Android ViewGroup.setDescendantFocusability函數

這個函數是在ViewGroup裏定義的，主要用於控制child View獲取焦點的能力，比如是否阻止child View獲取焦點。

 

他有三個常量可供設置

 

1. FOCUS_BEFORE_DESCENDANTS ViewGroup本身先對焦點進行處理，如果沒有處理則分發給child View進行處理
2. FOCUS_AFTER_DESCENDANTS 先分發給Child View進行處理，如果所有的Child View都沒有處理，則自己再處理
3. FOCUS_BLOCK_DESCENDANTS ViewGroup本身進行處理，不管是否處理成功，都不會分發給ChildView進行處理

我們看下這個方法的實現

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，只有這三個常量可以設置，不是這三個常量會拋出異常的。

 

 

設置後，會在requestFocus(int direction, Rect previouslyFocusedRect) 方法里根據設置進行相應的處理。來看下實現

 

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);
        }
    }

 

通過這裏的實現可以看到上面定義的三個常量設置的意思。。