flutter_ffi_uvc

This package is based on libuvc (Android) and Media Foundation (Windows).

It provides UVC camera access on Android and Windows — live preview on a Flutter Texture, frame access from Dart, camera controls, and stream diagnostics.

Supported Platforms

• Android(arm64-v8a, x86_64, armeabi-v7a)
• Windows(x64)
• Dart SDK: >=3.8.1 <4.0.0
• Android minSdk: 24

Installation

flutter pub add flutter_ffi_uvc

Usage

Typical lifecycle

1. Call uvcCamera.ensureCameraPermission() if your app requires the CAMERA permission (always returns true on Windows — there is no runtime dialog).
2. Call uvcCamera.listUsbDevices() to discover attached UVC cameras.
3. Call uvcCamera.openUsbDevice(deviceId) to open the device (on Android this also requests USB permission; on Windows it opens directly).
4. Read uvcCamera.supportedModes().
5. Pick a mode and call await uvcCamera.startPreview(mode) — starts the stream and verifies frame delivery.
6. On success, attach a Flutter Texture via attachPreviewTexture for live preview.
7. Use copyLatestFrame() when you need frame bytes in Dart, such as for capture or inspection.
8. Call uvcCamera.stopPreview() when preview is no longer needed.
9. When finished, call uvcCamera.closeUsbDevice().

Single-camera model

This plugin is designed around a single, shared global uvcCamera instance. It supports one connected camera at a time:

import 'package:flutter_ffi_uvc/flutter_ffi_uvc.dart';

class UvcPreviewPage extends StatefulWidget {
  UvcPreviewPage({
    super.key,
    UvcCamera? camera,
  }) : camera = camera ?? uvcCamera;

  final UvcCamera camera;
}

USB Device discovery and opening

// List attached UVC cameras
final List<UvcUsbDevice> devices = await uvcCamera.listUsbDevices();

// Open a device — on Android this requests USB permission if not already granted
final int result = await uvcCamera.openUsbDevice(devices.first.deviceId);
if (result != 0) {
  print('Open failed: ${uvcCamera.lastError}');
}

On Android, openUsbDevice goes through the Android USB layer to acquire permission and a file descriptor, then passes it to libusb to open the session. On Windows it resolves the id to the camera's Media Foundation source and opens it directly — no permission flow. In both cases it throws a PlatformException if the platform layer fails, and returns a non-zero code if the native session fails to initialize.

If another device is already open, openUsbDevice safely tears down the current session first (stopping any running preview and closing the previous device), so switching between cameras is just another openUsbDevice call — no manual closeUsbDevice needed in between.

To close and release the USB connection:

await uvcCamera.closeUsbDevice();

Attach / detach events

deviceEvents reports when a UVC-capable device is plugged in or unplugged:

StreamSubscription<UvcDeviceEvent>? _deviceEventSub;

_deviceEventSub = uvcCamera.deviceEvents.listen((UvcDeviceEvent event) {
  if (event.type == UvcDeviceEventType.detached) {
    // If this was the opened device, the native session lost its transport.
    uvcCamera.stopPreview();
    uvcCamera.closeUsbDevice();
  } else {
    // A camera was plugged in — refresh the device list, offer to open it, …
  }
});

This is a broadcast stream; the underlying platform listener (an Android broadcast receiver / Windows device notifications) is only registered while at least one listener is subscribed. Cancel the subscription on dispose.

Alternative: opening by file descriptor (Android only)

If your app manages USB access independently on Android, pass the file descriptor directly to skip the Android layer:

// fd: int from UsbDeviceConnection.fileDescriptor
uvcCamera.openFd(fd);

openFd/closeFd throw UnsupportedError on Windows — there is no file descriptor concept there; use openUsbDevice/closeUsbDevice instead.

Preview & Capture

Live preview with Texture

Create a texture, start preview, then attach the texture once the stream is confirmed running:

final int textureId = await uvcCamera.createPreviewTexture();

// stableFrames (default): verifies both frame delivery and frame validity.
// sequenceOnly: verifies frame delivery only — frame validity is not checked.
final UvcPreviewStartResult result = await uvcCamera.startPreview(
  mode,
  policy: UvcPreviewPolicy.stableFrames,
);
if (result.success) {
  await uvcCamera.attachPreviewTexture(
    textureId,
    width: mode.width,
    height: mode.height,
  );
}

Display it with Flutter's Texture widget:

AspectRatio(
  aspectRatio: mode.width / mode.height,
  child: Texture(textureId: textureId),
)

On teardown:

uvcCamera.stopPreview();
await uvcCamera.disposePreviewTexture(textureId);

Automatic mode selection

Descriptor-reported modes are candidates, not guaranteed-safe defaults — a mode may negotiate but never deliver decodable frames. startPreviewAuto() encodes the recommended fallback loop: it tries candidate modes in order and keeps the first one that streams and verifies successfully.

final UvcAutoPreviewResult result = await uvcCamera.startPreviewAuto();
if (result.success) {
  final UvcCameraMode mode = result.mode!; // now streaming in this mode
  await uvcCamera.attachPreviewTexture(
    textureId,
    width: mode.width,
    height: mode.height,
  );
} else {
  // Inspect per-mode failures:
  for (final UvcPreviewStartResult attempt in result.attempts) {
    print('${attempt.mode.label}: ${attempt.lastError}');
  }
}

By default candidates come from supportedModes() ordered MJPEG-first (compressed modes are far less likely to exceed USB bandwidth), then by resolution and frame rate according to preference, capped at maxCandidates (default 8):

• UvcAutoPreviewPreference.reliability (default) — smaller resolutions first; attaches fastest and is least likely to hit bandwidth limits.
• UvcAutoPreviewPreference.quality — larger resolutions first; picks the best-looking mode that actually streams.

On Windows, supportedModes() lists every format × resolution × fps combination the camera advertises (H264 native types excluded), so the candidate pool is larger than on Android.

final UvcAutoPreviewResult result = await uvcCamera.startPreviewAuto(
  preference: UvcAutoPreviewPreference.quality,
);

Pass candidates to control the order yourself; preference is then ignored.

Preview transform

Rotation and flip are applied to the Flutter Texture output only.

// Absolute: set rotation and flip in one call
uvcCamera.setPreviewTransform(
  const UvcPreviewTransform(rotation: 90, flipHorizontal: true),
);

// Incremental helpers
uvcCamera.rotatePreviewClockwise();          // +90° each call
uvcCamera.rotatePreviewCounterClockwise();   // -90° each call
uvcCamera.togglePreviewFlipHorizontal();     // mirror left-right
uvcCamera.togglePreviewFlipVertical();       // mirror top-bottom

// Read current state
final UvcPreviewTransform t = uvcCamera.previewTransform;

rotation accepts 0, 90, 180, or 270 (clockwise degrees). Values outside this set are normalised to 0 by the native layer.

For 90° and 270° rotations the output dimensions are swapped. Use applyToSize() to get the correct dimensions for the AspectRatio widget:

final (int w, int h) = uvcCamera.previewTransform.applyToSize(mode.width, mode.height);
AspectRatio(
  aspectRatio: w / h,
  child: Texture(textureId: textureId),
)

Capture

To get frame bytes in Dart — call copyLatestFrame() while preview is running:

final UvcPreviewFrame? frame = uvcCamera.copyLatestFrame();
if (frame != null) {
  // frame.rgbaBytes: RGBA pixel data (width * height * 4 bytes)
  // frame.width, frame.height: frame dimensions
}

To capture with the current preview transform applied:

final UvcPreviewFrame? frame = uvcCamera.copyLatestFrameTransformed(
  uvcCamera.previewTransform,
);

frame.width and frame.height reflect the post-transform dimensions.

Controls

supportedControls() returns the UvcCameraControl list exposed by the currently opened device, including min/max/default/current values and a UvcControlKind (integer, boolean, or enum-like) describing how the value behaves. getControl(...) and setControl(...) use typed UvcControlId values instead of raw integer IDs.
For device debugging, debugBmControls() returns the UvcBmControlInfo list advertised by descriptor bmControls without GET_CUR probing. This is useful when a device reports a control bit but rejects or mishandles GET_CUR. (Android only — the Windows backend has no raw descriptor access and returns an empty list.)

Control labels are for display only. Use UvcControlId to identify controls in code:

final int? autoFocus = uvcCamera.getControl(UvcControlId.focusAuto);
await Future<void>.delayed(const Duration(milliseconds: 100));
uvcCamera.setControl(UvcControlId.focusAuto, autoFocus == 0 ? 1 : 0);

Compound UVC controls are exposed as typed APIs instead of a single integer:

final UvcPanTiltAbsoluteControl? panTilt =
    uvcCamera.getPanTiltAbsoluteControl();

if (panTilt != null) {
  uvcCamera.setPanTiltAbsoluteControl(
    UvcPanTiltAbsoluteControl(
      pan: panTilt.pan + 10,
      tilt: panTilt.tilt,
    ),
  );
}

Diagnostics

Preview state

uvcCamera.isPreviewing returns true while the native stream callback is active — that is, after a successful startPreview() and before stopPreview() or device close. Use it to guard UI state or skip work when preview is not running.

Frame drop behavior

When the native pipeline is still processing a frame, incoming frames are dropped rather than queued — the preview always shows the latest frame. Drops are visible in getStreamStats().

Stream stats

Use getStreamStats() to read a UvcStreamStats snapshot of cumulative native stats for the current preview session, including input/delivered FPS, decode failures, dropped frames, inter-frame gap timing, and first-frame latency.

Stats reset when a new startPreview() session begins.

Streaming error reporting

Frame pipeline errors — decode failures, undersized frames, buffer allocation failures — are delivered proactively via streamErrors rather than being silently stored in lastError.

Subscribe once when the widget is initialised and cancel on dispose:

StreamSubscription<UvcStreamError>? _streamErrorSub;

@override
void initState() {
  super.initState();
  _streamErrorSub = uvcCamera.streamErrors.listen((UvcStreamError error) {
    // handle error, e.g. show a SnackBar
    print(error.message);
  });
}

@override
void dispose() {
  _streamErrorSub?.cancel();
  super.dispose();
}

streamErrors is a broadcast stream, so multiple subscribers are allowed.

Stall detection and recovery

Some devices keep the stream "running" while silently delivering no frames. Enable the watchdog to detect this and optionally recover:

uvcCamera.enableStallDetection(
  const UvcStallDetectionConfig(
    stallTimeout: Duration(seconds: 2),
    autoRestart: true,
    maxRestartAttempts: 3,
  ),
);

uvcCamera.stallEvents.listen((UvcStallEvent event) {
  switch (event.type) {
    case UvcStallEventType.stalled:
      // No frames for `event.silence` while previewing.
      break;
    case UvcStallEventType.restartSucceeded:
      // Preview is running again (attempt `event.restartAttempt`).
      break;
    case UvcStallEventType.restartFailed:
      // `event.restartResult` holds the failed verification details.
      break;
  }
});

A stall is declared when the delivered frame sequence stops advancing for stallTimeout while isPreviewing is true, and is reported once per stall episode. With autoRestart, the preview is stopped and restarted with the parameters of the most recent startPreview call; the attempt counter resets once frames flow again. Detection stays enabled across preview sessions until disableStallDetection().

Typed error codes

APIs that return raw int codes pass through libuvc uvc_error_t values (the Windows backend maps its failures into the same code space). UvcErrorCode gives them names, and UvcException is available for throw-style handling in app code:

final UvcPreviewStartResult result = await uvcCamera.startPreview(mode);
if (!result.success) {
  if (result.errorCode == UvcErrorCode.noDevice) {
    // Device disconnected or never opened.
  }
  // Or wrap it:
  throw UvcException.fromNativeCode(
    result.nativeErrorCode,
    message: result.lastError ?? '',
  );
}

UvcPreviewStartResult.nativeErrorCode is non-zero only when stream startup itself failed; verification failures keep it at 0 — inspect lastError and the frame counters instead.

Logging

You can change the log level for the underlying native layer at runtime:

uvcCamera.setLogLevel(UvcLogLevel.warn);

Available levels are:

• UvcLogLevel.error
• UvcLogLevel.warn
• UvcLogLevel.info
• UvcLogLevel.debug
• UvcLogLevel.trace

If you do not call uvcCamera.setLogLevel(...), the package defaults to UvcLogLevel.info. Native logs are emitted on Android (logcat); the Windows backend reports problems through streamErrors and lastError instead of log output.

Example app

A demo app lives in the example/ directory at the root of this repository.

RoadMap

For upcoming work areas and current planning direction, see ROADMAP.md.

Licensing

This package is licensed under the BSD 3-Clause License. Bundled third-party components keep their own licenses.

See THIRD_PARTY_NOTICES.md for bundled dependency license notices, including libuvc, libusb, and libjpeg-turbo