On UI composition, embedded view controllers, weak vars, multi-cast delegates.

Interface Builder is great for composing plain interfaces, where each view controller is represented by a single instance of view in a view hierarchy.

Embedded view controllers are different.

Even though you can create multiple Embed segues connecting different places in a storyboard to the same embedded view controller - during app initialization the framework will create as many instances of that controller as there are segues.

This is counter-intuitive at first. A collection of views is an ordered tree, where each node has a reference to its single parent. But in a storyboard the view controllers can be connected by Embed segues arbitrarily and can form cycles.

It appears Apple gets away with this by forcing each Embed segue to create a new instance of a destination view controller.

For example, the following storyboard with embedded view controllers (part of a project in a coursera.org course) has many parent-child Embed relations:




Note the three parent Container views, and two of them share a view controller.

A parent view controller will have outlets to instances of child view controllers. Those outlets are declared weak and fished out of the storyboard in an override of prepareForSegue() method. If there are multiple instances of the same embedded view controller (as in the screenshot) - you’d want them stored in an array of weak references. But the challenge with Swift 2 is that it does not support collections of weak references.

In a simple case - having separate weak refs is ok. The prepareForSegue() implementation might look like this:

    // MARK: - Navigation

    override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {

        // !!! Embed segues are processed before viewDidLoad().
        switch segue.destinationViewController {
        case let vc as MyHobbiesCollectionViewController:
            // Note that this embedded VC has multiple instances,
            // need additional id check (i.e. by segue.identifier).
            switch segue.identifier {
                case .Some("My1stHobbiesEmbedSegue"):
                    my1stHobbiesCVC = vc;
            default:
                my2ndHobbiesCVC = vc;
            }
           
            // Also prepare non-embed segues.
        case let vc as EditHobbiesViewController:
            break;
        default: break;
        }

    }

In other more advanced cases, a recommended solution is to declare a weak ref in a structure and then have an array of those structures. Hopefully Apple is working to fix this minor annoyance.

A few words about how a hierarchy of view controllers should communicate with the rest of the tree.

KVO and NSNotificationCenter are ok in general case, but they lack type-safety of messages, and require not very nice symmetry of subscrbe/unsubscribe calls. Instead, I prefer declaring protocols for each child view controller, then a parent would implement those protocols, and assign itself to weak ref delegates of its children.

Events of the child controllers will first travel up a hierarchy, where a parent will route them to peers.

This messaging system could make use of a collection of weak ref delegates, which would be referred to as multi-cast delegates. But this will have to wait till a version of Swift that implements weak ref collections natively.

Modal Views in iOS 7 Storyboards

Walkthrough to make a secondary scene in a storyboard act as an animated modal dialog, similar to the Alert view.

Prerequisites:

1. Create a single-view app project.
2. In Project Settings/General/Deployment Info make sure the storyboard name (Main.storyboard) appears in Main Interface popover.

3. Add/remove remove view controllers (VCs) to the storyboard as needed.
4. To make a VC the very first controller - click it to highlight blue, then in Attributes Inspector check the Initial Scene. This will set origin arrow.

5. Create a modal Manual Segue ("seg-way"). In the IB zoom out by double-click on an empty space. Then Ctrl-drag from the presenting VC to the modal VC. Select "modal" in a dialog.

6. To make sure the transition is animated select the segue, check the Animates attribute in the Attribute Inspector.

Now, to use the default animation provided by Xcode - do the following:

1. Add a new View Controller (VC), set it as a CustomClass/Class attribute in Identity Inspector for the presenting view (from-view). Add a button and a tap event handler to the presenting view.

//
//  ALYViewController.m
//

#import "ALYViewController.h"
#import "ALYZoomAnimator.h"

NSString *const MySegueID = @"MySegueID";

@interface ALYViewController ()
@property (strong, nonatomic) ALYZoomAnimator *zoomAnimator;

- (IBAction)showModalViewTapped:(id)sender;
@end


@implementation ALYViewController

- (IBAction)showModalViewTapped:(id)sender {
    [self performSegueWithIdentifier: MySegueID sender: nil];
}
@end

2. Do the same for the modal view, except the code will be dismissing this dialog.

//
//  ALYModalViewController.m
//

#import "ALYModalViewController.h"

@interface ALYModalViewController ()

- (IBAction)dismissTapped:(id)sender;
@end

@implementation ALYModalViewController

- (IBAction)dismissTapped:(id)sender {
    [self dismissViewControllerAnimated: YES completion: nil];
}
@end

3. Run the app. Click the button to reveal the modal dialog.


To add a custom transition with added benefit of controlling size and appearance of the modal dialog (for interactivity, cancellation, etc. in later posts), do the following:

1. Implement <UIViewControllerTransitioningDelegate> and <UIViewControllerAnimatedTransitioning> in a separate animator class. This will handle in/out transitions for a modal dialog. Unfortunately there is no context property to tell which transition is being initiated, in or out. One workaround is to record a VC being presented and then compare it against the to-VC of the transitionContext.

//
//  ALYZoomAnimator.h
//

#import <Foundation/Foundation.h>

@interface ALYZoomAnimator : NSObject <UIViewControllerTransitioningDelegate, UIViewControllerAnimatedTransitioning>

@end 

//
//  ALYZoomAnimator.m
//

#import "ALYZoomAnimator.h"

#define IN_DURATION 1.0
#define OUT_DURATION 0.3

@interface ALYZoomAnimator()
@property (weak, nonatomic) UIViewController *presentedVC;

@end


@implementation ALYZoomAnimator

// Returns true while the IN animation is active to reveal the modal sub-dialog.
- (BOOL) isBeingPresented: (id<UIViewControllerContextTransitioning>) transitionContext
{
 UIViewController *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    return toVC == self.presentedVC;
}


#pragma mark - <UIViewControllerTransitioningDelegate>

- (id<UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source
{
    self.presentedVC = presented;
    return self;
}

- (id<UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed
{
    return self;
}


#pragma mark - UIViewControllerAnimatedTransitioning

- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext
{
    return [self isBeingPresented: transitionContext] ? IN_DURATION : OUT_DURATION;
}

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
    UIView *container = transitionContext.containerView;
 
 UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    UIView *fromView = fromVC.view;
    UIView *toView = toVC.view;
    
    CGRect cb = container.bounds;
    CGRect startFrame = CGRectMake(cb.size.width / 2, cb.size.height / 2 - 50, 0, 0);
 CGRect endFrame = CGRectInset(startFrame, -100, -100);
    UIView *snapshot;
    
 if ([self isBeingPresented: transitionContext]) {
        fromView.userInteractionEnabled = NO;
        fromView.tintColor = [UIColor grayColor];
        toView.frame = endFrame;
  snapshot = [toView snapshotViewAfterScreenUpdates:YES];
  snapshot.frame = startFrame;
        snapshot.alpha = 0.0;
        [container addSubview: snapshot];
        [UIView animateWithDuration: [self transitionDuration: transitionContext]
                              delay: 0
             usingSpringWithDamping: 500 initialSpringVelocity: 15
                            options: 0
                         animations: ^{
                             fromView.tintAdjustmentMode = UIViewTintAdjustmentModeDimmed;
                             snapshot.frame = endFrame;
                             snapshot.alpha = 1.0;
                         }
                         completion: ^(BOOL finished) {
                             toView.frame = endFrame;
                             [container addSubview: toView];
                             [snapshot removeFromSuperview];
                             [transitionContext completeTransition:YES];
                         }];
 } else {
  snapshot = [fromView snapshotViewAfterScreenUpdates:YES];
        snapshot.frame = endFrame;
        [container addSubview: snapshot];
        [fromView removeFromSuperview];
        [UIView animateWithDuration: [self transitionDuration: transitionContext]
                              delay: 0
             usingSpringWithDamping: 500 initialSpringVelocity: 15
                            options: 0
                         animations: ^{
                             toView.tintAdjustmentMode = UIViewTintAdjustmentModeNormal;
                             snapshot.frame = startFrame;
                             snapshot.alpha = 0.0;
                         }
                         completion: ^(BOOL finished) {
                             [snapshot removeFromSuperview];
                             toView.userInteractionEnabled = YES;
                             [transitionContext completeTransition:YES];
                         }];
 }
}

@end

2. Provide -prepareForSegue method in the presenting VC.

//
//  ALYViewController.m
//

#import "ALYViewController.h"
#import "ALYZoomAnimator.h"

NSString *const MySegueID = @"MySegueID";

@interface ALYViewController ()
@property (strong, nonatomic) ALYZoomAnimator *zoomAnimator;

- (IBAction)showModalViewTapped:(id)sender;
@end


@implementation ALYViewController

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if (![segue.identifier isEqualToString: MySegueID]) {
        return;
    }
    if (!self.zoomAnimator) {
        self.zoomAnimator = [[ALYZoomAnimator alloc] init];
    }
    
    [segue.destinationViewController setTransitioningDelegate: self.zoomAnimator];
    [segue.destinationViewController setModalPresentationStyle: UIModalPresentationCustom];
}

- (IBAction)showModalViewTapped:(id)sender {
    [self performSegueWithIdentifier: MySegueID sender: nil];
}
@end

3. Run the app. Observe the modal dialog animations, transparency, presenting view in the background providing a context to help users keep track of their position in app. This is unlike the normal transitions that will hide the presenting view.