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 mOnWindowFocusListeners;

private CopyOnWriteArrayList mOnWindowAttachListeners;

private CopyOnWriteArrayList mOnGlobalFocusListeners;

private CopyOnWriteArrayList mOnTouchModeChangeListeners;

private CopyOnWriteArrayList mOnEnterAnimationCompleteListeners;

// Non-recursive listeners use CopyOnWriteArray

// Any listener invoked from ViewRootImpl.performTraversals() should not be recursive

private CopyOnWriteArray mOnGlobalLayoutListeners;

private CopyOnWriteArray mOnComputeInternalInsetsListeners;

private CopyOnWriteArray mOnScrollChangedListeners;

private CopyOnWriteArray mOnPreDrawListeners;

private CopyOnWriteArray mOnWindowShownListeners;

// These listeners cannot be mutated during dispatch

private ArrayList 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();

}

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 listeners = mOnGlobalLayoutListeners;

if (listeners != null && listeners.size() > 0) {

CopyOnWriteArray.Access 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.

*/

@SuppressWarnings("unchecked")

public final boolean dispatchOnPreDraw() {

boolean cancelDraw = false;

final CopyOnWriteArray listeners = mOnPreDrawListeners;

if (listeners != null && listeners.size() > 0) {

CopyOnWriteArray.Access access = listeners.start();

try {

int count = access.size();

for (int i = 0; i < count; i++) {

cancelDraw |= !(access.get(i).onPreDraw());

}

} finally {

listeners.end();

}

}

return cancelDraw;

}

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;

}

});