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Observing ViewModel Changes

In the Unreal runtime, ViewModels are the supported way for Rive content to communicate back to Unreal. Legacy State Machine events and direct callback mechanisms are deprecated.
New integrations should observe ViewModel property changes instead.
All runtime output from Rive should flow through a ViewModel Instance.

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How Observation Works

During the Artboard tick:
  1. Unreal sets ViewModel values
  2. The State Machine evaluates transitions
  3. The State Machine may modify ViewModel values
  4. Property change callbacks are emitted
  5. Rendering occurs
Observation happens synchronously during this update cycle.

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Registering Callbacks

Each property on a ViewModel Instance can be observed. When a property changes:
  • The callback is invoked
  • The updated value is available
  • Logic can react immediately
Callbacks should be:
  • Registered after creating the ViewModel Instance
  • Unregistered before destroying the instance
  • Owned by the same system that owns the instance You can use the Add Field Value Changed Delegate to trigger events when a value is changed.
    Duelist Delegate

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Observing Trigger Properties

A Trigger Property can be used as a one-shot signal. Common use cases:
  • Button click notifications
  • Animation milestones
  • State transition signals
  • Gameplay triggers
When fired:
  • The trigger activates during the next tick
  • The property resets automatically
  • The observer receives the callback
Use Trigger Properties for actions.
Use boolean or numeric properties for persistent state.

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Observing Structured Data

Because ViewModels may contain nested structures:
  • Nested ViewModel Instances can also be observed
  • Changes propagate through the same callback mechanism
  • Observation remains consistent regardless of hierarchy depth
This allows complex UI or gameplay state to remain structured and predictable.

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Lifetime and Safety

Observation follows the lifetime of the ViewModel Instance. Important rules:
  • Do not observe destroyed instances
  • Unbind callbacks before destroying instances
  • Do not assume callbacks persist after Artboard reinitialization
Callbacks are synchronous and not asynchronous events.

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Summary

ViewModels are both the input and output boundary of the runtime. Unreal writes values.
The State Machine evaluates.
Unreal observes changes.