m-kuhn
commented
8 years ago Issue #8 proposes to remove singleton behavior from QgsProject. I would propose to do the same to the map layer registry and make it a child of QgsProject.
This in turn would mean that there is no longer a single registry where a map layer would be registered to, but in turn a map layer could be registered to be part of one of several open projects.
So to sum it up, I'd prefer to reduce the usage of the map layer registry (mostly by passing pointer instead of layer ids to methods and classes) instead of making the registry do more magic than it already does.
wonder-sk
Currently, a map layer needs to be added to QgsMapLayerRegistry before it can be used for various tasks (e.g. in layer tree, map rendering) - however not for all tasks (e.g. get vector features, raster blocks). This is confusing to the users of the API (and makes the API less elegant, too).
The solution would be do automatically add map layers to registry in their constructor and remove map layers in their destructor. That does not mean that all layers would be automatically owned by the registry - but the registry would be at least aware of them all the time. (The transfer of ownership to the registry would still need to be explicit.)