Android强行进阶—按键事件&焦点事件攻略
对于Android手机APP普通开发者来说，KeyEvent接触相对较少，相反接触较多的应该是TouchEvent。而Android TV开发者对KeyEvent的接触就非常频繁。这也是手机应用和TV应用的主要区别：一个主要响应手指触摸事件，一个响应遥控器按键事件。
本文主要基于Android 9.0的源码，踏着巨人的肩膀，进行分析的，个人能力有限，有误请多多指正。篇幅也比较长，对于流程不感兴趣的，可以直接看文末总结。
带着疑问学习本文：
- KeyEvent在Activity、View层次结构是如何分发的？
- 我们设置View的OnKeyListener和重写onKeyDown()函数，能否同时生效？
- OnKeyListener与onKeyDown()的调用顺序？
- 如果所有View都不消费事件，最后KeyEvent如何处理？
ViewRootImpl
由于TouchEvent和KeyEvent等等相关Event的源头是ViewRootImpl，所以需要先了解什么是ViewRootImpl。在Android中，View之间的关系是以View树的形式组织的，也是说，可以通过根View查找到所有的 View，进而可以判断view树是否有view消费相关事件。在Activity中，根View就是 DecorView。ViewRootImpl 本身并不是一个View,而是View树的管理者，ViewRootImpl可以对View进行布局（layout），测量(measure)和绘制(draw)，以及分发事件。同时也是View和WindowManager的桥梁。
关于ViewRootImpl的更多信息
ViewPostImeInputStage
ViewRootImpl以不同的InputStage来管理不同的事件(Event)，界面相关的事件（触摸事件、按键事件、轨迹事件和手势事件）由ViewRootImpl的内部类ViewPostImeInputStage来完成。ViewPostImeInputStage对象内的onProcess()函数是触摸事件和焦点事件的源头。
@Override
protected int onProcess(QueuedInputEvent q) {
if (q.mEvent instanceof KeyEvent) {
//按键事件（焦点事件）
return processKeyEvent(q);
} else {
final int source = q.mEvent.getSource();
if ((source & InputDevice.SOURCE_CLASS_POINTER) != 0) {
//触摸事件
return processPointerEvent(q);
} else if ((source & InputDevice.SOURCE_CLASS_TRACKBALL) != 0) {
//轨迹事件，现在基本不用
return processTrackballEvent(q);
} else {
//通用触摸事件
return processGenericMotionEvent(q);
}
}
}
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这里只关心processKeyEvent()函数对按键（焦点）事件做了什么处理。
private int processKeyEvent(QueuedInputEvent q) {
final KeyEvent event = (KeyEvent)q.mEvent;
if (mUnhandledKeyManager.preViewDispatch(event)) {
return FINISH_HANDLED;
}
//在View层次结构分发按键事件
//如果View树中有View消费事件dispatchKeyEvent（）函数返回true，
后续步骤不再执行
if (mView.dispatchKeyEvent(event)) {
return FINISH_HANDLED;
}
//一些保护措施
//在View层次结构不消费事件，判断窗口是否有输入事件或者已经停止和销毁
if (shouldDropInputEvent(q)) {
return FINISH_NOT_HANDLED;
}
if (mUnhandledKeyManager.dispatch(mView, event)) {
return FINISH_HANDLED;
}
//用来保存焦点事件方向
int groupNavigationDirection = 0;
//处理tab键，判断焦点的方向
if (event.getAction() == KeyEvent.ACTION_DOWN
&& event.getKeyCode() == KeyEvent.KEYCODE_TAB) {
//metaStateHasModifiers()根据指定的meta状态按下特定时返回true
//如果按的是组合键则返回false
if (KeyEvent.metaStateHasModifiers(event.getMetaState(), KeyEvent.META_META_ON)) {
groupNavigationDirection = View.FOCUS_FORWARD;
} else if (KeyEvent.metaStateHasModifiers(event.getMetaState(),
KeyEvent.META_META_ON | KeyEvent.META_SHIFT_ON)) {
groupNavigationDirection = View.FOCUS_BACKWARD;
}
}
// If a modifier is held, try to interpret the key as a shortcut.
if (event.getAction() == KeyEvent.ACTION_DOWN
&& !KeyEvent.metaStateHasNoModifiers(event.getMetaState())
&& event.getRepeatCount() == 0
&& !KeyEvent.isModifierKey(event.getKeyCode())
&& groupNavigationDirection == 0) {
if (mView.dispatchKeyShortcutEvent(event)) {
return FINISH_HANDLED;
}
if (shouldDropInputEvent(q)) {
return FINISH_NOT_HANDLED;
}
}
// 应用 fallback 策略
// 具体实现见PhoneFallbackEventHandler中dispatchKeyEvent()方法
// 主要是对媒体键，音量键，通话键等做处理
if (mFallbackEventHandler.dispatchKeyEvent(event)) {
return FINISH_HANDLED;
}
if (shouldDropInputEvent(q)) {
return FINISH_NOT_HANDLED;
}
//View层次结构不处理KeyEvent,那么变成了寻找焦点的过程
if (event.getAction() == KeyEvent.ACTION_DOWN) {
if (groupNavigationDirection != 0) {
if (performKeyboardGroupNavigation(groupNavigationDirection)) {
return FINISH_HANDLED;
}
} else {
if (performFocusNavigation(event)) {
return FINISH_HANDLED;
}
}
}
return FORWARD;
}
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在processKeyEvent()函数中，我们主要关心按键事件在View层次结构的分发（mView.dispatchKeyEvent(event)）和函数最后自动处理焦点事件两大过程。
KeyEvent在View层次结构的分发
在上一小节中，会调用ViewPostImeInputStage对象内processKeyEvent()函数，使KeyEvent在View的层次结构中分发。 其中mView是DecorView对象，并调用其dispatchKeyEvent()函数。
// DecorView
@Override
public boolean dispatchKeyEvent(KeyEvent event) {
final int keyCode = event.getKeyCode();
final int action = event.getAction();
final boolean isDown = action == KeyEvent.ACTION_DOWN;
//第一次按下，处理panel快捷键
if (isDown && (event.getRepeatCount() == 0)) {
if ((mWindow.mPanelChordingKey > 0) && (mWindow.mPanelChordingKey != keyCode)) {
boolean handled = dispatchKeyShortcutEvent(event);
if (handled) {
return true;
}
}
if ((mWindow.mPreparedPanel != null) && mWindow.mPreparedPanel.isOpen) {
if (mWindow.performPanelShortcut(mWindow.mPreparedPanel, keyCode, event, 0)) {
return true;
}
}
}
//重点分析的地方
//当Window未destoryed且callback非null,
//交给Window对象的callback处理
if (!mWindow.isDestroyed()) {
final Window.Callback cb = mWindow.getCallback();
// mFeatureId < 0 表示是application的DecorView，比如Activity、Dialog
final boolean handled = cb != null && mFeatureId < 0 ? cb.dispatchKeyEvent(event)//事件派发给callback对象
: super.dispatchKeyEvent(event);//派发给ViewGroup（View层次结构）
if (handled) {
return true;
}
}
//只有View不消费事件，才将事件交给Window对象
return isDown ? mWindow.onKeyDown(mFeatureId, event.getKeyCode(), event)
: mWindow.onKeyUp(mFeatureId, event.getKeyCode(), event);
}
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我们知道Activity和Dialog都是实现了CallBack接口，因此这里先分析KeyEvent在Activity中分发，再看看View层次结构分发。因为PhoneWindow是Window的唯一实现类，所以最后需要看看PhoneWindow的onKeyDown()和onKeyUp()函数。
//Activity.java
public boolean dispatchKeyEvent(KeyEvent event) {
onUserInteraction();
final int keyCode = event.getKeyCode();
//处理KEYCODE_MENU键，Activity有ActionBar且消费该按键
if (keyCode == KeyEvent.KEYCODE_MENU &&
mActionBar != null && mActionBar.onMenuKeyEvent(event)) {
return true;
}
Window win = getWindow();
//这里KeyEvent交给Window对象分发
if (win.superDispatchKeyEvent(event)) {
return true;
}
View decor = mDecor;
if (decor == null) decor = win.getDecorView();
//Window对象win不消费Key事件,则将事件交给KeyEvent自身
return event.dispatch(this, decor != null
? decor.getKeyDispatcherState() : null, this);
}
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通过后面的学习，可以KeyEvent在Window对象的分发其实是在View的层次结构分发。在View层次结构中不消费KeyEvent事件，那么会交给KeyEvent自身处理，会调用Activity相关方法，如onKeyDown()。
我们知道PhoneWindow是Window的具体实现，所以看看PhoneWindow对象的superDispatchKeyEvent()函数。
//Window
/**
* Used by custom windows, such as Dialog, to pass the key press event
* further down the view hierarchy. Application developers should
* not need to implement or call this.
*
*/
public abstract boolean superDispatchKeyEvent(KeyEvent event);
//PhoneWindow
@Override
public boolean superDispatchKeyEvent(KeyEvent event) {
return mDecor.superDispatchKeyEvent(event);
}
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可以看到，又回到DecorView对象的superDispatchKeyEvent()函数。
// DecorView
public boolean superDispatchKeyEvent(KeyEvent event) {
//优先处理KEYCODE_BACK的事件
if (event.getKeyCode() == KeyEvent.KEYCODE_BACK) {
final int action = event.getAction();
// Back cancels action modes first.
if (mPrimaryActionMode != null) {
if (action == KeyEvent.ACTION_UP) {
mPrimaryActionMode.finish();
}
return true;
}
}
//在View层次结构进行派发
if (super.dispatchKeyEvent(event)) {
return true;
}
//如果View层次结构不消费，且ViewRootImpl不为空，
//则在ViewRootImpl对象处理
return (getViewRootImpl() != null) && getViewRootImpl().dispatchUnhandledKeyEvent(event);
}
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View层次结构大概如下图所示，这里先看看KeyEvent在View层次结构的分发。
我们知道，DecorView继承至FrameLayout,而FrameLayout是ViewGroup的子类。 ViewGroup类的dispatchKeyEvent()函数如下：
//ViewGroup
@Override
public boolean dispatchKeyEvent(KeyEvent event) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onKeyEvent(event, 1);
}
//ViewGroup是focused或者设置了具体的大小，则交给它实现
if ((mPrivateFlags & (PFLAG_FOCUSED | PFLAG_HAS_BOUNDS))
== (PFLAG_FOCUSED | PFLAG_HAS_BOUNDS)) {
//在View中的实现
if (super.dispatchKeyEvent(event)) {
return true;
}
} else if (mFocused != null && (mFocused.mPrivateFlags & PFLAG_HAS_BOUNDS)
== PFLAG_HAS_BOUNDS) {
//mFocused表示当前ViewGroup中获得焦点或者包含焦点的View（子View）
if (mFocused.dispatchKeyEvent(event)) {
return true;
}
}
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 1);
}
return false;
}
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从上面代码可以看出，子View想要获得焦点,处理KeyEvent，需要设置focusable属性为true。KeyEvent会优先派发给符合条件的ViewGroup处理，而不是子View。mFocused.dispatchKeyEvent(event)中的dispatchKeyEvent()可能会迭代调用，因为子View也可能是ViewGroup。
这里看看View中事件分发。
//View
/**
* Dispatch a key event to the next view on the focus path. This path runs
* from the top of the view tree down to the currently focused view. If this
* view has focus, it will dispatch to itself. Otherwise it will dispatch
* the next node down the focus path. This method also fires any key
* listeners.
*
* @param event The key event to be dispatched.
* @return True if the event was handled, false otherwise.
*/
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;
//如果我们给View设置了OnKeyListener且View是ENABLED状态,
//则会回调我们的了OnKeyListener
if (li != null && li.mOnKeyListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnKeyListener.onKey(this, event.getKeyCode(), event)) {
return true;
}
//调用KeyEvent.dispatch方法，并将view对象本身作为参数传递进去，view的各种callback方法在这里被触发
if (event.dispatch(this, mAttachInfo != null
? mAttachInfo.mKeyDispatchState : null, this)) {
return true;
}
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
return false;
}
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ViewGroup和View的dispatchKeyEvent()就构成了View层次结构的KeyEvent分发，且总是从树根DecorView开始到具体的View。注意到此处在View不消费KeyEvent会调用KeyEvent.dispatch方法，在Activity也会调用该方法。
//KeyEvent
public final boolean dispatch(Callback receiver, DispatcherState state,
Object target) {
switch (mAction) {
case ACTION_DOWN: {
mFlags &= ~FLAG_START_TRACKING;
//回调Callback对象receiver的onKeyDown函数，上文知道Activity和View都实现Callback
boolean res = receiver.onKeyDown(mKeyCode, this);
if (state != null) {//通常成立
if (res && mRepeatCount == 0 && (mFlags&FLAG_START_TRACKING) != 0) {//判断是否轨迹事件
state.startTracking(this, target);
} else if (isLongPress() && state.isTracking(this)) {
try {
//处理长按事件
if (receiver.onKeyLongPress(mKeyCode, this)) {
state.performedLongPress(this);
res = true;//消费该事件
}
} catch (AbstractMethodError e) {
}
}
}
return res;
}
case ACTION_UP:
if (state != null) {
//reset state的内部状态，也改变了KeyEvent的某些状态
state.handleUpEvent(this);
}
//回调Callback对象receiver的onKeyUp函数
return receiver.onKeyUp(mKeyCode, this);
case ACTION_MULTIPLE:
final int count = mRepeatCount;
final int code = mKeyCode;
if (receiver.onKeyMultiple(code, count, this)) {
return true;
}
if (code != KeyEvent.KEYCODE_UNKNOWN) {
mAction = ACTION_DOWN;
mRepeatCount = 0;
boolean handled = receiver.onKeyDown(code, this);
if (handled) {
mAction = ACTION_UP;
receiver.onKeyUp(code, this);
}
mAction = ACTION_MULTIPLE;
mRepeatCount = count;
return handled;
}
return false;
}
return false;
}
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在上面代码中，可以看到Callback对象的onKeyDown(),onKeyUp(),onKeyLongPress()函数被回调。而Activity和View是Callback接口的实现，因此调用Activity和View对应的方法。
先看看Activity对几个方法的实现：
//Activity
public boolean onKeyDown(int keyCode, KeyEvent event) {
//处理返回键
if (keyCode == KeyEvent.KEYCODE_BACK) {
if (getApplicationInfo().targetSdkVersion
>= Build.VERSION_CODES.ECLAIR) {
// 标记追踪这个key event
event.startTracking();
} else {
//手机APP常在Activity重写该方法，
//要求用户双击两次来退出APP,而不是一次就退出APP
onBackPressed();
}
return true;
}
if (mDefaultKeyMode == DEFAULT_KEYS_DISABLE) {
return false;
} else if (mDefaultKeyMode == DEFAULT_KEYS_SHORTCUT) {
Window w = getWindow();
if (w.hasFeature(Window.FEATURE_OPTIONS_PANEL) &&
w.performPanelShortcut(Window.FEATURE_OPTIONS_PANEL, keyCode, event,
Menu.FLAG_ALWAYS_PERFORM_CLOSE)) {
return true;
}
return false;
} else if (keyCode == KeyEvent.KEYCODE_TAB) {
return false;
} else {
boolean clearSpannable = false;
boolean handled;
if ((event.getRepeatCount() != 0) || event.isSystem()) {
clearSpannable = true;
handled = false;
} else {
handled = TextKeyListener.getInstance().onKeyDown(
null, mDefaultKeySsb, keyCode, event);
if (handled && mDefaultKeySsb.length() > 0) {
final String str = mDefaultKeySsb.toString();
clearSpannable = true;
switch (mDefaultKeyMode) {
case DEFAULT_KEYS_DIALER:
Intent intent = new Intent(Intent.ACTION_DIAL, Uri.parse("tel:" + str));
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
startActivity(intent);
break;
case DEFAULT_KEYS_SEARCH_LOCAL:
startSearch(str, false, null, false);
break;
case DEFAULT_KEYS_SEARCH_GLOBAL:
startSearch(str, false, null, true);
break;
}
}
}
if (clearSpannable) {
mDefaultKeySsb.clear();
mDefaultKeySsb.clearSpans();
Selection.setSelection(mDefaultKeySsb,0);
}
return handled;
}
}
public boolean onKeyLongPress(int keyCode, KeyEvent event) {
return false;
}
public boolean onKeyUp(int keyCode, KeyEvent event) {
if (getApplicationInfo().targetSdkVersion
>= Build.VERSION_CODES.ECLAIR) {
if (keyCode == KeyEvent.KEYCODE_BACK && event.isTracking()
&& !event.isCanceled()) {
onBackPressed();
return true;
}
}
return false;
}
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而View中实现：
//View
public boolean onKeyDown(int keyCode, KeyEvent event) {
//KEYCODE_DPAD_CENTER、KEYCODE_ENTER、
//KEYCODE_SPACE、KEYCODE_NUMPAD_ENTER都返回ture,其他返回false
if (KeyEvent.isConfirmKey(keyCode)) {
//当前View是DISABLED状态直接返回false
if ((mViewFlags & ENABLED_MASK) == DISABLED) {
return true;
}
if (event.getRepeatCount() == 0) {
final boolean clickable = (mViewFlags & CLICKABLE) == CLICKABLE
|| (mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE;
if (clickable || (mViewFlags & TOOLTIP) == TOOLTIP) {
final float x = getWidth() / 2f;
final float y = getHeight() / 2f;
if (clickable) {
//标记为Pressed，例如根据状态改变背景
setPressed(true, x, y);
}
//长按检测
checkForLongClick(0, x, y);
return true;
}
}
}
return false;
}
public boolean onKeyUp(int keyCode, KeyEvent event) {
if (KeyEvent.isConfirmKey(keyCode)) {
if ((mViewFlags & ENABLED_MASK) == DISABLED) {
return true;
}
if ((mViewFlags & CLICKABLE) == CLICKABLE && isPressed()) {
setPressed(false);
if (!mHasPerformedLongPress) {
removeLongPressCallback();
if (!event.isCanceled()) {
return performClickInternal();
}
}
}
}
return false;
}
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在DecorView的superDispatchKeyEvent()函数最后一行，如果View层次结构不消费事件，那么会调用ViewRootImpl对象的dispatchUnhandledKeyEvent()函数，这里主要是将未被消费的KeyEvent分发给注册了处理所有KeyEvent的处理者（监听器）
//ViewRootImpl.java
private final UnhandledKeyManager mUnhandledKeyManager
=new UnhandledKeyManager();
public boolean dispatchUnhandledKeyEvent(KeyEvent event) {
return mUnhandledKeyManager.dispatch(mView, event);
}
boolean dispatch(View root, KeyEvent event) {
if (mDispatched) {
return false;
}
View consumer;
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "UnhandledKeyEvent dispatch");
mDispatched = true;
//将未处理的KeyEvent进行分发,如果有View消费该事件，则返回该
//View
consumer = root.dispatchUnhandledKeyEvent(event);
//用于追踪该事件
if (event.getAction() == KeyEvent.ACTION_DOWN) {
int keycode = event.getKeyCode();
if (consumer != null && !KeyEvent.isModifierKey(keycode)) {
mCapturedKeys.put(keycode, new WeakReference<>(consumer));
}
}
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
return consumer != null;
}
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这里的root是DecorView,但从参数的类型来说，这里root.dispatchUnhandledKeyEvent(event)应该是在View中实现的。
//View.java
View dispatchUnhandledKeyEvent(KeyEvent evt) {
if (onUnhandledKeyEvent(evt)) {
return this;
}
return null;
}
boolean onUnhandledKeyEvent(@NonNull KeyEvent event) {
if (mListenerInfo != null && mListenerInfo.mUnhandledKeyListeners != null) {
//mListenerInfo通过栈的方式保存是否有View设置UnhandledKeyListener，如果有且消费事件，则该DecorView消费该事件
for (int i = mListenerInfo.mUnhandledKeyListeners.size() - 1; i >= 0; --i) {
if (mListenerInfo.mUnhandledKeyListeners.get(i).onUnhandledKeyEvent(this, event)) {
return true;
}
}
}
return false;
}
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通过上文代码可以知道，我们可以在View中添加addOnUnhandledKeyEventListener(OnUnhandledKeyEventListener)来监听所有未被处理的KeyEvent。会在KeyEvent正常分发未被消费前，且早于Window的Callback被回调。
在DecorView的dispatchKeyEvent()函数，如果View层次结构不消费事件，那么会交给PhoneWindow的onKeyDown()或onKeyUp()函数。
//PhoneWindow
protected boolean onKeyDown(int featureId, int keyCode, KeyEvent event) {
final KeyEvent.DispatcherState dispatcher =
mDecor != null ? mDecor.getKeyDispatcherState() : null;
Integer.toHexString(event.getFlags()));
switch (keyCode) {
case KeyEvent.KEYCODE_VOLUME_UP:
case KeyEvent.KEYCODE_VOLUME_DOWN:
case KeyEvent.KEYCODE_VOLUME_MUTE: {
if (mMediaController != null) {
mMediaController.dispatchVolumeButtonEventAsSystemService(event);
} else {
getMediaSessionManager().dispatchVolumeKeyEventAsSystemService(event,
mVolumeControlStreamType);
}
return true;
}
case KeyEvent.KEYCODE_MEDIA_PLAY:
case KeyEvent.KEYCODE_MEDIA_PAUSE:
case KeyEvent.KEYCODE_MEDIA_PLAY_PAUSE:
case KeyEvent.KEYCODE_MUTE:
case KeyEvent.KEYCODE_HEADSETHOOK:
case KeyEvent.KEYCODE_MEDIA_STOP:
case KeyEvent.KEYCODE_MEDIA_NEXT:
case KeyEvent.KEYCODE_MEDIA_PREVIOUS:
case KeyEvent.KEYCODE_MEDIA_REWIND:
case KeyEvent.KEYCODE_MEDIA_RECORD:
case KeyEvent.KEYCODE_MEDIA_FAST_FORWARD: {
if (mMediaController != null) {
if (mMediaController.dispatchMediaButtonEventAsSystemService(event)) {
return true;
}
}
return false;
}
case KeyEvent.KEYCODE_MENU: {
onKeyDownPanel((featureId < 0) ? FEATURE_OPTIONS_PANEL : featureId, event);
return true;
}
case KeyEvent.KEYCODE_BACK: {
if (event.getRepeatCount() > 0) break;
if (featureId < 0) break;
if (dispatcher != null) {
dispatcher.startTracking(event, this);
}
return true;
}
}
return false;
}
protected boolean onKeyUp(int featureId, int keyCode, KeyEvent event) {
final KeyEvent.DispatcherState dispatcher =
mDecor != null ? mDecor.getKeyDispatcherState() : null;
if (dispatcher != null) {
dispatcher.handleUpEvent(event);
}
switch (keyCode) {
case KeyEvent.KEYCODE_VOLUME_UP:
case KeyEvent.KEYCODE_VOLUME_DOWN: {
if (mMediaController != null) {
mMediaController.dispatchVolumeButtonEventAsSystemService(event);
} else {
getMediaSessionManager().dispatchVolumeKeyEventAsSystemService(
event, mVolumeControlStreamType);
}
return true;
}
case KeyEvent.KEYCODE_VOLUME_MUTE: {
getMediaSessionManager().dispatchVolumeKeyEventAsSystemService(
event, AudioManager.USE_DEFAULT_STREAM_TYPE);
return true;
}
case KeyEvent.KEYCODE_MEDIA_PLAY:
case KeyEvent.KEYCODE_MEDIA_PAUSE:
case KeyEvent.KEYCODE_MEDIA_PLAY_PAUSE:
case KeyEvent.KEYCODE_MUTE:
case KeyEvent.KEYCODE_HEADSETHOOK:
case KeyEvent.KEYCODE_MEDIA_STOP:
case KeyEvent.KEYCODE_MEDIA_NEXT:
case KeyEvent.KEYCODE_MEDIA_PREVIOUS:
case KeyEvent.KEYCODE_MEDIA_REWIND:
case KeyEvent.KEYCODE_MEDIA_RECORD:
case KeyEvent.KEYCODE_MEDIA_FAST_FORWARD: {
if (mMediaController != null) {
if (mMediaController.dispatchMediaButtonEventAsSystemService(event)) {
return true;
}
}
return false;
}
case KeyEvent.KEYCODE_MENU: {
onKeyUpPanel(featureId < 0 ? FEATURE_OPTIONS_PANEL : featureId,
event);
return true;
}
case KeyEvent.KEYCODE_BACK: {
if (featureId < 0) break;
if (event.isTracking() && !event.isCanceled()) {
if (featureId == FEATURE_OPTIONS_PANEL) {
PanelFeatureState st = getPanelState(featureId, false);
if (st != null && st.isInExpandedMode) {
reopenMenu(true);
return true;
}
}
closePanel(featureId);
return true;
}
break;
}
case KeyEvent.KEYCODE_SEARCH: { /*
if (isNotInstantAppAndKeyguardRestricted()) {
break;
}
if ((getContext().getResources().getConfiguration().uiMode
& Configuration.UI_MODE_TYPE_MASK) == Configuration.UI_MODE_TYPE_WATCH) {
break;
}
if (event.isTracking() && !event.isCanceled()) {
launchDefaultSearch(event);
}
return true;
}
case KeyEvent.KEYCODE_WINDOW: {
if (mSupportsPictureInPicture && !event.isCanceled()) {
getWindowControllerCallback().enterPictureInPictureModeIfPossible();
}
return true;
}
}
return false;
}
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从上面可以看到，PhoneWindow也只对一些物理按键做了处理，如果PhoneWindow和View、Activity都没有消费事件，那么ViewPostImeInputStage对象通过系统算法自动寻找焦点了。
总结一下：
- 监听器的优先级高于Callback的回调，也就是说OnKeyListener的函数优先Callback的onKeyDown等等函数的回调。
- View的Callback回调要早于Activity，Activity的回调早于PhoneWindow。
- 优先级高的消费KeyEvent,优先级低的不再受理该事件。
下面通过时序图对上文KeyEvent做一个整体流程的阐释（虽然不能准备表达意思）
在图表示在KeyEvent在DecorView开始不拦截最终在View的OnKeyListener或Callback对象被消费的过程。
参考文章
系统自动寻找焦点
回到梦开始的地方ViewPostImeInputStage对象的processKeyEvent()函数的末尾，在自动处理焦点的地方，会调用performFocusNavigation()函数来寻找下个获得焦点的View。
//ViewRootImpl.java
private boolean performFocusNavigation(KeyEvent event) {
int direction = 0;
//从下面代码可以看出，switch语句在此的主要作用是判断焦点的方向
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) {
//分析一
//mView在这里是DecorView对象，查找出当前获得焦点的View
View focused = mView.findFocus();
if (focused != null) {
//分析二
//在当前获得焦点View通过指定方向搜索下一个获得焦点的View
View v = focused.focusSearch(direction);
if (v != null && v != focused) {
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 true;
}
}
//给DecorView最后一次处理焦点的机会
if (mView.dispatchUnhandledMove(focused, direction)) {
return true;
}
} else {
//如果没有View获得焦点
if (mView.restoreDefaultFocus()) {
return true;
}
}
}
return false;
}
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在分析一处，通过DecorView对象mView来查找已获得焦点的View，findFocus()函数在ViewGroup和View都有实现，而DecorView继承自ViewGroup。这里其实进入了View的层次结果查找已获得焦点的View.
//ViewGroup.java
@Override
public View findFocus() {
if (isFocused()) {
return this;
}
if (mFocused != null) {
//mFocused表示获得焦点的View,有可能是ViewGroup,也有可能是具体的View
return mFocused.findFocus();
}
return null;
}
//View.java
public View findFocus() {
return (mPrivateFlags & PFLAG_FOCUSED) != 0 ? this : null;
}
public boolean isFocused() {
return (mPrivateFlags & PFLAG_FOCUSED) != 0;
}
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对于ViewGroup来说，如果本身获得焦点则直接返回自身即可，否则继续通过mFocused.findFocus()函数继续查找已获得焦点的View。具体的View是否获得焦点与ViewGroup的判断条件是一致的，判断PFLAG_FOCUSED标志位，也就是我们调用View的focuesabe=true，如果设置，则返回该View，否则返回null，表示没有View获得焦点。
假设寻找到了已获得焦点的View，那么下面就是寻找下一个获得焦点的View。也就是ViewPostImeInputStage对象的performFocusNavigation()函数分析二的代码。由于focused对象有可能是ViewGroup，也有可能是具体的View。一起看看它两的实现。
//View
//具体View的实现非常的简单，如果有父视图ViewGroup，则在俯视图在寻找，
//否则返回null
public View focusSearch(@FocusRealDirection int direction) {
if (mParent != null) {
return mParent.focusSearch(this, direction);
} else {
return null;
}
}
//ViewGroup.java
@Override
public View focusSearch(View focused, int direction) {
if (isRootNamespace()) { //判断当前ViewGroup是否顶层View，即DecorView
//调用FocusFinder实例findNextFocus进行查找
return FocusFinder.getInstance().findNextFocus(this, focused, direction);
} else if (mParent != null) {
//递归到顶层View
return mParent.focusSearch(focused, direction);
}
return null;
}
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通过递归方式，从内到外，到布局最外层View，准确说是ViewGroup。然后调用FocusFinder的实例方法findNextFocus()来寻找下一个获得焦点的View。 FocusFinder类通过算法，根据当前获得焦点的View和按键方向来寻找下一个获得焦点的View。
//FocusFinder.java
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;
ViewGroup effectiveRoot = getEffectiveRoot(root, focused);
if (focused != null) {
//寻找当前获得焦点view是否设置了指定方向的下一个获得焦点的View
next = findNextUserSpecifiedFocus(effectiveRoot, focused, direction);
}
if (next != null) {
return next;
}
ArrayList focusables = mTempList;
try {
focusables.clear();
effectiveRoot.addFocusables(focusables, direction);
if (!focusables.isEmpty()) {
//通过算法寻找最近可获得焦点的View
next = findNextFocus(effectiveRoot, focused, focusedRect, direction, focusables);
}
} finally {
focusables.clear();
}
return next;
}
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findNextFocus()函数主要通过两种方式来寻找指定方向的下一个获得焦。的View：我们在XML布局或者代码设置特定方向获得焦点的View;通过算法来寻找特定方向可以获得焦点最近的View。
对于方式一，findNextUserSpecifiedFocus()函数的实现如下：
//View.java
//主要作用是判断当前获得焦点的View有没有指定下一个获得焦点View的ID
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 test(View t) {
return t.mNextFocusForwardId == id;
}
});
}
}
return null;
}
//MatchIdPredicate匹配到相同ID会返回true
private View findViewInsideOutShouldExist(View root, int id) {
if (mMatchIdPredicate == null) {
mMatchIdPredicate = new MatchIdPredicate();
}
mMatchIdPredicate.mId = id;
View result = root.findViewByPredicateInsideOut(this, mMatchIdPredicate);
if (result == null) {
Log.w(VIEW_LOG_TAG, "couldn't find view with id " + id);
}
return result;
}
//从当前View开始，遍历View的层次结构来的下一个获得焦点的View
public final T findViewByPredicateInsideOut(
View start, Predicate predicate) {
View childToSkip = null;
for (;;) {
T view = start.findViewByPredicateTraversal(predicate, childToSkip);
if (view != null || start == this) {
return view;
}
ViewParent parent = start.getParent();
if (parent == null || !(parent instanceof View)) {
return null;
}
childToSkip = start;
start = (View) parent;
}
}
//匹配相同ID，返回该View
protected T findViewByPredicateTraversal(Predicate predicate,
View childToSkip) {
if (predicate.test(this)) {
return (T) this;
}
return null;
}
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在上面的相关方法，主要是通过在View的层次结构中去寻找到和指定id匹配的View。
那么方式二，通过算法来寻找下一个焦点又是如何的呢？
//FocusFinder.java
private View findNextFocus(ViewGroup root, View focused, Rect focusedRect,
int direction, ArrayList 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;
}
}
}
}
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对FocusFinder就不作进一步分析了。感兴趣同学可以自己看看源码。
看到这里，估计都很累了，我们想了解也都知道了。
总结
所有的KeyEvent都会优先在View的层次结构分发，然后再通过自动寻找焦点来查找下一个获得焦点的View。这就是为什么在OnKeyListener或Callback相关回调方法返回true消费KeyEvent,下一个View无法获得焦点。
KeyEvent在View的层次结构分发总是从外到里，外层ViewGroup消费KeyEvent,内层的View是无法获得焦点的。这就是为什么我们不想要EditText弹出软键盘，在根布局设置focusable为true的原因。
OnKeyListener的调用要早于KeyEvent.Callback的调用，如果设置了OnKeyListener并消费了KeyEvent,那么Callback相关函数不会再被调用。
另外，想要在KeyEvent分发前处理KeyEvent,例如TV开发处理特殊的按键，可以修改PhoneWindowManager的interceptKeyBeforeDispatching()函数。
最后的最后，能回答开头的问题么？
作者：新小梦
链接：https://juejin.im/post/5d5d35e3e51d4561ae4da62c