Question

I would like to be able to move a subview on and off the screen much like you browse between images in the iPhone’s build in Photos app, so if the subview is more than 1/2 off screen when I let go with my finger it must animate off screen, but it must also support swipe so if the swipe/pan velocity is high enough it must animate off screen even though is might be less than 1/2 off screen.

My idea was to use UIPanGestureRecognizer and then test on the velocity. This works, but how do I set a correct animation duration for the moving of the UIView based on the view’s current location and velocity of the pan so that it seems smooth? If I set a fixed value, the animation either starts to slow or to fast compared to my fingers swipe speed.

Answer 1

The docs say

The velocity of the pan gesture, which is expressed in points per second. The velocity is broken into horizontal and vertical components.

So I’d say, given you want to move your view xPoints (measured in pt) to let it go off-screen, you could calculate the duration for that movement like so:

CGFloat xPoints = 320.0;
CGFloat velocityX = [panRecognizer velocityInView:aView].x;
NSTimeInterval duration = xPoints / velocityX;
CGPoint offScreenCenter = moveView.center;
offScreenCenter.x += xPoints;
[UIView animateWithDuration:duration animations:^{
    moveView.center = offScreenCenter;
}];

You might want to use + (void)animateWithDuration:(NSTimeInterval)duration delay:(NSTimeInterval)delay options:(UIViewAnimationOptions)options animations:(void (^)(void))animations completion:(void (^)(BOOL finished))completion instead though and try out different UIViewAnimationOptions.