Android事件分发底层原理

作者: GB_speak | 来源:发表于2017-04-13 19:16 被阅读25次

1.简介
1.1事件构成
在Android中，事件（TouchEvent）主要包括点按、长按、拖拽、滑动等，所有的事件都由如下三个部分组成
按下（ACTION_DOWN）

移动（ACTION_MOVE）

抬起（ACTION_UP）

一般来说，一次完整的Touch事件，应该是由一个Down、一个Up和若干个Move组成。
1.2View的分发事件
public boolean dispatchTouchEvent（MotionEvent ev）如果事件能够传递到当前的View，那么此方法一定会被调用。

public boolean onInterceptTouchEvent（MotionEvent ev）用来判断是否拦截某个事件

public boolean onTouchEvent(MotionEvent ev)用来处理点击事件

ViewGroup的相关事件有三个：onInterceptTouchEvent、dispatchTouchEvent、onTouchEvent。View的相关事件只有两个：dispatchTouchEvent、onTouchEvent。
1.3滑动冲突处理：
（1）外部拦截方法（在父容器的 onInterceptTouchEvent 进行控制，是否分发到子元素），遵循Android规范
（2）内部拦截方法（在子元素通过控制父容器的 requestDisallowInterceptTouchEvent 进行控制）
2.流程
Android中事件传递按照从上到下再从下到上进行层级传递，事件处理从Activity开始到ViewGroup再到View，如果View没有消费事件会再次从View到ViewGroup再到Activity最后事件被抛出消费掉。流转的流程图如下：

Paste_Image.png

dispatchTouchEvent和TouchEvnet return false的时候，事件都会回传给父控件的onTouchEvent处理

onInterceptTouchEvent默认为false不进行事件拦截，OnDispatchTouchEvnet默认为true进行事件分发，onTouchEvent需要根据具体的View是否设置了listener及具体View的实现进行区分是否为true

Activity的dispatchTouchEvent不管返回true或false都会进行分发

如果事件在具体的View或者ViewGroup的onTouchEvent返回true，则表明事件被消费，事件传递机制就会结束
3.示例
具体可以根据流程图修改对应方法返回值进行日志验证。这里就不一一进行日志输出显示。

public class MyView extends Button {
    private String TAG = "MyView";
    public MyView(Context context, AttributeSet attrs) {
        super(context, attrs);
        TAG += getTag();
    }
    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        switch (ev.getAction()) {
            case MotionEvent.ACTION_DOWN:
                Log.i(TAG, "MyView dispatchTouchEvent--ACTION_DOWN");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.i(TAG, "MyView dispatchTouchEvent--ACTION_MOVE");
                break;
            case MotionEvent.ACTION_UP:
                Log.i(TAG, "MyView dispatchTouchEvent--ACTION_UP");
                break;
        }
        return super.dispatchTouchEvent(ev);
    }
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                Log.i(TAG, "MyView onTouchEvent--ACTION_DOWN");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.i(TAG, "MyView onTouchEvent--ACTION_MOVE");
                break;
            case MotionEvent.ACTION_UP:
                Log.i(TAG, "MyView onTouchEvent--ACTION_UP");
                break;
        }
        boolean touch = super.onTouchEvent(event);
        Log.i(TAG, "MyView onTouchEvent--touch" + touch);
        return touch;
    }
}```

public class MyViewGroup extends RelativeLayout {
private String TAG = "MyViewGroup";
public MyViewGroup(Context context, AttributeSet attrs) {
super(context, attrs);
TAG+=getTag();
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
switch (ev.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i(TAG, "MyViewGroup dispatchTouchEvent--ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.i(TAG, "MyViewGroup dispatchTouchEvent--ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.i(TAG, "MyViewGroup dispatchTouchEvent--ACTION_UP");
break;
}
return super.dispatchTouchEvent(ev) ;
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
switch (ev.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.d(TAG, "MyViewGroup onInterceptTouchEvent--ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.d(TAG, "MyViewGroup onInterceptTouchEvent--ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.d(TAG, "MyViewGroup onInterceptTouchEvent--ACTION_UP");
break;
}
return super.onInterceptTouchEvent(ev);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i(TAG, "MyViewGroup onTouchEvent--ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.i(TAG, "MyViewGroup onTouchEvent--ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.i(TAG, "MyViewGroup onTouchEvent--ACTION_UP");
break;
}
boolean touch = super.onTouchEvent(event);
Log.i(TAG, "MyViewGroup onTouchEvent--touch" + touch);
return touch;
}
}```

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.jd.test.myapplication.MainActivity">
    <com.jd.test.myapplication.view.MyViewGroup
        android:id="@+id/mg1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:tag="mg1">
        <com.jd.test.myapplication.view.MyView
            android:id="@+id/v3"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:tag="btn3"
            android:text="btn3" />
    </com.jd.test.myapplication.view.MyViewGroup>
</RelativeLayout>```

public class ViewEventActivity extends Activity {
Button btn3;
MyViewGroup myViewGroup1;
private String TAG="ViewEventActivity";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_view_event);
btn3= (Button) findViewById(R.id.v3);
myViewGroup1= (MyViewGroup) findViewById(R.id.mg1);
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
switch (ev.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i(TAG, "DecorView dispatchTouchEvent--ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.i(TAG, "DecorView dispatchTouchEvent--ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.i(TAG, "DecorView dispatchTouchEvent--ACTION_UP");
break;
}
return super.dispatchTouchEvent(ev);
//return true;
}
@Override
public boolean onTouchEvent(MotionEvent event) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
Log.i(TAG, "DecorView onTouchEvent--ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.i(TAG, "DecorView onTouchEvent--ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.i(TAG, "DecorView onTouchEvent--ACTION_UP");
break;
}
boolean touch=super.onTouchEvent(event);
Log.d(TAG, "DecorView touch boolean---"+touch);
return touch;
}```
4.源码解析
1.Activity的TouchEvent流程
首先看一下Activity的dispatchTouchEvnet源码，实际是调用了Window的 superDispatchTouchEvent();
Activity:

public boolean dispatchTouchEvent(MotionEvent ev) {
    if (ev.getAction() == MotionEvent.ACTION_DOWN) {
        onUserInteraction();
    }
    if (getWindow().superDispatchTouchEvent(ev)) {
        return true;
    }
    return onTouchEvent(ev);
}```
Window是一个抽象类，他的实现类是PhoneWindow，而PhoneWidow的最底层视图是mDecorView，是一个FrameLayout。其实也就是ViewGroup。
PhoneWindow:

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

public boolean superDispatchTouchEvent(MotionEvent event) {
return super.dispatchTouchEvent(event);
}```
如果Activity有实现 dispatchTouchEvent回调则调用改回调，若没有则调用super的 dispatchTouchEvent，继续往子View进行事件分发

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

ViewGroup：

 /**
  * {@inheritDoc}
  */
  @Override
  public boolean dispatchTouchEvent(MotionEvent ev) {
  if (mInputEventConsistencyVerifier != null) {
  mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
  }
//对于辅助功能的事件处理
  if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
  ev.setTargetAccessibilityFocus(false);
  }
  boolean handled = false;
  if (onFilterTouchEventForSecurity(ev)) {
  final int action = ev.getAction();
  final int actionMasked = action & MotionEvent.ACTION_MASK;
  // 处理原始的DOWN事件
  if (actionMasked == MotionEvent.ACTION_DOWN) {
  //这里主要是在新事件开始时处理完上一个事件
  // due to an app switch, ANR, or some other state change.
  cancelAndClearTouchTargets(ev);
  resetTouchState();
  }
  //检查事件拦截
  final boolean intercepted;
  if (actionMasked == MotionEvent.ACTION_DOWN
  || mFirstTouchTarget != null) {
  final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
  if (!disallowIntercept) {
  intercepted = onInterceptTouchEvent(ev);
  ev.setAction(action); //恢复事件防止其改变
  } else {
  intercepted = false;
  }
  } else {
  // There are no touch targets and this action is not an initial down
  // so this view group continues to intercept touches.
  intercepted = true;
  }
  //如果事件被拦截了，则进行正常的事件分发
  if (intercepted || mFirstTouchTarget != null) {
  ev.setTargetAccessibilityFocus(false);
  }
  // 检查事件是否取消
  final boolean canceled = resetCancelNextUpFlag(this)
  || actionMasked == MotionEvent.ACTION_CANCEL;
  // 如果有必要的话，为DOWN事件检查所有的目标对象
  final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
  TouchTarget newTouchTarget = null;
  boolean alreadyDispatchedToNewTouchTarget = false;
//如果事件未被取消并未被拦截
  if (!canceled && !intercepted) {
  //如果有辅助功能的参与，则直接将事件投递到对应的View，否则将事件分发给所有的子View
  View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
  ? findChildWithAccessibilityFocus() : null;
  if (actionMasked == MotionEvent.ACTION_DOWN
  || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
  || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
  final int actionIndex = ev.getActionIndex(); // always 0 for down
  final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
  : TouchTarget.ALL_POINTER_IDS;
  // Clean up earlier touch targets for this pointer id in case they
  // have become out of sync.
  removePointersFromTouchTargets(idBitsToAssign);
  final int childrenCount = mChildrenCount;
//如果TouchTarget为空并且子元素不为0
  if (newTouchTarget == null && childrenCount != 0) {
  final float x = ev.getX(actionIndex);
  final float y = ev.getY(actionIndex);
  //由上至下去寻找一个可以接收改事件的子View
  final ArrayList<View> preorderedList = buildOrderedChildList();
  final boolean customOrder = preorderedList == null
  && isChildrenDrawingOrderEnabled();
  final View[] children = mChildren;
//遍历子元素
  for (int i = childrenCount - 1; i >= 0; i--) {
  final int childIndex = customOrder
  ? getChildDrawingOrder(childrenCount, i) : i;
  final View child = (preorderedList == null)
  ? children[childIndex] : preorderedList.get(childIndex);
  // If there is a view that has accessibility focus we want it
  // to get the event first and if not handled we will perform a
  // normal dispatch. We may do a double iteration but this is
  // safer given the timeframe.
  if (childWithAccessibilityFocus != null) {
  if (childWithAccessibilityFocus != child) {
  continue;
  }
  childWithAccessibilityFocus = null;
  i = childrenCount - 1;
  }
//如果这个子元素无法接收Pointer Event或这个事件电压根本就没有在子元素的边界范围内
  if (!canViewReceivePointerEvents(child)
  || !isTransformedTouchPointInView(x, y, child, null)) {
  ev.setTargetAccessibilityFocus(false);
//那么就跳出该次循环继续遍历
  continue;
  }
//找到Event该由那个子元素持有
  newTouchTarget = getTouchTarget(child);
  if (newTouchTarget != null) {
  // Child is already receiving touch within its bounds.
  // Give it the new pointer in addition to the ones it is handling.
  newTouchTarget.pointerIdBits |= idBitsToAssign;
  break;
  }
  resetCancelNextUpFlag(child);
//投递事件执行触摸操作
//如果子元素还是一个ViewGroup，则递归调用重复此过程
//如果子元素还是一个View，那么则会调用View的dispatTouchEvent，
//并最终由onTouchEvent处理
  if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
  // 子View在其边界范围内接收事件
  mLastTouchDownTime = ev.getDownTime();
  if (preorderedList != null) {
  // childIndex points into presorted list, find original index
  for (int j = 0; j < childrenCount; j++) {
  if (children[childIndex] == mChildren[j]) {
  mLastTouchDownIndex = j;
  break;
  }
  }
  } else {
  mLastTouchDownIndex = childIndex;
  }
  mLastTouchDownX = ev.getX();
  mLastTouchDownY = ev.getY();
  newTouchTarget = addTouchTarget(child, idBitsToAssign);
  alreadyDispatchedToNewTouchTarget = true;
  break;
  }
  // The accessibility focus didn't handle the event, so clear
  // the flag and do a normal dispatch to all children.
  ev.setTargetAccessibilityFocus(false);
  }
  if (preorderedList != null) preorderedList.clear();
  }
//如果发现没有子元素可以持有该次事件
  if (newTouchTarget == null && mFirstTouchTarget != null) {
  // Did not find a child to receive the event.
  // Assign the pointer to the least recently added target.
  newTouchTarget = mFirstTouchTarget;
  while (newTouchTarget.next != null) {
  newTouchTarget = newTouchTarget.next;
  }
  newTouchTarget.pointerIdBits |= idBitsToAssign;
  }
  }
  }
  // Dispatch to touch targets.
  if (mFirstTouchTarget == null) {
  // No touch targets so treat this as an ordinary view.
  handled = dispatchTransformedTouchEvent(ev, canceled, null,
  TouchTarget.ALL_POINTER_IDS);
  } else {
  // Dispatch to touch targets, excluding the new touch target if we already
  // dispatched to it. Cancel touch targets if necessary.
  TouchTarget predecessor = null;
  TouchTarget target = mFirstTouchTarget;
  while (target != null) {
  final TouchTarget next = target.next;
  if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
  handled = true;
  } else {
  final boolean cancelChild = resetCancelNextUpFlag(target.child)
  || intercepted;
  if (dispatchTransformedTouchEvent(ev, cancelChild,
  target.child, target.pointerIdBits)) {
  handled = true;
  }
  if (cancelChild) {
  if (predecessor == null) {
  mFirstTouchTarget = next;
  } else {
  predecessor.next = next;
  }
  target.recycle();
  target = next;
  continue;
  }
  }
  predecessor = target;
  target = next;
  }
  }
  // Update list of touch targets for pointer up or cancel, if needed.
  if (canceled
  || actionMasked == MotionEvent.ACTION_UP
  || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
  resetTouchState();
  } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
  final int actionIndex = ev.getActionIndex();
  final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
  removePointersFromTouchTargets(idBitsToRemove);
  }
  }
  if (!handled && mInputEventConsistencyVerifier != null) {
  mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
  }
  return handled;
  }```
View:

/**

Dispatches a key shortcut event.
@param event The key event to be dispatched.
@return True if the event was handled by the view, false otherwise.
/
public boolean dispatchKeyShortcutEvent(KeyEvent event) {
return onKeyShortcut(event.getKeyCode(), event);
}
/*
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 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;
}```
3.onInterceptTouchEvent

public boolean onInterceptTouchEvent(MotionEvent ev) {
 return false;
 }```
**4.TouchEvent**

/**

Implement this method to handle touch screen motion events.
<p>
If this method is used to detect click actions, it is recommended that
the actions be performed by implementing and calling
{@link #performClick()}. This will ensure consistent system behavior,
including:
<ul>
<li>obeying click sound preferences
<li>dispatching OnClickListener calls
<li>handling {@link AccessibilityNodeInfo#ACTION_CLICK ACTION_CLICK} when
accessibility features are enabled
</ul>
@param event The motion event.
@return True if the event was handled, false otherwise.
*/
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;
}```

Android事件分发底层原理

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