单选button

定义一个全局button, 用来记录上次选中的button

    if (!self.oldButton) {
        sender.selected = YES;
        self.oldButton = sender;
    }
    else if (self.oldButton !=nil && self.oldButton == sender)
    {
        sender.selected = YES;
    }
    else if (self.oldButton!=sender && self.oldButton!=nil)
    {
        self.oldButton.selected = NO;
        sender.selected=YES;
        self.oldButton = sender;
    }