Android Touch事件分发过程

虽然网络上已经有非常多关于这个话题的优秀文章了，但还是写了这篇文章，主要还是为了加强自己的记忆吧，自己过一遍总比看别人的分析要深刻得多。那就走起吧。

简单演示样例

先看一个演示样例 :

布局文件 :

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_gravity="center"
    tools:context="com.example.touch_event.MainActivity"
    tools:ignore="MergeRootFrame" >

    <Button
        android:id="@+id/my_button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />

</FrameLayout>

MainActivity文件: 

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button mBtn = (Button) findViewById(R.id.my_button);
        mBtn.setOnTouchListener(new OnTouchListener() {

            @Override
            public boolean onTouch(View v, MotionEvent event) {
                Log.d("", "### onTouch : " + event.getAction());
                return false;
            }
        });
        mBtn.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                Log.d("", "### onClick : " + v);
            }
        });

    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.d("", "### activity dispatchTouchEvent");
        return super.dispatchTouchEvent(ev);
    }
}

当用户点击按钮时会输出例如以下Log, 

08-31 03:03:56.116: D/(1560): ### activity dispatchTouchEvent
08-31 03:03:56.116: D/(1560): ### onTouch : 0
08-31 03:03:56.196: D/(1560): ### activity dispatchTouchEvent
08-31 03:03:56.196: D/(1560): ### onTouch : 1
08-31 03:03:56.196: D/(1560): ### onClick : android.widget.Button{52860d98 VFED..C. ...PH... 0,0-1080,144 #7f05003d app:id/my_button}

我们能够看到首先运行了Activity中的dispatchTouchEvent方法，然后运行了onTouch方法，然后再是dispatchTouchEvent --> onTouch， 最后才是运行按钮的点击事件。

这里我们可能有个疑问，为什么dispatchTouchEvent和onTouch都运行了两次。而onClick才运行了一次 ？ 为什么两次的Touch事件的action不一样，action 0 和 action 1究竟代表了什么 ？

覆写过onTouchEvent的朋友知道。一般来说我们在该方法体内都会处理集中touch类型的事件。有ACTION_DOWN、ACTION_MOVE、ACTION_UP等，只是上面我们的样例中并没有移动。仅仅是单纯的按下、抬起。

因此。我们的触摸事件也仅仅有按下、抬起，因此有2次touch事件，而action分别为0和1。我们看看MotionEvent中的一些变量定义吧:

public final class MotionEvent extends InputEvent implements Parcelable {
    // 代码省略

    public static final int ACTION_DOWN             = 0;    // 按下事件

    public static final int ACTION_UP               = 1;    // 抬起事件 

    public static final int ACTION_MOVE             = 2;    // 手势移动事件

    public static final int ACTION_CANCEL           = 3;    // 取消
  // 代码省略
}

能够看到，代表按下的事件为0。抬起事件为1，也证实了我们上面所说的。

在看另外一个场景:

1、我们在onTouch函数中返回true, 而且点击按钮，输出Log例如以下 :

08-31 03:06:04.764: D/(1612): ### activity dispatchTouchEvent
08-31 03:06:04.764: D/(1612): ### onTouch : 0
08-31 03:06:04.868: D/(1612): ### activity dispatchTouchEvent
08-31 03:06:04.868: D/(1612): ### onTouch : 1

能够看到，按钮的点击事件并没有得到运行，为什么会这样呢 ？   我们继续往下看吧。

Android Touch事件分发

那么整个事件分发的流程是如何的呢 ？

简单来说就是用户触摸手机屏幕会产生一个触摸消息，终于这个触摸消息会被传送到ViewRoot ( 看4.2的源代码时这个类改成了ViewRootImpl )的InputHandler。ViewRoot是GUI管理系统与GUI呈现系统之间的桥梁，依据ViewRoot的定义，发现它并非一个View类型。而是一个Handler。InputHandler是一个接口类型，用于处理KeyEvent和TouchEvent类型的事件，我们看看源代码 :

public final class ViewRoot extends Handler implements ViewParent,
        View.AttachInfo.Callbacks {
            // 代码省略
    private final InputHandler mInputHandler = new InputHandler() {
        public void handleKey(KeyEvent event, Runnable finishedCallback) {
            startInputEvent(finishedCallback);
            dispatchKey(event, true);
        }
        public void handleMotion(MotionEvent event, Runnable finishedCallback) {
            startInputEvent(finishedCallback);
            dispatchMotion(event, true);      // 1、handle 触摸消息
        }
    };
       // 代码省略
    // 2、分发触摸消息
    private void dispatchMotion(MotionEvent event, boolean sendDone) {
        int source = event.getSource();
        if ((source & InputDevice.SOURCE_CLASS_POINTER) != 0) {
            dispatchPointer(event, sendDone);      // 分发触摸消息
        } else if ((source & InputDevice.SOURCE_CLASS_TRACKBALL) != 0) {
            dispatchTrackball(event, sendDone);
        } else {
            // TODO
            Log.v(TAG, "Dropping unsupported motion event (unimplemented): " + event);
            if (sendDone) {
                finishInputEvent();
            }
        }
    }
    // 3、通过Handler投递消息
    private void dispatchPointer(MotionEvent event, boolean sendDone) {
        Message msg = obtainMessage(DISPATCH_POINTER);
        msg.obj = event;
        msg.arg1 = sendDone ?

1 : 0;
        sendMessageAtTime(msg, event.getEventTime());
    }
    @Override
    public void handleMessage(Message msg) {           // ViewRoot覆写handlerMessage来处理各种消息
        switch (msg.what) {
            // 代码省略
        case DO_TRAVERSAL:// 重绘整个View Tree的消息
            if (mProfile) {
                Debug.startMethodTracing("ViewRoot");
            }

            performTraversals();// 遍历整个View Tree。运行measure,layout,draw这几个过程.

            if (mProfile) {
                Debug.stopMethodTracing();
                mProfile = false;
            }
            break;

        case DISPATCH_POINTER: {        // 4、处理DISPATCH_POINTER类型的消息，即触摸屏幕的消息
            MotionEvent event = (MotionEvent) msg.obj;
            try {
                deliverPointerEvent(event); // 5、处理触摸消息
            } finally {
                event.recycle();
                if (msg.arg1 != 0) {
                    finishInputEvent();
                }
                if (LOCAL_LOGV || WATCH_POINTER) Log.i(TAG, "Done dispatching!");
            }
        } break;
        // 代码省略
    }
    // 6、真正的处理事件
    private void deliverPointerEvent(MotionEvent event) {
        if (mTranslator != null) {
            mTranslator.translateEventInScreenToAppWindow(event);
        }
        boolean handled;
        if (mView != null && mAdded) {
            // enter touch mode on the down
            boolean isDown = event.getAction() == MotionEvent.ACTION_DOWN;
            if (isDown) {
                ensureTouchMode(true);    // 假设是ACTION_DOWN事件则进入触摸模式。否则为按键模式。
            }
            if(Config.LOGV) {
                captureMotionLog("captureDispatchPointer", event);
            }
            if (mCurScrollY != 0) {
                event.offsetLocation(0, mCurScrollY);   // 物理坐标向逻辑坐标的转换
            }
            if (MEASURE_LATENCY) {
                lt.sample("A Dispatching TouchEvents", System.nanoTime() - event.getEventTimeNano());
            }
            // 7、分发事件。假设是窗体类型。则这里的mView相应的就是PhonwWindow中的DecorView，否则为根视图的ViewGroup。
            handled = mView.dispatchTouchEvent(event);
            // 8、假设终于事件没有被处理,且是ACTION_DOWN事件。那么就会交给mView，即DecorView类来处理.
            if (!handled && isDown) {
                int edgeSlop = mViewConfiguration.getScaledEdgeSlop();

                final int edgeFlags = event.getEdgeFlags();
                int direction = View.FOCUS_UP;
                int x = (int)event.getX();
                int y = (int)event.getY();
                final int[] deltas = new int[2];

                if ((edgeFlags & MotionEvent.EDGE_TOP) != 0) {
                    direction = View.FOCUS_DOWN;
                    if ((edgeFlags & MotionEvent.EDGE_LEFT) != 0) {
                        deltas[0] = edgeSlop;
                        x += edgeSlop;
                    } else if ((edgeFlags & MotionEvent.EDGE_RIGHT) != 0) {
                        deltas[0] = -edgeSlop;
                        x -= edgeSlop;
                    }
                } else if ((edgeFlags & MotionEvent.EDGE_BOTTOM) != 0) {
                    direction = View.FOCUS_UP;
                    if ((edgeFlags & MotionEvent.EDGE_LEFT) != 0) {
                        deltas[0] = edgeSlop;
                        x += edgeSlop;
                    } else if ((edgeFlags & MotionEvent.EDGE_RIGHT) != 0) {
                        deltas[0] = -edgeSlop;
                        x -= edgeSlop;
                    }
                } else if ((edgeFlags & MotionEvent.EDGE_LEFT) != 0) {
                    direction = View.FOCUS_RIGHT;
                } else if ((edgeFlags & MotionEvent.EDGE_RIGHT) != 0) {
                    direction = View.FOCUS_LEFT;
                }

                if (edgeFlags != 0 && mView instanceof ViewGroup) {
                    View nearest = FocusFinder.getInstance().findNearestTouchable(
                            ((ViewGroup) mView), x, y, direction, deltas);
                    if (nearest != null) {
                        event.offsetLocation(deltas[0], deltas[1]);
                        event.setEdgeFlags(0);
                        // 看到,mView处理了该事件,也就是DecorView或者最*的ViewGroup.
                        mView.dispatchTouchEvent(event);
                    }
                }
            }
           }
        }
    }
   // 代码省略
} 

经过层层迷雾。无论代码7处的mView是DecorView还是非窗体界面的根视图，其本质都是ViewGroup，即触摸事件终于被根视图ViewGroup进行分发。！！

我们就以Activity为例来分析这个过程，我们知道显示出来的Activity有一个顶层窗体。这个窗体的实现类是PhoneWindow, PhoneWindow中的内容区域是一个DecorView类型的View，这个View这就是我们在手机上看到的内容，这个DecorView是FrameLayout的子类，Activity的的dispatchTouchEvent实际上就是调用PhoneWindow的dispatchTouchEvent。我们看看源代码吧，进入Activity的dispatchTouchEvent函数 :

  public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }
        if (getWindow().superDispatchTouchEvent(ev)) {     // 1、调用的是PhoneWindow的superDispatchTouchEvent(ev)

            return true;
        }
        return onTouchEvent(ev);
    }

    public void onUserInteraction() {
    }

能够看到，假设事件为按下事件，则会进入到onUserInteraction()这个函数，该函数为空实现，我们暂且无论它。

继续看，发现touch事件的分发调用了getWindow().superDispatchTouchEvent(ev)函数。getWindow()获取到的实例的类型为PhoneWindow类型。你能够在你的Activity类中使用例如以下方式查看getWindow()获取到的类型：

 Log.d("", "### Activiti中getWindow()获取的类型是 : " + this.getWindow()) ;

输出:

08-31 03:40:17.036: D/(1688): ### Activiti中getWindow()获取的类型是 : com.android.internal.policy.impl.PhoneWindow@5287fe38

OK，废话不多说。我们还是继续看PhoneWindow中的superDispatchTouchEvent函数吧。

    @Override
    public boolean superDispatchTouchEvent(MotionEvent event) {
        return mDecor.superDispatchTouchEvent(event);
    }

恩，调用的是mDecor的superDispatchTouchEvent(event)函数，这个mDecor就是我们上面所说的DecorView类型，也就是我们看到的Activity上的全部内容的一个顶层ViewGroup，即整个ViewTree的根节点。看看它的声明吧。

    // This is the top-level view of the window, containing the window decor.
    private DecorView mDecor;

DecorView

那么我继续看看DecorView究竟是个什么玩意儿吧。

   private final class DecorView extends FrameLayout implements RootViewSurfaceTaker {
        /* package */int mDefaultOpacity = PixelFormat.OPAQUE;

        /** The feature ID of the panel, or -1 if this is the application's DecorView */
        private final int mFeatureId;

        private final Rect mDrawingBounds = new Rect();

        private final Rect mBackgroundPadding = new Rect();

        private final Rect mFramePadding = new Rect();

        private final Rect mFrameOffsets = new Rect();

        private boolean mChanging;

        private Drawable mMenuBackground;
        private boolean mWatchingForMenu;
        private int mDownY;

        public DecorView(Context context, int featureId) {
            super(context);
            mFeatureId = featureId;
        }

        @Override
        public boolean dispatchKeyEvent(KeyEvent event) {
            final int keyCode = event.getKeyCode();
            // 代码省略
            return isDown ?

PhoneWindow.this.onKeyDown(mFeatureId, event.getKeyCode(), event)
                    : PhoneWindow.this.onKeyUp(mFeatureId, event.getKeyCode(), event);
        }

        @Override
        public boolean dispatchTouchEvent(MotionEvent ev) {
            final Callback cb = getCallback();
            return cb != null && mFeatureId < 0 ? cb.dispatchTouchEvent(ev) : super
                    .dispatchTouchEvent(ev);
        }

        @Override
        public boolean dispatchTrackballEvent(MotionEvent ev) {
            final Callback cb = getCallback();
            return cb != null && mFeatureId < 0 ?

cb.dispatchTrackballEvent(ev) : super
                    .dispatchTrackballEvent(ev);
        }

        public boolean superDispatchKeyEvent(KeyEvent event) {
            return super.dispatchKeyEvent(event);
        }

        public boolean superDispatchTouchEvent(MotionEvent event) {
            return super.dispatchTouchEvent(event);
        }

        public boolean superDispatchTrackballEvent(MotionEvent event) {
            return super.dispatchTrackballEvent(event);
        }

        @Override
        public boolean onTouchEvent(MotionEvent event) {
            return onInterceptTouchEvent(event);
        }
// 代码省略
}

能够看到，DecorView继承自FrameLayout， 它对于touch事件的分发( dispatchTouchEvent )、处理都是交给super类来处理。也就是FrameLayout来处理，我们在FrameLayout中没有看到相应的实现，那继续跟踪到FrameLayout的父类。即ViewGroup，我们看到了dispatchTouchEvent的实现，那我们就先看ViewGroup (Android 2.3 源代码)是如何进行事件分发的吧。

ViewGroup的Touch事件分发

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (!onFilterTouchEventForSecurity(ev)) {
            return false;
        }

        final int action = ev.getAction();
        final float xf = ev.getX();
        final float yf = ev.getY();
        final float scrolledXFloat = xf + mScrollX;
        final float scrolledYFloat = yf + mScrollY;
        final Rect frame = mTempRect;
        // 是否禁用拦截，假设为true表示不能拦截事件；反之，则为能够拦截事件
        boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
        // ACTION_DOWN事件。即按下事件
        if (action == MotionEvent.ACTION_DOWN) {
            if (mMotionTarget != null) {
                // this is weird, we got a pen down, but we thought it was
                // already down!
                // XXX: We should probably send an ACTION_UP to the current
                // target.
                mMotionTarget = null;
            }
            // If we're disallowing intercept or if we're allowing and we didn't
            // intercept。假设不同意事件拦截或者不拦截该事件，那么运行以下的操作
            if (disallowIntercept || !onInterceptTouchEvent(ev))         // 1、是否禁用拦截、是否拦截事件的推断
                // reset this event's action (just to protect ourselves)
                ev.setAction(MotionEvent.ACTION_DOWN);
                // We know we want to dispatch the event down, find a child
                // who can handle it, start with the front-most child.
                final int scrolledXInt = (int) scrolledXFloat;
                final int scrolledYInt = (int) scrolledYFloat;
                final View[] children = mChildren;
                final int count = mChildrenCount;

                for (int i = count - 1; i >= 0; i--)        // 2、迭代全部子view，查找触摸事件在哪个子view的坐标范围内
                    final View child = children[i];
                    // 该child是可见的
                    if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE
                            || child.getAnimation() != null) {
                        // 3、获取child的坐标范围
                        child.getHitRect(frame);
                        // 4、推断发生该事件坐标是否在该child坐标范围内
                        if (frame.contains(scrolledXInt, scrolledYInt))
                            // offset the event to the view's coordinate system
                            final float xc = scrolledXFloat - child.mLeft;
                            final float yc = scrolledYFloat - child.mTop;
                            ev.setLocation(xc, yc);
                            child.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
                            // 5、child处理该事件，假设返回true，那么mMotionTarget为该child。正常情况下,
                            // dispatchTouchEvent(ev)的返回值即onTouchEcent的返回值。因此onTouchEcent假设返回为true，
                            // 那么mMotionTarget为触摸事件所在位置的child。
                            if (child.dispatchTouchEvent(ev))
                                // Event handled, we have a target now.
                                mMotionTarget = child;
                                return true;
                            }

                        }
                    }
                }
            }
        }// end if

        boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) ||
                (action == MotionEvent.ACTION_CANCEL);

        if (isUpOrCancel) {
            // Note, we've already copied the previous state to our local
            // variable, so this takes effect on the next event
            mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
        }

        // The event wasn't an ACTION_DOWN, dispatch it to our target if
        // we have one.
        final View target = mMotionTarget;
        // 6、假设mMotionTarget为空。那么运行super.super.dispatchTouchEvent(ev)，
        // 即View.dispatchTouchEvent(ev),就是该View Group自己处理该touch事件，仅仅是又走了一遍View的分发过程而已.
        // 拦截事件或者在不拦截事件且target view的onTouchEvent返回false的情况都会运行到这一步.
        if (target == null) {
            // We don't have a target, this means we're handling the
            // event as a regular view.
            ev.setLocation(xf, yf);
            if ((mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {
                ev.setAction(MotionEvent.ACTION_CANCEL);
                mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
            }
            return super.dispatchTouchEvent(ev);
        }

        // if have a target, see if we're allowed to and want to intercept its
        // events
        // 7、假设没有禁用事件拦截。而且onInterceptTouchEvent(ev)返回为true，即进行事件拦截.
        if (!disallowIntercept && onInterceptTouchEvent(ev)) {
            final float xc = scrolledXFloat - (float) target.mLeft;
            final float yc = scrolledYFloat - (float) target.mTop;
            mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
            ev.setAction(MotionEvent.ACTION_CANCEL);
            ev.setLocation(xc, yc);
            //
            if (!target.dispatchTouchEvent(ev)) {
                // target didn't handle ACTION_CANCEL. not much we can do
                // but they should have.
            }
            // clear the target
            mMotionTarget = null;
            // Don't dispatch this event to our own view, because we already
            // saw it when intercepting; we just want to give the following
            // event to the normal onTouchEvent().
            return true;
        }

        if (isUpOrCancel) {
            mMotionTarget = null;
        }

        // finally offset the event to the target's coordinate system and
        // dispatch the event.
        final float xc = scrolledXFloat - (float) target.mLeft;
        final float yc = scrolledYFloat - (float) target.mTop;
        ev.setLocation(xc, yc);

        if ((target.mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {
            ev.setAction(MotionEvent.ACTION_CANCEL);
            target.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
            mMotionTarget = null;
        }
        // 事件不拦截，且target view在ACTION_DOWN时返回true，那么兴许事件由target来处理事件
        return target.dispatchTouchEvent(ev);
    }

这个函数代码比較长，我们仅仅看上文中标注的几个关键点。

首先在代码1处能够看到一个条件推断，假设disallowIntercept和!onInterceptTouchEvent(ev)两者有一个为true。就会进入到这个条件推断中。disallowIntercept是指是否禁用掉事件拦截的功能，默认是false，也能够通过调用requestDisallowInterceptTouchEvent方法对这个值进行改动。

那么当第一个值为false的时候就会全然依赖第二个值来决定能否够进入到条件推断的内部，第二个值是什么呢？onInterceptTouchEvent就是ViewGroup对事件进行拦截的一个函数。返回该函数返回false则表示不拦截事件。反之则表示拦截。第二个条件是是对onInterceptTouchEvent方法的返回值取反。也就是说假设我们在onInterceptTouchEvent方法中返回false。就会让第二个值为true。从而进入到条件推断的内部，假设我们在onInterceptTouchEvent方法中返回true，就会让第二个值的总体变为false，从而跳出了这个条件推断。比如我们须要实现ListView滑动删除某一项的功能。那么能够通过在onInterceptTouchEvent返回true,而且在onTouchEvent中实现相关的推断逻辑，从而实现该功能。

进入代码1内部的if后。有一个for循环。遍历了当前ViewGroup下的全部子child view。假设触摸该事件的坐标在某个child view的坐标范围内，那么该child view来处理这个触摸事件,即调用该child view的dispatchTouchEvent。

假设该child view是ViewGroup类型，那么继续运行上面的推断。而且遍历子view。假设该child view不是ViewGroup类型，那么直接调用的是View中的dispatchTouchEvent方法,除非这个child view的类型覆写了该方法。我们看看View中的dispatchTouchEvent函数：

View的Touch事件分发

    /**
     * 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);
    }

该函数中，首先推断该事件是否符合安全策略，然后推断该view是否是enable的 ，以及是否设置了Touch Listener，mOnTouchListener即我们通过setOnTouchListener设置的。

    /**
     * Register a callback to be invoked when a touch event is sent to this view.
     * @param l the touch listener to attach to this view
     */
    public void setOnTouchListener(OnTouchListener l) {
        mOnTouchListener = l;
    }

假设mOnTouchListener.onTouch(this, event)返回false则继续运行onTouchEvent(event)；假设mOnTouchListener.onTouch(this, event)返回true，则表示该事件被消费了，不再传递，因此也不会运行onTouchEvent(event)。这也验证了我们上文中留下的场景2。当onTouch函数返回true时，点击按钮，但我们的点击事件没有运行。

那么我们还是先来看看onTouchEvent(event)函数究竟做了什么吧。

    /**
     * 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)        // 1、推断该view是否enable
            // A disabled view that is clickable still consumes the touch
            // events, it just doesn't respond to them.
            return (((viewFlags & CLICKABLE) == CLICKABLE ||
                    (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE));
        }

        if (mTouchDelegate != null) {
            if (mTouchDelegate.onTouchEvent(event)) {
                return true;
            }
        }

        if (((viewFlags & CLICKABLE) == CLICKABLE ||
                (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) // 2、是否是clickable或者long clickable
            switch (event.getAction()) {
                case MotionEvent.ACTION_UP:                    // 抬起事件
                    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();       // 获取焦点
                        }

                        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))     // post
                                    performClick();          // 3、点击事件处理
                                }
                            }
                        }

                        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:
                    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;
    }

我们看到，在onTouchEvent函数中就是对ACTION_UP、ACTION_DOWN、ACTION_MOVE等几个事件进行处理。而最重要的就是UP事件了。由于这个里面包括了对用户点击事件的处理，或者是说对于用户而言相对重要一点，因此放在了第一个case中。

在ACTION_UP事件中会推断该view是否enable、是否clickable、是否获取到了焦点。然后我们看到会通过post方法将一个PerformClick对象投递给UI线程。假设投递失败则直接调用performClick函数运行点击事件。

    /**
     * Causes the Runnable to be added to the message queue.
     * The runnable will be run on the user interface thread.
     *
     * @param action The Runnable that will be executed.
     *
     * @return Returns true if the Runnable was successfully placed in to the
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.
     */
    public boolean post(Runnable action) {
        Handler handler;
        if (mAttachInfo != null) {
            handler = mAttachInfo.mHandler;
        } else {
            // Assume that post will succeed later
            ViewRoot.getRunQueue().post(action);
            return true;
        }

        return handler.post(action);
    }

我们看看PerformClick类吧。

    private final class PerformClick implements Runnable {
        public void run() {
            performClick();
        }
    }

能够看到，其内部就是包装了View类中的performClick()方法。再看performClick()方法：

   /**
     * Register a callback to be invoked when this view is clicked. If this view is not
     * clickable, it becomes clickable.
     *
     * @param l The callback that will run
     *
     * @see #setClickable(boolean)
     */
    public void setOnClickListener(OnClickListener l) {
        if (!isClickable()) {
            setClickable(true);
        }
        mOnClickListener = l;
    }

    /**
     * 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;
    }

代码非常easy，主要就是调用了mOnClickListener.onClick(this);方法，即运行用户通过setOnClickListener设置进来的点击事件处理Listener。

总结

用户触摸屏幕产生一个触摸消息。系统底层将该消息转发给ViewRoot ( ViewRootImpl )，ViewRoot产生一个DISPATCHE_POINTER的消息，而且在handleMessage中处理该消息，终于会通过deliverPointerEvent(MotionEvent event)来处理该消息。

在该函数中会调用mView.dispatchTouchEvent(event)来分发消息,该mView是一个ViewGroup类型，因此是ViewGroup的dispatchTouchEvent(event)。在该函数中会遍历全部的child view，找到该事件的触发的左边与每一个child view的坐标进行对照，假设触摸的坐标在该child view的范围内，则由该child view进行处理。假设该child view是ViewGroup类型。则继续上一步的查找过程；否则运行View中的dispatchTouchEvent(event)函数。

在View的dispatchTouchEvent(event)中首先推断该控件是否enale以及mOnTouchListent是否为空，假设mOnTouchListener不为空则运行mOnTouchListener.onTouch(event)方法，假设该方法返回false则再运行View中的onTouchEvent(event)方法，而且在该方法中运行mOnClickListener.onClick(this, event) ;方法。 假设mOnTouchListener.onTouch(event)返回true则不会运行onTouchEvent方法，因此点击事件也不会被运行。

粗略的流程图例如以下 :

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