官方文档的描述ViewTreeObserver是用来监听一些全局变化的。
在 ViewTreeObserver 中，包含了以下几个接口：
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interface ViewTreeObserver.OnGlobalFocusChangeListener
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interface ViewTreeObserver.OnGlobalLayoutListener
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interface ViewTreeObserver.OnPreDrawListener
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interface ViewTreeObserver.OnScrollChangedListener
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interface ViewTreeObserver.OnTouchModeChangeListener
ViewTreeObserver 注册一个观察者来监听视图树，当视图树的布局、视图树的焦点、视图树将要绘制、视图树滚动等发生改变时，ViewTreeObserver都会收到通知，ViewTreeObserver不能被实例化，可以调用View.getViewTreeObserver()来获得。
ViewTreeObserver继承关系：
public final class ViewTreeObserverextendsObject
java.lang.Object
android.view.ViewTreeObserver
ViewTreeObserver直接继承自Object.
ViewTreeObserver提供了View的多种监听，每一种监听都有一个内部类接口与之对应，内部类接口全部保存在CopyOnWriteArrayList中，通过ViewTreeObserver.addXXXListener()来添加这些监听，源码如下：
public final class ViewTreeObserver { // Recursive listeners use CopyOnWriteArrayList private CopyOnWriteArrayList<OnWindowFocusChangeListener> mOnWindowFocusListeners; private CopyOnWriteArrayList<OnWindowAttachListener> mOnWindowAttachListeners; private CopyOnWriteArrayList<OnGlobalFocusChangeListener> mOnGlobalFocusListeners; private CopyOnWriteArrayList<OnTouchModeChangeListener> mOnTouchModeChangeListeners; private CopyOnWriteArrayList<OnEnterAnimationCompleteListener> mOnEnterAnimationCompleteListeners; // Non-recursive listeners use CopyOnWriteArray // Any listener invoked from ViewRootImpl.performTraversals() should not be recursive private CopyOnWriteArray<OnGlobalLayoutListener> mOnGlobalLayoutListeners; private CopyOnWriteArray<OnComputeInternalInsetsListener> mOnComputeInternalInsetsListeners; private CopyOnWriteArray<OnScrollChangedListener> mOnScrollChangedListeners; private CopyOnWriteArray<OnPreDrawListener> mOnPreDrawListeners; private CopyOnWriteArray<OnWindowShownListener> mOnWindowShownListeners; // These listeners cannot be mutated during dispatch private ArrayList<OnDrawListener> mOnDrawListeners; } 以OnGlobalLayoutListener为例，首先是定义接口：
public interface OnGlobalLayoutListener { /** * Callback method to be invoked when the global layout state or the visibility of views * within the view tree changes */ public void onGlobalLayout(); } 将OnGlobalLayoutListener 添加到CopyOnWriteArray数组中：
/** * Register a callback to be invoked when the global layout state or the visibility of views * within the view tree changes * * @param listener The callback to add * * @throws IllegalStateException If {@link #isAlive()} returns false */ public void addOnGlobalLayoutListener(OnGlobalLayoutListener listener) { checkIsAlive(); if (mOnGlobalLayoutListeners == null) { mOnGlobalLayoutListeners = new CopyOnWriteArray<OnGlobalLayoutListener>(); } mOnGlobalLayoutListeners.add(listener); } 移除OnGlobalLayoutListener，当视图树布局发生变化时不会再收到通知了：
/** * Remove a previously installed global layout callback * * @param victim The callback to remove * * @throws IllegalStateException If {@link #isAlive()} returns false * * @deprecated Use #removeOnGlobalLayoutListener instead * * @see #addOnGlobalLayoutListener(OnGlobalLayoutListener) */ @Deprecated public void removeGlobalOnLayoutListener(OnGlobalLayoutListener victim) { removeOnGlobalLayoutListener(victim); } /** * Remove a previously installed global layout callback * * @param victim The callback to remove * * @throws IllegalStateException If {@link #isAlive()} returns false * * @see #addOnGlobalLayoutListener(OnGlobalLayoutListener) */ public void removeOnGlobalLayoutListener(OnGlobalLayoutListener victim) { checkIsAlive(); if (mOnGlobalLayoutListeners == null) { return; } mOnGlobalLayoutListeners.remove(victim); } 其他常用方法：
dispatchOnGlobalLayout()：视图树发生改变时通知观察者，如果想在View Layout 或 View hierarchy 还未依附到Window时，或者在View处于GONE状态时强制布局，这个方法也可以手动调用。
/** * Notifies registered listeners that a global layout happened. This can be called * manually if you are forcing a layout on a View or a hierarchy of Views that are * not attached to a Window or in the GONE state. */ public final void dispatchOnGlobalLayout() { // NOTE: because of the use of CopyOnWriteArrayList, we *must* use an iterator to // perform the dispatching. The iterator is a safe guard against listeners that // could mutate the list by calling the various add/remove methods. This prevents // the array from being modified while we iterate it. final CopyOnWriteArray<OnGlobalLayoutListener> listeners = mOnGlobalLayoutListeners; if (listeners != null && listeners.size() > 0) { CopyOnWriteArray.Access<OnGlobalLayoutListener> access = listeners.start(); try { int count = access.size(); for (int i = 0; i < count; i++) { access.get(i).onGlobalLayout(); } } finally { listeners.end(); } } } dispatchOnPreDraw()：通知观察者绘制即将开始，如果其中的某个观察者返回 true，那么绘制将会取消，并且重新安排绘制，如果想在View Layout 或 View hierarchy 还未依附到Window时，或者在View处于GONE状态时强制绘制，可以手动调用这个方法。
/** * Notifies registered listeners that the drawing pass is about to start. If a * listener returns true, then the drawing pass is canceled and rescheduled. This can * be called manually if you are forcing the drawing on a View or a hierarchy of Views * that are not attached to a Window or in the GONE state. * * @return True if the current draw should be canceled and resceduled, false otherwise. */ ViewTreeObserver常用内部类：
| 内部类接口 | 备注 |
|---|---|
| ViewTreeObserver.OnPreDrawListener | 当视图树将要被绘制时，会调用的接口 |
| ViewTreeObserver.OnGlobalLayoutListener | 当视图树的布局发生改变或者View在视图树的可见状态发生改变时会调用的接口 |
| ViewTreeObserver.OnGlobalFocusChangeListener | 当一个视图树的焦点状态改变时，会调用的接口 |
| ViewTreeObserver.OnScrollChangedListener | 当视图树的一些组件发生滚动时会调用的接口 |
| ViewTreeObserver.OnTouchModeChangeListener | 当视图树的触摸模式发生改变时，会调用的接口 |
获得View高度的几种方式：
我们应该都遇到过在onCreate()方法里面调用view.getWidth()和view.getHeight()获取到的view的宽高都是0的情况，这是因为在onCreate()里还没有执行测量，需要在onResume()之后才能得到正确的高度，那么可不可以在onCreate()里就得到宽高呢？答：可以！常用的有下面几种方式：
1、通过设置View的MeasureSpec.UNSPECIFIED来测量：
int w = View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED); int h = View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED); view.measure(w, h); //获得宽高 int viewWidth=view.getMeasuredWidth(); int viewHeight=view.getMeasuredHeight(); 设置我们的SpecMode为UNSPECIFIED，然后去调用onMeasure测量宽高，就可以得到宽高。
2、通过ViewTreeObserver .addOnGlobalLayoutListener来获得宽高，当获得正确的宽高后，请移除这个观察者，否则回调会多次执行：
//获得ViewTreeObserver ViewTreeObserver observer=view.getViewTreeObserver(); //注册观察者，监听变化 observer.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() { @Override public void onGlobalLayout() { //判断ViewTreeObserver 是否alive，如果存活的话移除这个观察者 if(observer.isAlive()){ observer.removeGlobalOnLayoutListener(this); //获得宽高 int viewWidth=view.getMeasuredWidth(); int viewHeight=view.getMeasuredHeight(); } } }); 3、通过ViewTreeObserver .addOnPreDrawListener来获得宽高，在执行onDraw之前已经执行了onLayout()和onMeasure()，可以得到宽高了，当获得正确的宽高后，请移除这个观察者，否则回调会多次执行
//获得ViewTreeObserver ViewTreeObserver observer=view.getViewTreeObserver(); //注册观察者，监听变化 observer.addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() { @Override public boolean onPreDraw() { if(observer.isAlive()){ observer.removeOnDrawListener(this); } //获得宽高 int viewWidth=view.getMeasuredWidth(); int viewHeight=view.getMeasuredHeight(); return true; } }); 案例分析
xml布局如下
<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:gravity="center" android:layout_height="match_parent" android:orientation="vertical" android:id="@+id/layout" tools:context="trs.com.viewtreeobserverdemo.MainActivity"> <TextView android:id="@+id/tv_show" android:layout_width="match_parent" android:layout_height="wrap_content" /> <EditText android:hint="et1" android:tag="et1" android:id="@+id/et_1" android:layout_width="match_parent" android:layout_height="wrap_content" /> <EditText android:tag="et2" android:hint="et2" android:layout_marginTop="10dp" android:id="@+id/et_2" android:layout_width="match_parent" android:layout_height="wrap_content" /> <Button android:text="test" android:id="@+id/btn" android:layout_width="wrap_content" android:layout_height="wrap_content" /> </LinearLayout> 让MainActivity实现相应接口
public class MainActivity extends AppCompatActivity implements ViewTreeObserver.OnGlobalLayoutListener, ViewTreeObserver.OnPreDrawListener, ViewTreeObserver.OnGlobalFocusChangeListener, ViewTreeObserver.OnTouchModeChangeListener, View.OnClickListener 在onCreat中添加监听
EditText et_1,et_2; TextView tv_show; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); ViewTreeObserver vto = findViewById(R.id.layout).getViewTreeObserver(); et_1= (EditText) findViewById(R.id.et_1); et_2= (EditText) findViewById(R.id.et_2); vto.addOnGlobalLayoutListener(this); vto.addOnPreDrawListener(this); vto.addOnGlobalFocusChangeListener(this); vto.addOnTouchModeChangeListener(this); findViewById(R.id.btn).setOnClickListener(this); tv_show= (TextView) findViewById(R.id.tv_show); } 一.OnGlobalFocusChangeListener
首先测试ViewTreeObserver.OnGlobalFocusChangeListener,实现接口方法
代码
@Override public void onGlobalFocusChanged(View oldFocus, View newFocus) { if(oldFocus!=null) { tv_show.setText("Focus change from " + oldFocus.getTag() + " to " + newFocus.getTag()); }else{ tv_show.setText( newFocus.getTag()+"get focus"); } } 注意：在第一次进入页面的时候没有oldFoucs
效果
这个接口很简单就是监听focus的变化：
image.png
二.OnPreDrawListener
OnPreDrawListener接口是在绘制界面前调用
代码
@Override public boolean onPreDraw() { et_1.setHint("set hint on onPreDraw "); //Return true to proceed with the current drawing pass, or false to cancel. //返回 true 继续绘制，返回false取消。 return true; } 效果
image.png
如果返回false的话，效果是这样的，界面没有绘制。
image.png
关于OnPreDrawListener的使用，有一个例子就是CoordinatorLayout调用Behavior的onDependentViewChanged就是通过注册OnPreDrawListener接口，在绘制的时候检查界面是否发生变化，如果变化就调用Behavior的onDependentViewChanged。
源码
@Override public void onAttachedToWindow() { super.onAttachedToWindow(); resetTouchBehaviors(); if (mNeedsPreDrawListener) { if (mOnPreDrawListener == null) { mOnPreDrawListener = new OnPreDrawListener(); } //注册OnPreDrawListener final ViewTreeObserver vto = getViewTreeObserver(); vto.addOnPreDrawListener(mOnPreDrawListener); } if (mLastInsets == null && ViewCompat.getFitsSystemWindows(this)) { // We're set to fitSystemWindows but we haven't had any insets yet... // We should request a new dispatch of window insets ViewCompat.requestApplyInsets(this); } mIsAttachedToWindow = true; } class OnPreDrawListener implements ViewTreeObserver.OnPreDrawListener { @Override //分发OnDependentViewChanged dispatchOnDependentViewChanged(false); return true; } } void dispatchOnDependentViewChanged(final boolean fromNestedScroll) { final int layoutDirection = ViewCompat.getLayoutDirection(this); final int childCount = mDependencySortedChildren.size(); for (int i = 0; i < childCount; i++) { final View child = mDependencySortedChildren.get(i); final LayoutParams lp = (LayoutParams) child.getLayoutParams(); // Check child views before for anchor for (int j = 0; j < i; j++) { final View checkChild = mDependencySortedChildren.get(j); if (lp.mAnchorDirectChild == checkChild) { offsetChildToAnchor(child, layoutDirection); } } //判断是否发生变化 // Did it change? if not continue final Rect oldRect = mTempRect1; final Rect newRect = mTempRect2; getLastChildRect(child, oldRect); getChildRect(child, true, newRect); if (oldRect.equals(newRect)) { continue; } recordLastChildRect(child, newRect); // Update any behavior-dependent views for the change for (int j = i + 1; j < childCount; j++) { final View checkChild = mDependencySortedChildren.get(j); final LayoutParams checkLp