traitCollectionDidChange not called when multitasking - ios

Question

I'm working on an adopting multitasking to support split view for app, but I find the traitCollectionDidChange not called when app is on the right.
Does anyone have idea about this?

Answer 1

You may override traitCollectionDidChange in your ViewController.
However.
traitCollectionDidChange called when you change from one split view mode to another. For example from 50/50 view to 33/66. It so NOT called when you enter multitasking mode or exit it.
If you need to handle all events including entering and exiting multitasking mode, use viewWillTransitionToSize:withTransitionCoordinator: instead:
// put this in your ViewController code
-(void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator {
[super viewWillTransitionToSize:size withTransitionCoordinator:coordinator];
// TODO: put your code here (runs BEFORE transition complete)
}
If you want your code called AFTER the transition compelete:
-(void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator {
[coordinator animateAlongsideTransition:nil completion:^(id<UIViewControllerTransitionCoordinatorContext> _Nonnull context) {
// TODO: put your code here (runs AFTER transition complete)
}];
}

Answer 2

Have you tried the viewWillTransitionToSize method? This is used to notify the container that the size of its view is about to change.
Objective-C
- (void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator
Swift
func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {
coordinator.animateAlongsideTransition(nil, completion: {
if UIDevice.currentDevice().orientation.isLandscape.boolValue {
print("landscape")
} else {
print("portrait")
}
}

Answer 3

If anyone still doubt that is the point:
// This method called every time user changes separator position or when user rotates device
-(void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator {
// Always call super in those methods
[super viewWillTransitionToSize:size withTransitionCoordinator:coordinator];
// Before this call your layout is old, status bar orientation will return value before rotation
[coordinator animateAlongsideTransition:^(id <UIViewControllerTransitionCoordinatorContext> context) {
// Code here will be executed during transform. Status bar orientation is new, your view size already changed (in general).
// Setup required animations or custom views transitions
}
completion:^(id<UIViewControllerTransitionCoordinatorContext> _Nonnull context) {
// Split view transition complete or device rotated
}];
}
Also there is a method traitCollectionDidChange:, but it will be called only when horizontal size class actually changed. For example, if your app presenting from right side in split view mode, traitCollectionDidChange: will not be called when user changed separator position. But if your app presenting from left, it will be called always in portrait mode, and in landscape for transitions (50/50) <-> (66/33)

Answer 4

In view controller, we should call
- (void)willTransitionToTraitCollection:(UITraitCollection *)newCollection withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator {
to get notified when the orientation/multi window view changes.

Answer 5

As per documentation of UITraitEnvironment:
The system calls this method when the iOS interface environment changes. Implement this method in view controllers and views, according to your app’s needs, to respond to such changes. For example, you might adjust the layout of the subviews of a view controller when an iPhone is rotated from portrait to landscape orientation. The default implementation of this method is empty.
At the beginning of your implementation, call super to ensure that interface elements higher in the view hierarchy have an opportunity to adjust their layout first.