placeholderBuilder and errorBuilder callbacks for MobileScanner have an extra argument which serves no purpose.

[remonke]

This is what the build method of the MobileScanner widget looks like:

@override
Widget build(BuildContext context) {
  return ValueListenableBuilder<MobileScannerState>(
    valueListenable: controller,
    builder: (BuildContext context, MobileScannerState value, Widget? child) {
      if (!value.isInitialized) {
        const Widget defaultPlaceholder = ColoredBox(color: Colors.black);

        return widget.placeholderBuilder?.call(context, child) ??
            defaultPlaceholder;
      }

      final MobileScannerException? error = value.error;

      if (error != null) {
        const Widget defaultError = ColoredBox(
          color: Colors.black,
          child: Center(child: Icon(Icons.error, color: Colors.white)),
        );

        return widget.errorBuilder?.call(context, error, child) ??
            defaultError;
      }

      return LayoutBuilder(
        builder: (context, constraints) {
          _maybeUpdateScanWindow(value, constraints);

          final Widget? overlay =
              widget.overlayBuilder?.call(context, constraints);
          final Size cameraPreviewSize = value.size;

          final Widget scannerWidget = ClipRect(
            child: SizedBox.fromSize(
              size: constraints.biggest,
              child: FittedBox(
                fit: widget.fit,
                child: SizedBox(
                  width: cameraPreviewSize.width,
                  height: cameraPreviewSize.height,
                  child: MobileScannerPlatform.instance.buildCameraView(),
                ),
              ),
            ),
          );

          if (overlay == null) {
            return scannerWidget;
          }

          return Stack(
            alignment: Alignment.center,
            children: <Widget>[
              scannerWidget,
              overlay,
            ],
          );
        },
      );
    },
  );
}

Since child is not passed to the ValueListenableBuilder, it will always be null. There is no point in making placeholderBuilder and errorBuilder callbacks take an extra argument.

[navaronbracke]

Good catch! Should we provide the child to the ValueListenable builder? Or should we omit the argument?

I would go with adding the child to the value listenable builder, since then you can optimize rebuilds for that child, if desired.
The child argument of ValueListenableBuilder is there for performance reasons, so I think we should use it.

[remonke]

I think it's the best option as it wouldn't be a breaking change.