UIButton的selected设为TRUE时在按下时显示自己定义的背景图

在UIButton的selected设为TRUE后。须要在按钮高亮时，显示自己定义的背景图。
经研究hightLighted和selected这两个状态是能够重叠的,就是button能够同一时候处于selectec和highlighted两个状态下。
从UIControlState的定义也能够看出：
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
};


设置这样的状态重叠情景时的背景图：
 [button setBackgroundImage:[UIImage imageNamed:@"seled_highLight.png"] forState:UIControlStateSelected| UIControlStateHighlighted];