Android焦点事件分发与传递机制
Posted 单灿灿
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篇首语：本文由小常识网(cha138.com)小编为大家整理，主要介绍了Android焦点事件分发与传递机制相关的知识，希望对你有一定的参考价值。
如果您对TouchEvent事件分发机制不太了解的，可以参考我的这篇文章——安卓TounchEvent事件分发机制。
问题：TV端焦点满天飞，如何解决和处理？
记得初入TV开发，以为很简单。TV的这些界面与布局太简单了，分分钟就可以把页面搭建出来，处理好，然后就没有然后了。。。。
下面我们就从源码来带大家进行安卓TV焦点事件的传递
这里先给出android系统View的绘制流程：
依次执行View类里面的如下三个方法：
- measure(int ,int) :测量View的大小
- layout(int ,int ,int ,int) ：设置子View的位置
- draw(Canvas) ：绘制View内容到Canvas画布上
ViewRootImpl的主要作用如下（此处不多讲，如有意图，看源码）：
A：链接WindowManager和DecorView的纽带，更广一点可以说是Window和View之间的纽带。
B：完成View的绘制过程，包括measure、layout、draw过程。
C：向DecorView分发收到的用户发起的event事件，如按键，触屏等事件。
ViewRootImpl不再多余叙述，进入正题：
Android焦点分发的主要方法以及拦截方法的讲解。
在RootViewImpl中的函数通道是各种策略（InputStage）的组合，各策略负责的任务不同，如SyntheticInputStage、ViewPostImeInputStage、NativePostImeInputStage等等，这些策略以链表结构结构起来，当一个策略者没有消费事件时，就传递个下一个策略者。其中触摸和按键事件由ViewPostImeInputStage处理。
@Override
protected int onProcess(QueuedInputEvent q)
if (q.mEvent instanceof KeyEvent)
return processKeyEvent(q);//如果是按键事件走此处，处理按键和焦点问题了
else
final int source = q.mEvent.getSource();
if ((source & InputDevice.SOURCE_CLASS_POINTER) != 0)
return processPointerEvent(q);//如果是触摸事件走此处
else if ((source & InputDevice.SOURCE_CLASS_TRACKBALL) != 0)
return processTrackballEvent(q);
else
return processGenericMotionEvent(q);
processKeyEvent(QueuedInputEvent q)源码如下：
@Override
protected void onDeliverToNext(QueuedInputEvent q)
if (mUnbufferedInputDispatch
&& q.mEvent instanceof MotionEvent
&& ((MotionEvent)q.mEvent).isTouchEvent()
&& isTerminalInputEvent(q.mEvent))
mUnbufferedInputDispatch = false;
scheduleConsumeBatchedInput();
super.onDeliverToNext(q);
private int processKeyEvent(QueuedInputEvent q)
final KeyEvent event = (KeyEvent)q.mEvent;
// Deliver the key to the view hierarchy.
if (mView.dispatchKeyEvent(event))
return FINISH_HANDLED;
if (shouldDropInputEvent(q))
return FINISH_NOT_HANDLED;
// If the Control modifier is held, try to interpret the key as a shortcut.
if (event.getAction() == KeyEvent.ACTION_DOWN
&& event.isCtrlPressed()
&& event.getRepeatCount() == 0
&& !KeyEvent.isModifierKey(event.getKeyCode()))
if (mView.dispatchKeyShortcutEvent(event))
return FINISH_HANDLED;
if (shouldDropInputEvent(q))
return FINISH_NOT_HANDLED;
// Apply the fallback event policy.
if (mFallbackEventHandler.dispatchKeyEvent(event))
return FINISH_HANDLED;
if (shouldDropInputEvent(q))
return FINISH_NOT_HANDLED;
// Handle automatic focus changes.
if (event.getAction() == KeyEvent.ACTION_DOWN)
int direction = 0;
switch (event.getKeyCode())
case KeyEvent.KEYCODE_DPAD_LEFT:
if (event.hasNoModifiers())
direction = View.FOCUS_LEFT;
break;
case KeyEvent.KEYCODE_DPAD_RIGHT:
if (event.hasNoModifiers())
direction = View.FOCUS_RIGHT;
break;
case KeyEvent.KEYCODE_DPAD_UP:
if (event.hasNoModifiers())
direction = View.FOCUS_UP;
break;
case KeyEvent.KEYCODE_DPAD_DOWN:
if (event.hasNoModifiers())
direction = View.FOCUS_DOWN;
break;
case KeyEvent.KEYCODE_TAB:
if (event.hasNoModifiers())
direction = View.FOCUS_FORWARD;
else if (event.hasModifiers(KeyEvent.META_SHIFT_ON))
direction = View.FOCUS_BACKWARD;
break;
if (direction != 0)
View focused = mView.findFocus();
if (focused != null)
View v = focused.focusSearch(direction);
if (v != null && v != focused)
// do the math the get the interesting rect
// of previous focused into the coord system of
// newly focused view
focused.getFocusedRect(mTempRect);
if (mView instanceof ViewGroup)
((ViewGroup) mView).offsetDescendantRectToMyCoords(
focused, mTempRect);
((ViewGroup) mView).offsetRectIntoDescendantCoords(
v, mTempRect);
if (v.requestFocus(direction, mTempRect))
playSoundEffect(SoundEffectConstants
.getContantForFocusDirection(direction));
return FINISH_HANDLED;
// Give the focused view a last chance to handle the dpad key.
if (mView.dispatchUnhandledMove(focused, direction))
return FINISH_HANDLED;
else
// find the best view to give focus to in this non-touch-mode with no-focus
View v = focusSearch(null, direction);
if (v != null && v.requestFocus(direction))
return FINISH_HANDLED;
return FORWARD;
进入源码讲解：
(1) 首先由dispatchKeyEvent进行焦点的分发
如果dispatchKeyEvent方法返回true，那么下面的焦点查找步骤就不会继续了。
dispatchKeyEvent方法返回true代表事件（包括焦点和按键）被消费了。
dispatchKeyEvent(event)如果不了解，看我上一篇文章安卓TounchEvent事件分发机制。
mView的dispatchKeyEvent方法，
mView是是Activity的顶层容器DecorView，它是一FrameLayout。
所以这里的dispatchKeyEvent方法应该执行的是ViewGroup的dispatchKeyEvent()方法，而不是View的dispatchKeyEvent方法。
@Override
public boolean dispatchKeyEvent(KeyEvent event)
if (mInputEventConsistencyVerifier != null)
mInputEventConsistencyVerifier.onKeyEvent(event, 1);
if ((mPrivateFlags & (PFLAG_FOCUSED | PFLAG_HAS_BOUNDS))
== (PFLAG_FOCUSED | PFLAG_HAS_BOUNDS))
if (super.dispatchKeyEvent(event))
return true;
else if (mFocused != null && (mFocused.mPrivateFlags & PFLAG_HAS_BOUNDS)
== PFLAG_HAS_BOUNDS)
if (mFocused.dispatchKeyEvent(event))
return true;
if (mInputEventConsistencyVerifier != null)
mInputEventConsistencyVerifier.onUnhandledEvent(event, 1);
return false;
ViewGroup的dispatchKeyEvent简略执行流程
首先ViewGroup会执行父类的dispatchKeyEvent方法，如果返回true那么父类的dispatchKeyEvent方法就会返回true，也就代表父类消费了该焦点事件，那么焦点事件自然就不会往下进行分发。
然后ViewGroup会判断mFocused这个view是否为空，如果为空就会****return false，焦点继续往下传递；如果不为空，那就会return mFocused的dispatchKeyEvent方法返回的结果。这个mFocused是ViewGroup中当前获取焦点的子View，这个可以从requestChildFocus方法中得到答案。
requestChildFocus()的源码如下：
@Override
public void requestChildFocus(View child, View focused)
if (DBG)
System.out.println(this + " requestChildFocus()");
if (getDescendantFocusability() == FOCUS_BLOCK_DESCENDANTS)
return;
// Unfocus us, if necessary
super.unFocus(focused);
// We had a previous notion of who had focus. Clear it.
if (mFocused != child)
if (mFocused != null)
mFocused.unFocus(focused);
mFocused = child;
if (mParent != null)
mParent.requestChildFocus(this, focused);
居然有这个彩蛋？
View的dispatchKeyEvent简略执行流程
public boolean dispatchKeyEvent(KeyEvent event)
if (mInputEventConsistencyVerifier != null)
mInputEventConsistencyVerifier.onKeyEvent(event, 0);
// Give any attached key listener a first crack at the event.
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnKeyListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnKeyListener.onKey(this, event.getKeyCode(), event))
return true;
if (event.dispatch(this, mAttachInfo != null
? mAttachInfo.mKeyDispatchState : null, this))
return true;
if (mInputEventConsistencyVerifier != null)
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
return false;
要修改ViewGroup焦点事件的分发：
- 重写view的dispatchKeyEvent方法
- 给某个子view设置onKeyListener监听
焦点没有被dispatchKeyEvent拦截的情况下的继续代码中的处理过程，还是进入ViewRootImpl源码
// Handle automatic focus changes.
if (event.getAction() == KeyEvent.ACTION_DOWN)
int direction = 0;
switch (event.getKeyCode())
case KeyEvent.KEYCODE_DPAD_LEFT:
if (event.hasNoModifiers())
direction = View.FOCUS_LEFT;
break;
case KeyEvent.KEYCODE_DPAD_RIGHT:
if (event.hasNoModifiers())
direction = View.FOCUS_RIGHT;
break;
case KeyEvent.KEYCODE_DPAD_UP:
if (event.hasNoModifiers())
direction = View.FOCUS_UP;
break;
case KeyEvent.KEYCODE_DPAD_DOWN:
if (event.hasNoModifiers())
direction = View.FOCUS_DOWN;
break;
case KeyEvent.KEYCODE_TAB:
if (event.hasNoModifiers())
direction = View.FOCUS_FORWARD;
else if (event.hasModifiers(KeyEvent.META_SHIFT_ON))
direction = View.FOCUS_BACKWARD;
break;
if (direction != 0)
View focused = mView.findFocus();
if (focused != null)
View v = focused.focusSearch(direction);
if (v != null && v != focused)
// do the math the get the interesting rect
// of previous focused into the coord system of
// newly focused view
focused.getFocusedRect(mTempRect);
if (mView instanceof ViewGroup)
((ViewGroup) mView).offsetDescendantRectToMyCoords(
focused, mTempRect);
((ViewGroup) mView).offsetRectIntoDescendantCoords(
v, mTempRect);
if (v.requestFocus(direction, mTempRect))
playSoundEffect(SoundEffectConstants
.getContantForFocusDirection(direction));
return FINISH_HANDLED;
// Give the focused view a last chance to handle the dpad key.
if (mView.dispatchUnhandledMove(focused, direction))
return FINISH_HANDLED;
else
// find the best view to give focus to in this non-touch-mode with no-focus
View v = focusSearch(null, direction);
if (v != null && v.requestFocus(direction))
return FINISH_HANDLED;
dispatchKeyEvent方法返回false后，先得到按键的方向direction一个int值。direction值是后面来进行焦点查找的。
接着会调用DecorView的findFocus()方法一层一层往下查找已经获取焦点的子View。
DecorView则是PhoneWindow类的一个内部类，继承于FrameLayout，由此可知它是一个ViewGroup。
那么，DecroView到底充当了什么样的角色呢？
其实，DecorView是整个ViewTree的最顶层View，它是一个FrameLayout布局，代表了整个应用的界面。在该布局下面，有标题view和内容view这两个子元素。
@Override
public View findFocus()
if (DBG)
System.out.println("Find focus in " + this + ": flags="
+ isFocused() + ", child=" + mFocused);
if (isFocused())
return this;
if (mFocused != null)
return mFocused.findFocus();
return null;
View的findFocus方法
/**
* Find the view in the hierarchy rooted at this view that currently has
* focus.
*
* @return The view that currently has focus, or null if no focused view can
* be found.
*/
public View findFocus()
return (mPrivateFlags & PFLAG_FOCUSED) != 0 ? this : null;
View的hasFocus()方法和isFocused()方法对比
Stackoverflow解释来了：
hasFocus() is different from isFocused(). hasFocus() == true means that the View or one of its descendants is focused. If you look closely, there’s a chain of hasFocused Views till you reach the View that isFocused.
/**
* Returns true if this view has focus itself, or is the ancestor of the
* view that has focus.
*
* @return True if this view has or contains focus, false otherwise.
*/
@ViewDebug.ExportedProperty(category = "focus")
public boolean hasFocus()
return (mPrivateFlags & PFLAG_FOCUSED) != 0;
/**
* Returns true if this view has focus
*
* @return True if this view has focus, false otherwise.
*/
@ViewDebug.ExportedProperty(category = "focus")
public boolean isFocused()
return (mPrivateFlags & PFLAG_FOCUSED) != 0;
接着，如果mView.findFocus()方法返回的mFocused不为空，说明找到了当前获取焦点的view（mFocused），接着focusSearch会把direction（遥控器按键按下的方向）作为参数，找到特定方向下一个将要获取焦点的view，最后如果该view不为空，那么就让该view获取焦点。
我们来看一下focusSearch方法的源码以及具体实现。
@Override
public View focusSearch(View focused, int direction)
if (isRootNamespace())
// root namespace means we should consider ourselves the top of the
// tree for focus searching; otherwise we could be focus searching
// into other tabs. see LocalActivityManager and TabHost for more info
return FocusFinder.getInstance().findNextFocus(this, focused, direction);
else if (mParent != null)
return mParent.focusSearch(focused, direction);
return null;
focusSearch其实是一层一层地网上调用父View的focusSearch方法，直到当前view是根布局（isRootNamespace()方法），通过注释可以知道focusSearch最终会调用DecorView的focusSearch方法。而DecorView的focusSearch方法找到的焦点view是通过FocusFinder来找到的。
FocusFinder是什么？
根据给定的按键方向，通过当前的获取焦点的View，查找下一个获取焦点的view这样算法的类。焦点没有被拦截的情况下，Android焦点的查找最终都是通过FocusFinder类来实现的。
FocusFinder是如何通过findNextFocus方法寻找焦点的？
public final View findNextFocus(ViewGroup root, View focused, int direction)
return findNextFocus(root, focused, null, direction);
private View findNextFocus(ViewGroup root, View focused, Rect focusedRect, int direction)
View next = null;
if (focused != null)
next = findNextUserSpecifiedFocus(root, focused, direction);
if (next != null)
return next;
ArrayList<View> focusables = mTempList;
try
focusables.clear();
root.addFocusables(focusables, direction);
if (!focusables.isEmpty())
next = findNextFocus(root, focused, focusedRect, direction, focusables);
finally
focusables.clear();
return next;
private View findNextFocus(ViewGroup root, View focused, Rect focusedRect,
int direction, ArrayList<View> focusables)
if (focused != null)
if (focusedRect == null)
focusedRect = mFocusedRect;
// fill in interesting rect from focused
focused.getFocusedRect(focusedRect);
root.offsetDescendantRectToMyCoords(focused, focusedRect);
else
if (focusedRect == null)
focusedRect = mFocusedRect;
// make up a rect at top left or bottom right of root
switch (direction)
case View.FOCUS_RIGHT:
case View.FOCUS_DOWN:
setFocusTopLeft(root, focusedRect);
break;
case View.FOCUS_FORWARD:
if (root.isLayoutRtl())
setFocusBottomRight(root, focusedRect);
else
setFocusTopLeft(root, focusedRect);
break;
case View.FOCUS_LEFT:
case View.FOCUS_UP:
setFocusBottomRight(root, focusedRect);
break;
case View.FOCUS_BACKWARD:
if (root.isLayoutRtl())
setFocusTopLeft(root, focusedRect);
else
setFocusBottomRight(root, focusedRect);
break;
private View findNextUserSpecifiedFocus(ViewGroup root, View focused, int direction)
// check for user specified next focus
View userSetNextFocus = focused.findUserSetNextFocus(root, direction);
if (userSetNextFocus != null && userSetNextFocus.isFocusable()
&& (!userSetNextFocus.isInTouchMode()
|| userSetNextFocus.isFocusableInTouchMode()))
return userSetNextFocus;
return null;
FocusFinder类通过findNextFocus来找焦点的。一层一层往寻找，后面会执行findNextUserSpecifiedFocus()方法，这个方法会执行focused(即当前获取焦点的View)的findUserSetNextFocus方法，如果该方法返回的View不为空，
且isFocusable = true && isInTouchMode() = true的话。
FocusFinder找到的焦点就是findNextUserSpecifiedFocus()返回的View。
findNextFocus会优先根据XML里设置的下一个将获取焦点的View的ID值来寻找将要获取焦点的View。
View findUserSetNextFocus(View root, @FocusDirection int direction)
switch (direction)
case FOCUS_LEFT:
if (mNextFocusLeftId == View.NO_ID) return null;
return findViewInsideOutShouldExist(root, mNextFocusLeftId);
case FOCUS_RIGHT:
if (mNextFocusRightId == View.NO_ID) return null;
return findViewInsideOutShouldExist(root, mNextFocusRightId);
case FOCUS_UP:
if (mNextFocusUpId == View.NO_ID) return null;
return findViewInsideOutShouldExist(root, mNextFocusUpId);
case FOCUS_DOWN:
if (mNextFocusDownId == View.NO_ID) return null;
return findViewInsideOutShouldExist(root, mNextFocusDownId);
case FOCUS_FORWARD:
if (mNextFocusForwardId == View.NO_ID) return null;
return findViewInsideOutShouldExist(root, mNextFocusForwardId);
case FOCUS_BACKWARD:
if (mID == View.NO_ID) return null;
final int id = mID;
return root.findViewByPredicateInsideOut(this, new Predicate<View>()
@Override
public boolean apply(View t)
return t.mNextFocusForwardId == id;
);
return null;
焦点事件分发步骤：
DecorView会调用dispatchKey一层一层进行焦点的分发，如果dispatchKeyEvent方法返回true的话，那么焦点或者按键事件就不会往下分发了。
如果你想拦截某个子View，对其设置OnKeyListener进行焦点的拦截。
如果焦点没有被拦截的话，那么焦点就会交给系统来处理,还是会继续分发，直到找到那个获取焦点的View
Android底层先会记录按键的方向，后面DecorView会一层一层往下调用findFocus方法找到当前获取焦点的View
后面系统又会根据按键的方向，执行focusSearch方法来寻找下一个将要获取焦点的View
focusSearch内部其实是通过FocusFinder来查找焦点的。FocusFinder会优先通过View在XML布局设置的下一个焦点的ID来查找焦点。
最终如果找到将要获取焦点的View，就让其requestFocus。如果请求无效，将其放在onWindowFocusChanged（）这个方法中去请求。这是在Activity寻找到焦点的时候。
我的前一篇文章，主要是介绍了TouchEvent事件分发机制，省略了焦点分发传递机制的代码，这篇文章与此相反。如果将两个结合起来，太繁杂，冗长了。分开反而有利于您的理解。至此，事件分发机制，你也了解的差不多了，給个粉吧！
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