View Controller Advancements in iOS 8

Support for Adaptive User Interfaces

Before iOS 8, most of your application interface/structure was determined by looking at:

• Device type

• Interface Orientation

• Size

From iOS 8, the first two have been reshaped into traits and trait collection. The new way to structure the app is via:

• Traits and trait collection

• Size

What’s a trait collection?

A collection of traits, more specifically:

• horizontalSizeClass

• verticalSizeClass

• userInterfaceIdiom

• displayScale

What’s a (horizontal/vertical) size class? A trait that coarsely defines the space available for that axis (possible values: .compact or .regular)

Most UIKit objects conform to a new UITraitEnvironment that both provide the current trait collection and also lets us get notified when the trait environment changes:

@protocol UITraitEnvironment <NSObject>
  @property UITraitCollection *traitCollection;
  - (void)traitCollectionDidChange:
@end

A parent view controller can override the trait collection for its child. For example, a UISplitViewController might give different traits to its primary and secondary view controllers.

How to:

@interface UIViewController <UITraitEnvironment>
  - (void)setOverrideTraitCollection: forChildViewController:
  - (UITraitCollection *)overrideTraitCollectionForChildViewController:
@end

Presentation Controllers

For finer control on custom transitions, from iOS 8 we have UIPresentationController, which takes care of calling and retaining references to various parts of each transition (e.g., the containerView, the presentedView). Note that the presented view might not be the presented view controller view, as UIKit might add a dimming view/drop shadow around that view because that’s what the custom presentation demanded.

To overcome this UIViewControllerContextTransitioning now offers a new view(forKey:) besides the old viewController(forKey:), make sure to use this new function to figure out which views are actually participating in the animation.

@protocol UIViewControllerContextTransitioning
  - (UIView *)viewForKey:(NSString *)key AVAILABLE_IOS(8_0);
@end

@interface UIPresentationController : NSObject
          <UIAppearanceContainer, UITraitEnvironment, UIContentContainer>

  @property(nonatomic, readonly) UIView *containerView;
  - (UIView *)presentedView;

  // we can decide whether we want the removal of the presenter views
  - (BOOL)shouldRemovePresentersView;

  // we can also decide whether the presented view will take over the screen.
  // If you set this to NO, your presentation will no longer adapt.
  - (BOOL)shouldPresentInFullScreen;
@end

Previous iPad-only styles are available on the iPhone (by default they adapt to full screen presentations when the horizontal trait is .compact).

New Presentation Styles:

• UIModalPresentationOverFullscreen

• UIModalPresentationOverCurrentContext

• UIModalPresentationPopover

All presentation styles have an associated presentation controller that we can access to (to set their delegate):

-[UIViewController presentationController]
-[UIViewController popoverPresentationController]

Thanks to the UIAdaptivePresentationControllerDelegate delegate we can

@protocol UIAdaptivePresentationControllerDelegate <NSObject>

  @optional
  // here for example we can tell the presentation to not adapt to full screen when the horizontal trait is compact
  - (UIModalPresentationStyle)adaptivePresentationStyleForPresentationController:

  // allows you to return a whole new view controller that should be presented in that style.
  - (UIViewController *)presentationController:viewControllerForAdaptivePresentationStyle:
@end

Transition Coordinators

• Objects conforming to UIViewControllerTransitionCoordinator

• Every transition coordinator has an associated transition

• This coordinator can add animation blocks to the original animation

• from iOS 8, transition coordinators also conform to UIContentContainer (UIViewControllers also conform to this)

@protocol UIContentContainer <NSObject>
  @property (nonatomic, assign) CGSize preferredContentSize;
  - (void)preferredContentSizeDidChangeForChildContentContainer:
  - (void)systemLayoutFittingSizeDidChangeForChildContentContainer:
  - (void)sizeForChildContentContainer:withParentContainerSize:
  - (void)willTransitionToTraitCollection:withTransitionCoordinator:
  - (void)viewWillTransitionToSize:withTransitionCoordinator:
@end

Use - (void)willTransitionToTraitCollection:withTransitionCoordinator: to react to resizing (replaces of old deprecated rotation callbacks).

When a trait changes, you can check UIViewControllerTransitionCoordinatorContext’s targetTransform to participate in the rotation animation (if there’s a rotation).

@protocol UIViewControllerTransitionCoordinatorContext
  - (CGAffineTransform)targetTransform;
  - (UIView *)viewForKey:(NSString *)key;
@end

For example:

- (void) viewWillTransitionToSize:(CGSize)s
         withTransitionCoordinator:(UIVCTC)t {
  orientation = [self orientationFromTransform: [t targetTransform]]; 
  oldOrientation = [[UIApplication sharedApplication] statusBarOrientation];

  [self myWillRotateToInterfaceOrientation:orientation duration: duration];

  [t animateAlongsideTransition:^(id <UIVCTCContext>) {
    [self myWillAnimateRotationToInterfaceOrientation:orientation
                                             duration:duration];
  }
  completion: ^(id <UIVCTCContext>) {
    [self myDidAnimateFromInterfaceOrientation:oldOrientation];
  }];
}

Note:

• The legacy rotation methods are still available (just don’t implement the new methods)

• Most view controller transitions are immediate when called from within the dynamic scope of this method (e.g., if you do a push here, the push will be done immediately without animation)

• Call super in order to forward to descendant view controllers

• Only necessary when you need to do a special size transition