UIKit Dynamics in a Custom Segue - ios

Question

I'm trying to make a view fall as a custom segue transition, but when the perform method is called in the UIStoryboardSegue implementation, it does not fall. I have tried moving the view to be dropped into the source's view to see if it does anything, but it doesn't.
-(void)perform {
UIViewController *src = (UIViewController *)self.sourceViewController;
UIViewController *dest = (UIViewController *)self.destinationViewController;
UIView *viewdrop = [dest.view snapshotViewAfterScreenUpdates:YES];
viewdrop.frame = CGRectMake(0, -src.view.frame.size.height, dest.view.frame.size.width, dest.view.frame.size.height);
[src.view addSubview:viewdrop];
animator = [[UIDynamicAnimator alloc] initWithReferenceView:src.view];
UIGravityBehavior* gravityBehavior = [[UIGravityBehavior alloc] initWithItems:#[viewdrop]];
[animator addBehavior:gravityBehavior];
}

Answer 1

The reason it doesn't drop is because the gravity behavior takes time, but the segue itself is deallocated as soon as the perform method finishes. So, you need a way to keep the segue alive at least until the movement is complete. One way to do this is to make a strong property for the segue in the source view controller, and set its value in prepareForSegue,
-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
self.dropSegue = segue;
}
I made a modified version of your segue that also adds a collision behavior, and sets the source view controller as the delegate of the collision behavior, so I can use the delegate method, collisionBehavior:endedContactForItem:withBoundaryIdentifier:, to set the dropSegue property to nil (after a slight delay) which causes the segue to be deallocated,
-(void)collisionBehavior:(UICollisionBehavior *)behavior endedContactForItem:(id<UIDynamicItem>)item withBoundaryIdentifier:(id<NSCopying>)identifier {
NSLog(#"collision ended with %#", identifier);
[self performSelector:#selector(setDropSegue:) withObject:nil afterDelay:1];
}
Here is my version of the gravity drop segue,
#interface RDSegue ()
#property (strong, nonatomic) UIDynamicAnimator *animator;
#end
#implementation RDSegue
-(id)initWithIdentifier:(NSString *)identifier source:(UIViewController *)source destination:(UIViewController *)destination {
if (self = [super initWithIdentifier:identifier source:source destination:destination]) {
UIViewController *src = self.sourceViewController;
UIViewController *dest = self.destinationViewController;
self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:src.view];
[src addChildViewController:dest];
[dest didMoveToParentViewController:src];
dest.view.frame = CGRectMake(0, -src.view.bounds.size.height, src.view.bounds.size.width, src.view.bounds.size.height);
[src.view addSubview:dest.view];
}
return self;
}
-(void)perform {
UIGravityBehavior* gravityBehavior = [[UIGravityBehavior alloc] initWithItems:#[[self.destinationViewController view]]];
UICollisionBehavior *collide = [[UICollisionBehavior alloc] initWithItems:#[[self.destinationViewController view]]];
CGPoint left = CGPointMake(self.animator.referenceView.bounds.origin.x, self.animator.referenceView.bounds.origin.y + self.animator.referenceView.bounds.size.height);
CGPoint right = CGPointMake(self.animator.referenceView.bounds.origin.x + self.animator.referenceView.bounds.size.width, self.animator.referenceView.bounds.origin.y + self.animator.referenceView.bounds.size.height);
[collide addBoundaryWithIdentifier:#"bottom" fromPoint:left toPoint:right];
[collide setCollisionDelegate:self.sourceViewController];
[self.animator addBehavior:gravityBehavior];
[self.animator addBehavior:collide];
}
-(void)dealloc {
NSLog(#"In dealloc");
}