It’s possible to extend Framework with custom component templates. They’re developed using Vue 3 and TypeScript. Once transpiled, they can be used by copying them to the extensions/ folder of any project.
Custom components behave exactly like built-in ones. They are just as performant, can contain other components, and offer the same the Builder experience. They only differ from built-in components in the way that they’re bundled and imported.

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Architecture

Framework front-end compiles to a collection of static assets that is distributed in the Python package. These static assets are then served via FastAPI. During initialisation time, the server scans the extensions/ folder in the project folder and looks for .css and .js files. This folder is also served, similarly to static/. If it finds any valid files in extensions/, it shares the list with clients and tells them to dynamically import these files during runtime. Extensions and custom templates are currently synonyms, but this might change in order to accommodate other extension capabilities. Custom Components - Architecture Dependencies are provided using injection symbols and can be injected to be used by the component template. These include evaluatedFields, which contain the current values of the editable fields. Injected dependencies are fully typed, making development easier. Rollup’s external feature, invoked via Vite, allows for extensions to be compiled without dependencies and link those during runtime. Therefore, extensions aren’t bundled to be standalone, but rather to work as a piece of a puzzle. Custom Components - External

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Anatomy of a template

A template defines how a certain component is rendered. For example, corebutton defines how Button components are rendered. Framework component templates are purely front-end. They are Vue 3 templates that extend the Vue specification via a custom option, writer. This custom option defines all the Framework-specific behaviour of the component. For example, its fields property establishes which fields will be editable via the Builder.

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Simple example

This example shows a template for Bubble Message, a simple demo component with one editable field, text. 
<template>
	<div class="BubbleMessage">
        <div class="triangle"></div>
        <div class="message">

            <!-- Shows the current value of the field "text" -->

            {{ fields.text.value }}
        </div>
	</div>
</template>

<script lang="ts">
export default {
    writer: {
		name: "Bubble Message",
		description: "Shows a message in the shape of a speech bubble.",
		category: "Content",

        // Fields will be editable via Framework Builder

        fields: {
			text: {
				name: "Text",
				type: FieldType.Text,
			},
		},

        // Preview field is used in the Component Tree

		previewField: "text",
	},
};
</script>
<script setup lang="ts">
import { FieldType } from "@/writerTypes";
import injectionKeys from "../injectionKeys";
import { inject } from "vue";

/*
The values for the fields defined earlier in the custom option
will be available using the evaluatedFields injection symbol.
*/

const fields = inject(injectionKeys.evaluatedFields);
</script>

<style scoped>
/* ... */
</style>
The code above will make Bubble Message available in the Builder. Custom Components - Bubble Message

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Developing templates

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Run a local server

1

Clone the Framework Repository

To get started, clone the Framework repository from GitHub.
2

Set Up the Development Environment

To develop custom templates in a developer-friendly way, ensure you have a front-end development server with instant reload capabilities. The front-end code for Framework is located in the ui folder. With Node and npm installed on your system, run npm install to install dependencies. Then, start the server with support for custom component templates using npm run custom.dev.
cd ui npm install # links templates in
"custom_components/" # "dev" runs the server without them npm run custom.dev
3

Start the Back-End Server

The command npm run custom.dev starts a front-end server, which requires a back-end to function fully. Start Framework via command line, specifying the option --port 5000, to provide a back-end on that port. It’s recommended to create a new app for testing the template you’re developing. sh writer create customtester writer edit customtester --port 5000
4

Access Framework and Test Custom Component

You should now be able to access Framework via the URL provided by Vite, e.g. http://localhost:5174. In the Builder’s Toolkit, you should see the sample component, Balloon Message. Add it to your tester application.

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Create a new component

You can also have a look at the built-in component templates, since their syntax is equivalent. They can be found in the ui/src/components/core folder.
Go to ui/src/components/custom and open the Vue single-file components, i.e. the .vue files. These files contain comments that will help you get started. Try editing the provided templates, you should see changes reflected. You can get started by duplicating one of these examples. Make sure you add the new template to the entrypoint, as discussed below.

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Define entrypoint

For custom component templates to be taken into account, they need to be accessible from the entrypoint. Edit ui/src/components/custom/index.ts to define which templates you wish to export and under which identifiers. 
// Import the templates

import BubbleMessage from "./BubbleMessage.vue";
import BubbleMessageAdvanced from "./BubbleMessageAdvanced.vue";

// Export an object with the ids and the templates as default

export default {
  bubblemessage: BubbleMessage,
  bubblemessageadvanced: BubbleMessageAdvanced,
};
A single or multiple templates can be specified. Take into account that they will all be exported, and later imported, together.

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Bundling templates

Execute npm run custom.build into src/ui, this will generate the output .js and .css files into ./custom_components_dist. 
# "build" builds the entire front-end
# "custom.build" only builds the custom templates

npm run custom.check # Optional: checks certain issues on custom components
npm run custom.build
Collect the files from ./custom_components_dist and pack them in a folder such as my_custom_bubbles. The folder containing the generated files, e.g. my_custom_bubbles, can now be placed in the extensions/ folder of any Framework project. It’ll be automatically detected during server startup.
The custom.check command is optional, but it’s recommended to run it before building the custom components. It checks for common issues in the custom components, such as invalid key declaration, …