Transition events

Transition events inform you of the changes in a transition’s state.

UI Toolkit uses transitions when a VisualElement’s style property is modified. Changes to VisualElement property are immediately reflected visually. However, you can use the transition USS property to interpolate between the initial and end results gradually.

The transition’s lifecycle

A transition’s lifecycle has the following stages:

1. A VisualElement’s property is modified when you:

   • Add or remove a class with C# methods. For example: element.ToggleInClassList() (where element is any VisualElement).
   • Use USS with selectors like :hover.
   • Manipulate the element’s style property. For example: element.style.backgroundColor = Color.red; (where element is any VisualElement).

2. A TransitionRunEvent is sent.

3. If the resolved transition-delay property for the changing property has a value other than 0, nothing happens for the duration of the delay.

4. After the delay, a TransitionStartEvent is sent, and the transition starts with the property at its initial value.

5. For the length of time set by transition-duration, the transition occurs. During that time, the property goes from its initial to its final value.

6. If the property is changed to a new value during the transition, TransitionCancelEvent is sent. The transition process restarts at step 2.

7. After the transition-duration elapses, the property sets to its final value. A TransitionEndEvent is sent.

Transition events reference table

The following table describes the transition events and their propagation phases:

Behavior

Each transition property has its own lifecycle and its own transition events. You can access the current property with an event’s stylePropertyNames property.

If a shorthand USS property is changed, every component also gets its own lifecycle. For example, if you change margin, margin-left, margin-right, margin-top and margin-bottom, they all get their own TransitionRunEvent, TransitionStartEvent and TransitionEndEvent, for a total of 12 separate events.

If you set transition-delay to 0, the TransitionRunEvent and TransitionStartEvent are sent one after the other within a few milliseconds.

If you set transition-delay to a value below 0, the transition won’t start at the beginning. For example, with a transition-delay of -3 seconds and transition-duration of 5 seconds, the TransitionRunEvent and TransitionStartEvent is sent with an elapsedTime property set to 3 seconds and the transition effectively starts at the third second of a five-second animation.

Event list

This section describes the target, stylePropertyNames, and elapsedTime of each transition event.

TransitionRunEvent

A TransitionRunEvent event is sent when a transition is created.

• target: The element that executes the transition.
• stylePropertyNames: The list of properties modified by the transition.
• elapsedTime: The time since the start of the transition.

TransitionStartEvent

A TransitionStartEvent event is sent when the transition’s delay phase ends and the transition begins.

• target: The element that executes the transition.
• stylePropertyNames: The list of properties modified by the transition.
• elapsedTime: The time since the start of the transition.

TransitionEndEvent

A TransitionEndEvent event is sent when a transition ends.

• target: The element that executes the transition.
• stylePropertyNames: The list of properties modified by the transition.
• elapsedTime: The time since the start of the transition.

TransitionCancelEvent

A TransitionCancelEvent event is sent when a transition is interrupted by the property being changed again.

• target: The element that executes the transition.
• stylePropertyNames: The list of properties modified by the transition.
• elapsedTime: The time since the start of the transition.

Examples

• Create a transition event: This example demonstrates the lifecycle of a transition event.
• Create looping transitions: This example demonstrates how to leverage the TransitionEndEvent to create transitions that loop.
• Create a simple transition with UI Builder and C# script: This example demonstrates how to create simple transitions with the UI(User Interface) Allows a user to interact with your application. Unity currently supports three UI systems. More info
  See in Glossary Builder and C# script.

Additional resources

• USS transition