Navigating UIViewControllers with gestures in iOS

I have several view controllers embedded in a UINavigationController (some modal, some pushed) and am navigating through them using swipe gestures as such:

// Gesture recognizers
UISwipeGestureRecognizer *downGesture = [[UISwipeGestureRecognizer alloc]initWithTarget:self action:@selector(dismissButton)];
downGesture.direction = UISwipeGestureRecognizerDirectionDown;
[downGesture setCancelsTouchesInView:NO];
[self.view addGestureRecognizer:downGesture];

This works fine, however I want the user to be able to physically drag the modally presented view controller, for example, down and off the screen instead of just a flick and an animation doing the rest, or dragging right across the screen and snapping to the previous view instead of tapping the back button.

I've tried implementing this using a pan gesture on the view but of course the previous view controller isn't visible behind it, which it needs to be. How is this effect achieved properly? With view controller containment? If so how would that work when pushing a few view controllers on to the stack? An example of the type of navigation I'm talking about can be found in the LetterPress app.

thanks.

Yes, custom container view is the way to go (iOS 5 and greater). You basically write your own custom container, using the built-in childViewControllers property to keep track of all of the child view controllers. You may want your own property, say currentChildIndex, to keep track of which child controller you're currently on:

@property (nonatomic) NSInteger currentChildIndex;

Your parent controller should probably have some push and pop methods for non-swipe related navigation, such as:

- (void)pushChildViewController:(UIViewController *)newChildController
{
    // remove any other children that we've popped off, but are still lingering about

    for (NSInteger index = [self.childViewControllers count] - 1; index > self.currentChildIndex; index--)
    {
        UIViewController *childController = self.childViewControllers[index];
        [childController willMoveToParentViewController:nil];
        [childController.view removeFromSuperview];
        [childController removeFromParentViewController];
    }

    // get reference to the current child controller

    UIViewController *currentChildController = self.childViewControllers[self.currentChildIndex];

    // set new child to be off to the right

    CGRect frame = self.containerView.bounds;
    frame.origin.x += frame.size.width;
    newChildController.view.frame = frame;

    // add the new child

    [self addChildViewController:newChildController];
    [self.containerView addSubview:newChildController.view];
    [newChildController didMoveToParentViewController:self];

    [UIView animateWithDuration:0.5
                     animations:^{
                         CGRect frame = self.containerView.bounds;
                         newChildController.view.frame = frame;
                         frame.origin.x -= frame.size.width;
                         currentChildController.view.frame = frame;
                     }];

    self.currentChildIndex++;
}

- (void)popChildViewController
{
    if (self.currentChildIndex == 0)
        return;

    UIViewController *currentChildController = self.childViewControllers[self.currentChildIndex];
    self.currentChildIndex--;
    UIViewController *previousChildController = self.childViewControllers[self.currentChildIndex];

    CGRect onScreenFrame = self.containerView.bounds;

    CGRect offToTheRightFrame = self.containerView.bounds;
    offToTheRightFrame.origin.x += offToTheRightFrame.size.width;

    [UIView animateWithDuration:0.5
                     animations:^{
                         currentChildController.view.frame = offToTheRightFrame;
                         previousChildController.view.frame = onScreenFrame;
                     }];
}

Personally, I have a protocol defined for these two methods, and make sure that my parent controller is configured to conform to that protocol:

@protocol ParentControllerDelegate <NSObject>

- (void)pushChildViewController:(UIViewController *)newChildController;
- (void)popChildViewController;

@end

@interface ParentViewController : UIViewController <ParentControllerDelegate>
...
@end

Then, when a child wants to push a new child on, it can do it like so:

ChildViewController *controller = ... // instantiate and configure your next controller however you want to do that

id<ParentControllerDelegate> parent = (id)self.parentViewController;
NSAssert([parent conformsToProtocol:@protocol(ParentControllerDelegate)], @"Parent must conform to ParentControllerDelegate");

[parent pushChildViewController:controller];

When a child wants to pop itself off, it can do it like so:

id<ParentControllerDelegate> parent = (id)self.parentViewController;
NSAssert([parent conformsToProtocol:@protocol(ParentControllerDelegate)], @"Parent must conform to ParentControllerDelegate");

[parent popChildViewController];

And then the parent view controller has a pan gesture set up, to handle the user panning from one child to another:

- (void)handlePan:(UIPanGestureRecognizer *)gesture
{
    static UIView *currentView;
    static UIView *previousView;
    static UIView *nextView;

    if (gesture.state == UIGestureRecognizerStateBegan)
    {
        // identify previous view (if any)

        if (self.currentChildIndex > 0)
        {
            UIViewController *previous = self.childViewControllers[self.currentChildIndex - 1];
            previousView = previous.view;
        }
        else
        {
            previousView = nil;
        }

        // identify next view (if any)

        if (self.currentChildIndex < ([self.childViewControllers count] - 1))
        {
            UIViewController *next = self.childViewControllers[self.currentChildIndex + 1];
            nextView = next.view;
        }
        else
        {
            nextView = nil;
        }

        // identify current view

        UIViewController *current = self.childViewControllers[self.currentChildIndex];
        currentView = current.view;
    }

    // if we're in the middle of a pan, let's adjust the center of the views accordingly

    CGPoint translation = [gesture translationInView:gesture.view.superview];

    previousView.transform = CGAffineTransformMakeTranslation(translation.x, 0.0);
    currentView.transform = CGAffineTransformMakeTranslation(translation.x, 0.0);
    nextView.transform = CGAffineTransformMakeTranslation(translation.x, 0.0);

    // if we're all done, let's animate the completion (or if we didn't move far enough,
    // the reversal) of the pan gesture

    if (gesture.state == UIGestureRecognizerStateEnded ||
        gesture.state == UIGestureRecognizerStateCancelled ||
        gesture.state == UIGestureRecognizerStateFailed)
    {

        CGPoint center = currentView.center;
        CGPoint currentCenter = CGPointMake(center.x + translation.x, center.y);
        CGPoint offRight = CGPointMake(center.x + currentView.frame.size.width, center.y);
        CGPoint offLeft = CGPointMake(center.x - currentView.frame.size.width, center.y);

        CGPoint velocity = [gesture velocityInView:gesture.view.superview];

        if ((translation.x + velocity.x * 0.5) < (-self.containerView.frame.size.width / 2.0) && nextView)
        {
            // if we finished pan to left, reset transforms

            previousView.transform = CGAffineTransformIdentity;
            currentView.transform = CGAffineTransformIdentity;
            nextView.transform = CGAffineTransformIdentity;

            // set the starting point of the animation to pick up from where
            // we had previously transformed the views

            CGPoint nextCenter = CGPointMake(nextView.center.x + translation.x, nextView.center.y);
            currentView.center = currentCenter;
            nextView.center = nextCenter;

            // and animate the moving of the views to their final resting points,
            // adjusting the currentChildIndex appropriately

            [UIView animateWithDuration:0.25
                                  delay:0.0
                                options:UIViewAnimationOptionCurveEaseOut
                             animations:^{
                                 currentView.center = offLeft;
                                 nextView.center = center;
                                 self.currentChildIndex++;
                             }
                             completion:NULL];
        }
        else if ((translation.x + velocity.x * 0.5) > (self.containerView.frame.size.width / 2.0) && previousView)
        {
            // if we finished pan to right, reset transforms

            previousView.transform = CGAffineTransformIdentity;
            currentView.transform = CGAffineTransformIdentity;
            nextView.transform = CGAffineTransformIdentity;

            // set the starting point of the animation to pick up from where
            // we had previously transformed the views

            CGPoint previousCenter = CGPointMake(previousView.center.x + translation.x, previousView.center.y);
            currentView.center = currentCenter;
            previousView.center = previousCenter;

            // and animate the moving of the views to their final resting points,
            // adjusting the currentChildIndex appropriately

            [UIView animateWithDuration:0.25
                                  delay:0.0
                                options:UIViewAnimationOptionCurveEaseOut
                             animations:^{
                                 currentView.center = offRight;
                                 previousView.center = center;
                                 self.currentChildIndex--;
                             }
                             completion:NULL];
        }
        else
        {
            [UIView animateWithDuration:0.25
                                  delay:0.0
                                options:UIViewAnimationOptionCurveEaseInOut
                             animations:^{
                                 previousView.transform = CGAffineTransformIdentity;
                                 currentView.transform = CGAffineTransformIdentity;
                                 nextView.transform = CGAffineTransformIdentity;
                             }
                             completion:NULL];
        }
    }
}

It looks like you're doing up and down panning, rather than the left-right panning that I used above, but hopefully you get the basic idea.

By the way, in iOS 6, the user interface you're asking about (the sliding between views using gestures), could probably be done more efficiently using a built-in container controller, UIPageViewController. Just use a transition style of UIPageViewControllerTransitionStyleScroll and a navigation orientation of UIPageViewControllerNavigationOrientationHorizontal. Unfortunately, iOS 5 only allows page curl transitions, and Apple only introduced the scrolling transitions that you want in iOS 6, but if that's all you need, UIPageViewController gets the job done even more efficiently than what I've laid out above (you don't have to do any custom container calls, no writing of gesture recognizers, etc).

For example, you can drag a "page view controller" onto your storyboard, create a UIPageViewController subclass and then in viewDidLoad, you need to configure the first page:

UIViewController *firstPage = [self.storyboard instantiateViewControllerWithIdentifier:@"1"]; // use whatever storyboard id your left page uses
self.viewControllerStack = [NSMutableArray arrayWithObject:firstPage];

[self setViewControllers:@[firstPage]
               direction:UIPageViewControllerNavigationDirectionForward
                animated:NO
              completion:NULL];

self.dataSource = self;

Then you need to define the following UIPageViewControllerDataSource methods:

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController
{
    if ([viewController isKindOfClass:[LeftViewController class]])
        return [self.storyboard instantiateViewControllerWithIdentifier:@"2"];

    return nil;
}

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController
{
    if ([viewController isKindOfClass:[RightViewController class]])
        return [self.storyboard instantiateViewControllerWithIdentifier:@"1"];

    return nil;
}

Your implementation will vary (at the very least different class names and different storyboard identifiers; I'm also letting the page view controller instantiate the next page's controller when the user asks for it and because I'm not retaining any strong reference to them, the'll be released when I'm done transitioning to the other page ... you could alternatively just instantiate both at startup and then these before and after routines would obviously not instantiate, but rather look them up in an array), but hopefully you get the idea.

But the key issue is that I don't have any gesture code, no custom container view controller code, etc. Much simpler.