Android(安卓)ViewGroup.setDescendantFocusability函数

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

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

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。。

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