关于ios flag 做按标识钮切换的问题

先定义bool _flag 然后如下图中代码，一组按钮！

_flag = !_flag;
 if (_flag) {
 [_btn1 setBackgroundImage:[UIImage imageNamed:@"buttn2.png"] forState:UIControlStateNormal];
 [_btn2 setBackgroundImage:[UIImage imageNamed:@"buttn1.png"] forState:UIControlStateNormal];
 [_btn3 setBackgroundImage:[UIImage imageNamed:@"buttn1.png"] forState:UIControlStateNormal];
 [_btn4 setBackgroundImage:[UIImage imageNamed:@"buttn1.png"] forState:UIControlStateNormal];
 [_btn5 setBackgroundImage:[UIImage imageNamed:@"buttn1.png"] forState:UIControlStateNormal];
 [_btn6 setBackgroundImage:[UIImage imageNamed:@"buttn1.png"] forState:UIControlStateNormal];
 [_btn7 setBackgroundImage:[UIImage imageNamed:@"buttn1.png"] forState:UIControlStateNormal];
 [_btn8 setBackgroundImage:[UIImage imageNamed:@"buttn1.png"] forState:UIControlStateNormal];
 [_btn9 setBackgroundImage:[UIImage imageNamed:@"buttn1.png"] forState:UIControlStateNormal];
 [_btn10 setBackgroundImage:[UIImage imageNamed:@"buttn1.png"] forState:UIControlStateNormal];
 [_btn11 setBackgroundImage:[UIImage imageNamed:@"buttn1.png"] forState:UIControlStateNormal];
 [_btn12 setBackgroundImage:[UIImage imageNamed:@"buttn1.png"] forState:UIControlStateNormal];
 }
 else
 {
 [_btn1 setBackgroundImage:[UIImage imageNamed:@"buttn1.png"] forState:UIControlStateNormal];
}

是为了，选中按钮后 按钮会显示不一样的图片 好出分开来，但是有个问题，比如说按钮1在显示buttn2的时候，点击别的按钮， 按钮1是会显示buttn1！ 但是当再次点击按钮1的时候，按钮1会先显示1次buttn1,再才能达到我想要的效果！中间就会有2次显示buttn1 请问这怎么再定义下