doc: Add QT MVC structure to style guide

CONTRIBUTING.md

@@ -93,19 +93,7 @@ Mypy is also available as a VS Code [extension](https://marketplace.visualstudio
 -   Run all tests by running `pytest tests/` in the repository root.
 
 ## Code Style
-
-Most of the style guidelines can be checked, fixed, and enforced via Ruff. Older code may not be adhering to all of these guidelines, in which case _"do as I say, not as I do"..._
-
--   Do your best to write clear, concise, and modular code.
--   Keep a maximum column with of no more than **100** characters.
--   Code comments should be used to help describe sections of code that can't speak for themselves.
--   Use [Google style](https://google.github.io/styleguide/pyguide.html#s3.8-comments-and-docstrings) docstrings for any classes and functions you add.
-    -   If you're modifying an existing function that does _not_ have docstrings, you don't _have_ to add docstrings to it... but it would be pretty cool if you did ;)
--   Imports should be ordered alphabetically.
--   Lists of values should be ordered using their [natural sort order](https://en.wikipedia.org/wiki/Natural_sort_order).
-    -   Some files have their methods ordered alphabetically as well (i.e. [`thumb_renderer`](https://github.com/TagStudioDev/TagStudio/blob/main/src/tagstudio/qt/widgets/thumb_renderer.py)). If you're working in a file and notice this, please try and keep to the pattern.
--   When writing text for window titles or form titles, use "[Title Case](https://apastyle.apa.org/style-grammar-guidelines/capitalization/title-case)" capitalization. Your IDE may have a command to format this for you automatically, although some may incorrectly capitalize short prepositions. In a pinch you can use a website such as [capitalizemytitle.com](https://capitalizemytitle.com/) to check.
--   If it wasn't mentioned above, then stick to [**PEP-8**](https://peps.python.org/pep-0008/)!
+See the [Style Guide](/STYLE.md)
 
 ### Modules & Implementations
 

STYLE.md

@@ -0,0 +1,82 @@
+# Code Style
+
+Most of the style guidelines can be checked, fixed, and enforced via Ruff. Older code may not be adhering to all of these guidelines, in which case _"do as I say, not as I do"..._
+
+-   Do your best to write clear, concise, and modular code.
+    - This should include making methods private by default (e.g. `__method()`)
+    - Methods should only be protected (e.g. `_method()`) or public (e.g. `method()`) when needed and warranted
+-   Keep a maximum column width of no more than **100** characters.
+-   Code comments should be used to help describe sections of code that can't speak for themselves.
+-   Use [Google style](https://google.github.io/styleguide/pyguide.html#s3.8-comments-and-docstrings) docstrings for any classes and functions you add.
+    -   If you're modifying an existing function that does _not_ have docstrings, you don't _have_ to add docstrings to it... but it would be pretty cool if you did ;)
+-   Imports should be ordered alphabetically.
+-   Lists of values should be ordered using their [natural sort order](https://en.wikipedia.org/wiki/Natural_sort_order).
+    -   Some files have their methods ordered alphabetically as well (i.e. [`thumb_renderer`](https://github.com/TagStudioDev/TagStudio/blob/main/src/tagstudio/qt/widgets/thumb_renderer.py)). If you're working in a file and notice this, please try and keep to the pattern.
+-   When writing text for window titles or form titles, use "[Title Case](https://apastyle.apa.org/style-grammar-guidelines/capitalization/title-case)" capitalization. Your IDE may have a command to format this for you automatically, although some may incorrectly capitalize short prepositions. In a pinch you can use a website such as [capitalizemytitle.com](https://capitalizemytitle.com/) to check.
+-   If it wasn't mentioned above, then stick to [**PEP-8**](https://peps.python.org/pep-0008/)!
+
+## QT
+As of writing this section, the QT part of the code base is quite unstructured and the View and Controller parts are completely intermixed[^1]. This makes maintenance, fixes and general understanding of the code base quite challenging, because the interesting parts you are looking for are entangled in a bunch of repetitive UI setup code. To address this we are aiming to more strictly separate the view and controller aspects of the QT frontend.
+
+The general structure of the QT code base should look like this:
+```
+qt
+├── controller
+│   ├── widgets
+│   │   └── preview_panel_controller.py
+│   └── main_window_controller.py
+├── view
+│   ├── widgets
+│   │   └── preview_panel_view.py
+│   └── main_window_view.py
+├── ts_qt.py
+└── mixed.py
+```
+
+In this structure there are the `view` and `controller` sub-directories. They have the exact same structure and for every `<component>_view.py` there is a `<component>_controller.py` at the same location in the other subdirectory and vice versa.
+
+Typically the classes should look like this:
+```py
+# my_cool_widget_view.py
+class MyCoolWidgetView(QWidget):
+    def __init__(self):
+        super().__init__()
+        self.__button = QPushButton()
+        self.__color_dropdown = QComboBox()
+        # ...
+        self.__connect_callbacks()
+
+    def __connect_callbacks(self):
+        self.__button.clicked.connect(self._button_click_callback)
+        self.__color_dropdown.currentIndexChanged.connect(
+            lambda idx: self._color_dropdown_callback(self.__color_dropdown.itemData(idx))
+        )
+
+    def _button_click_callback(self):
+        raise NotImplementedError()
+```
+```py
+# my_cool_widget_controller.py
+class MyCoolWidget(MyCoolWidgetView):
+    def __init__(self):
+        super().__init__()
+
+    def _button_click_callback(self):
+        print("Button was clicked!")
+
+    def _color_dropdown_callback(self, color: Color):
+        print(f"The selected color is now: {color}")
+```
+
+Observe the following key aspects of this example:
+- The Controller is just called `MyCoolWidget` instead of `MyCoolWidgetController` as it will be directly used by other code
+- The UI elements are in private variables
+  - This enforces that the controller shouldn't directly access UI elements
+  - Instead the view should provide a protected API (e.g. `_get_color()`) for things like setting/getting the value of a dropdown, etc.
+  - Instead of `_get_color()` there could also be a `_color` method marked with `@property` 
+- The callback methods are already defined as protected methods with NotImplementedErrors
+  - Defines the interface the callbacks
+  - Enforces that UI events be handled
+
+
+[^1]: For an explanation of the Model-View-Controller (MVC) Model, checkout this article: [MVC Framework Introduction](https://www.geeksforgeeks.org/mvc-framework-introduction/).