Microphone
Microphone capture is sensitive and should be explicit. The shared app asks for a bounded recording; the host owns device selection, permission, capture, encoding, and cancellation.
By the end of this guide, you will know how to build a Record voice note button that captures five seconds of audio and stores the stream handle in app state. The same pattern applies to larger apps: declare the capability when native configuration is needed, store the result in typed app state, request the host operation from a reducer, and test both success and failure with a memory host.
Do not start recording from build code or background state changes. Use a visible user action, make the recording duration clear, and handle cancellation or denied permission.
1. Start with the state the screen needs
Add the smallest state fields the screen needs to render honestly. For this flow, useful state is:
#[derive(Default)]
struct MicrophoneState {
// Add these fields to your existing app state.
voice_note_stream: Option<DataStreamId>,
microphone_error: Option<String>,
recording: bool,
}
impl GlobalState for MicrophoneState {}
State is the source of truth for the UI. Do not store whether the host "probably" succeeded; wait for the typed success action and store the returned value.
2. Configure the target and host
Run this once from the project root:
fission add-capability microphone --project-dir .
That command records the capability in fission.toml and updates generated platform files where Fission can do so safely. It does not replace product-specific review of native manifests, usage text, entitlements, service identifiers, domains, or store policy.
The CLI adds Android RECORD_AUDIO and iOS NSMicrophoneUsageDescription. Packaged macOS apps need microphone usage text, and Windows packages may need microphone capability metadata.
This is the difference between the Fission API, a provider, and packaging configuration:
•
The Fission API is the typed Rust contract your reducer calls, such as ctx.effects.camera().capture_photo(...).
•
The provider is the shell-side Rust implementation that turns that typed request into the correct OS, browser, or hardware call.
•
Packaging configuration is the manifest, plist, entitlement, service-worker, protocol, domain, or store metadata that lets the platform permit that call in a real installed app.
If the API exists but the provider is missing, the request returns a typed unsupported error. If the provider exists but packaging is missing, the operating system or browser may deny the request before the provider can complete it.
3. Define reducers for success and failure
Capability work finishes later, after the reducer that started it has returned. You therefore need one reducer for the successful result and one reducer for the error path.
#[fission_reducer(AudioCaptured)]
fn on_audio_captured(state: &mut MicrophoneState, capture: MicrophoneCapture) {
state.recording = false;
state.voice_note_stream = Some(capture.stream);
state.microphone_error = None;
}
#[fission_reducer(AudioCaptureFailed)]
fn on_audio_capture_failed(state: &mut MicrophoneState, error: MicrophoneError) {
state.recording = false;
state.microphone_error = Some(error.message);
}
Keep both paths explicit. A denied permission, unavailable device, unsupported host, or cancelled prompt is not an exceptional mystery; it is normal product behavior the UI should explain.
4. Request the capability from a reducer
Call the capability from the reducer that handles the user's intent. The reducer queues a host effect and returns. The runtime later dispatches either the success reducer or the failure reducer.
state.recording = true;
ctx.effects
.microphone()
.capture_audio(MicrophoneCaptureRequest {
duration_ms: 5_000,
sample_rate_hz: Some(44_100),
channels: Some(1),
format: AudioSampleFormat::Pcm16,
..Default::default()
})
.on_ok(ctx.effects.bind(
AudioCaptured(MicrophoneCapture::default()),
reduce_with!(on_audio_captured),
))
.on_err(ctx.effects.bind(
AudioCaptureFailed(MicrophoneError::default()),
reduce_with!(on_audio_capture_failed),
));
The call does not run the OS API directly. It records a typed host request. That keeps shared app logic portable across Fission targets where the capability applies, plus static-site previews and tests.
5. Trigger the reducer from the UI
Wire the user-facing control with the normal Fission action pattern. This example uses a small widget so the binding has one clear home.
struct RecordVoiceNoteButton;
impl From<RecordVoiceNoteButton> for Widget {
fn from(component: RecordVoiceNoteButton) -> Self {
let (ctx, view) = fission::build::current::<MicrophoneState>();
let run = with_reducer!(ctx, RecordVoiceNote, on_record_voice_note);
Button::new("Record voice note")
.disabled(view.state().recording)
.on_press(run)
.into()
}
}
Rename the widget and action for your product, but keep the shape: the widget dispatches one typed action, the reducer starts the capability request, and the success or failure reducer updates state.
6. Provide and test the host behavior
Register a MicrophoneHost with .with_microphone_host(...). Use MemoryMicrophoneHost in tests. Providers should keep capture bounded and return typed errors for denied permission, no input devices, and unsupported formats.
Test permission denial, no microphone, successful capture, unsupported format, and cancellation. Reducers should clear recording state on both success and failure.
A useful first test creates the app with the memory host, dispatches the start action, feeds the queued effect through the host, and asserts that the reducer updates state from the returned payload. Add a second test for the error path before you ship the screen.
Platform expectations
Android, iOS, web, Windows, and macOS have microphone APIs. Linux depends on portals and audio services such as PulseAudio or PipeWire. Browser capture requires a secure origin and user permission.
For a cross-platform summary, read the capability matrix. For exact request fields, provider traits, and generated configuration, see the Microphone reference.
Finished capability flow
A complete Microphone integration has all of these pieces in place:
Piece | What should exist |
|---|---|
Configuration | fission.toml declares the capability and generated platform files contain the permission text, manifest entry, entitlement, domain, or protocol metadata the target needs. |
State | App state records the in-flight flag, the successful value, and the user-facing error separately. |
Reducers | One reducer starts the host request, one handles success, and one handles denial, unsupported hosts, cancellation, or provider errors. |
Provider | The shell registers a real provider for production and a memory provider for tests. |
Tests | At least one test covers success and one covers permission denial or unsupported host behavior. |
Diagnose capability failures
Capability failures should be safe to retry and easy to explain. Start by checking fission.toml, then run fission doctor, then inspect the generated target files. If the provider is missing, the request should return an unsupported error. If the provider exists but the platform configuration is wrong, the OS or browser will usually deny the request before the provider can complete it. Keep those cases distinct in the UI so users know whether they can retry, change settings, connect hardware, or choose another path.
Fission
A cross-platform, GPU-accelerated user interface framework for Rust. MIT licensed.
Copyright (c) 2026 Fission
Ready to use today. Widget APIs are expected to remain stable; some runtime and shell APIs may change before 1.0.0.
Fission 0.7.0