UIButton的状态

UIButton的状态
一般如果是由Touch Up Inside触发的事件：

1. 手指按到控件，UIControl更新ControlState到Highlighted
   
2. 手指离开控件，一般UIControl更新ControlState到Selected
3. Highlighted -> Selected之后的状态取决于具体的UIControl子类，其实连UITableViewCell也有这三种状态，Cell还是UIView的子类，这种特殊的状态默认就不会更新回Normal了～

4.至于在UIbutton中，那就是非常自然的时序：Normal -> Highlighted -> Selected -> Normal啦。

在Button的变化过程中就可以做一些可爱的动画了。

NSString *leftImageName = [NSString stringWithFormat:@"%@-1.png", _leftTurnip.turnipImageName];

    UIImage *leftImage = IMAGE(leftImageName);

    _leftTurnipButton.bounds = CGRectMake(0, 0, leftImage.size.width / 2.5 * PROPORTION, leftImage.size.height / 2.5 * PROPORTION);

    [_leftTurnipButton setImage:leftImage forState:UIControlStateNormal];

    leftImageName = [NSString stringWithFormat:@"%@-2.png", _leftTurnip.turnipImageName];

    leftImage = IMAGE(leftImageName);

    [_leftTurnipButton setImage:leftImage forState:UIControlStateHighlighted];