Android ViewTreeObserver使用总结及获得View高度的几种方法

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.
*/
@SuppressWarnings("unchecked")
public final boolean dispatchOnPreDraw() {
boolean cancelDraw = false;
final CopyOnWriteArray<OnPreDrawListener> listeners = mOnPreDrawListeners;
if (listeners != null && listeners.size() > 0) {
CopyOnWriteArray.Access<OnPreDrawListener> 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;
}
});