Android(安卓)浅析View的事件分发机制

对于一个Android开发者来说View事件分发机制是不得不了解的.不了解事件分发机制的话很多自定义控件都无法写出来.今天我们一起来看看View的事件分发机制是如何实现的.

首先来看个栗子:

自定义一个MyButton继承Button并重写dispatchTouchEvent和onTouchEvent方法

@Override    public boolean dispatchTouchEvent(MotionEvent ev) {        switch (ev.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;        }        return super.dispatchTouchEvent(ev);    }    @Override    public boolean onTouchEvent(MotionEvent event) {        switch (event.getAction()){            case MotionEvent.ACTION_DOWN:                Log.e(TAG, "onTouchEvent ACTION_DOWN");                break;            case MotionEvent.ACTION_MOVE:                Log.e(TAG, "onTouchEvent ACTION_MOVE");                break;            case MotionEvent.ACTION_UP:                Log.e(TAG, "onTouchEvent ACTION_UP");                break;        }        return super.onTouchEvent(event);    }

xml里面调用

        MyButton mButton= (MyButton) findViewById(R.id.my_bt);        mButton.setOnTouchListener(new View.OnTouchListener() {            @Override            public boolean onTouch(View v, MotionEvent event) {                switch (event.getAction()) {                    case MotionEvent.ACTION_DOWN:                        Log.e(TAG, "onTouch ACTION_DOWN");                        break;                    case MotionEvent.ACTION_MOVE:                        Log.e(TAG, "onTouch ACTION_MOVE");                        break;                    case MotionEvent.ACTION_UP:                        Log.e(TAG, "onTouch ACTION_UP");                        break;                }                return false;            }        });        mButton.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                Log.e(TAG, "onClick");            }        });        mButton.setOnLongClickListener(new View.OnLongClickListener() {            @Override            public boolean onLongClick(View v) {                Log.e(TAG, "onLongClick");                return false;            }        });

当我们只点击一下的时候日志打出

08-14 11:47:38.173 15712-15712/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_DOWN08-14 11:47:38.173 15712-15712/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_DOWN08-14 11:47:38.174 15712-15712/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_DOWN08-14 11:47:38.199 15712-15712/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_UP08-14 11:47:38.199 15712-15712/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_UP08-14 11:47:38.199 15712-15712/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_UP08-14 11:47:38.200 15712-15712/fun.hxy.com.testalg E/MainActivity: onClick

我们发现时间都是从dispatchTouchEvent开始的然后onTouch->onTouchEvent->onClick

为什么会这样呢?我们来看一下View的源码:

public boolean dispatchTouchEvent(MotionEvent event) {1      ....2       if (li != null && li.mOnTouchListener != null                    && (mViewFlags & ENABLED_MASK) == ENABLED                    && li.mOnTouchListener.onTouch(this, event)) {                result = true;            }3            if (!result && onTouchEvent(event)) {                result = true;            }               .....        return result;    }

在View的dispatchTouchEvent里面我们可以发现View先判断li.mOnTouchListener.onTouch(this, event)先执行了onTouch()事件在到第三行的onTouchEvent(),并且如果onTouch()的返回值返回了true的话就不会在往下执行onTouchEvent()方法了;

我们在看看onTouchEvent()的源码:

public boolean onTouchEvent(MotionEvent event) {                ......            switch (action) {                case MotionEvent.ACTION_UP:                     if (!focusTaken) {                                                  if (!post(mPerformClick)) {                             performClick();                         }                     }                                          break;                       }            return true;        }        return false;    }

我们发现onClick的方法是在onTouch()方法的up里面执行的.这就是为什么View点击的时候会先调用onTouch() -> onTouchEvent() -> onClick(),面试的时候经常会问到

我们在来看一下长按事件:

现在我们长按button按钮

08-14 14:16:16.725 11084-11084/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_DOWN08-14 14:16:16.726 11084-11084/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_DOWN08-14 14:16:16.726 11084-11084/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_DOWN08-14 14:16:16.875 11084-11084/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_MOVE08-14 14:16:16.875 11084-11084/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_MOVE08-14 14:16:16.875 11084-11084/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_MOVE08-14 14:16:16.890 11084-11084/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_MOVE08-14 14:16:16.890 11084-11084/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_MOVE08-14 14:16:16.890 11084-11084/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_MOVE08-14 14:16:16.906 11084-11084/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_MOVE......08-14 14:16:17.142 11084-11084/fun.hxy.com.testalg E/MainActivity: onLongClick08-14 14:16:17.189 11084-11084/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_MOVE08-14 14:16:17.190 11084-11084/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_MOVE08-14 14:16:17.190 11084-11084/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_MOVE.....08-14 14:16:17.948 11084-11084/fun.hxy.com.testalg E/MyButton: dispatchTouchEvent ACTION_UP08-14 14:16:17.949 11084-11084/fun.hxy.com.testalg E/MainActivity: onTouch ACTION_UP08-14 14:16:17.949 11084-11084/fun.hxy.com.testalg E/MyButton: onTouchEvent ACTION_UP08-14 14:16:17.954 11084-11084/fun.hxy.com.testalg E/MainActivity: onClick

我们发现长按的时候会执行ACTION_DOWN因为手指按住可能会有一点抖,会执行不等次的ACTION_MOVE,然后先执行onLongClick后面放手在执行onClick.

再次翻看onTouchEvent()的源码:

public boolean onTouchEvent(MotionEvent event) {        switch (action) {            case MotionEvent.ACTION_UP:                ......                                if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {                    if (!post(mPerformClick)) {                        performClick();                    }                }                ......                break;            case MotionEvent.ACTION_DOWN:                ......                checkForLongClick(0, x, y);                ......                break;        }        return false;    }

我们发现在ACTION_DOWN的时候,系统就会执行checkForLongClick方法

private void checkForLongClick(int delayOffset, float x, float y) {        if (mPendingCheckForLongPress == null) {                mPendingCheckForLongPress = new CheckForLongPress();            }            postDelayed(mPendingCheckForLongPress,                    ViewConfiguration.getLongPressTimeout() - delayOffset);            }

这个checkForLongClick里面执行了一个延迟操作延迟五百毫秒时候就会执行CheckForLongPress这个延迟事件

@Override        public void run() {            if (isPressed() && (mParent != null)                    && mOriginalWindowAttachCount == mWindowAttachCount) {                if (performLongClick(mX, mY)) {                    mHasPerformedLongPress = true;                }            }        }

这个方法里执行了

    public boolean performLongClick(float x, float y) {        mLongClickX = x;        mLongClickY = y;        final boolean handled = performLongClick();        mLongClickX = Float.NaN;        mLongClickY = Float.NaN;        return handled;    }

我们可以看到performLongClick这个方法的返回值为boolean类型,所以当onLongClickListener方法返回值改为true的时候,mHasPerformedLongPress 被置为true,从上面又可以看到只有在mHasPerformedLongPress为false的时候performClick()方法才会执行.

好了,到这里View的事件机制差不多就看完一遍了,View的事件机制还是比较简单的,下一篇我将跟大家一起学习一下ViewGroup的事件机制.希望大家都能有所收获.

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