- (CGRect)backgroundRectForBounds:(CGRect)bounds;        //返回背景边界 (image + title)
- (CGRect)contentRectForBounds:(CGRect)bounds;               //
- (CGRect)titleRectForContentRect:(CGRect)contentRect;      //返回title边界
- (CGRect)imageRectForContentRect:(CGRect)contentRect;  //返回image边界

自定义按钮