Android View事件的分发机制

Android View的事件分发机制是比较重要的知识点，大家有空要抽点时间出来整理一下，分析一下它的原理。
写一个简单的例子，先看看运行结果，再从源码分析。
自定义一个Button类，名字叫CustomButton
在onTouchEvent和dispatchTouchEvent两个方法中打印相应的事件日志；

com.example.mc_viewtouch.viewtouch;import android.content.Context;import android.util.AttributeSet;import android.util.Log;import android.view.MotionEvent;import android.widget.Button;/** * Created by Michael on 2016/4/17. */public class CustomButton extends Button{    private static final String TAG = CustomButton.class.getSimpleName();    public CustomButton(Context context) {        super(context);    }    public CustomButton(Context context, AttributeSet attrs) {        super(context, attrs);    }    public CustomButton(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        switch (event.getAction()){            case MotionEvent.ACTION_DOWN:                Log.w(TAG, "[onTouchEvent] ACTION_DOWN");                break;            case MotionEvent.ACTION_MOVE:                Log.w(TAG, "[onTouchEvent] ACTION_MOVE");                break;            case MotionEvent.ACTION_UP:                Log.w(TAG, "[onTouchEvent] ACTION_UP");                break;            default:                break;        }        return super.onTouchEvent(event);    }    @Override    public boolean dispatchTouchEvent(MotionEvent event) {        switch (event.getAction()){            case MotionEvent.ACTION_DOWN:                Log.e(TAG, "[dispatchTouchEvent] ACTION_DOWN");                break;            case MotionEvent.ACTION_MOVE:                Log.e(TAG, "[dispatchTouchEvent] ACTION_MOVE");                break;            case MotionEvent.ACTION_UP:                Log.e(TAG, "[dispatchTouchEvent] ACTION_UP");                break;            default:                break;        }        return super.dispatchTouchEvent(event);    }}

布局文件就是一个线性布局包含刚才写的CustomButton。

<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:gravity="center"    android:orientation="vertical">    <com.example.mc_viewtouch.viewtouch.CustomButton        android:id="@+id/mButton"        android:background="#14fe14"        android:gravity="center"        android:layout_width="100dp"        android:layout_height="50dp"        android:text="点我!" /></LinearLayout>

在MainActivity里面设置customButton的一个setOnTouchListener方法和setOnClickListener方法，在他们的方法里面输出相应的事件日志；

public class MainActivity extends AppCompatActivity {    protected static final String TAG = "CustomButton";    private Button mButton ;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        mButton = (Button) findViewById(R.id.mButton);        mButton.setOnTouchListener(new View.OnTouchListener() {            @Override            public boolean onTouch(View v, MotionEvent event) {                switch (event.getAction()) {                    case MotionEvent.ACTION_DOWN:                        Log.d(TAG, "[onTouch] ACTION_DOWN");                        break;                    case MotionEvent.ACTION_MOVE:                        Log.d(TAG, "[onTouch] ACTION_MOVE");                        break;                    case MotionEvent.ACTION_UP:                        Log.d(TAG, "[onTouch] ACTION_UP");                        break;                    default:                        break;                }                return false;            }        });        mButton.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                Log.v(TAG, "[onClick]");            }        });    }}

我们点击CustomButton，输出以下的日志：

注意：这里的Move可能不止执行一次，手指滑了一下，会执行多次，我这里刚好执行一次而已；

通过打印的日志，我们可以得知这三个方法的执行顺序：dispatchTouchEvent -> onTouch -> onTouchEvent

二.分析源码
接下来我们要通过分析源码来深入了解View事件的分发机制。
我是通过grepcode 在线查看android 2.3的代码，这个版本的代码相比6.0简单一点，大家的核心代码也差不多，这里我就选取简单的，方便阅读。
我们首先看看dispatchTouchEvent的源码：

/**     * Pass the touch screen motion event down to the target view, or this     * view if it is the target.     *     * @param event The motion event to be dispatched.     * @return True if the event was handled by the view, false otherwise.     */    public boolean dispatchTouchEvent(MotionEvent event) {        if (!onFilterTouchEventForSecurity(event)) {            return false;        }        if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED &&                mOnTouchListener.onTouch(this, event)) {            return true;        }        return onTouchEvent(event);    }

我们来看看if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED && mOnTouchListener.onTouch(this, event))
当这个if语句的表达式结果为true的时候，就不会执行onTouchEvent,
1.那么首先看看第一个条件mOnTouchListener != null ，mOnTouchListener是从哪里的赋值的呢？跟一下源码，发现是在View`的setOnTouchListener方法赋值的，那就是说我们给该view设置一个setOnTouchListener方法的，这个mOnTouchListener就肯定不为空了。

public void setOnTouchListener(OnTouchListener l) {    mOnTouchListener = l;}

2.第二个条件(mViewFlags & ENABLED_MASK) == ENABLED 意思是判断view是否为enabled状态
3.第三个条件mOnTouchListener.onTouch(this, event)，回调注册view的Touch事件的onTouch方法，例如我们刚才在MainActivity的customButton设置setOnTouchListener，如果它的onTouch返回true,则View的onTouchEvent方法将不会执行；

所以这里可以总结一下：如果调用者将设置了view.setOnTouchListener监听，但在回调方法onTouch返回 true, 那么view的onTouchEvent将不会执行。

接下来，我们再看看View的onTouchEvent源码：

/**     * Implement this method to handle touch screen motion events.     *     * @param event The motion event.     * @return True if the event was handled, false otherwise.     */    public boolean onTouchEvent(MotionEvent event) {        final int viewFlags = mViewFlags;        if ((viewFlags & ENABLED_MASK) == DISABLED) {            // A disabled view that is clickable still consumes the touch            // events, it just doesn't respond to them.            //一个不可用状态下的view会消费touch事件，尽管它看起来不可用            return (((viewFlags & CLICKABLE) == CLICKABLE ||                    (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE));        }       //  如果View有设置代理，它会交给代理去处理touch事件        if (mTouchDelegate != null) {            if (mTouchDelegate.onTouchEvent(event)) {                return true;            }        }        // 如果view可以单击或者长按        if (((viewFlags & CLICKABLE) == CLICKABLE ||                (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {            switch (event.getAction()) {                case MotionEvent.ACTION_UP:                    boolean prepressed = (mPrivateFlags & PREPRESSED) != 0;                    if ((mPrivateFlags & PRESSED) != 0 || prepressed) {                        // take focus if we don't have it already and we should in                        // touch mode.                        boolean focusTaken = false;                        if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {                            focusTaken = requestFocus();                        }                        if (!mHasPerformedLongPress) {                            // This is a tap, so remove the longpress check                            removeLongPressCallback();                            // Only perform take click actions if we were in the pressed state                            if (!focusTaken) {                                // Use a Runnable and post this rather than calling                                // performClick directly. This lets other visual state                                // of the view update before click actions start.                                if (mPerformClick == null) {                                    mPerformClick = new PerformClick();                                }                                if (!post(mPerformClick)) {                                    performClick();                                }                            }                        }                        if (mUnsetPressedState == null) {                            mUnsetPressedState = new UnsetPressedState();                        }                        if (prepressed) {                            mPrivateFlags |= PRESSED;                            refreshDrawableState();                            postDelayed(mUnsetPressedState,                                    ViewConfiguration.getPressedStateDuration());                        } else if (!post(mUnsetPressedState)) {                            // If the post failed, unpress right now                            mUnsetPressedState.run();                        }                        removeTapCallback();                    }                    break;                case MotionEvent.ACTION_DOWN:                    //  延时执行CheckForTap的run方法                    if (mPendingCheckForTap == null) {                        mPendingCheckForTap = new CheckForTap();                    }                    mPrivateFlags |= PREPRESSED;                    mHasPerformedLongPress = false;                    postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());                    break;                case MotionEvent.ACTION_CANCEL:                    mPrivateFlags &= ~PRESSED;                    refreshDrawableState();                    removeTapCallback();                    break;                case MotionEvent.ACTION_MOVE:                    final int x = (int) event.getX();                    final int y = (int) event.getY();                    // Be lenient about moving outside of buttons                    int slop = mTouchSlop;                    if ((x < 0 - slop) || (x >= getWidth() + slop) ||                            (y < 0 - slop) || (y >= getHeight() + slop)) {                        // Outside button                        removeTapCallback();                        if ((mPrivateFlags & PRESSED) != 0) {                            // Remove any future long press/tap checks                            removeLongPressCallback();                            // Need to switch from pressed to not pressed                            mPrivateFlags &= ~PRESSED;                            refreshDrawableState();                        }                    }                    break;            }            return true;        }        return false;    }

我在源码里写了一点中文的注释，
我们重点留意一下ACTION_UP的PerformClick()方法

    /**     * Call this view's OnClickListener, if it is defined.     *     * @return True there was an assigned OnClickListener that was called, false     *         otherwise is returned.     */    public boolean performClick() {        sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);        if (mOnClickListener != null) {            playSoundEffect(SoundEffectConstants.CLICK);            mOnClickListener.onClick(this);            return true;        }        return false;    }

当我们为View设置OnClickListener监听的话，performClick()在onTouchEvent的ACTION_UP会执行，当mOnClickListener 不为空，会回调onClick()方法，所以在我们刚才的小程序中可以看出有打印【onClick】的日志。

总结：
View事件的分发流程大致如下：
dispatchEvent-> setOnTouchListener-> onTouchEvent