How to create a custom dock in the Godot editor with basic functionality. Custom docks are made of Control nodes, they run in the editor, and any behavior must be done through @tool scripts. For more information, see this documentation article: https://docs.godotengine.org/en/latest/tutorials/plugins/editor/making_plugins.html#a-custom-dock
How to create a custom Resource type, and provide logic for serializing, deserializing, and converting this Resource type.
Editor integration with classes for loading, saving, and importing this Resource type, including optional import customization.

For a more comprehensive example, see the GLTF module in Godot's source code.

For a less comprehensive example, see the "simple_import_plugin" folder.

Importing vs Loading

The custom Resource type in this demo is supplemented by two different sets of editor classes:

EditorImportPlugin in the importers/ folder, which allow customizing how files are imported into Godot as Resources of different types and optionally with import settings. Imported files have .import files generated next to them.
ResourceFormatLoader and ResourceFormatSaver in the load_and_save/ folder, which allow easily editing files in the inspector and saving them back. Resource files have .uid files generated next to them.

These two approaches are mutually exclusive. You may only use one approach at a time for a given file extension. This demo showcases both by using 2 different file extensions.

In actual projects, you should either choose EditorImportPlugin(s) for a configurable import, OR ResourceFormatLoader for a writeable resource load. The choice depends on if you treat the file as an external source asset which should be imported and may be customized at import (EditorImportPlugin), or if you treat the file as an internal Godot resource meant to be natively and directly edited within Godot (ResourceFormatLoader).

For example, a glTF file may be generated by Blender, and therefore is not intended to be edited directly in Godot, so it should use EditorImportPlugin. Similarly, a PNG file is typically generated by an image editor, and Godot needs to convert it to a different internal format, like a .ctex file for a VRAM-compressed texture, so it should use EditorImportPlugin. However, files like .tres and .tscn are Godot-native formats meant to be edited directly in Godot, so they should use ResourceFormatLoader.

Once you choose one approach, create scripts deriving the appropriate classes, override their callback functions, and register them in your plugin's *_plugin.gd script.

Example Files

The examples/ folder contains several example files:

blue.tres: Directly saving a SillyMaterialResource using Godot's built-in .tres format, without any custom loader/saver logic or import/export logic, available for all Resource types. This can be edited in Godot's inspector and saved back.
cyan.silly_mat_loadable: Storing a SillyMaterialResource as a custom format, such as via ResourceFormatSaver, which is loaded back using a custom ResourceFormatLoader. This can be edited in Godot's inspector and saved back.
green_as_standard_mat.silly_mat_importable: Storing a SillyMaterialResource as a custom format, which is imported as a StandardMaterial3D using custom import/export logic. This shows how importers can import files as any Resource type, converting custom files to data Godot can use. Imported files are read-only and cannot be edited in the inspector.
yellow.silly_mat_importable: Storing a SillyMaterialResource as a custom format, which is imported as a SillyMaterialResource using custom import/export logic. Imported files are read-only and cannot be edited in the inspector.
yellow_tinted_red.silly_mat_importable: Storing a SillyMaterialResource as a custom format, which is imported as a SillyMaterialResource using custom import/export logic.
- This file has the same exact contents as yellow.silly_mat_importable, but the corresponding .import file has a flag set to tint the albedo color towards red, which makes the material appear orange instead of yellow. This demonstrates how importers can use import settings to modify data during import. After import, imported files are read-only and cannot be edited in the inspector.
- If you try to load this file using "Load Imported Material (EditorImportPlugin)" in the editor, or call ResourceLoader.load() in GDScript, it will load the imported version, which includes the red tint, so the albedo color will be orange. If you try to import this file using "Import Material (directly at runtime)" in the editor, or call SillyMaterialResource.read_from_file() in GDScript, it will directly read the original file's contents, ignoring the import process, so the albedo color will be yellow. This demonstrates how files can be loaded within Godot's import process (editor only), or bypass the import process entirely (works in editor and at runtime).

Editor Buttons

The material creator dock has these 6 buttons:

"Apply Material": Applies the current material to all selected MeshInstance3D nodes in the editor.
"Save Material (ResourceFormatSaver)": Saves the current material to a .silly_mat_loadable file using the custom ResourceFormatSaver, or to a .tres file using Godot's built-in ResourceFormatSaverText. This can be edited in Godot's inspector and saved back.
"Export Material (directly at runtime)": Exports the current material to a .silly_mat_* file using the functions on SillyMaterialResource. This works for files outside of the res:// folder, and can be done at runtime.
"Load Material (ResourceFormatLoader)": Loads a .silly_mat_loadable file using the custom ResourceFormatLoader, or loads a .tres file using Godot's built-in ResourceFormatLoaderText. This can be edited in Godot's inspector and saved back.
"Load Imported Material (EditorImportPlugin)": Loads a .silly_mat_importable that was imported by an EditorImportPlugin. The loaded data actually comes from the corresponding imported file saved as res://.godot/imported/something.silly_mat_importable-hash.res. Imported files are read-only and cannot be edited in the inspector.
"Import Material (directly at runtime)": Imports a .silly_mat_* directly from the source file, performing an import on request instead of loading data the editor imported earlier. This ignores any editor import settings, works for files outside of the res:// folder, and can be done at runtime.