setDescendantFocusability,在父View和子View间处理焦点关系

android:descendantFocusability用法简析

    开发中很常见的一个问题，项目中的listview不仅仅是简单的文字，常常需要自己定义listview，自己的Adapter去继承BaseAdapter，在adapter中按照需求进行编写，问题就出现了，可能会发生点击每一个item的时候没有反应，无法获取的焦点。原因多半是由于在你自己定义的Item中存在诸如ImageButton，Button，CheckBox等子控件(也可以说是Button或者Checkable的子类控件)，此时这些子控件会将焦点获取到，所以常常当点击item时变化的是子控件，item本身的点击没有响应。

    这时候就可以使用descendantFocusability来解决啦，API描述如下：

android:descendantFocusability

Defines the relationship between the ViewGroup and its descendants when looking for a View to take focus.

Must be one of the following constant values.

 

该属性是当一个为view获取焦点时，定义viewGroup和其子控件两者之间的关系。

属性的值有三种：

        beforeDescendants：viewgroup会优先其子类控件而获取到焦点

        afterDescendants：viewgroup只有当其子类控件不需要获取焦点时才获取焦点

        blocksDescendants：viewgroup会覆盖子类控件而直接获得焦点

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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。。

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增加xml中的配置方法，一劳永逸

adapter的item根布局添加属性  android:descenadantFocusability="blockDesceadant"

ImageButton / Button/ checkBox中添加android:focusability="false"

可以解决item不相应点击事件问题