flutter_ffi_uvc

This package is based on libuvc.

It provides Android UVC camera access, including USB device handling, preview streaming to a Flutter Texture, frame access from Dart, preview transforms, stream diagnostics, and UVC camera controls.

Supported Platforms

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

Installation

flutter pub add flutter_ffi_uvc

How it works

This package combines three layers:

• Android USB Host API — device discovery, USB permission, and acquiring the file descriptor for the connected device.
• libusb — wraps that file descriptor and handles the actual USB communication.
• libuvc — sits on top of libusb and handles the UVC protocol: mode negotiation, frame streaming, and camera controls.

Usage

Typical lifecycle

1. Call uvcCamera.ensureCameraPermission() if your app requires the CAMERA permission.
2. Call uvcCamera.listUsbDevices() to discover attached UVC cameras.
3. Call uvcCamera.openUsbDevice(deviceId) to request USB permission and open the device.
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 on Android.
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 — requests USB permission if not already granted
final int result = await uvcCamera.openUsbDevice(devices.first.deviceId);
if (result != 0) {
  print('Open failed: ${uvcCamera.lastError}');
}

openUsbDevice goes through the Android USB layer to acquire permission and a file descriptor, then passes it to libusb to open the session. It throws a PlatformException if the Android layer fails, and returns a non-zero code if libusb/libuvc fails to initialize.

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 Android receiver is only registered while at least one listener is subscribed. Cancel the subscription on dispose.

Alternative: opening by file descriptor

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

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

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.

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.

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 callback is already processing a frame, incoming callbacks are dropped rather than queued.

Dropped callbacks are visible at UvcLogLevel.trace:

dropping frame callback because previous callback is still processing

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.
Errors are only emitted while a native error listener is active.

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. 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.

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.

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,
    ),
  );
}

Logging

You can change the log level for the underlying libuvc 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.

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.