This temporary layer store (a QgsProject) is used as a
store for layers that are added if a parameter that
is evaluated to a layer requires that a new, non-active-project
layer is loaded. It means that these layers will remain accessible
for the duration of the algorithm's execution (or models
execution if an algorithm is run as part of a model), before
being automatically discarded when the QgsProcessingContext
used to run the algorithm/model goes out of scope.
This approach has several benefits:
- it means that algorithms (including c++ algorithms) are able
to use both project and non-project layers without needing
to handle any memory management themselves.
- it means that layers are guaranteed to last for the duration
of a model execution. This is currently an issue where models
which use memory layers as intermediate outputs do not
function correctly as the memory layers are destroyed before
the model has finished executing
- there should be no leakage of layers remaining open
after an algorithm exits
