[QGIS-Developer] Understanding plugin management in QGIS - exposing QgsPluginRegistry to Python?

[Borys Jurgiel]

Hi Sebastian,

The reason the whole infrastructure is split between Plugin Manager and Python 
Plugin Installer (and Python utils) is Python support is optional in QGIS. 

If QGIS is compiled without Python Support, the Plugin Manager still displays 
all C++ plugins, it just lacks Python ones (and of course install/deinstall 
funcions).

I admit the the infrastructure became a bit complicated ;) However, porting 
the Plugin Manager to Python would require making Python a hard dependency of 
QGIS GUI (or we should drop C++ plugins by porting them all to core, what is 
discussed from time to time and doesn't seem to may happen).

Regards,

Borys


Dnia sobota, 14 marca 2020 21:10:24 CET Sebastian M. Ernst pisze:
> Hi folks,
> 
> out of curiosity I have started poking around in QGIS' code related to
> plugin management. It's an interesting mix of C++ and Python code. Even
> the plugin management GUI itself is partially C++ (the main window,
> QgsPluginManager[Interface], and part of its logic) and partially Python
> (all further dialogs and their logic). As an experiment for now, I am
> looking into how I could port at least the entire GUI to Python and
> disentangle some of the knots. The rational is that plugin management is
> not performance-critical and could become much more easy to maintain,
> among a lot of other interesting benefits that a potentially fully
> Python-based solution could offer. But that's a different story. For
> now, I think I have hit a roadblock and was wondering whether anyone
> could enlighten me. I'd greatly appreciate any help.
> 
> I *believe* I have a fairly good understanding of what happens
> underneath `/python/pyplugin_installer/`. The class `QgsPluginInstaller`
> from `installer.py` appears to be a Python API that is called from the
> C++-interface (`QgsPluginManagerInterface`) through `QgsPythonRunner`.
> Most of the C++ appears to be located underneath
> `/src/app/pluginmanager/` (with a single ui-file elsewhere,
> `/src/ui/qgspluginmanagerbase.ui`). Having analyzed the C++ class
> `QgsPluginManager` I believe most of the interface can easily ported to
> Python.
> 
> The tricky bit appears to be `/src/app/qgspluginregistry.cpp`. It offers
> a class named `QgsPluginRegistry` (which is being used by
> `QgsPluginManager`). I have not found a way to access this class (or
> an/the instance of it) from Python. I figure it is required to handle
> both Python *and* C++ plugins. `QgsPluginRegistry` makes heavy use of
> `mPythonUtils`, which appears to be a C++ wrapper around
> `/python/utils.py` (through `/src/python/qgspythonutilsimpl.cpp`). So
> this portion of `QgsPluginRegistry` (about 50%) can be rewritten in
> Python rather quickly. Methods such as `loadCppPlugin` or
> `unloadCppPlugin` are a lot more problematic. As far as I can tell,
> those would need to remain in C++ and they would need to be exposed to
> Python somehow (if they are not already).
> 
> Have I found all relevant components of QGIS responsible for plugin
> management? Does my analysis contain any major flaws? Is there a way to
> manage C++ plugins (i.e. to call `loadCppPlugin` and `unloadCppPlugin`)
> from Python?
> 
> Best regards,
> Sebastian
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