发布于 2016-07-04 10:24:58 | 28 次阅读 | 评论: 0 | 来源: 网友投递

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Android移动端操作系统

Android是一种基于Linux的自由及开放源代码的操作系统,主要使用于移动设备,如智能手机和平板电脑,由Google公司和开放手机联盟领导及开发。尚未有统一中文名称,中国大陆地区较多人使用“安卓”或“安致”。


这篇文章主要为大家详细介绍了Android事件的分发机制,感兴趣的小伙伴们可以参考一下

在分析Android事件分发机制前,明确android的两大基础控件类型:View和ViewGroup。View即普通的控件,没有子布局的,如Button、TextView. ViewGroup继承自View,表示可以有子控件,如Linearlayout、Listview这些。今天我们先来了解View的事件分发机制。
先看下代码,非常简单,只有一个Button,分别给它注册了OnClick和OnTouch的点击事件。


btn.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        Log.i("Tag", "This is button onClick event");
      }
    });
    btn.setOnTouchListener(new View.OnTouchListener() {
      @Override
      public boolean onTouch(View v, MotionEvent event) {
        Log.i("Tag", "This is button onTouch action" + event.getAction());
        return false;
      }
    });

运行一下项目,结果如下:
 I/Tag: This is button onTouch action0
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action1
 I/Tag: This is button onClick event 
可以看到,onTouch是有先于onClick执行的,因此事件的传递顺序是先onTouch,在到OnClick。具体为什么这样,下面会通过源码来说明。这时,我们可能注意到了,onTouch的方法是有返回值,这里是返回false,我们将它改为true再运行一次,结果如下:
 I/Tag: This is button onTouch action0
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action1

对比两次结果,我们发现onClick方法不再执行,为什么会这样,下面我将通过源码给大家一步步理清这个思路。
查看源码时,首先要知道所有View类型控件事件入口都是dispatchTouchEvent(),所以我们直接进入到View这个类里面的dispatchTouchEvent()方法看一下。 


public boolean dispatchTouchEvent(MotionEvent event) {
    // If the event should be handled by accessibility focus first.
    if (event.isTargetAccessibilityFocus()) {
      // We don't have focus or no virtual descendant has it, do not handle the event.
      if (!isAccessibilityFocusedViewOrHost()) {
        return false;
      }
      // We have focus and got the event, then use normal event dispatch.
      event.setTargetAccessibilityFocus(false);
    }
    boolean result = false;
    if (mInputEventConsistencyVerifier != null) {
      mInputEventConsistencyVerifier.onTouchEvent(event, 0);
    }
    final int actionMasked = event.getActionMasked();
    if (actionMasked == MotionEvent.ACTION_DOWN) {
      // Defensive cleanup for new gesture
      stopNestedScroll();
    }
    if (onFilterTouchEventForSecurity(event)) {
      //noinspection SimplifiableIfStatement
      ListenerInfo li = mListenerInfo;
      if (li != null && li.mOnTouchListener != null
          && (mViewFlags & ENABLED_MASK) == ENABLED
          && li.mOnTouchListener.onTouch(this, event)) {
        result = true;
      }
      if (!result && onTouchEvent(event)) {
        result = true;
      }
    }
    if (!result && mInputEventConsistencyVerifier != null) {
      mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
    }
    // Clean up after nested scrolls if this is the end of a gesture;
    // also cancel it if we tried an ACTION_DOWN but we didn't want the rest
    // of the gesture.
    if (actionMasked == MotionEvent.ACTION_UP ||
        actionMasked == MotionEvent.ACTION_CANCEL ||
        (actionMasked == MotionEvent.ACTION_DOWN && !result)) {
      stopNestedScroll();
    }
    return result;
  }

从源码第25行处可以看到,mOnTouchListener.onTouch()的方法首先被执行,如果li != null && li.mOnTouchListener != null&& (mViewFlags & ENABLED_MASK) == ENABLED&& li.mOnTouchListener.onTouch(this, event)都为真的话,result赋值为true,否则就执行onTouchEvent(event)方法。

从上面可以看到要符合条件有四个,
 1、ListenerInfo li,它是view中的一个静态类,里面定义view的事件的监听等等,所以有涉及到view的事件,ListenerInfo都会被实例化,因此li不为null
 2、mOnTouchiListener是在setOnTouchListener方法里面赋值的,只要touch事件被注册,mOnTouchiListener一定不会null
 3、 (mViewFlags & ENABLED_MASK) == ENABLED,是判断当前点击的控件是否是enable的,button默认为enable,这个条件也恒定为true,
 4、重点来了,li.mOnTouchListener.onTouch(this, event)就是回调控件onTouch方法,当这个条件也为true时,result=true,onTouchEvent(event)将不会被执行。如果onTouch返回false,就会再执行onTouchEvent(event)方法。
我们接着再进入到onTouchEvent方法查看源码。


public boolean onTouchEvent(MotionEvent event) {
    final float x = event.getX();
    final float y = event.getY();
    final int viewFlags = mViewFlags;
    final int action = event.getAction();
    if ((viewFlags & ENABLED_MASK) == DISABLED) {
      if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
        setPressed(false);
      }
      // 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)
          || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);
    }
    if (mTouchDelegate != null) {
      if (mTouchDelegate.onTouchEvent(event)) {
        return true;
      }
    }
    if (((viewFlags & CLICKABLE) == CLICKABLE ||
        (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
        (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
      switch (action) {
        case MotionEvent.ACTION_UP:
          boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
          if ((mPrivateFlags & PFLAG_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 (prepressed) {
              // The button is being released before we actually
              // showed it as pressed. Make it show the pressed
              // state now (before scheduling the click) to ensure
              // the user sees it.
              setPressed(true, x, y);
            }
            if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
              // 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) {
              postDelayed(mUnsetPressedState,
                  ViewConfiguration.getPressedStateDuration());
            } else if (!post(mUnsetPressedState)) {
              // If the post failed, unpress right now
              mUnsetPressedState.run();
            }
            removeTapCallback();
          }
          mIgnoreNextUpEvent = false;
          break;
        case MotionEvent.ACTION_DOWN:
          mHasPerformedLongPress = false;
          if (performButtonActionOnTouchDown(event)) {
            break;
          }
          // Walk up the hierarchy to determine if we're inside a scrolling container.
          boolean isInScrollingContainer = isInScrollingContainer();
          // For views inside a scrolling container, delay the pressed feedback for
          // a short period in case this is a scroll.
          if (isInScrollingContainer) {
            mPrivateFlags |= PFLAG_PREPRESSED;
            if (mPendingCheckForTap == null) {
              mPendingCheckForTap = new CheckForTap();
            }
            mPendingCheckForTap.x = event.getX();
            mPendingCheckForTap.y = event.getY();
            postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
          } else {
            // Not inside a scrolling container, so show the feedback right away
            setPressed(true, x, y);
            checkForLongClick(0);
          }
          break;
        case MotionEvent.ACTION_CANCEL:
          setPressed(false);
          removeTapCallback();
          removeLongPressCallback();
          mInContextButtonPress = false;
          mHasPerformedLongPress = false;
          mIgnoreNextUpEvent = false;
          break;
        case MotionEvent.ACTION_MOVE:
          drawableHotspotChanged(x, y);
          // Be lenient about moving outside of buttons
          if (!pointInView(x, y, mTouchSlop)) {
            // Outside button
            removeTapCallback();
            if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
              // Remove any future long press/tap checks
              removeLongPressCallback();
              setPressed(false);
            }
          }
          break;
      }
      return true;
    }
    return false;
  }

从源码的21行我们可以看出,该控件可点击就会进入到switch判断中,当我们触发了手指离开的实际,则会进入到MotionEvent.ACTION_UP这个case当中。我们接着往下看,在源码的50行,调用到了mPerformClick()方法,我们继续进入到这个方法的源码看看。 


public boolean performClick() {
    final boolean result;
    final ListenerInfo li = mListenerInfo;
    if (li != null && li.mOnClickListener != null) {
      playSoundEffect(SoundEffectConstants.CLICK);
      li.mOnClickListener.onClick(this);
      result = true;
    } else {
      result = false;
    }
    sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
    return result;
  }

现在我们可以看到,只要ListenerInfo和mOnClickListener不为null就会调用onClick这个方法,之前说过,只要有监听事件,ListenerInfo就不为null,带mOnClickListener又是在哪里赋值呢?我们再继续看下它的源码。


public void setOnClickListener(@Nullable OnClickListener l) {
    if (!isClickable()) {
      setClickable(true);
    }
    getListenerInfo().mOnClickListener = l;
  }

看到这里一切就清楚了,当我们调用setOnClickListener方法来给按钮注册一个点击事件时,就会给mOnClickListener赋值。整个分发事件的顺序是onTouch()-->onTouchEvent(event)-->performClick()-->OnClick()。
 现在我们可以解决之前的问题。
1、onTouch方法是优先于OnClick,所以是执行了onTouch,再执行onClick。 
2、无论是dispatchTouchEvent还是onTouchEvent,如果返回true表示这个事件已经被消费、处理了,不再往下传了。在dispathTouchEvent的源码里可以看到,如果onTouchEvent返回了true,那么它也返回true。如果dispatchTouchEvent在执行onTouch监听的时候,onTouch返回了true,那么它也返回true,这个事件提前被onTouch消费掉了。就不再执行onTouchEvent了,更别说onClick监听了。

以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持phperz。



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