button selected 状态

　　　　UIButton * btn;

                   [btn setImage:[UIImage imageNamed:@"a.png"] forState:UIControlStateNormal];

                   [btn setImage:[UIImage imageNamed:@"b.png"] forState:UIControlStateSelected];

　　　　　　btn.method(rmPwd:);

- (IBAction)rmbPwd:(UIButton *)sender {

    sender.selected = !sender.selected;