Add page on magicgui type registration (#399)

# References and relevant issues
Came about after #68

Details `magicgui` type registration, in particular the `reset_choices`
connection to layers we do for all widgets that have that method.

docs/_toc.yml

@@ -163,6 +163,7 @@ subtrees:
           - file: developers/architecture/dir_organization
           - file: developers/architecture/napari_models
           - file: developers/architecture/app_model
+          - file: developers/architecture/magicgui_type_reg
       - file: naps/index
         subtrees:
         - maxdepth: 1

docs/developers/architecture/app_model.md

@@ -311,6 +311,8 @@ at runtime (see [issue 6600](https://github.com/napari/napari/issues/6600)
 for more on this). Note that it is likely that this pull request will be split into
 smaller pull requests for ease of review and better git history.
 
+(app_model_dep_inj_result)=
+
 ## Dependency injection and result processing
 
 Dependency injection allows to write functions using parameter type annotations,

docs/developers/architecture/index.md

@@ -11,3 +11,6 @@ code base. For advanced napari usage documentation, see [](explanations).
   connected to Qt classes and Vispy classes.
 - [](app-model): Explains the napari application model, a declarative schema for
   keeping track of commands, menus and keybindings of the napari GUI.
+- [](magicgui_type_registration): Explains how `magicgui` widgets declared by users
+  or plugins are automatically created, inputs updated and outputs added to the
+  `Viewer` for registered `napari` types.

docs/developers/architecture/magicgui_type_reg.md

@@ -0,0 +1,80 @@
+(magicgui_type_registration)=
+
+# `magicgui` type registration
+
+[`magicgui`](https://pyapp-kit.github.io/magicgui/) uses
+[type hints](https://peps.python.org/pep-0484/) to infer the appropriate widget type
+for a given function parameter. It allows third party packages
+(like `napari`) to [register](https://pyapp-kit.github.io/magicgui/type_map/#registering-support-for-custom-types) support for their types using
+{func}`~magicgui.type_map.register_type`. `napari` registers
+a number of types, additionally specifying, where appropriate:
+
+* widget type for the parameter type
+* function for updating inputs for the widget
+* return callback, for return types
+
+This enables `magicgui` widgets to be easily created via type annotations.
+
+```{note}
+This page provides implementation details on `napari`-specific type registration
+in `magicgui` and is aimed at `napari` developers.
+
+For information about using `magicgui` (for users and plugin developers) see
+[](creating-widgets).
+```
+
+## Registration details
+
+`napari` types are either registered via the
+{func}`@register_type <magicgui.type_map.register_type>` decorator when they are
+defined or in
+[`napari/types.py`](https://github.com/napari/napari/blob/main/napari/types.py).
+For the full list of types registered, see [](magicgui-parameter-annotations).
+
+### Layer types
+
+The 'layer' types registered are {class}`~napari.layers.Layer` and its subclasses, and
+`<LayerType>Data` types.
+All these types provide a `choices` callable when they are registered with
+`magicgui`. This means that annotating with these types creates a
+{class}`~magicgui.widgets.bases.CategoricalWidget`, which will have a dropdown
+selection whose options will be updated via the
+`choices` callable.
+
+This callable is either `get_layers_data` or `get_layers`.
+These functions retrieve the closest parent `Viewer` of the native
+{class}`~magicgui.widgets.bases.CategoricalWidget` widget and returns a list of
+{class}`~napari.layers.Layer` or tuple of format `('layer name', <LayerType>Data)`.
+This callable is set to the `choices` attribute of the
+{class}`~magicgui.widgets.bases.CategoricalWidget` in its
+{meth}`~magicgui.widgets.bases.CategoricalWidget.reset_choices` method and thus
+gets called via {class}`~magicgui.widgets.bases.CategoricalWidget`'s `choices` setter.
+`napari` {meth}`~napari.qt.Window.add_dock_widget` checks if the dock widget has
+a `reset_choices` attribute and if so, connects it to layer events.
+
+Note that `magicgui` {class}`~magicgui.widgets.bases.ContainerWidget`'s will call
+`reset_choices` on all subwidgets. This means that when the dock widget is a
+{class}`~magicgui.widgets.bases.ContainerWidget`, any subwidgets (e.g.,
+{class}`~magicgui.widgets.bases.CategoricalWidget`s) are updated
+whenever layers change.
+**Note that {meth}`~napari.qt.Window.add_dock_widget` will
+connect any existing `reset_choices` widget attribute to layer events for all widgets,
+not just `magicgui` widgets.**
+
+The 'layer' types also specify a `return_callback` function that adds the layer
+to the closest parent `Viewer` of the native widget when a 'layer' type is a return
+annotation.
+
+### `Viewer` type
+
+{class}`~napari.viewer.Viewer` differs from the layer types. `napari` simply specifies
+that the closest parent {class}`~napari.viewer.Viewer` (technically a public proxy of
+the {class}`~napari.viewer.Viewer` that prevents private attribute access) be bound to
+the widget (technically it's bound to a hidden child
+{class}`~magicgui.widgets.EmptyWidget`). This allows the {class}`~napari.viewer.Viewer`
+to be used in the `magicgui` widget.
+
+```{note}
+When widget is not a `magicgui` widget, the {class}`~napari.viewer.Viewer` is provided
+via a wrapper `napari` adds around widget contributions.
+```