官方文檔的描述ViewTreeObserver是用來監(jiān)聽一些全局變化的样漆。

在 ViewTreeObserver 中怒见，包含了以下幾個接口：

• interface ViewTreeObserver.OnGlobalFocusChangeListener

• interface ViewTreeObserver.OnGlobalLayoutListener

• interface ViewTreeObserver.OnPreDrawListener

• interface ViewTreeObserver.OnScrollChangedListener

• interface ViewTreeObserver.OnTouchModeChangeListener

ViewTreeObserver 注冊一個觀察者來監(jiān)聽視圖樹盯拱，當視圖樹的布局事甜、視圖樹的焦點订歪、視圖樹將要繪制穿肄、視圖樹滾動等發(fā)生改變時年局，ViewTreeObserver都會收到通知，ViewTreeObserver不能被實例化咸产，可以調(diào)用View.getViewTreeObserver()來獲得矢否。

ViewTreeObserver繼承關系：

public final class ViewTreeObserverextendsObject
java.lang.Object
android.view.ViewTreeObserver

ViewTreeObserver直接繼承自Object.

ViewTreeObserver提供了View的多種監(jiān)聽，每一種監(jiān)聽都有一個內(nèi)部類接口與之對應脑溢，內(nèi)部類接口全部保存在CopyOnWriteArrayList中僵朗，通過ViewTreeObserver.addXXXListener()來添加這些監(jiān)聽，源碼如下：

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;
}

以OnGlobalLayoutListener為例屑彻，首先是定義接口：

 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();
    }

將OnGlobalLayoutListener 添加到CopyOnWriteArray數(shù)組中：

  /**
     * 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验庙，當視圖樹布局發(fā)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);
    }

其他常用方法：

dispatchOnGlobalLayout()：視圖樹發(fā)生改變時通知觀察者，如果想在View Layout 或 View hierarchy 還未依附到Window時社牲，或者在View處于GONE狀態(tài)時強制布局粪薛，這個方法也可以手動調(diào)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()：通知觀察者繪制即將開始搏恤，如果其中的某個觀察者返回 true违寿，那么繪制將會取消让禀，并且重新安排繪制，如果想在View Layout 或 View hierarchy 還未依附到Window時陨界，或者在View處于GONE狀態(tài)時強制繪制，可以手動調(diào)用這個方法痛阻。

 /**
     * 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常用內(nèi)部類：

獲得View高度的幾種方式：

我們應該都遇到過在onCreate()方法里面調(diào)用view.getWidth()和view.getHeight()獲取到的view的寬高都是0的情況俏扩，這是因為在onCreate()里還沒有執(zhí)行測量，需要在onResume()之后才能得到正確的高度弊添，那么可不可以在onCreate()里就得到寬高呢录淡？答：可以！常用的有下面幾種方式：

1油坝、通過設置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();

設置我們的SpecMode為UNSPECIFIED嫉戚，然后去調(diào)用onMeasure測量寬高，就可以得到寬高澈圈。

2彬檀、通過ViewTreeObserver .addOnGlobalLayoutListener來獲得寬高，當獲得正確的寬高后瞬女，請移除這個觀察者窍帝，否則回調(diào)會多次執(zhí)行：

//獲得ViewTreeObserver 
ViewTreeObserver observer=view.getViewTreeObserver();
//注冊觀察者，監(jiān)聽變化
observer.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
     @Override
     public void onGlobalLayout() {
            //判斷ViewTreeObserver 是否alive诽偷，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();
           }
        }
   });

3坤学、通過ViewTreeObserver .addOnPreDrawListener來獲得寬高，在執(zhí)行onDraw之前已經(jīng)執(zhí)行了onLayout()和onMeasure()报慕，可以得到寬高了深浮，當獲得正確的寬高后，請移除這個觀察者卖子，否則回調(diào)會多次執(zhí)行

//獲得ViewTreeObserver 
ViewTreeObserver observer=view.getViewTreeObserver();
//注冊觀察者略号，監(jiān)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;
     }
   });

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>

讓MainActivity實現(xiàn)相應接口

public class MainActivity extends AppCompatActivity implements 
ViewTreeObserver.OnGlobalLayoutListener, 
ViewTreeObserver.OnPreDrawListener, ViewTreeObserver.OnGlobalFocusChangeListener, 
ViewTreeObserver.OnTouchModeChangeListener, View.OnClickListener

在onCreat中添加監(jiān)聽

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

首先測試ViewTreeObserver.OnGlobalFocusChangeListener,實現(xiàn)接口方法

代碼

    @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：在第一次進入頁面的時候沒有oldFoucs

效果

這個接口很簡單就是監(jiān)聽focus的變化：

image.png

二.OnPreDrawListener

OnPreDrawListener接口是在繪制界面前調(diào)用

代碼

  @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 繼續(xù)繪制，返回false取消洋闽。
        return true;
    }

效果

image.png

如果返回false的話玄柠，效果是這樣的橱鹏，界面沒有繪制搭幻。

image.png

關于OnPreDrawListener的使用，有一個例子就是CoordinatorLayout調(diào)用Behavior的onDependentViewChanged就是通過注冊OnPreDrawListener接口序苏，在繪制的時候檢查界面是否發(fā)生變化刊懈，如果變化就調(diào)用Behavior的onDependentViewChanged这弧。

源碼

 @Override
    public void onAttachedToWindow() {
        super.onAttachedToWindow();
        resetTouchBehaviors();
        if (mNeedsPreDrawListener) {
            if (mOnPreDrawListener == null) {
                mOnPreDrawListener = new OnPreDrawListener();
            }
            //注冊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
        //分發(fā)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);
                }
            }
            //判斷是否發(fā)生變化
            // 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 = (LayoutParams) checkChild.getLayoutParams();
                final Behavior b = checkLp.getBehavior();

                if (b != null && b.layoutDependsOn(this, checkChild, child)) {
                    if (!fromNestedScroll && checkLp.getChangedAfterNestedScroll()) {
                        // If this is not from a nested scroll and we have already been changed
                        // from a nested scroll, skip the dispatch and reset the flag
                        checkLp.resetChangedAfterNestedScroll();
                        continue;
                    }
                //調(diào)用onDependentViewChanged
                    final boolean handled = b.onDependentViewChanged(this, checkChild, child);

                 ...
                }
            }
        }
    }

三.OnGlobalLayoutListener

Interface definition for a callback to be invoked when the global layout state
 or the visibility of views within the view tree changes.

當在一個視圖樹中全局布局發(fā)生改變或者視圖樹中的某個視圖的可視狀態(tài)發(fā)生改變時娃闲，所要調(diào)用的回調(diào)函數(shù)的接口類

代碼

1.在點擊時改變EditText的可視性。

    @Override
    public void onClick(View v) {
        if(et_1.isShown()){
            et_1.setVisibility(View.GONE);
        }else{
            et_1.setVisibility(View.VISIBLE);
        }
    }

2.在onGlobalLayout顯示EditText的可見性

  @Override
    public void onGlobalLayout() {
                if(et_1.isShown()){
                    tv_show.setText("EditText1 顯示");
                }else{
                    tv_show.setText("EditText1 隱藏");
                }
    }

效果

這里寫圖片描述

注意：在測試的時候發(fā)現(xiàn)使用

et_1.setVisibility(View.INVISIBLE);

時并不會觸發(fā)OnGlobalLayoutListener而只能使用

  et_1.setVisibility(View.GONE);

補充

可以使用OnGlobalLayoutListener獲取控件寬高匾浪。

private int mHeaderViewHeight;
private View mHeaderView;

.....

mHeaderView.getViewTreeObserver().addOnGlobalLayoutListener(
    new OnGlobalLayoutListener() {
        @Override
        public void onGlobalLayout() {

            mHeaderViewHeight = mHeaderView.getHeight();
            mHeaderView.getViewTreeObserver()
                    .removeGlobalOnLayoutListener(this);
        }
});