Android Button 原理

核心提示：9. * @param stateSet - An array of resource Ids to associate with the image.10. * Switch to this image by calling setState().11. * @param drawable -The image to

9. * @param stateSet - An array of resource Ids to associate with the image.

10. * Switch to this image by calling setState().

11. * @param drawable -The image to show.

12. */

13. public void addState(int[] stateSet, Drawable drawable) {

14. if (drawable != null) {

15. mStateListState.addStateSet(stateSet, drawable);

16. // in case the new state matches our current state...

17. onStateChange(getState());

18. }

19. }

20. ...

21. }

xml里面每一个Item就对应一种状态，而每一个有state_的属性就是一个状态的描述，drawable则是真正的drawable图片(这个其实也可以是其他类型的Drawable实例)。

当把这个实例付给View作为Background的时候，View会根据不同的state来切换不同状态的图片，从而实现了Press等诸多效果。简单看一下View中有关状态切换的代码如下：

各种View的状态列表：

1. /**

2. * The order here is very important to {@link #getDrawableState()}

3. */

4. private static final int[][] VIEW_STATE_SETS = {

5. EMPTY_STATE_SET, // 0 0 0 0 0

6. WINDOW_FOCUSED_STATE_SET, // 0 0 0 0 1

7. SELECTED_STATE_SET, // 0 0 0 1 0

8. SELECTED_WINDOW_FOCUSED_STATE_SET, // 0 0 0 1 1

9. FOCUSED_STATE_SET, // 0 0 1 0 0

10. FOCUSED_WINDOW_FOCUSED_STATE_SET, // 0 0 1 0 1

11. FOCUSED_SELECTED_STATE_SET, // 0 0 1 1 0

12. FOCUSED_SELECTED_WINDOW_FOCUSED_STATE_SET, // 0 0 1 1 1

13. ENABLED_STATE_SET, // 0 1 0 0 0

14. ENABLED_WINDOW_FOCUSED_STATE_SET, // 0 1 0 0 1

15. ENABLED_SELECTED_STATE_SET, // 0 1 0 1 0

16. ENABLED_SELECTED_WINDOW_FOCUSED_STATE_SET, // 0 1 0 1 1

17. ENABLED_FOCUSED_STATE_SET, // 0 1 1 0 0

18. ENABLED_FOCUSED_WINDOW_FOCUSED_STATE_SET, // 0 1 1 0 1

19. ENABLED_FOCUSED_SELECTED_STATE_SET, // 0 1 1 1 0