The `pre` handler is called after blender internal code is done populating
the link/append context with data to be processed, and before this data
starts being linked from library files.
The `post` handler is called after blender is done linking, and
potentailly appending and/or instantiating, the requested data and all
of their dependencies.
Both handlers are called with a single argument, the link/append
context.
An new RNA sets of wrappers have been added to expose relevant info from
these internal C++ structures.
NOTE: !113658 is very similar (but tied to asset drag & drop), whereas
this PR is more general (these could probably live hand-in-hand / side-
by-side).
Implements #122357
Pull Request: https://projects.blender.org/blender/blender/pulls/128279
-----------------
Some quick py example code:
```python
import bpy
def my_handler_pre(lapp_context):
print("About to {}:\n\t".format("link" if "LINK" in lapp_context.options else "append"),
"\n\t".join("{} '{}', from libs ['{}']".format(item.id_type, item.name,
"', '".join([l.filepath for l in item.source_libraries]))
for item in lapp_context.import_items))
def my_handler_post(lapp_context):
print("{}:\n\t".format("Linked" if "LINK" in lapp_context.options else "Appended"),
"\n\t".join("{} '{}', from lib '{}'".format(item.id.id_type, item.id.name, item.source_library.filepath)
for item in lapp_context.import_items))
bpy.app.handlers.link_append_pre.append(my_handler_pre)
bpy.app.handlers.link_append_post.append(my_handler_post)
```
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