System

System-level APIs and environment constraints.

Runtime baseline

This page is aligned to the June 4, 2026 compatibility baseline: public npm runtime line 2.37.8, with source pins tracked in Runtime Compatibility Baseline.

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Listener

Callback that observes changes on an object. Supports two calling conventions:

Usage

-- Simple callback
property:addListener(function()
    print("Value changed to", property.value)
end)

-- Bound object callback
property:addListener(self, function(obj)
    print("Changed on", obj)
end)

See Also: Property, Input

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Output

Output<T> appears in upstream docs, but at the current C++ runtime baseline it is not exposed in Luau bindings.

Current guidance

function advance(self: MyScript, seconds: number): boolean
    -- Use regular typed fields on self for script state
    self.result = math.sin(seconds)
    self.label = "Hello"
    return true
end

Compatibility note

Do not rely on Output<T> in course code until a future runtime baseline confirms exposure. Track changes in Runtime Compatibility Baseline.

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EnumValues

Collection of enum values. Supports length operator and indexed access.

Operators

Length (#)

Returns the number of enum values.

local count = #enumValues

Index ([])

Access individual enum values by index.

local firstOption = enumValues[1]

Example

local enum = vm:getEnum("status")
local values = enum:values()
for i = 1, #values do
    print(i, values[i])
end

See Also: PropertyEnum

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Promise, async, and await

Promise utilities are available in the runtime and support chaining, cancellation, and coroutine-style async flows.

Promise static methods

Promise.new(executor: (resolve: (any) -> (), reject: (any) -> (), onCancel: ((() -> ())?) -> ()) -> ()): Promise
Promise.resolve(value: any?): Promise
Promise.reject(reason: any?): Promise
Promise.all(promises: {Promise}): Promise

Promise instance methods

promise:andThen(onFulfilled: (any) -> any, onRejected: ((any) -> any)?): Promise
promise:catch(onRejected: (any) -> any): Promise
promise:finally(callback: () -> ()): Promise
promise:cancel()
promise:onCancel(callback: () -> ())
promise:getStatus(): "pending" | "fulfilled" | "rejected" | "cancelled"

async(fn) and await(promise)

await(promise) must be called inside an async(...) function.

local run = async(function()
    local ok, result = await(fetchSomething())
    if ok then
        print("resolved:", result)
    else
        print("failed:", result)
    end
end)

run:andThen(function()
    print("done")
end)

Await return shape

await(...) is non-throwing and returns (ok, valueOrError). Handle both branches explicitly.

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Sandbox Limitations

Not available in Rive's Luau environment:

Library	Status
io	Not available
os	Partial (os.clock(), os.time(), os.difftime() only)
debug	Not available
package	Not available
loadfile, dofile, loadstring	Not available

The __gc metamethod is also disabled.

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late()

Deferred initialization marker for script fields. Use in factory return tables for fields that will be assigned later (e.g., in init()).

declare function late(): any

Example

return function(): Node<MyScript>
    return {
        init = init,
        advance = advance,
        myPath = late(),     -- Will be assigned in init()
        myPaint = late(),
    }
end

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Audio

Global API for audio playback. Used with AudioSource assets.

Methods

Audio.time()

Returns the current audio system time in seconds.

Audio.time(): number

Audio.sampleRate()

Returns the audio sample rate.

Audio.sampleRate(): number

Audio.timeFrame()

Returns the current audio system time in frames.

Audio.timeFrame(): number

Audio.play(source)

Plays an audio source immediately. Returns an AudioSound for controlling playback.

Audio.play(source: AudioSource): AudioSound

Audio.playAtTime(source, time)

Schedules playback at a specific audio time.

Audio.playAtTime(source: AudioSource, time: number): AudioSound

Audio.playInTime(source, delay)

Schedules playback after a delay in seconds.

Audio.playInTime(source: AudioSource, delay: number): AudioSound

Audio.playAtFrame(source, frame)

Schedules playback at a specific frame.

Audio.playAtFrame(source: AudioSource, frame: number): AudioSound

Audio.playInFrame(source, frames)

Schedules playback after a delay in frames.

Audio.playInFrame(source: AudioSource, frames: number): AudioSound

Example

function init(self: MyScript, context: Context): boolean
    local source = context:audio("click_sound")
    if source then
        local sound = Audio.play(source)
        sound.volume = 0.8
    end
    return true
end

See Also: AudioSource, AudioSound, Context

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Knowledge Check

Q:What does await(promise) return in this runtime?

Next Steps

• Continue to Quick Reference
• Verify surface status in Runtime Compatibility Baseline
• Review key terms in Glossary