Button 文本显示闪动问题解决

- (void)viewDidLoad {
    [super viewDidLoad];
    
    _btn = [UIButton buttonWithType:UIButtonTypeSystem];
    [_btn setFrame:CGRectMake(100, 100, 100, 30)];
    _btn.backgroundColor = [UIColor redColor];
    [self.view addSubview:_btn];
    _countDown = 60;
    [NSTimer scheduledTimerWithTimeInterval:1.0
                                     target:self
                                   selector:@selector(timeCount:)
                                   userInfo:nil repeats:YES];
}

#pragma mark ------ timeCount

- (void)timeCount:(NSTimer *)timer{

    if (_countDown > 0) {
        
        _countDown --;
        // 同时设置
        _btn.titleLabel.text = [NSString stringWithFormat:@"%ld 计时",(unsigned long)_countDown];
        [_btn setTitle:[NSString stringWithFormat:@"%ld 计时",(unsigned long)_countDown] forState:UIControlStateNormal];
        
    } else {
        
        [timer invalidate];
        timer = nil;
    }
    
    
}