[PATCH RFC] There should be a method to know when a DataSet's DataSources change

[taylor.smock]

In the attached patch, I've created a new listener that the DataSet triggers when it modifies the DataSource list.

Possible use cases:

• Mapillary
• Microsoft Streetside (the interesting code is literally the same as Mapillary's)
• OpenStreetCam
• MapWithAI

Mapillary currently uses the following in src/main/java/org/openstreetmap/josm/plugins/mapillary/MapillaryLayer.java:

    new DataSetListenerAdapter(e -> {
      if (e instanceof DataChangedEvent && MapillaryDownloader.getMode() == DOWNLOAD_MODE.OSM_AREA) {
        // When more data is downloaded, a delayed update is thrown, in order to
        // wait for the data bounds to be set.
        MainApplication.worker.execute(MapillaryDownloader::downloadOSMArea);
      }
    });

While it works, it is prone to being called when the user didn't download an additional area.

[taylor.smock]

Add listeners for data source change events (for DataSets) -- class/method names may need to be changed for clarity

[taylor.smock]

Don't add duplicate DataSource entries to dataSources (DataSetMerger adds them, and then mergeFrom was adding them again). See source:trunk/src/org/openstreetmap/josm/data/osm/DataSet.java@15559:1084-1087#L1084​

[taylor.smock]

source:trunk/src/org/openstreetmap/josm/data/osm/DataSetMerger.java@15559:458-461#L458​ is where DataSetMerger adds the bounds.

[Don-vip]

In 15609/josm:

fix #18436 - add listeners to know when a DataSet's DataSources change (patch by taylor.smock)