[camera_web] Add onCameraClosing implementation

[bselwe]

Adds onCameraClosing implementation to the web camera platform.

• Added onEnded to Camera (based on MediaStreamTrack.onEnded).
  • The stream of the camera video tracks that have ended playing. This occurs when there is no more camera stream data, e.g. the user has stopped the stream by changing the camera device, revoked the camera permissions or ejected the camera device.
• Added onCameraClosing implementation.
  • Emits when the camera with the given id emits the ended event (Camera.onEnded).
• Moved the previous camera controller's disposal to the finally clause in the example app.

Part of flutter/flutter#45297.

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[bselwe]

@ditman @felangel @orestesgaolin

[bselwe]

I couldn't find a reason yet why CameraController.getMaxZoomLevel and CameraController.getMinZoomLevel throws a PlatformException even though this type of exception is always mapped to CameraException in these methods.

Added this to always dispose the previous camera controller even if an error occurred while initializing a new one.

[bselwe]

But maybe it should be added as a separate PR or as a part of #4240.

[ditman]

Yes, this should be part of #4240 instead of this one.

[ditman]

(added to 4240, and reverted here)

[bselwe]

Thanks!

[ditman]

I wonder if there's a possibility that we fire the onCameraClosing event twice (if the programmer calls "stop" and the user yanks the camera from the device, or similar). Probably not, because upon calling stop we clear the tracks, right?

[bselwe]

I wonder if there's a possibility that we fire the onCameraClosing event twice (if the programmer calls "stop" and the user yanks the camera from the device, or similar). Probably not, because upon calling stop we clear the tracks, right?

Yeah, I believe so. Stopping the camera programatically stops all stream tracks (both video and audio) so they should no longer emit events about the stream being stopped for another reason.

I have tested this, stopped the camera programmatically, then rejected the camera permissions. Normally, it emits an ended event but it didn't as the camera was stopped so it is fine I think.