Implemented reporting callbacks.

Both the dependency Tracer class and the Packer class now support a
callback object, where the latter is a subclass of the former.

For file transfers running in a separate thread, there is a thread-safe
wrapper for progress callbacks. This wrapper can be called from any thread,
and calls the wrapped callback object from the main thread. This way the
callback implementation itself doesn't have to worry about threading
issues.

blender_asset_tracer/trace/__init__.py

@@ -3,7 +3,7 @@ import pathlib
 import typing
 
 from blender_asset_tracer import blendfile
-from . import result, blocks2assets, file2blocks
+from . import result, blocks2assets, file2blocks, progress
 
 log = logging.getLogger(__name__)
 
@@ -17,18 +17,24 @@ codes_to_skip = {
 }
 
 
-def deps(bfilepath: pathlib.Path) -> typing.Iterator[result.BlockUsage]:
+def deps(bfilepath: pathlib.Path, progress_cb: typing.Optional[progress.Callback] = None) \
+        -> typing.Iterator[result.BlockUsage]:
     """Open the blend file and report its dependencies.
 
     :param bfilepath: File to open.
+    :param progress_cb: Progress callback object.
     """
 
     log.info('opening: %s', bfilepath)
     bfile = blendfile.open_cached(bfilepath)
 
+    bi = file2blocks.BlockIterator()
+    if progress_cb:
+        bi.progress_cb = progress_cb
+
+    ahb = asset_holding_blocks(bi.iter_blocks(bfile))
     # Sort the asset-holding blocks so that we can iterate over them
     # in disk order, which is slightly faster than random order.
-    ahb = asset_holding_blocks(file2blocks.iter_blocks(bfile))
     for block in sorted(ahb):
         yield from blocks2assets.iter_assets(block)