draggable-dom-with-lit
Learn how to create an interactive, draggable DOM using a Lit web component with CSS transforms and slots, enabling you to manipulate HTML and SVG elements within a canvas-like environment.
What this skill does
# Draggable DOM with Lit
In this article I will go over how to set up a [Lit](https://lit.dev/) web component and use it to create a interactive dom with CSS transforms and slots.
> **TLDR** The final source [here](https://github.com/rodydavis/lit-draggable-dom) and an online [demo](https://rodydavis.github.io/lit-draggable-dom/).
## Prerequisites
* Vscode
* Node >= 16
* Typescript
## Getting Started
We can start off by navigating in terminal to the location of the project and run the following:
```
npm init @vitejs/app --template lit-ts
```
Then enter a project name `lit-draggable-dom` and now open the project in vscode and install the dependencies:
```
cd lit-draggable-dom
npm i lit
npm i -D @types/node
code .
```
Update the `vite.config.ts` with the following:
```
import { defineConfig } from "vite";
import { resolve } from "path";
export default defineConfig({
base: "/lit-draggable-dom/",
build: {
rollupOptions: {
input: {
main: resolve(__dirname, "index.html"),
},
},
},
});
```
## Template
Open up the `index.html` and update it with the following:
```
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<link rel="icon" type="image/svg+xml" href="/src/favicon.svg" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Lit Draggable DOM</title>
<style>
body {
margin: 0;
padding: 0;
width: 100%;
height: 100vh;
}
</style>
<script type="module" src="/src/draggable-dom.ts"></script>
</head>
<body>
<draggable-dom>
<img
src="https://lit.dev/images/logo.svg"
alt="Lit Logo"
width="500"
height="333"
style="--dx: 59.4909px; --dy: 32.8429px"
/>
<svg width="400" height="110" style="--dx: 230.057px; --dy: 33.6257px">
<rect
width="400"
height="100"
style="fill: rgb(0, 0, 255); stroke-width: 3; stroke: rgb(0, 0, 0)"
/>
</svg>
<svg height="100" width="100">
<circle
cx="50"
cy="50"
r="40"
stroke="black"
stroke-width="3"
fill="red"
/>
</svg>
</draggable-dom>
</body>
</html>
```
We are setting up the `lit-element` to have a few slots which can be any valid HTML or SVG Elements.
It is optional to set the [css custom properties](https://developer.mozilla.org/en-US/docs/Web/CSS/--*) `--dx` and `--dy` as this is just the initial positions on the canvas.
## Web Component
Before we update our component we need to rename `my-element.ts` to `draggable-dom.ts`
Open up `draggable-dom.ts` and update it with the following:
```
import { html, css, LitElement } from "lit";
import { customElement, query } from "lit/decorators.js";
type DragType = "none" | "canvas" | "element";
type SupportedNode = HTMLElement | SVGElement;
@customElement("draggable-dom")
export class CSSCanvas extends LitElement {
@query("main") root!: HTMLElement;
@query("#children") container!: HTMLElement;
@query("canvas") canvas!: HTMLCanvasElement;
dragType: DragType = "none";
offset: Offset = { x: 0, y: 0 };
pointerMap: Map<number, PointerData> = new Map();
static styles = css`
:host {
--offset-x: 0;
--offset-y: 0;
--grid-background-color: white;
--grid-color: black;
--grid-size: 40px;
--grid-dot-size: 1px;
}
main {
overflow: hidden;
}
canvas {
background-size: var(--grid-size) var(--grid-size);
background-image: radial-gradient(
circle,
var(--grid-color) var(--grid-dot-size),
var(--grid-background-color) var(--grid-dot-size)
);
background-position: var(--offset-x) var(--offset-y);
z-index: 0;
}
.full-size {
width: 100%;
height: 100%;
position: fixed;
}
.child {
--dx: 0px;
--dy: 0px;
position: fixed;
flex-shrink: 1;
z-index: var(--layer, 0);
transform: translate(var(--dx), var(--dy));
}
@media (prefers-color-scheme: dark) {
main {
--grid-background-color: black;
--grid-color: grey;
}
}
`;
render() {
return html`
<main class="full-size">
<canvas class="full-size"></canvas>
<div id="children" class="full-size"></div>
</main>
`;
}
}
interface Offset {
x: number;
y: number;
}
interface PointerData {
id: number;
startPos: Offset;
currentPos: Offset;
}
```
Here we are just setting up some boilerplate to render a `main` element with a `canvas` element as a background and the `div` element to contain the canvas elements.
We are also making sure to clip and only render what is visible.
The `Offset` and `PointerData` interfaces will be used for storing the location of each pointer interacting with the screen.
When the user has dark mode enabled for the system it will change the colors of the canvas grid.
Now let's add the slot children to the canvas by adding the following to the class:
```
async firstUpdated() {
const items = Array.from(this.childNodes);
let i = 0;
for (const node of items) {
if (node instanceof SVGElement || node instanceof HTMLElement) {
const child = node as SupportedNode;
child.classList.add("child");
child.style.setProperty("--layer", `${i}`);
this.container.append(child);
child.addEventListener("pointerdown", (e: any) => {
// Pointer Down for Child
});
child.addEventListener("pointermove", (e: any) => {
// Pointer Move for Child
});
i++;
}
}
this.requestUpdate();
this.root.addEventListener("pointerdown", (e: any) => {
// Pointer Down for Canvas
});
this.root.addEventListener("pointermove", (e: any) => {
// Pointer Move for Canvas
});
this.root.addEventListener("pointerup", (e: any) => {
// Pointer Up for Canvas
});
}
```
The order of the slots defines what renders on top of each other. For each item in the slot it sets`--layer` and [`z-index`](https://developer.mozilla.org/en-US/docs/Web/CSS/z-index) to the current index.
Currently nothing is happening when we interact with the elements but things should be rendering.
Now let's add the event handlers for the [pointer events](https://developer.mozilla.org/en-US/docs/Web/API/Pointer_events) by appending the following to the class:
```
handleDown(event: PointerEvent, type: DragType) {
if (this.dragType === "none") {
event.preventDefault();
this.dragType = type;
(event.target as Element).setPointerCapture(event.pointerId);
this.pointerMap.set(event.pointerId, {
id: event.pointerId,
startPos: { x: event.clientX, y: event.clientY },
currentPos: { x: event.clientX, y: event.clientY },
});
}
}
handleMove(
event: PointerEvent,
type: DragType,
onMove: (delta: Offset) => void
) {
if (this.dragType === type) {
event.preventDefault();
const saved = this.pointerMap.get(event.pointerId)!;
const current = { ...saved.currentPos };
saved.currentPos = { x: event.clientX, y: event.clientY };
const delta = {
x: saved.currentPos.x - current.x,
y: saved.currentPos.y - current.y,
};
onMove(delta);
}
}
handleUp(event: PointerEvent) {
this.dragType = "none";
(event.target as Element).releasePointerCapture(event.pointerId);
}
```
For each event we want to check if the current event `canvas` or `element` so if we start moving an element it doesn't move the canvas and vice versa.
When we have a pointer interact with the screen we will add it to the pointer map (since itRelated in Design
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