计算当前CADisplayLink时间

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

-(void)displayAction:(id)sender{    if (_lastTime == 0) {        _lastTime = _displayLink.timestamp;        return;    }    _count++;    NSTimeInterval delta = _displayLink.timestamp - _lastTime;    if (delta < 1) return;    _lastTime = _displayLink.timestamp;    float fps = _count / delta;    _count = 0;    NSString *text = [[NSString alloc] initWithString:[NSString stringWithFormat:@"%d FPS", (int)round(fps)]];    _numLabel.text = text;}