这个函数是在ViewGroup里定义的，主要用于控制child View获取焦点的能力，比如是否阻止child View获取焦点。
他有三个常量可供设置
-
FOCUS_BEFORE_DESCENDANTS ViewGroup本身先对焦点进行处理，如果没有处理则分发给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都没有处理，则自己再处理
-
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
我们看下这个方法的实现
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public void setDescendantFocusability(int focusability) {
switch (focusability) {
case FOCUS_BEFORE_DESCENDANTS:
case FOCUS_AFTER_DESCENDANTS:
case FOCUS_BLOCK_DESCENDANTS:
break;
default:
throw new IllegalArgumentException("must be one of FOCUS_BEFORE_DESCENDANTS, "
+ "FOCUS_AFTER_DESCENDANTS, FOCUS_BLOCK_DESCENDANTS");
}
mGroupFlags &= ~FLAG_MASK_FOCUSABILITY;
mGroupFlags |= (focusability & FLAG_MASK_FOCUSABILITY);
}
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可以看到，只有这三个常量可以设置，不是这三个常量会抛出异常的。
设置后，会在requestFocus(int direction, Rect previouslyFocusedRect) 方法里根据设置进行相应的处理。来看下实现
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public boolean requestFocus(int direction, Rect previouslyFocusedRect) {
if (DBG) {
System.out.println(this + " ViewGroup.requestFocus direction="
+ direction);
}
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);
}
default:
throw new IllegalStateException("descendant focusability must be "
+ "one of FOCUS_BEFORE_DESCENDANTS, FOCUS_AFTER_DESCENDANTS, FOCUS_BLOCK_DESCENDANTS "
+ "but is " + descendantFocusability);
}
}
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通过这里的实现可以看到上面定义的三个常量设置的意思。。
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