Facebook纸张标题向上拖动然后向下拖动到屏幕底部

Question

我创建了一个类似Facebook纸张的应用程序，我试图找出如何将视图拖动/捕捉到底部，然后再拖回来。我很确定我完成了75％，但我不能为我的生活弄清楚如何让它突然到底。 这是代码：

- (IBAction)onDrag:(UIPanGestureRecognizer *)sender {
    CGPoint translation = [sender translationInView:self.view];

    CGPoint center = self.headlinesView.center;

    // this is how high the tray should go
    int upperThreshold = self.view.frame.size.height - (self.headlinesView.frame.size.height / 2);
    // this is how low the tray should go
    int lowerThreshold = self.view.frame.size.height + (self.headlinesView.frame.size.height / 2);

    if (sender.state == UIGestureRecognizerStateBegan) {
        NSLog(@"start");
        // reset the tray location to where the tray currently is
        self.originalTrayLocation = self.headlinesView.center;
    } else if (sender.state == UIGestureRecognizerStateChanged) {
        NSLog(@"changed");

        if (center.y < upperThreshold) {
            // if the tray location is past the point where it should go,
            // add some resistance
            center.y = upperThreshold + (translation.y * 0.1);
        } else if (center.y > lowerThreshold) {
            // if the user drags down the tray a lot, don't make the tray
            // go that low
            center.y = lowerThreshold;
        } else {
            // drag as normal
            center.y = self.originalTrayLocation.y + translation.y;
        }
        self.headlinesView.center = center;
    } else if (sender.state == UIGestureRecognizerStateEnded) {
        NSLog(@"ended");
        // if the user dragged too far up or down, animate the tray
        // back to its desired location
        [UIView animateWithDuration:0.3 animations:^{
            self.headlinesView.center = CGPointMake(160, upperThreshold);
        }];
    }
}

任何想法？