Add batch mode of screen location

[OttyLab]

Summary

toScreenLocationBatch is created for the better performance.

Background

I want to put a rich marker that is implemented as a Flutter Widget instead of using Symbol Layer. The idea is as same as that of Android MarkerView implementation.

This approach requires toScreenLocation query to update marker location on the screen every time the camera position is changed. toScreenLocation is implemented as a method channel and it is not performant.

The camera moving callback is called more than 10 times per second. Therefore, if the number of marker is 100, the API call should be less than 1ms. It's depends on the device but some fail to meet this requirement and the animation gets heavy.

Implementation

toScreenLocationBatch is newly defined. List<List<LatLng>> is a parameter and returned value is List<List<Point>>. The parameter and return value orders are garanteeed.

Sample code

1. camera tracking is enabled trackCameraPosition: true,
2. camera moving call back is handled in

    controller.addListener(() {
      if (controller.isCameraMoving) {
        _updateMarkerPosition();
      }
    });

  void _onCameraIdleCallback() {
    _updateMarkerPosition();
  }

3. new position is calculated

  void _updateMarkerPosition() {
    var param = <LatLng>[];
    for (var marker in markers) {
      param.add(marker.Coordinate);
    }

    mapController.toScreenLocationBatch(param).then((points){
      for (var i = 0; i < markers.length; i++){
        var point = Point(points[i].x, points[i].y);
        markers[i].Position = point;
      }
    });
  }

Result

x-axis shows the number of toScreenLocation or toScreenLocationBatch call and y-axis shows the time. Blue lines show the result of toScreenLocation while red lines show toScreenLocationBatch. According to the result, toScreenLocationBatch significantly improves the performance.

Screenshot

output.mp4

Review required items

• if the SDK minimum version update (to v2.1.0) is fine. (To my knowledge, flutter-mapbox-gl also requires v2.1.0)
• if the implementation toScreenLocationBatch is acceptable
• if method channel for each platform are correctly implemented
• if the sample code is efficient

[felix-ht]

LGTM besides the todo - very nice to have this feature! @shroff would be great if you could review

[shroff]

The code to perform the batch coords to position translation LGTM, but naming in the example code is quite confusing and should be fixed before landing.

[OttyLab]

@shroff
Thank you for the review. I'll update them.

[shroff]

Not tested after the changes, but code LGTM.

[shroff]

Same as in java, better to construct a list of the known size and insert at index rather than appending.

[OttyLab]

@shroff

Thanks, fixed.

[OttyLab]

@shroff @felix-ht
Can I merge this PR?

[tobrun]

Appreciate the PR @OttyLab! 🚀
The performance isn't that great but we can iterate on it.

[AAverin]

I am getting this on v11 release now

/flutter-mapbox-gl/lib/src/controller.dart:824:46: Error: The method 'toScreenLocationBatch' isn't defined for the class 'MapboxGlPlatform'.
 - 'MapboxGlPlatform' is from 'package:mapbox_gl_platform_interface/mapbox_gl_platform_interface.dart' ('/Users/aaverin/.pub-cache/git/flutter-mapbox-gl-5df934232034988c516da11c6c72f4eee119822c/mapbox_gl_platform_interface/lib/mapbox_gl_platform_interface.dart').
Try correcting the name to the name of an existing method, or defining a method named 'toScreenLocationBatch'.
    return MapboxGlPlatform.getInstance(_id).toScreenLocationBatch(latLngs);
                                             ^^^^^^^^^^^^^^^^^^^^^

[OttyLab]

@AAverin

Thank you for reporting the issue. How can I reproduce the error?

Running sample code in SDK nor creating a simple app from the scratch did not reproduce it.

[AAverin]

@OttyLab I get when I just update from 0.10.0 to 0.11.0. What could be important is that library is used as path, that points to my fork. In my fork I checkout 0.11.0 tag and then do pub get in the main project.

[OttyLab]

@AAverin

When I create a simple app with 0.10.0 (getting via Pub.dev) and upgrading to 0.11.0 does not reproduce the issue.

What could be important is that library is used as path, that points to my fork. In my fork I checkout 0.11.0 tag and then do pub get in the main project.

Does this mean you forked flutter-mapbox-gl and your application directly refers to the local flutter-mapbox-gl as a path? In that case, does your local fork implements toScreenLocationBatch?