ViewTreeObserver 註冊一個觀察者來監聽檢視樹，當檢視樹的佈局、檢視樹的焦點、檢視樹將要繪製、檢視樹滾動等發生改變時，ViewTreeObserver都會收到通知，ViewTreeObserver不能被例項化，可以呼叫View.getViewTreeObserver()來獲得。
ViewTreeObserver繼承關係：
public final class ViewTreeObserverextendsObject
java.lang.Object
↳android.view.ViewTreeObserver
複製程式碼ViewTreeObserver直接繼承自Object.
ViewTreeObserver提供了View的多種監聽，每一種監聽都有一個內部類介面與之對應，內部類介面全部儲存在CopyOnWriteArrayList中，通過ViewTreeObserver.addXXXListener()來新增這些監聽，原始碼如下：
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;
}
複製程式碼以OnGlobalLayoutListener為例，首先是定義介面：
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();
}
複製程式碼將OnGlobalLayoutListener 新增到CopyOnWriteArray陣列中：
/**
* 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);
}
複製程式碼移除OnGlobalLayoutListener，當檢視樹佈局發生變化時不會再收到通知了：
/**
* 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);
}
複製程式碼其他常用方法：
**dispatchOnGlobalLayout()：**檢視樹發生改變時通知觀察者，如果想在View Layout 或 View hierarchy 還未依附到Window時，或者在View處於GONE狀態時強制佈局，這個方法也可以手動呼叫。
/**
* 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();
}
}
}
複製程式碼**dispatchOnPreDraw()：**通知觀察者繪製即將開始，如果其中的某個觀察者返回 true，那麼繪製將會取消，並且重新安排繪製，如果想在View Layout 或 View hierarchy 還未依附到Window時，或者在View處於GONE狀態時強制繪製，可以手動呼叫這個方法。
/**
* 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;
}
複製程式碼ViewTreeObserver常用內部類：
| 內部類介面 | 備註 |
|---|---|
| ViewTreeObserver.OnPreDrawListener | 當檢視樹將要被繪製時，會呼叫的介面 |
| ViewTreeObserver.OnGlobalLayoutListener | 當檢視樹的佈局發生改變或者View在檢視樹的可見狀態發生改變時會呼叫的介面 |
| ViewTreeObserver.OnGlobalFocusChangeListener | 當一個檢視樹的焦點狀態改變時，會呼叫的介面 |
| ViewTreeObserver.OnScrollChangedListener | 當檢視樹的一些元件發生滾動時會呼叫的介面 |
| ViewTreeObserver.OnTouchModeChangeListener | 當檢視樹的觸控模式發生改變時，會呼叫的介面 |
獲得View高度的幾種方式：
我們應該都遇到過在onCreate()方法裡面呼叫view.getWidth()和view.getHeight()獲取到的view的寬高都是0的情況，這是因為在onCreate()裡還沒有執行測量，需要在onResume()之後才能得到正確的高度，那麼可不可以在onCreate()裡就得到寬高呢？答：可以！常用的有下面幾種方式：
1、通過設定View的MeasureSpec.UNSPECIFIED來測量：
int w = View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED);
int h = View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED);
view.measure(w, h);
//獲得寬高
int viewWidth=view.getMeasuredWidth();
int viewHeight=view.getMeasuredHeight();
複製程式碼設定我們的SpecMode為UNSPECIFIED，然後去呼叫onMeasure測量寬高，就可以得到寬高。
2、通過ViewTreeObserver .addOnGlobalLayoutListener來獲得寬高，當獲得正確的寬高後，請移除這個觀察者，否則回撥會多次執行：
//獲得ViewTreeObserver
ViewTreeObserver observer=view.getViewTreeObserver();
//註冊觀察者，監聽變化
observer.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
//判斷ViewTreeObserver 是否alive，如果存活的話移除這個觀察者
if(observer.isAlive()){
observer.removeGlobalOnLayoutListener(this);
//獲得寬高
int viewWidth=view.getMeasuredWidth();
int viewHeight=view.getMeasuredHeight();
}
}
});
複製程式碼3、通過ViewTreeObserver .addOnPreDrawListener來獲得寬高，在執行onDraw之前已經執行了onLayout()和onMeasure()，可以得到寬高了，當獲得正確的寬高後，請移除這個觀察者，否則回撥會多次執行
//獲得ViewTreeObserver
ViewTreeObserver observer=view.getViewTreeObserver();
//註冊觀察者，監聽變化
observer.addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
@Override
public boolean onPreDraw() {
if(observer.isAlive()){
observer.removeOnDrawListener(this);
}
//獲得寬高
int viewWidth=view.getMeasuredWidth();
int viewHeight=view.getMeasuredHeight();
return true;
}
});
複製程式碼