Add RFC: Separate context bar code into UI and plugins

Author: norihiro

text/0061-context-bar-in-plugins.md

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+# Summary
+
+<!-- Simple explanation of feature/changes -->
+
+Move the source-type specific implementation of the context-bar into plugins.
+
+## Implementation in libobs
+
+Add a new callback into `struct obs_source_info`.
+```c
+	obs_properties_t *(*get_context_bar_properties)(void *data, void *type_data);
+```
+This callback will return the property information of this source for the context bar.
+Unlike the existing callbacks `get_properties` and `get_properties2`,
+it is expected to return a few subset of the properties.
+
+Add a new API `obs_source_context_bar_properties` that returns the property list for the context bar.
+If `obs_source_info.get_context_bar_properties` is not defined, the API will return NULL.
+The interface is the same as the existing API `obs_source_properties`.
+```c
+obs_properties_t *obs_source_properties(const obs_source_t *source);
+obs_properties_t *obs_source_context_bar_properties(const obs_source_t *source);
+```
+
+The context-bar version of `obs_get_source_properties` won't be implemented
+since the context-bar will be created only when there is an instance.
+```c
+obs_properties_t *obs_get_source_properties(const char *id);
+```
+
+## Alternative implementation in libobs
+
+Add new callbacks `obs_property_set_context_bar` and `obs_property_context_bar`.
+These callbacks will set and get each property that should be available for the context bar.
+
+In the callback `get_properties` or `get_properties2` in the plugin should call `obs_property_set_context_bar` to indicate which properties should be shown in the context bar.
+
+The drawback of this alternative implementation is that the properties of the context bar should be fully subset of the properties of the original properties.
+For example, though the browser source has a button `Refresh cache of current page` in the property dialog, the context bar has a short button `Refresh`. This difference will be covered by a different description returned by `get_context_bar_properties`.
+
+
+## Implementation in UI
+
+The `class WidgetInfo` displays one property and handles its value and change.
+The class is currently expected to be instantiated from the `class OBSPropertiesView`.
+In the new implementation, the `class WidgetInfo` will be renamed to `class OBSPropertyWidget` and it will be generalized so that the class can be instantiated from the context bar.
+Major changes in the class will be changing direct access to the members of `class OBSPropertiesView` but will emit signals, which will be connected to slots on the parent instance.
+
+For the filter properties, the undo and redo handler is pushed into the stack using a 500-millisecond timer.
+The timer will be moved into `class OBSPropertiesView`.
+Hopefully, this change will fix [the minor bug on the redo](https://github.com/obsproject/obs-studio/issues/10628).
+
+## Implementation in plugins
+
+All these plugins will implement the callback `get_context_bar_properties`.
+
+- `browser_source`
+- `wasapi_input_capture`, `wasapi_output_capture`, `wasapi_process_output_capture`
+- `coreaudio_input_capture`, `coreaudio_output_capture`
+- `pulse_input_capture`, `pulse_output_capture`
+- `alsa_input_capture`
+- `window_capture`
+- `xcomposite_input`
+- `monitor_capture`
+- `display_capture`
+- `xshm_input`
+- `dshow_input`
+- `game_capture`
+- `image_source`
+- `color_source`
+- `text_ft2_source`
+- `text_gdiplus`
+
+# Motivation
+
+<!-- What problem is this solving? What are the common use cases? -->
+
+UI has the implementation of the context-bar for each source type.
+As a result, property names are hard-coded in both UI and plugins.
+In possible future modification, it might require to change both plugin and UI, which might lead to a potential bug.
+
+It is a good implementation to separate the plugin-specific implementation from the UI.
+
+This will also enable 3rd party plugins to have their context bar.
+
+# Drawbacks
+
+<!-- What is the potential detriment for adding this feature/change? -->
+
+This makes it unable to have features that are not supported by `obs_property_t`.
+So far, I don't see any features on the context bar that cannot be covered by this RFC.
+
+# Additional Information
+
+<!-- Any additional information that may not be covered above that you feel is relevant. External links, references, examples, etc. -->
+
+Preliminary implementation in libobs is as below.
+https://github.com/norihiro/obs-studio/commit/dbbb706f32ed5de73aabda4a788d7460ecec033f