docs(ui/solara): add metadata viewer to README (#49)

README.md

@@ -36,6 +36,15 @@ It has the following use cases:
 
 - Converting workflows from ComfyUI's web UI format to API format without the web UI.
 
+## [Documentation](docs/README.md)
+- [Runtime](docs/Runtime.md)
+- [Images](docs/Images/README.md)
+- [Models](docs/Models/README.md)
+- [Additional Nodes](docs/Nodes/README.md)
+- [Transpiler](docs/Transpiler.md)
+- UI
+  - [Solara UI](docs/UI/Solara.md)
+
 ## Installation
 ### With ComfyUI
 Install [ComfyUI](https://github.com/comfyanonymous/ComfyUI#installing) first. And then run the following commands:
@@ -297,14 +306,14 @@ Screenshot:
 
 ![](docs/imgs/README/select.png)
 
-## [Documentation](docs/README.md)
-- [Runtime](docs/Runtime.md)
-- [Images](docs/Images/README.md)
-- [Models](docs/Models/README.md)
-- [Additional Nodes](docs/Nodes/README.md)
-- [Transpiler](docs/Transpiler.md)
-- UI
-  - [Solara UI](docs/UI/Solara.md)
+## UI
+### [Solara UI](docs/UI/Solara.md)
+These Solara widgets can be used in Jupyter Notebook and in web pages.
+
+#### [MetadataViewer](docs/UI/Solara.md#metadataviewer)
+A widget for viewing the metadata of an image generated by ComfyScript / ComfyUI / Stable Diffusion web UI. Workflow JSON files are supported too, including both the web UI format and the API format.
+
+![](docs/UI/images/Solara/MetadataViewer.png)
 
 
 [^graph-gui]: [I hate nodes. (No offense comfyui) : StableDiffusion](https://www.reddit.com/r/StableDiffusion/comments/15cr5xx/i_hate_nodes_no_offense_comfyui/)

src/comfy_script/runtime/__init__.py

@@ -654,7 +654,7 @@ def when_empty(
           - `None` to remove the callback.
           - If the callback takes 0 argument, it will be called directly.
           - If the callback takes 1 argument, it will be passed an *entered* `Workflow`, so that you don't need to write it yourself and indent the code one more level. 
-          - If the callback returns `False`, it will be removed and the workflow will not be queued.
+          - If the callback returns `False`, it will be removed. The workflow will not be queued if the callback uses the passed `Workflow`, otherwise it depends.
           - If the callback raises an exception, the workflow will not be queued.
 
         Only one callback can be registered at a time. Use `add_queue_remaining_callback()` if you want to register multiple callbacks.