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.
A transition’s lifecycle has the following stages:
A VisualElement
’s property is modified when you:
element.ToggleInClassList()
(where element
is any VisualElement
).:hover
.style
property. For example: element.style.backgroundColor = Color.red;
(where element
is any VisualElement
).A TransitionRunEvent
is sent.
If the resolved transition-delay
property for the changing property has a value other than 0
, nothing happens for the duration of the delay.
After the delay, a TransitionStartEvent
is sent, and the transition starts with the property at its initial value.
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.
If the property is changed to a new value during the transition, TransitionCancelEvent
is sent. The transition process restarts at step 2.
After the transition-duration
elapses, the property sets to its final value. A TransitionEndEvent
is sent.
The following table describes the transition events and their propagation phases:
Event | Description | Trickles down | Bubbles up | Cancellable |
---|---|---|---|---|
TransitionRunEvent | Sent when a transition is created. | Yes | ||
TransitionStartEvent | Sent when a transition’s delay phase ends and the transition starts. | Yes | ||
TransitionEndEvent | Sent when a transition ends. | Yes | ||
TransitionCancelEvent | Sent when an a transition is canceled. | Yes |
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.
This section describes the target
, stylePropertyNames
, and elapsedTime
of each transition event.
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.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.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.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.TransitionEndEvent
to create transitions that loop.