SensorConfig.multiple crashes with "Multiple widgets used the same GlobalKey" — module-level previewWidgetKey collides between primary and PiP previews #646

Environment

• camerawesome: 2.5.0 (latest on pub.dev as of 2026-05-31)
• Flutter: 3.x (stable)
• Device: iPhone 16 Plus, iOS 18.x
• CamerawesomePlugin.isMultiCamSupported() returns true

Symptom

Using SensorConfig.multiple(...) on either CameraAwesomeBuilder.awesome or CameraAwesomeBuilder.custom floods the log with:

FlutterError: Multiple widgets used the same GlobalKey.
The key [GlobalKey#xxxxx] was used by multiple widgets. The parents of those widgets were:
- Padding(padding: EdgeInsets.zero, dependencies: [Directionality], ...)
- Padding(padding: EdgeInsets.zero, dependencies: [Directionality], ...)
A GlobalKey can only be specified on one widget at a time in the widget tree.

Visible effect: both the primary preview and the PiP preview render black (or one of them does). On capture, the primary sensor file is never written; only the secondary (PiP) sensor produces a file. The capture event's multiple.fileBySensor is missing the primary entry.

Root cause

lib/src/widgets/preview/awesome_preview_fit.dart:7:

final previewWidgetKey = GlobalKey();

This is a module-level singleton GlobalKey. It is then assigned to the wrapping InteractiveViewer on line 159 of the same file:

InteractiveViewer(
  key: previewWidgetKey,
  transformationController: transformController,
  scaleEnabled: false,
  constrained: false,
  panEnabled: false,
  ...
)

When SensorConfig.multiple is in effect, the package mounts PreviewFitWidget twice (once for the primary preview, once for the PiP overlay). Both instances attach the same module-level GlobalKey, which Flutter rejects: a GlobalKey must be unique in the widget tree at any given time.

The resulting tree-finalize assertion appears to be what prevents the second AVCaptureMultiCamSession output from binding correctly on iOS — only the secondary sensor produces a capture file.

Reproduction

The package's own example/lib/multi_camera.dart should reproduce this on any iOS 13+ device with multi-cam support (iPhone XS or later).

sensorConfig: SensorConfig.multiple(
  sensors: [
    Sensor.type(SensorType.telephoto),  // matches the example's iOS branch
    Sensor.position(SensorPosition.front),
  ],
  flashMode: FlashMode.none,
  aspectRatio: CameraAspectRatios.ratio_4_3,
),

Suggested fix

The InteractiveViewer's scaleEnabled and panEnabled are both false in this code path. The GlobalKey does not appear to be load-bearing for the InteractiveViewer's state — it can either be removed, or scoped per-instance by making it a final _previewWidgetKey = GlobalKey() inside _AnimatedPreviewFitState (or similar per-mount scope).

If the key is needed elsewhere (e.g., for transformationController coordination with another widget I haven't traced), the simplest correctness fix is to make it per-instance.

Happy to send a PR if a maintainer can confirm which scope is intended.