UICollectionViewTransitionLayout Class Reference

Inherits from
Conforms to
Availability
Available in iOS 7.0 and later.
Declared in
UICollectionViewTransitionLayout.h

Overview

The UICollectionViewTransitionLayout class is a special type of layout object that lets you implement behaviors when changing from one layout to another in your collection view. You can use this class as-is or subclass it to provide specialized behavior for your app. A common use for transition layouts is to create interactive transitions, such as those that are driven by gesture recognizers or touch events.

During a layout change, the collection view installs this layout object temporarily to manage the changeover. This layout object determines the layout of each item by interpolating between the layout values in the current and new layout objects. The interpolation is driven by the value in the transitionProgress property, which you update periodically from your code to drive the transition. For example, if you use this class in conjunction with a gesture recognizer, the handler for your gesture recognizer would update that property and invalidate the layout.

If you want to provide more than just a linear transition from the old to new layout over time, you need to subclass and provide the layout attributes for items yourself. Subclassing requires you to override all of the same methods you would override when subclassing UICollectionViewLayout. The difference is that your custom methods can work with your gesture recognizers or touch event code to change the layout based on input from the user. For example, you could use a custom layout object in conjunction with a gesture recognizer to make items track the location of the user’s finger on the screen. You also need to implement the collectionView:transitionLayoutForOldLayout:newLayout: method of your collection view delegate and return your custom layout object when asked for it.

Tasks

Initializing the Transition Layout Object

Updating the Transition Information

Accessing the Layout Objects

Properties

currentLayout

The collection view’s current layout object. (read-only)

@property (readonly, nonatomic) UICollectionViewLayout *currentLayout;
Discussion

This object provides the layout attributes representing the initial position of items in the collection view. If you ultimately cancel the transition, the collection view animates its items back to the positions provided by this object.

Availability
  • Available in iOS 7.0 and later.
Declared In
UICollectionViewTransitionLayout.h

nextLayout

The collection view’s new layout object. (read-only)

@property (readonly, nonatomic) UICollectionViewLayout *nextLayout;
Discussion

This object provides the layout attributes representing the new position of items in the collection view. If the transition completes as expected, the collection view animates its items to the positions provided by this object.

Availability
  • Available in iOS 7.0 and later.
Declared In
UICollectionViewTransitionLayout.h

transitionProgress

The completion percentage of the transition.

@property (assign, nonatomic) CGFloat transitionProgress;
Discussion

During the transition, you should set the value of this property periodically and call invalidateLayout to force the collection view to update item positions. If you are driving the transition with a gesture recognizer, you would likely set this property from the handler method of your gesture recognizer.

Availability
  • Available in iOS 7.0 and later.
Declared In
UICollectionViewTransitionLayout.h

Instance Methods

initWithCurrentLayout:nextLayout:

Initializes and returns a transition layout object.

- (id)initWithCurrentLayout:(UICollectionViewLayout *)currentLayout nextLayout:(UICollectionViewLayout *)newLayout
Parameters
currentLayout

The layout object currently in use by the collection view.

newLayout

The new layout object that is being installed into the collection view.

Return Value

An initialized transition layout object or nil if the object could not be created.

Discussion

This method initializes the transition layout object and saves references to the current and new layout objects so that you can access them later. If you subclass and implement your own initialization method, you must call this method to initialize the superclass.

Availability
  • Available in iOS 7.0 and later.
Declared In
UICollectionViewTransitionLayout.h

updateValue:forAnimatedKey:

Sets the value for an animatable key.

- (void)updateValue:(CGFloat)value forAnimatedKey:(NSString *)key
Parameters
value

The value you want to store for the specified key.

key

A key that you define for your custom transition layout.

Discussion

Use this method to store a floating-point value with a specific key that you define for your transition layout object. The name of the key should be one that has meaning to your layout object. For example, if you track the position of the user’s finger over time, you might define the keys "PositionX” and "PositionY” to track those changes. Each time you update the value of a key, the layout object records the change along with a timestamp value. When the layout is finalized or canceled, those values can then be used to determine the speed with which to perform the remaining animations.

For any keys you set using this method, you should get that value as part of the normal process of generating layout attribute information. Getting the value using the valueForAnimatedKey: method (as opposed to getting the value from a class variable) means that when the collection view performs its final animations, your layout methods provide the correct values.

Availability
  • Available in iOS 7.0 and later.
Declared In
UICollectionViewTransitionLayout.h

valueForAnimatedKey:

Returns the most recently set value for the specified key.

- (CGFloat)valueForAnimatedKey:(NSString *)key
Parameters
key

A key whose value you set using the updateValue:forAnimatedKey: method.

Return Value

The last value set for the key.

Discussion

Use this method to retrieve floating-point values that are useful when laying out the contents of your collection view. The key you specify is a string that you define and that has some meaning to your implementation. At points during an interactive transition, you can assign new values to that key using the updateValue:forAnimatedKey: method.

Availability
  • Available in iOS 7.0 and later.
Declared In
UICollectionViewTransitionLayout.h