深入理解：View和ViewGroup如何才能获取焦点

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简介

一般我们知道View请求获取焦点调用requestFocus方法

查找流程

我们看看requestFocus方法

public final boolean requestFocus() {return requestFocus(View.FOCUS_DOWN);}

requestFocus中调用了requestFocus(View.FOCUS_DOWN) 我们接着看

public final boolean requestFocus(int direction) {return requestFocus(direction, null);}

又调用了 requestFocus(direction, null) 我们接着看

public boolean requestFocus(int direction, Rect previouslyFocusedRect) {return requestFocusNoSearch(direction, previouslyFocusedRect);}

注意：ViewGroup重写了requestFocus(direction, null)此方法,在里面处理焦点先给子View还是先给自己处理

又调用了requestFocusNoSearch(direction, previouslyFocusedRect) 着看

private boolean requestFocusNoSearch(int direction, Rect previouslyFocusedRect) {// need to be focusableif (!canTakeFocus()) { //判断是否具可以获取焦点return false;}// need to be focusable in touch mode if in touch modeif (isInTouchMode() &&(FOCUSABLE_IN_TOUCH_MODE != (mViewFlags & FOCUSABLE_IN_TOUCH_MODE))) {return false;}// need to not have any parents blocking usif (hasAncestorThatBlocksDescendantFocus()) {return false;}if (!isLayoutValid()) {mPrivateFlags |= PFLAG_WANTS_FOCUS;} else {clearParentsWantFocus();}handleFocusGainInternal(direction, previouslyFocusedRect);return true;}

看到这个方法我们发现了一个方法 canTakeFocus 这个方法就是判断View是否可以聚焦，不可聚焦直接返回false

private boolean canTakeFocus() {return ((mViewFlags & VISIBILITY_MASK) == VISIBLE) //View可见&& ((mViewFlags & FOCUSABLE) == FOCUSABLE)//Viewfocuable&& ((mViewFlags & ENABLED_MASK) == ENABLED)//View enable&& (sCanFocusZeroSized || !isLayoutValid() || hasSize()); //sCanFocusZeroSized 这个值在Android P以下默认就是true,也就是在Android P以下一个View没有大小也是可聚焦的；}

看到这儿我们就明白了view获取焦点的条件是：

可见性为 VISIBLE 并且 focusable并且状态 enabled 并且有尺寸(有尺寸不管在哪个版本都是可聚焦的)。

ViewGroup的重写的方法

ViewGroup重写了 requestFocusNoSearch(direction, previouslyFocusedRect) 方法，看下

 @Overridepublic boolean requestFocus(int direction, Rect previouslyFocusedRect) {if (DBG) {System.out.println(this + " ViewGroup.requestFocus direction="+ direction);}int descendantFocusability = getDescendantFocusability(); //获取descendantFocusability 的值boolean result;switch (descendantFocusability) {case FOCUS_BLOCK_DESCENDANTS://是BLOCK则调用父类的方法，直接进入自己的聚焦流程result = super.requestFocus(direction, previouslyFocusedRect);break;case FOCUS_BEFORE_DESCENDANTS: {//如果是before则先自己尝试聚焦,如果自己不能聚焦则去查找子View是否可以聚焦final boolean took = super.requestFocus(direction, previouslyFocusedRect);result = took ? took : onRequestFocusInDescendants(direction,previouslyFocusedRect);break;}case FOCUS_AFTER_DESCENDANTS: {//如果是AFTER 先给子View去聚焦，子View没有处理焦点则自己去请求聚焦final boolean took = onRequestFocusInDescendants(direction, previouslyFocusedRect);result = took ? took : super.requestFocus(direction, previouslyFocusedRect);break;}default:throw new IllegalStateException("descendant focusability must be "+ "one of FOCUS_BEFORE_DESCENDANTS, FOCUS_AFTER_DESCENDANTS, FOCUS_BLOCK_DESCENDANTS "+ "but is " + descendantFocusability);}if (result && !isLayoutValid() && ((mPrivateFlags & PFLAG_WANTS_FOCUS) == 0)) {mPrivateFlags |= PFLAG_WANTS_FOCUS;}return result;}

首先获取descendantFocusability 的值

• 如果FOCUS_BLOCK_DESCENDANTS则调用父类的方法，直接进入自己的聚焦流程，不给子View处理焦点的机会
• 如果是FOCUS_BEFORE_DESCENDANTS则先自己尝试聚焦,如果自己不能聚焦则去查找子View聚焦
• 如果是FOCUS_AFTER_DESCENDANTS先给子View去聚焦，子View没有处理焦点则自己去请求聚焦

descendantFocusability 决定了ViewGroup和子View请求焦点的先后顺序

descendantFocusability
xml属性  android:descendantFocusability=""
方法：setDescendantFocusability(int focusability)
取值：FOCUS_BEFORE_DESCENDANTS或者FOCUS_AFTER_DESCENDANTS或者FOCUS_BLOCK_DESCENDANTSViewGroup中 initViewGroup方法设置了默认值 FOCUS_BEFORE_DESCENDANTS

结论

• 对于ViewGroup，我们调用ViewGroup的requestFocus方法时
  • 如果是FOCUS_BLOCK_DESCENDANTS，ViewGroup获取焦点和子View是一样的
  • 非FOCUS_BLOCK_DESCENDANTS的情况，焦点可能是ViewGroup获取到也可能是子View获取到

ViewGroup中descendantFocusability 决定了子View和ViewGroup获取焦点的先后顺序

无论是View还是ViewGroup，这个对象本身获取焦点：
必须同时满足三个条件：可见性Visible、focusable为true、状态为enable
对应代码就是setFocusable(true),setVisbility(View.Visible)、setEnable(true)。

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