CADisplayLink 定时器

-(void)startTimer {

[self invalidateTimer];
self.displayTimer = [CADisplayLink displayLinkWithTarget:self selector:@selector(captureView:)];
if (@available(iOS 10.0, *)) {
    self.displayTimer.preferredFramesPerSecond = desiredFrameRate;
} else {
    self.displayTimer.frameInterval = displayLinkFrameRate / desiredFrameRate;
}
[self.displayTimer addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSRunLoopCommonModes];
self.displayTimer.paused = [UIApplication sharedApplication].applicationState == UIApplicationStateBackground；

}
-(void)invalidateTimer{
if(self.displayTimer){
[self.displayTimer invalidate];
self.displayTimer = nil;
}
}