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Draggable DOM with Lit

基于Lit的可拖拽DOM

In this article I will go over how to set up a Lit web component and use it to create a interactive dom with CSS transforms and slots.
TLDR The final source here and an online demo.
在本文中，我将介绍如何搭建一个Lit Web组件，并使用它结合CSS变换和插槽创建可交互的DOM。
快速概览 最终源码可在这里获取，在线演示见这里。

Prerequisites 

前置要求

  • Vscode
  • Node >= 16
  • Typescript
  • Vscode
  • Node >= 16
  • Typescript

Getting Started 

开始上手

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"),
      },
    },
  },
});
我们可以先在终端导航到项目目录，然后运行以下命令：
npm init @vitejs/app --template lit-ts
然后输入项目名称
lit-draggable-dom
，现在在Vscode中打开项目并安装依赖：
cd lit-draggable-dom
npm i lit
npm i -D @types/node
code .
更新
vite.config.ts
为以下内容：
import { defineConfig } from "vite";
import { resolve } from "path";

export default defineConfig({
  base: "/lit-draggable-dom/",
  build: {
    rollupOptions: {
      input: {
        main: resolve(__dirname, "index.html"),
      },
    },
  },
});

Template 

模板

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 
--dx
 and 
--dy
 as this is just the initial positions on the canvas.
打开
index.html
并更新为以下内容：
<!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>
我们正在设置
lit-element
使其包含几个插槽，这些插槽可以是任何有效的HTML或SVG元素。
可以选择设置CSS自定义属性
--dx
和
--dy
，这只是元素在画布上的初始位置。

Web Component 

Web组件

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
 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 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 it can be multi touch) and start tracking the offset.
The delta is calculated to move the elements but a global offset is used for the canvas background.
We are setting the pointer capture events so if the mouse is not perfectly on the item it won't lose tracking.
Now let's add methods for moving the canvas and elements by appending the following to the class:
moveCanvas(delta: Offset) {
    this.offset.x += delta.x;
    this.offset.y += delta.y;
    this.root.style.setProperty("--offset-x", `${this.offset.x}px`);
    this.root.style.setProperty("--offset-y", `${this.offset.y}px`);
}

moveElement(child: SupportedNode, delta: Offset) {
    const getNumber = (key: "--dx" | "--dy", fallback: number) => {
      const saved = child.style.getPropertyValue(key);
      if (saved.length > 0) {
        return parseFloat(saved.replace("px", ""));
      }
      return fallback;
    };
    const dx = getNumber("--dx", 0) + delta.x;
    const dy = getNumber("--dy", 0) + delta.y;
    child.style.transform = `translate(${dx}px, ${dy}px)`;
    child.style.setProperty("--dx", `${dx}px`);
    child.style.setProperty("--dy", `${dy}px`);
}
For the canvas it will set a global offset for the CSS 
background-position
 and update the saved offset.
For the element we want to transform by the delta so the animation is smooth thanks to hardware acceleration. After the transform it will store the offset as CSS custom properties.
Now let's add the event handlers to the canvas and elements by adjusting the following:
async firstUpdated() {
    const items = Array.from(this.childNodes);
    let i = 0;
    for (const node of items) {
      if (node instanceof SVGElement || node instanceof HTMLElement) {
        const child = node as SupportedNode;
        child.classList.add("child");
        child.style.setProperty("--layer", `${i}`);
        this.container.append(child);
        child.addEventListener("pointerdown", (e: any) => {
          this.handleDown(e, "element");
        });
        child.addEventListener("pointermove", (e: any) => {
          this.handleMove(e, "element", (delta) => {
            this.moveElement(child, delta);
          });
        });
        i++;
      }
    }
    this.requestUpdate();
    this.root.addEventListener("pointerdown", (e: any) => {
      this.handleDown(e, "canvas");
    });
    this.root.addEventListener("pointermove", (e: any) => {
      this.handleMove(e, "canvas", (delta) => {
        this.moveCanvas(delta);
        for (const node of Array.from(this.container.children)) {
          if (node instanceof SVGElement || node instanceof HTMLElement) {
            this.moveElement(node, delta);
          }
        }
      });
    });
    this.root.addEventListener("pointerup", (e: any) => {
      this.handleUp(e);
    });
}
Everything should work as expected now and the final code should look like 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 DraggableDOM 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() {
    console.log("render");
    return html`
      <main class="full-size">
        <canvas class="full-size"></canvas>
        <div id="children" class="full-size"></div>
      </main>
    `;
  }

  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);
  }

  moveCanvas(delta: Offset) {
    this.offset.x += delta.x;
    this.offset.y += delta.y;
    this.root.style.setProperty("--offset-x", `${this.offset.x}px`);
    this.root.style.setProperty("--offset-y", `${this.offset.y}px`);
  }

  moveElement(child: SupportedNode, delta: Offset) {
    const getNumber = (key: "--dx" | "--dy", fallback: number) => {
      const saved = child.style.getPropertyValue(key);
      if (saved.length > 0) {
        return parseFloat(saved.replace("px", ""));
      }
      return fallback;
    };
    const dx = getNumber("--dx", 0) + delta.x;
    const dy = getNumber("--dy", 0) + delta.y;
    child.style.transform = `translate(${dx}px, ${dy}px)`;
    child.style.setProperty("--dx", `${dx}px`);
    child.style.setProperty("--dy", `${dy}px`);
  }

  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) => {
          this.handleDown(e, "element");
        });
        child.addEventListener("pointermove", (e: any) => {
          this.handleMove(e, "element", (delta) => {
            this.moveElement(child, delta);
          });
        });
        child.setAttribute("draggable", "false");
        i++;
      }
    }
    this.requestUpdate();
    this.root.addEventListener("pointerdown", (e: any) => {
      this.handleDown(e, "canvas");
    });
    this.root.addEventListener("pointermove", (e: any) => {
      this.handleMove(e, "canvas", (delta) => {
        this.moveCanvas(delta);
        for (const node of Array.from(this.container.children)) {
          if (node instanceof SVGElement || node instanceof HTMLElement) {
            this.moveElement(node, delta);
          }
        }
      });
    });
    this.root.addEventListener("touchstart", function (e) {
      e.preventDefault();
    });
    this.root.addEventListener("pointerup", (e: any) => {
      this.handleUp(e);
    });
  }
}

interface Offset {
  x: number;
  y: number;
}

interface PointerData {
  id: number;
  startPos: Offset;
  currentPos: Offset;
}
在更新组件之前，我们需要将
my-element.ts
重命名为
draggable-dom.ts
打开
draggable-dom.ts
并更新为以下内容：
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;
}
这里我们只是设置一些基础代码，渲染一个包含
canvas
元素作为背景的
main
元素，以及一个用于容纳画布元素的
div
元素。
我们还确保只裁剪并渲染可见的内容。
Offset
和
PointerData
接口将用于存储与屏幕交互的每个指针的位置。
当用户系统启用暗色模式时，画布网格的颜色会随之改变。
现在让我们通过向类中添加以下代码，将插槽子元素添加到画布中：
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
    });
}
插槽的顺序决定了元素的层级关系。对于插槽中的每个元素，我们会设置
--layer
和
z-index
为当前索引值。
目前当我们与元素交互时还没有任何响应，但元素应该已经可以正常渲染了。
现在让我们通过向类中添加以下代码，为指针事件添加事件处理程序：
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);
}
对于每个事件，我们需要检查当前事件是针对
canvas
还是
element
，这样当我们开始移动元素时，就不会同时移动画布，反之亦然。
当指针与屏幕交互时，我们会将其添加到指针映射中（因为支持多点触控）并开始跟踪偏移量。
通过计算增量来移动元素，而画布背景则使用全局偏移量。
我们设置了指针捕获事件，这样即使鼠标没有完全停留在元素上，也不会失去跟踪。
现在让我们通过向类中添加以下代码，添加移动画布和元素的方法：
moveCanvas(delta: Offset) {
    this.offset.x += delta.x;
    this.offset.y += delta.y;
    this.root.style.setProperty("--offset-x", `${this.offset.x}px`);
    this.root.style.setProperty("--offset-y", `${this.offset.y}px`);
}

moveElement(child: SupportedNode, delta: Offset) {
    const getNumber = (key: "--dx" | "--dy", fallback: number) => {
      const saved = child.style.getPropertyValue(key);
      if (saved.length > 0) {
        return parseFloat(saved.replace("px", ""));
      }
      return fallback;
    };
    const dx = getNumber("--dx", 0) + delta.x;
    const dy = getNumber("--dy", 0) + delta.y;
    child.style.transform = `translate(${dx}px, ${dy}px)`;
    child.style.setProperty("--dx", `${dx}px`);
    child.style.setProperty("--dy", `${dy}px`);
}
对于画布，我们会为CSS
background-position
设置全局偏移量，并更新保存的偏移值。
对于元素，我们希望通过增量进行变换，借助硬件加速让动画更流畅。变换完成后，我们会将偏移量存储为CSS自定义属性。
现在让我们调整以下代码，为画布和元素添加事件处理程序：
async firstUpdated() {
    const items = Array.from(this.childNodes);
    let i = 0;
    for (const node of items) {
      if (node instanceof SVGElement || node instanceof HTMLElement) {
        const child = node as SupportedNode;
        child.classList.add("child");
        child.style.setProperty("--layer", `${i}`);
        this.container.append(child);
        child.addEventListener("pointerdown", (e: any) => {
          this.handleDown(e, "element");
        });
        child.addEventListener("pointermove", (e: any) => {
          this.handleMove(e, "element", (delta) => {
            this.moveElement(child, delta);
          });
        });
        i++;
      }
    }
    this.requestUpdate();
    this.root.addEventListener("pointerdown", (e: any) => {
      this.handleDown(e, "canvas");
    });
    this.root.addEventListener("pointermove", (e: any) => {
      this.handleMove(e, "canvas", (delta) => {
        this.moveCanvas(delta);
        for (const node of Array.from(this.container.children)) {
          if (node instanceof SVGElement || node instanceof HTMLElement) {
            this.moveElement(node, delta);
          }
        }
      });
    });
    this.root.addEventListener("pointerup", (e: any) => {
      this.handleUp(e);
    });
}
现在所有功能都应该正常工作了，最终代码如下所示：
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 DraggableDOM 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() {
    console.log("render");
    return html`
      <main class="full-size">
        <canvas class="full-size"></canvas>
        <div id="children" class="full-size"></div>
      </main>
    `;
  }

  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);
  }

  moveCanvas(delta: Offset) {
    this.offset.x += delta.x;
    this.offset.y += delta.y;
    this.root.style.setProperty("--offset-x", `${this.offset.x}px`);
    this.root.style.setProperty("--offset-y", `${this.offset.y}px`);
  }

  moveElement(child: SupportedNode, delta: Offset) {
    const getNumber = (key: "--dx" | "--dy", fallback: number) => {
      const saved = child.style.getPropertyValue(key);
      if (saved.length > 0) {
        return parseFloat(saved.replace("px", ""));
      }
      return fallback;
    };
    const dx = getNumber("--dx", 0) + delta.x;
    const dy = getNumber("--dy", 0) + delta.y;
    child.style.transform = `translate(${dx}px, ${dy}px)`;
    child.style.setProperty("--dx", `${dx}px`);
    child.style.setProperty("--dy", `${dy}px`);
  }

  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) => {
          this.handleDown(e, "element");
        });
        child.addEventListener("pointermove", (e: any) => {
          this.handleMove(e, "element", (delta) => {
            this.moveElement(child, delta);
          });
        });
        child.setAttribute("draggable", "false");
        i++;
      }
    }
    this.requestUpdate();
    this.root.addEventListener("pointerdown", (e: any) => {
      this.handleDown(e, "canvas");
    });
    this.root.addEventListener("pointermove", (e: any) => {
      this.handleMove(e, "canvas", (delta) => {
        this.moveCanvas(delta);
        for (const node of Array.from(this.container.children)) {
          if (node instanceof SVGElement || node instanceof HTMLElement) {
            this.moveElement(node, delta);
          }
        }
      });
    });
    this.root.addEventListener("touchstart", function (e) {
      e.preventDefault();
    });
    this.root.addEventListener("pointerup", (e: any) => {
      this.handleUp(e);
    });
  }
}

interface Offset {
  x: number;
  y: number;
}

interface PointerData {
  id: number;
  startPos: Offset;
  currentPos: Offset;
}

Conclusion 

总结

If you want to learn more about building with Lit you can read the docs here. There is also an example on the Lit playground here.
The source for this example can be found here.
如果你想了解更多关于使用Lit进行开发的内容，可以查看这里的文档。Lit playground上也有一个示例，地址是这里.
本示例的源码可在这里找到。