Android事件分发机制详解
我们通过一个示例来分析Touch事件的分发过程。
示例:
布局文件:
<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:id="@+id/rootview" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" tools:context="com.example.maimingliang.test.view.TestTouchActivity"> <TextView android:id="@+id/txt" android:layout_width="match_parent" android:gravity="center" android:layout_height="55dp" android:text="textView"/> <ImageView android:id="@+id/img" android:layout_marginTop="20dp" android:layout_width="wrap_content" android:layout_height="wrap_content" android:background="@mipmap/ic_launcher"/></LinearLayout>
Activity:
public class TestTouchActivity extends AppCompatActivity { private static final String TAG = "TestTouchActivity"; @Bind(R.id.txt) TextView tv; @Bind(R.id.img) ImageView img; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_test_touch); ButterKnife.bind(this); initView(); } private void initView() { tv.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { Log.d(TAG,"-------> tv Onclick"); } }); tv.setOnTouchListener(new View.OnTouchListener() { @Override public boolean onTouch(View v, MotionEvent event) { Log.d(TAG, "-------> tv onTouch"); return false; } }); img.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { Log.d(TAG, "--------> img onClick"); } }); img.setOnTouchListener(new View.OnTouchListener() { @Override public boolean onTouch(View v, MotionEvent event) { Log.d(TAG, "--------> img onTouch"); return true; } }); }
点击图片,现象
可以看到onTouch事件比onClick事件优先级高。
再看看把setOnTouchListener事件的返回值改为true:
可以看到onClick事件没有了。这是为什么?我们透过源码来看看这个现象。
事件分发机制源码分析
当我们触摸屏幕上的某个控件时,底层的设备硬件传递给InputManager经过一 定的处理后,传递给AmS,再经过AmS的处理后就传递到我们的Activity,接着传递Window,最后传递到顶级View。
触摸事件的分发过程有三个重要的方法:
public boolean dispatchTouchEvent(MotionEvent ev)
用来分发事件的,如果当前事件能传递到该View,该 方法一定调用,View的onTouchEvent方法会调用,而该方法的返回值所onTouchEvent影响。
public boolean onInterceptHoverEvent(MotionEvent event)
用来拦截事件的,如果返回值为true,表示拦截。否则不拦截。
public boolean onTouchEvent(MotionEvent event)
处理当前事件的。如果返回值为true表示消耗该事件。否则无法再接收同一个序列的事件。
同一个序列的事件是;DOWN事件--》多个MOVE事件--》UP事件。
Activity触摸事件分发过程
当触摸事件传递到Activity,Activity的dispatchTouchEvent()方法就会调用,我们去看看:
public boolean dispatchTouchEvent(MotionEvent ev) { if (ev.getAction() == MotionEvent.ACTION_DOWN) { onUserInteraction(); } if (getWindow().superDispatchTouchEvent(ev)) { return true; } return onTouchEvent(ev); }
如果当前事件是DOWN事件,调用了onUserInteraction方法,该方法是一个空方法,我们可以重载该方法,在DOWN事件做一些处理。接着就把事件传递给Window来处理该事件。如果返回true,表示有View处理该事件,onTouch Event()方法返回了true,整个事件处理完成。否则Activity的onTouchEvent方法就会被调用。
Window触摸事件的分发过程
Window类是abstract的,唯一的具体实现类是PhoneWindow类,我们去看看PhoneWindow的superDispatchTouchEvent()方法:
@Override public boolean superDispatchTouchEvent(MotionEvent event) { return mDecor.superDispatchTouchEvent(event); } private DecorView mDecor;
DecorView类继承于FrameLayout:
private final class DecorView extends FrameLayout implements RootViewSurfaceTaker {....}
因此就是调用了DecorView的superDispatchTouchEvent方法:
public boolean superDispatchTouchEvent(MotionEvent event) { return super.dispatchTouchEvent(event); }
可以看到,其实就是调用了父类的dispatchTouchEvent()方法。DecorView继承于FrameLayout,FrameLayout继承于ViewGroup。因此就是调用了ViewGroup的dispatchTouchEvent()方法。
DecorView就是我们的顶层View,当我们通过setContentView()方法设置的是顶层View的一个子View。DecorView组成为:
可以看出,事件传递的大概过程:
Activity--》Window--》View。某个View的onTouchEvent()方法被调用。如果返回true,传递会Window,Window再传递会Activity,事件处理结束。否则返回false,再同样的传递会Activity。
顶层View事件分发的过程
DecorView继承与FrameLayout,是一个ViewGroup,ViewGroup继承于View,继承图:
ViewGroup重载了dispatchTouchEvent()方法。那我们去看看该方法:
@Override public boolean dispatchTouchEvent(MotionEvent ev) { .... boolean handled = false; if (onFilterTouchEventForSecurity(ev)) { final int action = ev.getAction(); final int actionMasked = action & MotionEvent.ACTION_MASK; 1. // Handle an initial down. if (actionMasked == MotionEvent.ACTION_DOWN) { // Throw away all previous state when starting a new touch gesture. // The framework may have dropped the up or cancel event for the previous gesture // due to an app switch, ANR, or some other state change. cancelAndClearTouchTargets(ev); resetTouchState(); } 2. // Check for interception. 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); // restore action in case it was changed } 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; } .... 3. if (!canceled && !intercepted) { .... final int childrenCount = mChildrenCount; if (newTouchTarget == null && childrenCount != 0) { final float x = ev.getX(actionIndex); final float y = ev.getY(actionIndex); .... final View[] children = mChildren; 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 (!canViewReceivePointerEvents(child) || !isTransformedTouchPointInView(x, y, child, null)) { ev.setTargetAccessibilityFocus(false); continue; } .... if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) { // Child wants to receive touch within its bounds. 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; } .... } } .... } } .... return handled; }
这个方法很长我们分几部分来分析。代码中标有1.2.3…..。
1.ViewGroup对DOWN事件重置状态的操作。
private void resetTouchState() { clearTouchTargets(); resetCancelNextUpFlag(this); mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT; mNestedScrollAxes = SCROLL_AXIS_NONE; }
标志FLAG_DISALLOW_INTERCEPT可以通过requestDisallowInterceptTouchEvent方法设置。因此在DOWN事件该方法不影响该标志,简单来说,就是不影响ViewGroup处理DOWN事件的操作。
2.判断是否拦截事件。
首先判断是否DOWN事件或者mFirstTouchTarget != null。
mFirstTouchTarget的意思是,如果ViewGroup的有子元素成功处理,mFirstTouchTarget就会指向该元素。
如果当前事件是DOWN:FLAG_DISALLOW_INTERCEPT不影响ViewGroup对DOWN事件的处理,因此调用了onInterceptTouchEvent()方法。是否拦截取决于该方法的返回值。
如果onInterceptTouchEvent()返回true,说明ViewGroup拦截事件,mFirstTouchTarget为null,同一序列的事件都由它处理,onInterceptTouchEvent也不会再调用了,因为actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != null条件都不满足。如果子 View调用了requestDisallowInterceptTouchEvent()方法后,ViewGroup将无法拦截除DOWN事件以外的其他事件。该方法不影响ViewGroup的DOWN事件。
3.如果ViewGroup不拦截,ViewGroup遍历所有的子View,判断子View是否满足当前的事件。满足的条件有两个:子View是否播放动画和事件的坐标是否在子View的区域。
如果满足条件,调用了dispatchTransformedTouchEvent()方法。去看看:
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, View child, int desiredPointerIdBits) { final boolean handled; // Canceling motions is a special case. We don't need to perform any transformations // or filtering. The important part is the action, not the contents. final int oldAction = event.getAction(); if (cancel || oldAction == MotionEvent.ACTION_CANCEL) { event.setAction(MotionEvent.ACTION_CANCEL); if (child == null) { handled = super.dispatchTouchEvent(event); } else { handled = child.dispatchTouchEvent(event); } event.setAction(oldAction); return handled; } ....}
其实就是调用了子View的dispatchTouchEvent()方法。如果返回了true,就会通过addTouchTarget()方法对mFirstTouchTarget赋值并停止遍历子View。
private TouchTarget addTouchTarget(View child, int pointerIdBits) { TouchTarget target = TouchTarget.obtain(child, pointerIdBits); target.next = mFirstTouchTarget; mFirstTouchTarget = target; return target; }
可以看到,mFirstTouchTarget是一个单链表的数据结构。
如果遍历全部的子View都没有成功处理的,mFirstTouchTarget成员变量为null,当该成员变量为null,就会调用:
if (mFirstTouchTarget == null) { // No touch targets so treat this as an ordinary view. handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS); }
因为第三个参数为null,就会调用super.dispatchTouchEvent()方法,调用到了View的dispatchTouchEvent()方法。
View的事件分发过程
dispatchTouchEvent方法如下:
/** * 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)) { //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; } } .... return result; }
从上面的代码可以看出,判断了是否设置了setOnTouchListener,是否为ENABLED,onTouch是否返回了true。
ENABLED对这个判断没有影响。
但onTouch返回true,onTouchEvent方法就不会执行了。而onClick的方法是在onTouchEvent()方法执行的。因此onTouch事件的优先级比onClick事件高,而且还当onTouch方法返回了true,onClick事件就不会调用了。说明了上面的示例的现象。
我们去看看onClick事件是否在onTouch Event方法中执行的。
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; ..... ..... } return true; } return false; }
从上述代码看到,判断了viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE判断是否可点击或者长点击。只要有一个为true,就会返回true,表示消耗此事件。
CLICKABLE和LONG_CLICKABLE的值可以在清单文件中通过android:clickable和 android:longClickable属性设置,也可以通过setOnclickListener()和setLongClickListener()方法设置。
当设置了点击事件调用了performClick()方法:
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; }
可以看到回调了我们设置的onClick方法。由此看出onClick事件是在onTouch Event方法执行的。
这就是事件分发的大概流程。
我们根据上面的示例走一下整个触摸事件的分发流程。
我们从顶View开始分析:
整个View树的结构如下:
上面的示例,我们点击了图片。
首先由顶层View(FrameLayout)的dispatchTouch()方法根据点击图片等坐标首先分发到第一个LinearLayout的,然后调用了ViewGroup的dispatchTouch()方法,又根据点击图片等坐标,把事件分发给图片来处理这个事件。
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