Android ViewGroup.setDescendantFocusability函数

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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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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);
    }
}