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 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;

}

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：

@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;

}

(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：

@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);

}

}

(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 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;

}

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：

@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

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 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;

}

}

}

(4)focusSearch方法的具体实现。

focusSearch方法的源码如下：

@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;

}

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。

@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;

}

(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 focus

View 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() {

@Override

public 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。