Android ViewGroup.setDescendantFocusability函数

转载：http://flysnow.iteye.com/blog/1762556

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

Setdescendantfocusability(int focusability)代码


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);

}





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



Requestfocus(int direction, rect previouslyfocusedrect)代码


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);

}

}



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。。