Interactive Transitions

InteractiveTransition.h

@interface InteractiveTransition.h : NSObject <UIViewControllerAnimatedTransitioning>

@end

InteractiveTransition.m

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
  UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
  
  CGRect initialFrame = fromVC.view.frame;
  CGRect finalFrame = CGRectMake(
      initialFrame.origin.x,
      initialFrame.size.height + 16,
      initialFrame.size.width,
      initialFrame.size.height
    );
  
  // use this option to make it stick under the user's finger:
  UIViewAnimationOptions *opts = UIViewAnimationOptionCurveLinear;
  
  [UIView animateWithDuration:1.0
    delay:0.0
    options:opts
    animations:^{
      fromVC.view.frame = finalFrame;
    }
    completion:^(BOOL finished) {
      [transitionContext completeTransition:![transitionContext transitionWasCancelled]];
    }
  ];
}

- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext
{
  // useless but necessary
  return 1.0f;
}

RootVC.h

@interface RootVC : UIViewController <UIViewControllerTransitioningDelegate>

@property (strong, nonatomic) UIPercentDrivenInteractiveTransition *myInteractiveDismissal;
@proprety (strong, nonatomic) BOOL interactive;

@end

RootVC.m

#import "InteractiveTransition.h"
#import "NonInteractiveTransition.h"

-(void)openButtonPressed
{
  ModalVC *modalVC = [[ModalVC alloc] init];
  modalVC.modalPresentationStyle = UIModalPresentationCustom;
  modalVC.transitioningDelegate = self;
  [self presentViewController:modalVC
      animated:YES
      completion:^{
        UIPanGestureRecognizer *gesture;
        gesture = [[UIPanGestureRecognizer alloc] init];
        // This method gets called in RootVC:
        [gesture addTarget:self action:@selector(handleGesture:)];
        // The gesture object gets added to modalVC.view:
        [loginVC.view addGestureRecognizer:gesture];
      }
  ];
}

- (void)handleGesture:(UIPanGestureRecognizer *)gesture
{
  switch (gesture.state) {
    case UIGestureRecognizerStateBegan:{
      self.interactive = YES;
      [self dismissViewControllerAnimated:YES completion:^{
        self.interactive = NO;
      }];
      break;
    }
    case UIGestureRecognizerStateChanged:{
      // Gives us the transition context's container view
      UIView *view = gesture.view.superview;
      // This gets us the point of where the gesture is in view:
      CGPoint translation = [gesture translationInView:view];
      CGFloat percentTransitioned = (translation.y / (CGRectGetHeight(view.frame)));
      
      [self.myInteractiveDismissal updateInteractiveTransition:percentTransitioned];
      
      break;
    }
    case UIGestureRecognizerStateEnded:{
      if (self.myInteractiveDismissal.percentComplete > 0.25) {
        [self.myInteractiveDismissal finishInteractiveTransition];
      } else {
        [self.myInteractiveDismissal cancelInteractiveTransition];
      }
      break;
    }
    case UIGestureRecognizerStateCancelled:{
      [self.myInteractiveDismissal cancelInteractiveTransition];
    }
  }
}

- (id <UIViewControllerInteractiveTransitioning>)interactionControllerForDismissal:(id<UIViewControllerAnimatedTransitioning>)animator
{
  // In animated transitions, we had to create out own classes (eg BouncyTransition) to make the 
  // transition work. In this case, we just use UIPercentDrivenInteractiveTransition.
  if (self.interactive) {
    self.myInteractiveDismissal = [[UIPercentDrivenInteractiveTransition alloc] init];
    return self.myInteractiveDismissal;
  }
  return nil;
}

// You also need the usual <UIViewControllerTransitioningDelegate> stuff:
- (id <UIViewControllerAnimatedTransitioning>) animationControllerForDismissedController:(UIViewController *)dismissed
{
  id<UIViewControllerAnimatedTransitioning> transition;
  
  if (self.interactive) {
    transition = [[InteractiveTransition alloc] init];
  } else {
    transition = [[NonInteractiveTransition alloc] init];
  }
  
  return transition;
}