Android 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

 

1. 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
2. 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
3. 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。。