Android TV 焦点分发原理解析

前言

相信很多刚接触AndroidTV开发的开发者，都会被各种焦点问题给折磨的不行。不管是学技术还是学习其他知识，都要学习和理解其中原理，碰到问题我们才能得心应手。下面就来探一探Android的焦点分发的过程。

Android焦点分发，拦截过程的实现

Android焦点事件的分发是从ViewRootImpl的processKeyEvent开始的，源码如下：

        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 viewfocused.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-focusView v = focusSearch(null, direction);if (v != null && v.requestFocus(direction)) {return FINISH_HANDLED;}}}}return FORWARD;}

源码比较长，下面我就慢慢来讲解一下具体的每一个细节。
* (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代表焦点事件被消费了。

  // Deliver the key to the view hierarchy.if (mView.dispatchKeyEvent(event)) {return FINISH_HANDLED;}

首先会执行mView的dispatchKeyEvent方法，估计大家会好奇这个mView是个什么鬼？其实它就是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。

ViewGroup的dispatchKeyEvent()方法的源码如下：

 @Overridepublic 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;}

（2）ViewGroup的dispatchKeyEvent执行流程

首先ViewGroup会一层一层往上执行父类的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是什么呢？其实
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：

 @Overridepublic void requestChildFocus(View child, View focused) {if (DBG) {System.out.println(this + " requestChildFocus()");}if (getDescendantFocusability() == FOCUS_BLOCK_DESCENDANTS) {return;}// Unfocus us, if necessarysuper.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);}}

（3）下面再来瞧瞧view的dispatchKeyEvent方法的具体的执行过程

  public boolean dispatchKeyEvent(KeyEvent event) {if (mInputEventConsistencyVerifier != null) {mInputEventConsistencyVerifier.onKeyEvent(event, 0);}// Give any attached key listener a first crack at the event.//noinspection SimplifiableIfStatementListenerInfo 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;}

惊奇的发现执行了onKeyListener中的onKey方法，如果onKey方法返回true，那么dispatchKeyEvent方法也会返回true

可以得出结论：如果想要修改ViewGroup焦点事件的分发，可以这么干：

重写view的dispatchKeyEvent方法
给某个子view设置onKeyListener监听

注意：实际开发中，理论上所有焦点问题都可以通过给dispatchKeyEvent方法增加监听来来拦截来控制。

焦点没有被dispatchKeyEvent拦截的情况下的处理过程

（1）dispatchKeyEvent方法返回false后，先得到按键的方向direction值，这个值是一个int类型参数。这个direction值是后面来进行焦点查找的。

  // 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;}

（2）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。
ViewGroup的findFocus方法如下：

 @Overridepublic 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

 public View findFocus() {return (mPrivateFlags & PFLAG_FOCUSED) != 0 ? this : null;}

说明：判断view是否获取焦点的isFocused()方法， (mPrivateFlags & PFLAG_FOCUSED) != 0 和view 的isFocused()方法是一致的。

  @ViewDebug.ExportedProperty(category = "focus")public boolean isFocused() {return (mPrivateFlags & PFLAG_FOCUSED) != 0;}

其中isFocused()方法的作用是判断view是否已经获取焦点，如果viewGroup已经获取到了焦点，那么返回本身即可，否则通过mFocused的findFocus()方法来找焦点。mFocused其实就是ViewGroup中获取焦点的子view，如果mView不是ViewGourp的话，findFocus其实就是判断本身是否已经获取焦点，如果已经获取焦点了，返回本身。

（3）回到processKeyEvent方法中，如果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。

 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 viewfocused.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-focusView v = focusSearch(null, direction);if (v != null && v.requestFocus(direction)) {return FINISH_HANDLED;}}}

（4）focusSearch方法的具体实现。

focusSearch方法的源码如下：

  @Overridepublic 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 inforeturn FocusFinder.getInstance().findNextFocus(this, focused, direction);} else if (mParent != null) {return mParent.focusSearch(focused, direction);}return null;}

可以看出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。

  @Overridepublic 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 inforeturn FocusFinder.getInstance().findNextFocus(this, focused, direction);} else if (mParent != null) {return mParent.focusSearch(focused, direction);}return null;}

（5）FocusFinder是什么？

它其实是一个实现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框架焦点的查找最终都是通过FocusFinder类来实现的。

（6）FocusFinder是如何通过findNextFocus方法寻找焦点的。

下面就来看看FocusFinder类是如何通过findNextFocus来找焦点的。一层一层往下看，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。

   private View findNextUserSpecifiedFocus(ViewGroup root, View focused, int direction) {// check for user specified next focusView userSetNextFocus = focused.findUserSetNextFocus(root, direction);if (userSetNextFocus != null && userSetNextFocus.isFocusable()&& (!userSetNextFocus.isInTouchMode()|| userSetNextFocus.isFocusableInTouchMode())) {return userSetNextFocus;}return null;}

（7）findNextFocus会优先根据XML里设置的下一个将获取焦点的View ID值来寻找将要获取焦点的View。

看看View的findUserSetNextFocus方法内部都干了些什么，OMG不就是通过我们xml布局里设置的nextFocusLeft，nextFocusRight的viewId来找焦点吗，如果按下Left键，那么便会通过nextFocusLeft值里的View Id值去找下一个获取焦点的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>() {@Overridepublic boolean apply(View t) {return t.mNextFocusForwardId == id;}});}}return null;}

可以得出以下结论：

1. 如果一个View在XML布局中设置了focusable = true && isInTouchMode = true，那么这个View会优先获取焦点。

2. 通过设置nextFocusLeft，nextFocusRight，nextFocusUp，nextFocusDown值可以控制View的下一个焦点。

Android焦点的原理实现就这些。总结一下：

首先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，那么焦点就不会往下分发了。
中途可以给某个子View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
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。

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