iOS 性能监测之FPS

_displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(displayLinkTick:)];
       [_displayLink setPaused:YES];
       [_displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];

//这个方法的执行频率跟当前屏幕的刷新频率是一样的，屏幕每渲染刷新一次，就执行一次，那么1秒的时长执行刷新的次数就是当前的FPS值
- (void)displayLinkTick:(CADisplayLink *)link{
   
   //     duration 是只读的， 表示屏幕刷新的间隔 = 1/fps
   //     timestamp 是只读的， 表示上次屏幕渲染的时间点
   //    frameInterval 是表示定时器被触发的间隔， 默认值是1， 就是表示跟屏幕的刷新频率一致。
   //    NSLog(@"timestamp= %f  duration= %f frameInterval= %f",link.timestamp, link.duration, frameInterval);
   
   //初始化屏幕渲染的时间
   if (_beginTime == 0) {
       _beginTime = link.timestamp;
       return;
   }
   //刷新次数累加
   _count++;
   //刚刚屏幕渲染的时间与最开始幕渲染的时间差
   NSTimeInterval interval = link.timestamp - _beginTime;
   if (interval < 1) {
       //不足1秒，继续统计刷新次数
       return;
   }
   //刷新频率
   float fps = _count / interval;
   
   if (self.FPSBlock != nil) {
       self.FPSBlock(fps);
   }

   //1秒之后，初始化时间和次数，重新开始监测
   _beginTime = link.timestamp;
   _count = 0;
}

 _fpsLabel = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 100, 50)];
        _fpsLabel.textAlignment = NSTextAlignmentCenter;
        _fpsLabel.backgroundColor = [UIColor orangeColor];
        _fpsLabel.font = [UIFont systemFontOfSize:15];
        _fpsLabel.alpha = 0.7;
        
        UIViewController * viewc = [[UIViewController alloc] init];
        _window = [[UIWindow alloc] initWithFrame:CGRectMake(0, 0, 100, 50)];
        _window.center = CGPointMake([UIScreen mainScreen].bounds.size.width/2.0, 64);
        _window.windowLevel = UIWindowLevelAlert + 1;
        _window.layer.cornerRadius = 10;
        _window.clipsToBounds = YES;
        _window.rootViewController = viewc;
        [_window addSubview:_fpsLabel];
        [_window makeKeyAndVisible];
        
        UIPanGestureRecognizer *pan = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(handleGesture:)];
        _window.userInteractionEnabled = YES;
        [_window addGestureRecognizer:pan];