拖动滑动阻尼动画

if (displayLink) {
    [displayLink invalidate];
    displayLink = nil;
}
CGPoint p = [panGestureRecognizer translationInView:self];

switch (panGestureRecognizer.state) {
    case UIGestureRecognizerStateBegan:
    {
        _lastPoint = [panGestureRecognizer translationInView:self];
        break;
    }
    case UIGestureRecognizerStateChanged:
    {
        int dx = (int) (p.x - _lastPoint.x);
        int dy = (int) (p.y - _lastPoint.y);
        [self.contentView TransitionWithXValue:dx*_transitionFactor andYValue:dy*_transitionFactor];

        _lastPoint.x = p.x;
        _lastPoint.y = p.y;
        break;
    }
    case UIGestureRecognizerStateEnded: {
        velocity = [sender velocityInView:self];
        CGFloat magnitude = sqrtf((velocity.x * velocity.x) + (velocity.y * velocity.y));
        CGFloat slideMult = magnitude / 200;
        float slideFactor = 0.1 * slideMult;
        
        updateCount = slideFactor * 120 + 1;
        currentCount = 0;
        displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(updateView)];
        [displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
    }
    default:
        break;
}

-(void)updateView {
    currentCount++;
    //40ms
    if (currentCount>updateCount || currentCount>40) {
        [displayLink removeFromRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
        [displayLink invalidate];
        displayLink = nil;
    }else{
        //速度衰减以开方的比例递增
        float p = sqrtf(currentCount);
        CGPoint point = CGPointMake(velocity.x/p, velocity.y/p);
        int dx = point.x *0.1;
        int dy = point.y *0.1;
        [self.contentView TransitionWithXValue:dx andYValue:dy];
    }
}

CADisplayLink ,定时器的一种。以屏幕刷新率为触发时间，即1/60。
velocity.x为x轴速度，即点/秒。
p为速度衰减比例，以currentCount递增即越来越慢，但以currentCount递增过快且不平滑，故取开方。