Android焦点分发策略
[TOC]
焦点分发基础知识
获取焦点的前提
- View#isFocusable返回true, 如果在触摸模式, 则View#isFocusableInTouchMode也要返回true
- 控件必须可见
- 控件相关的父控件, 包括祖父控件等, ViewGroup#getDescendantFocusability()不能为ViewGroup#FOCUS_BLOCK_DESCENDANTS
焦点相关api
- requestFocus
主动请求焦点
- clearFocus
主动清除焦点
- focusSearch
递归搜索需要焦点的View
- ViewGroup.setDescendantFocusability
三个参数含义：
FOCUS_BEFORE_DESCENDANTS
ViewGroup本身相对焦点进行处理，如果沒有处理则分发给child View进行处理
FOCUS_AFTER_DESCENDANTS
先分发给Child View进行处理，如果所有的Child View都沒有处理，則自己再处理
FOCUS_BLOCK_DESCENDANTS
ViewGroup本身进行处理，不管是否处理成功，都不会分发给ChildView进行处理
- onRequestFocusInDescendants
可以传入方向来改变遍历的顺序, 默认是从0递增,遍历子控件,调用子控件的View#requestFocus来尝试把焦点给可见的子控件, 某个子控件成功获取到焦点后, 停止遍历
注: 重写该方法可以改变ViewGroup分发焦点给子控件的行为, 例如遍历顺
- requestChildFocus
当子控件主动放弃焦点的时候,回调这个方法通知父控件.
- clearChildFocus
当子控件获取了焦点后, 回调这个方法通知父控件
- focusableViewAvailable
通知父控件, 子控件的状态发生改变, 从不能获取焦点, 变成可能可以获取焦点.
- hasFocus
当前视图是否是焦点视图或子视图里面有焦点视图
- isFocused()
当前视图是否是焦点视图
- findFocus
查找焦点控件
- getFocusedChild
获取子控件的焦点view
焦点分发源码分析
在Android中任何事件都会经由ViewRootImpl$ViewPostImeInputStage的onProcess处理.在onProcess方法中会判断不同事件类型做不同的处理.这里我们只分析触发焦点转移的KeyEvent.事件分发机制参考
当我们触发按键,最终会由底层把事件传入onProcess,在onProcess中如果是KeyEvent类型就进入processKeyEvent方法中进行处理.
processKeyEvent方法源码如下:
private int processKeyEvent(QueuedInputEvent q) {
final KeyEvent event = (KeyEvent)q.mEvent;
if (event.getAction() != KeyEvent.ACTION_UP) {
// If delivering a new key event, make sure the window is
// now allowed to start updating.
handleDispatchDoneAnimating();
}
//1.向View树分发KeyEvent事件
// Deliver the key to the view hierarchy.
if (mView.dispatchKeyEvent(event)) {
return FINISH_HANDLED;
}
//........................此处省略部分代码
//2.处理焦点的转移
// Handle automatic focus changes.
//........................此处省略部分代码
if (direction != 0) {
//step1 找到当前View树中的焦点View
View focused = mView.findFocus();
if (focused != null) {
//step2 当前焦点View根据方向搜索下一个需要焦点的View并主动请求焦点
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
//step3 ViewRootImpl根据方向搜索下一个需要焦点的View并主动请求焦点
View v = focusSearch(null, direction);
if (v != null && v.requestFocus(direction)) {
return FINISH_HANDLED;
}
}
}
}
这个方法中主要做了两件事,1.向View树分发KeyEvent事件 2.处理焦点的转移,下面主要分析处理焦点的转移这条逻辑.
1.向View树分发KeyEvent事件
// Deliver the key to the view hierarchy.
if (mView.dispatchKeyEvent(event)) {
return FINISH_HANDLED;
}
这就是我们熟悉View层的事件分发,同理还有
-
dispatchHoverEvent
-
dispatchTouchEvent
2.处理焦点的转移
step1 : 当我们的按键有一个确定的方向,首先找到当前View树中的焦点view.
if (direction != 0) {
View focused = mView.findFocus();
}
step2 : 如果找到了当前焦点View,则调用当前焦点View的焦点搜索方法,并返回下一个需要焦点的View,最后需要焦点的View主动请求焦点
View v = focused.focusSearch(direction);
if (v.requestFocus(direction, mTempRect)) {
playSoundEffect(SoundEffectConstants
.getContantForFocusDirection(direction));
return FINISH_HANDLED;
}
最终View#focusSearch直接调用的是ViewPaent#focusSearch
ViewParent是一个接口,最后在ViewGroup中实现,后文分析ViewGroup#focusSearch,View的focusSearch方法如下:
public View focusSearch(@FocusRealDirection int direction) {
if (mParent != null) {
return mParent.focusSearch(this, direction);
} else {
return null;
}
}
step3 : 如果未找到当前焦点View,则调用ViewRootImpl的焦点搜索方法,并返回下一个需要焦点的View,最后需要焦点的View主动请求焦点
View v = focusSearch(null, direction);
if (v != null && v.requestFocus(direction)) {
return FINISH_HANDLED;
}
本质上ViewRootImpl#focusSearch方法重写的是ViewPaent#focusSearch,此方法内部直接调用焦点帮助类,返回一个需要焦点的View.方法如下:
@Override
public View focusSearch(View focused, int direction) {
checkThread();
if (!(mView instanceof ViewGroup)) {
return null;
}
return FocusFinder.getInstance().findNextFocus((ViewGroup) mView, focused, direction);
}
至此,焦点分发的逻辑就进入了View层,下面分析View层的逻辑.
ViewGroup#focusSearch
因为View的focusSearch直接调用的是ViewParent的focusSearch,所以我们只分析ViewGroup的focusSearch
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;
}
ViewGroup的focusSearch方法非常简单,如果是root View就调用焦点帮助类返回一个需要焦点的View,反之递归调用ViewGroup的focusSearch,直到找到一个需要焦点的View.
requestFocus
了解完focusSearch我们继续分析requestFocus,requestFocus在View和ViewGrope有各自不同的实现.
ViewGroup#requestFocus
@Override
public boolean requestFocus(int direction, Rect previouslyFocusedRect) {
int descendantFocusability = getDescendantFocusability();
switch (descendantFocusability) {
case FOCUS_BLOCK_DESCENDANTS:
return super.requestFocus(direction, previouslyFocusedRect);
case FOCUS_BEFORE_DESCENDANTS: {
final boolean took = super.requestFocus(direction, previouslyFocusedRect);
return took ? took : onRequestFocusInDescendants(direction, previouslyFocusedRect);
}
case FOCUS_AFTER_DESCENDANTS: {
final boolean took = onRequestFocusInDescendants(direction, previouslyFocusedRect);
return took ? took : super.requestFocus(direction, previouslyFocusedRect);
}
}
}
ViewGroup的requestFocus会根据三种分发策略决定是自己先请求还是child先请求,最终都会调用View的requestFocus
View#requestFocus
View的requestFocus会调用到requestFocusNoSearch
在requestFocusNoSearch中做一些位运算后最终调用handleFocusGainInternal
void handleFocusGainInternal(@FocusRealDirection int direction, Rect previouslyFocusedRect) {
if ((mPrivateFlags & PFLAG_FOCUSED) == 0) {
mPrivateFlags |= PFLAG_FOCUSED;
View oldFocus = (mAttachInfo != null) ? getRootView().findFocus() : null;
if (mParent != null) {
mParent.requestChildFocus(this, this);
}
if (mAttachInfo != null) {
mAttachInfo.mTreeObserver.dispatchOnGlobalFocusChange(oldFocus, this);
}
onFocusChanged(true, direction, previouslyFocusedRect);
refreshDrawableState();
}
}
handleFocusGainInternal中通知父容器自己请求了焦点,回调onFocusChanged,并刷新View
