Pre and post loader hooks

[kalisp]

Changelog Description

This PR introduces new concept of Pre/Post Loader Hooks which could be used by to enhance or modify loading processes without need of modifying official loaders.

The idea is for custom addons to be able to implement their own PrePostLoaderHookPlugin which would be triggered before/after Loader action.

Method to be implemented in PrePostLoaderHookPlugin:

• pre_load
• post_load
• pre_update
• post_update
• pre_remove
• post_remove

The function signature (arguments) to each of the pre/post methods matches the regular LoadPlugin except that post_* methods take an additional result argument which receives the output of the LoadPlugin method call, like simplified:

pre_load()
result = load()
post_load(x, y, z, result=result)

Additional info

Each addon must register its folder with pre/post hooks like this:

    register_loader_hook_plugin_path(loader_hook_dir)

I am testing it with this dummy custom addon which shows hooks for "CopyFile", "FileLoader", "LoadImage" Loaders.
my_custom_addon-1.0.0.zip (outdated example files)

Register example hook to just print all the hook methods

from ayon_core.pipeline.load.plugins import LoaderHookPlugin
from ayon_core.pipeline.load import register_loader_hook_plugin


class LoadHookTest(LoaderHookPlugin):
    @classmethod
    def is_compatible(cls, Loader) -> bool:
        return True

    def pre_load(self, context, name=None, namespace=None, data=None, plugin=None):
        self.log.info("pre_loadaa")

    def post_load(self, context, name=None, namespace=None, data=None, plugin=None, result=None):
        self.log.info("post_load")

    def pre_update(self, container, context, plugin=None):
        self.log.info("pre_update")

    def post_update(self, container, context, plugin=None, result=None):
        self.log.info("post_update")

    def pre_remove(self, container, plugin=None):
        self.log.info("pre_remove")

    def post_remove(self, container, plugin=None, result=None):
        self.log.info("post_remove")
		
		
register_loader_hook_plugin(LoadHookTest)

Or when targeting specific loaders and e.g. writing to a file on disk on each hook call:

from ayon_core.pipeline.load import get_loader_identifier
from ayon_core.pipeline.load.plugins import LoaderHookPlugin


class PrePostCopyFile(LoaderHookPlugin):
    """Example of hook which would be run before and after CopyFile.load()"""
    @classmethod
    def is_compatible(cls, Loader) -> bool:
        identifier = get_loader_identifier(Loader)
        return identifier in {"CopyFile", "FileLoader", "LoadImage"}

    def pre_load(self, context, name=None, namespace=None, data=None, plugin=None):
        self._write("pre_load")

    def post_load(self, context, name=None, namespace=None, data=None, plugin=None, result=None):
        self._write("post_load")

    def pre_update(self, container, context, plugin=None):
        self._write("pre_update")

    def post_update(self, container, context, plugin=None, result=None):
        self._write("post_update")

    def pre_remove(self, container, plugin=None):
        self._write("pre_remove")

    def post_remove(self, container, plugin=None, result=None):
        self._write("post_remove")

    def _write(self, value):
        with open("c:/projects/pre_post_load.txt", "a") as fp:
            fp.write(f"{value}\n")

Testing notes:

1. update copy_file_hook.py in my_custom_addon however you feel fit for your testing
2. follow this step

[ynbot]

Task linked: OP-6870 Plugin hooks - Loader and Scene Inventory

[BigRoy]

I wonder if new hook plug-ins here make more sense than just an EventSystem 😕 Seems somewhat overkill here.

[kalisp]

I wonder if new hook plug-ins here make more sense than just an EventSystem 😕 Seems somewhat overkill here.

Had the same question, but they should be blocking, I dont think you can achieve that with events.

[BigRoy]

I wonder if new hook plug-ins here make more sense than just an EventSystem 😕 Seems somewhat overkill here.

Had the same question, but they should be blocking, I dont think you can achieve that with events.

The Event system in AYON is blocking by default, right @iLLiCiTiT ?

[kalisp]

Reordered arguments in pre_, post_ methods.

[iLLiCiTiT]

One more question, is there a reason that it is required to implement all the methods? I would just remove the abstractmethod except is_compatible which is really required.

[kalisp]

I feel it forces developer to actually think about (mostly) update process, which might be a bit hidden.

[iLLiCiTiT]

But if I want to implement only pre methods then I also have to implement all post methods too, just keep them empty.

[iLLiCiTiT]

It is mergable, from my point of view the abstract methods are unnecessary, but I won't be the one who will use it.