UIButton的selected设为TRUE时在按下时显示自定义的背景图

在UIButton的selected设为TRUE后，需要在按钮高亮时，显示自定义的背景图。

经研究hightLighted和selected这两个状态是可以重叠的,就是button可以同时处于selectec和highlighted两个状态下。

从UIControlState的定义也可以看出：

typedef NS_OPTIONS(NSUInteger, UIControlState) {

    UIControlStateNormal       = 0,

    UIControlStateHighlighted  = 1 << 0,                  // used when UIControl isHighlighted is set

    UIControlStateDisabled     = 1 << 1,

    UIControlStateSelected     = 1 << 2,                  // flag usable by app (see below)

    UIControlStateApplication  = 0x00FF0000,              // additional flags available for application use

    UIControlStateReserved     = 0xFF000000               // flags reserved for internal framework use

};

设置这种状态重叠情景时的背景图：

 [button setBackgroundImage:[UIImage imageNamed:@"seled_highLight.png"] forState:UIControlStateSelected| UIControlStateHighlighted];