UiContourCrossfadeController<T> class

A reusable, generic cross-dissolve abstraction: when update receives a value whose identity differs from the current one, the old value fades out while the new value fades in over one shared progress timeline — both visible simultaneously mid-transition, neither morphing into the other's geometry or position. See doc/contour.md, "Two kinds of value change."

Use this when consecutive values occupy the same on-screen slot but are structurally unrelated (a different tab's accessory content, a different screen's toolbar actions) — there is nothing to geometrically morph between them, only a soft dissolve. For a slot whose existence changes (appears/disappears entirely), use UiContourPresenceController instead; the two compose when a persistent shell's existence is owned by UiContourPresenceController while its inner content, once visible, is owned by this controller.

identity defaults to next itself (via ==) when omitted, which suits plain value types. Widget-bearing types such as configuration records rarely override == in a way that's meaningful here — a fresh instance is typically rebuilt every frame even when nothing logically changed — so callers holding those should pass an explicit, stable identity (e.g. a label or index), the same pattern AnimatedSwitcher uses with child.key.

Inheritance

Constructors

UiContourCrossfadeController({required TickerProvider vsync})

Properties

current → T?
The value fading in, or already fully visible at rest.
no setter
hashCode → int
The hash code for this object.
no setterinherited
hasListeners → bool
Whether any listeners are currently registered.
no setterinherited
isDisposed → bool
no setter
isTransitioning → bool
Whether a transition is in flight — previous is non-null and still fading out.
no setter
previous → T?
The value fading out, or null once the transition settles.
no setter
progress → double
0 at the start of a transition (only previous visible), 1 at rest (only current visible).
no setter
runtimeType → Type
A representation of the runtime type of the object.
no setterinherited

Methods

addListener(VoidCallback listener) → void
Register a closure to be called when the object changes.
inherited
dispose() → void
Discards any resources used by the object.
override
noSuchMethod(Invocation invocation) → dynamic
Invoked when a nonexistent method or property is accessed.
inherited
notifyListeners() → void
Call all the registered listeners.
inherited
removeListener(VoidCallback listener) → void
Remove a previously registered closure from the list of closures that are notified when the object changes.
inherited
toString() → String
A string representation of this object.
inherited
update(BuildContext context, T? next, {Object? identity, UiMotionDuration? duration}) → void
Sets the target value. Starts a new cross-dissolve from whatever is currently visible only when identity (or next itself, if identity is omitted) differs from the current identity — interrupting an in-flight transition restarts cleanly from the visually-current blend rather than discarding it.

Operators

operator ==(Object other) → bool
The equality operator.
inherited