorLayout coordinatorLayout, @NonNull V child, @NonNull View target, int dx, int dy, @NonNull int[] consumed) {
// Do nothing
}
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接下來看看CoordinatorLayout的嵌套滑動讓Behavior代理，這裏分析只兩個方法，其餘的方法十分相似：
//CoordinatorLayout
@Override
public boolean onStartNestedScroll(View child, View target, int axes, int type) {
boolean handled = false;
final int childCount = getChildCount();
for (int i = 0; i < childCount; i++) {
final View view = getChildAt(i);
...
final LayoutParams lp = (LayoutParams) view.getLayoutParams();
final Behavior viewBehavior = lp.getBehavior();
if (viewBehavior != null) {
//Behavior代理onStartNestedScroll
final boolean accepted = viewBehavior.onStartNestedScroll(this, view, child,
target, axes, type);
handled |= accepted;
//在Behavior關聯的View的LayoutParams記錄是否接受嵌套滑動
lp.setNestedScrollAccepted(type, accepted);
} else {
lp.setNestedScrollAccepted(type, false);
}
}
return handled;
}
//CoordinatorLayout.LayoutParams
void setNestedScrollAccepted(int type, boolean accept) {
switch (type) {
case ViewCompat.TYPE_TOUCH:
mDidAcceptNestedScrollTouch = accept;
break;
case ViewCompat.TYPE_NON_TOUCH:
mDidAcceptNestedScrollNonTouch = accept;
break;
}
}
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在CoordinatorLayout的onStartNestedScroll()裏遍歷子View，獲取子View的Behavior並調用onStartNestedScroll()並在LayoutParams記錄是否接受嵌套滑動。
//CoordinatorLayout
@Override
public void onNestedPreScroll(View target, int dx, int dy, int[] consumed, int type) {
...
final int childCount = getChildCount();
for (int i = 0; i < childCount; i++) {
final View view = getChildAt(i);
...
final LayoutParams lp = (LayoutParams) view.getLayoutParams();
//判斷Behavior是否接受嵌套滑動
if (!lp.isNestedScrollAccepted(type)) {
continue;
}
final Behavior viewBehavior = lp.getBehavior();
if (viewBehavior != null) {
...
////Behavior代理onNestedPreScroll
viewBehavior.onNestedPreScroll(this, view, target, dx, dy, mTempIntPair, type);
...
}
}
...
}
複製代碼
在CoordinatorLayout的onNestedPreScroll()裏遍歷子View，獲取子View的LayoutParams判斷Behavior是否接受嵌套滑動，若接受則獲取子View的Behavior並調用onNestedPreScroll()。
小結
Behavior很強大，可是通常而言子View的測量、佈局這部分邏輯能夠放在自定義View內部處理，而CoordinatorLayout的分發WindowInsets、Touch事件給子View都有固定的順序，若是你在Behavior處理時應該注意其邏輯在CoordinatorLayout的合理性，不必爲了使用Behavior而是用它，嵌套滑動在實現神奇滑動的效果倒是十分有用，也能夠解耦自定義NestedScrollParent的邏輯。
Behavior的View依賴關係
創建View之間的依賴關係
<android.support.design.widget.CoordinatorLayout>
<android.support.design.widget.AppBarLayout android:id="@+id/app_bar"/>
<android.support.design.widget.FloatingActionButton app:layout_anchor="@id/app_bar" app:layout_anchorGravity="bottom|end" />
</android.support.design.widget.CoordinatorLayout>
複製代碼
還有一種就是在佈局文件添加layout_anchor設置錨點來創建依賴關係，不過這種依賴關係 只能監聽依賴View的位置、大小改變時回調onDependentViewChanged()。
//CoordinatorLayout.Behavior
/** * 返回值表示child是否依賴dependency */
public boolean layoutDependsOn(CoordinatorLayout parent, V child, View dependency) {
return false;
}
/** * 返回值表示Behavior是否改變child的大小或者位置 */
public boolean onDependentViewChanged(CoordinatorLayout parent, V child, View dependency) {
return false;
}
public void onDependentViewRemoved(CoordinatorLayout parent, V child, View dependency) {
}
複製代碼
排序View的依賴關係
CoordinatorLayout對View的依賴關係經過support包的DirectedAcyclicGraph有向無環圖進行拓撲排序。
在圖論中，若是一個有向圖從任意頂點出發沒法通過若干條邊回到該點，則這個圖是一個有向無環圖（DAG,directed acyclic graph）--維基百科
在CoordinatorLayout的onMeasure()裏的prepareChildren()就是對View依賴關係進行排序:
private final List<View> mDependencySortedChildren = new ArrayList<>();
private final DirectedAcyclicGraph<View> mChildDag = new DirectedAcyclicGraph<>();
private void prepareChildren() {
mDependencySortedChildren.clear();
mChildDag.clear();
for (int i = 0, count = getChildCount(); i < count; i++) {
final View view = getChildAt(i);
//找到View的Anchor錨點
final LayoutParams lp = getResolvedLayoutParams(view);
lp.findAnchorView(this, view);
//將view當節點添加進有向無環圖
mChildDag.addNode(view);
// Now iterate again over the other children, adding any dependencies to the graph
for (int j = 0; j < count; j++) {
if (j == i) {
continue;
}
final View other = getChildAt(j);
if (lp.dependsOn(this, view, other)) {//判斷view與other是否存在的依賴關係
if (!mChildDag.contains(other)) {
//(若是other沒在圖裏則添加才能確保view與other在圖創建依賴)
// Make sure that the other node is added
mChildDag.addNode(other);
}
//(將view與other在圖添加邊創建依賴)
// Now add the dependency to the graph
mChildDag.addEdge(other, view);
}
}
}
//(將圖節點以深度優先排序的list存放在list容器裏)
// Finally add the sorted graph list to our list
mDependencySortedChildren.addAll(mChildDag.getSortedList());
//(反轉list讓沒有依賴關係的view排在list的前面)
// We also need to reverse the result since we want the start of the list to contain
// Views which have no dependencies, then dependent views after that
Collections.reverse(mDependencySortedChildren);
}
複製代碼
- 1.CoordinatorLayout遍歷遍歷子view，調用CoordinatorLayout.LayoutParams.findAnchorView()找到View的Anchor錨點，並將當前view做爲節點添加到有向無環圖裏。
- 2.在循環裏在開啓循環遍歷其餘子View,經過CoordinatorLayout.LayoutParams.dependsOn()判斷與外層循環的view是否存在依賴關係，如有則創建在圖添加邊創建依賴。
- 3.兩層循壞執行完後，將有向無環圖的節點以深度優先排序的list存放在mDependencySortedChildren裏，而後反轉mDependencySortedChildren讓沒有依賴關係的view排在list的前面。
Behavior依賴View回調觸發過程
Behavior的onDependentViewChanged()和onDependentViewRemoved()被觸發在CoordinatorLayout的onChildViewsChanged()，這方法type參數有三個值：EVENT_PRE_DRAW(依賴view繪製以前事件類型)、EVENT_NESTED_SCROLL(依賴view嵌套滑動事件類型)、EVENT_VIEW_REMOVED(依賴view從佈局移除事件類型)。
final void onChildViewsChanged(@DispatchChangeEvent final int type) {
...
final int childCount = mDependencySortedChildren.size();
...
for (int i = 0; i < childCount; i++) {
final View child = mDependencySortedChildren.get(i);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
...
for (int j = 0; j < i; j++) {
final View checkChild = mDependencySortedChildren.get(j);
if (lp.mAnchorDirectChild == checkChild) {
//檢測view的anchor錨點位置是否發生變化來調整依賴view的位置
offsetChildToAnchor(child, layoutDirection);
}
}
...
for (int j = i + 1; j < childCount; j++) {
final View checkChild = mDependencySortedChildren.get(j);
final LayoutParams checkLp = (LayoutParams) checkChild.getLayoutParams();
final Behavior b = checkLp.getBehavior();
//判斷checkChild是否依賴child
if (b != null && b.layoutDependsOn(this, checkChild, child)) {
...
final boolean handled;
switch (type) {
case EVENT_VIEW_REMOVED:
//(分發依賴view從佈局移除事件給Behavior)
b.onDependentViewRemoved(this, checkChild, child);
handled = true;
break;
default:
//(分發依賴view繪製以前事件或嵌套滑動事件給Behavior)
handled = b.onDependentViewChanged(this, checkChild, child);
break;
}
}
}
}
...
}
void offsetChildToAnchor(View child, int layoutDirection) {
...
//注意：這裏view和anchor錨點位置都調整了，將這變化通知給Behavior
// If we have needed to move, make sure to notify the child's Behavior
final Behavior b = lp.getBehavior();
if (b != null) {
b.onDependentViewChanged(this, child, lp.mAnchorView);
}
...
}
複製代碼
在CoordinatorLayout的onNestedFling()、onNestedPreScroll()、onNestedPreScroll()裏若是NestedScrollingChild處理了嵌套滑動都會經過onChildViewsChanged(EVENT_NESTED_SCROLL)將依賴view嵌套滑動事件分發給Behavior,下面以onNestedScroll代碼爲例。
//CoordiantorLayout.java
@Override
public void onNestedScroll(View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed, int type) {
...
if (accepted) {
onChildViewsChanged(EVENT_NESTED_SCROLL);
}
}
複製代碼
在CoordinatorLayout的構造方法裏經過setOnHierarchyChangeListener()註冊OnHierarchyChangeListener監聽添加或移除View的層級變化，而CoordinatorLayout.OnHierarchyChangeListener在View被移除回調中調用onChildViewsChanged(EVENT_VIEW_REMOVED)將依賴view從佈局移除事件類型分發給Behavior。
public CoordinatorLayout(Context context, AttributeSet attrs, int defStyleAttr) {
...
super.setOnHierarchyChangeListener(new HierarchyChangeListener();
}
private class HierarchyChangeListener implements OnHierarchyChangeListener {
...
@Override
public void onChildViewRemoved(View parent, View child) {
//將依賴view從佈局移除事件類型分發給Behavior
onChildViewsChanged(EVENT_VIEW_REMOVED);
...
}
}
複製代碼
在CoordinatorLayout的onAttachedToWindow()中往ViewTreeObserver註冊個CoordinatorLayout.OnPreDrawListener，它會在每次刷新肯定各View大小位置後並繪製以前回調，而在回調裏調用onChildViewsChanged()將依賴view繪製以前事件類型分發給對應的Behavior。
//是否須要註冊mOnPreDrawListener標識
private boolean mNeedsPreDrawListener;
//是否已經執行onAttachedToWindow()標識
private boolean mIsAttachedToWindow;
private OnPreDrawListener mOnPreDrawListener;
@Override
public void onAttachedToWindow() {
...
if (mNeedsPreDrawListener) {
if (mOnPreDrawListener == null) {
mOnPreDrawListener = new OnPreDrawListener();
}
final ViewTreeObserver vto = getViewTreeObserver();
vto.addOnPreDrawListener(mOnPreDrawListener);
}
...
mIsAttachedToWindow = true;
}
class OnPreDrawListener implements ViewTreeObserver.OnPreDrawListener {
@Override
public boolean onPreDraw() {
//分發依賴view繪製以前事件類型
onChildViewsChanged(EVENT_PRE_DRAW);
return true;
}
複製代碼
雖然onAttachedToWindow()會被調用在onDraw()以前，但也可能在onMeasure()以前調用，若是View之間不存在依賴關係則mOnPreDrawListener從ViewTree移除防止內存泄露，因此在onMeasure()的ensurePreDrawListener()裏檢測View之間是否存在依賴關係對mOnPreDrawListener進行註冊或註銷。
void ensurePreDrawListener() {
boolean hasDependencies = false;
final int childCount = getChildCount();
//遍歷子View，看它們是否存在依賴關係
for (int i = 0; i < childCount; i++) {
final View child = getChildAt(i);
if (hasDependencies(child)) {
hasDependencies = true;
break;
}
}
if (hasDependencies != mNeedsPreDrawListener) {
if (hasDependencies) {
//存在依賴，註冊mOnPreDrawListener
addPreDrawListener();
} else {
////不存在依賴，註銷mOnPreDrawListener
removePreDrawListener();
}
}
}
void addPreDrawListener() {
//若是已經執行onAttachedToWindow()
if (mIsAttachedToWindow) {
// Add the listener
if (mOnPreDrawListener == null) {
mOnPreDrawListener = new OnPreDrawListener();
}
final ViewTreeObserver vto = getViewTreeObserver();
vto.addOnPreDrawListener(mOnPreDrawListener);
}
//(由於onMeasure()與onAttachedToWindow()調用順序不肯定，
//因此這裏標識mNeedsPreDrawListener變量來處理註冊mOnPreDrawListener)
// Record that we need the listener regardless of whether or not we're attached.
// We'll add the real listener when we become attached.
mNeedsPreDrawListener = true;
}
void removePreDrawListener() {
if (mIsAttachedToWindow) {
if (mOnPreDrawListener != null) {
final ViewTreeObserver vto = getViewTreeObserver();
vto.removeOnPreDrawListener(mOnPreDrawListener);
}
}
mNeedsPreDrawListener = false;
}
}
複製代碼
自定義Behavior
-
1.在自定義Behavior以前您能夠參考系統自帶的Behavior可否知足需求，如FloatActionButton內部的Behavior能保證Snackbar彈出的時候不被FAB遮擋等：
-
2.是否有必要爲子View的測量、佈局、分發WindowInsets和Touch事件而使用CoordinatorLayout+Behavior，這部分邏輯是否能夠放在自定義View內部處理。
-
3.Behavior的View依賴關係與NestedScrolling結合實現滑動更爲方便。
總結
CoordinatorLayout和Behavior結合很強大，但本文偏向概念性內容，不免有些枯燥，下篇文章實踐自定義Behavior，因爲本人水平有限僅給各位提供參考，但願可以拋磚引玉，若是有什麼能夠討論的問題能夠在評論區留言或聯繫本人。
參考
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