# 快速开始

> 几行代码把 Fabric.js 画布保存为 JSON 再加载回来。安装后连接你的画布，还能直接用上撤销重做和自动保存。

Source: https://fabricjs-document-engine.jscrate.dev/zh/docs/overview/quick-start
Last updated: 2026-09-28

这篇指南演示如何用 Fabric.js 保存 JSON，再把它加载回来。画布、工具栏和界面都还是你自己的，引擎只负责它们周围的文档工作。

## 安装

这个包支持 Fabric.js 6 和 7。用 npm 把它和 `fabric` 一起安装：

```bash
npm install fabricjs-document-engine fabric
```

## 保存和加载

把引擎连接到你已有的画布。`toDocument()` 会把整个画布返回为普通 JSON，`loadDocument()` 再把它放回去：

```ts
import { Canvas, Rect } from "fabric";
import { createDocumentEngine } from "fabricjs-document-engine";

const canvas = new Canvas("editor", { width: 800, height: 600 });
const engine = createDocumentEngine({ canvas });

canvas.add(new Rect({ width: 100, height: 80, fill: "tomato" }));

const document = engine.toDocument();
localStorage.setItem(document.id, JSON.stringify(document));

await engine.loadDocument(JSON.parse(localStorage.getItem(document.id)!));
```

在下面试一试。先加几个图形，点按钮用 Fabric.js 保存 JSON，再随便移动一下，最后点「Load」从 JSON 回显保存时的样子。这就是加载 JSON 的完整过程。

```tsx
"use client";

import type { FabricDocument } from "fabricjs-document-engine";
import { useDocumentEngine } from "fabricjs-document-engine/react";
import { CircleIcon, FolderOpenIcon, SaveIcon, SquareIcon } from "lucide-react";
import { useTranslations } from "next-intl";
import { useEffect, useState } from "react";

import {
  DemoCanvas,
  DemoFrame,
  newShape,
  seedShapes,
  Status,
  Toolbar,
  ToolButton,
} from "./demo-ui";
import { useFabricCanvas } from "./use-fabric-canvas";

export default function SaveLoadDemo() {
  const t = useTranslations("demos");
  const { elementRef, canvas } = useFabricCanvas();
  const engine = useDocumentEngine(canvas);
  const [saved, setSaved] = useState<FabricDocument>();

  useEffect(() => {
    if (!engine) return;
    engine.canvas.add(...seedShapes());
    engine.clearHistory();
  }, [engine]);

  function save() {
    // The whole canvas as plain JSON. Store it anywhere.
    setSaved(engine?.toDocument());
  }

  async function load() {
    if (saved) await engine?.loadDocument(saved);
  }

  return (
    <DemoFrame>
      <DemoCanvas elementRef={elementRef} label={t("canvas")} />

      <Toolbar>
        <ToolButton
          icon={SquareIcon}
          label={t("square")}
          onClick={() => canvas?.add(newShape("square"))}
        />
        <ToolButton
          icon={CircleIcon}
          label={t("circle")}
          onClick={() => canvas?.add(newShape("circle"))}
        />
        <span className="ml-auto flex gap-1.5">
          <ToolButton
            icon={SaveIcon}
            label={t("saveLoad.save")}
            primary
            disabled={!engine}
            onClick={save}
          />
          <ToolButton
            icon={FolderOpenIcon}
            label={t("saveLoad.load")}
            disabled={!saved}
            onClick={() => void load()}
          />
        </span>
      </Toolbar>

      <Status tone={saved ? "ok" : "idle"}>
        {saved
          ? t("saveLoad.saved", { count: saved.objects.length })
          : t("saveLoad.hint")}
      </Status>

      <pre className="h-32 overflow-auto rounded-lg border bg-muted p-3 font-mono text-xs">
        {saved ? JSON.stringify(saved, null, 2) : "{}"}
      </pre>
    </DemoFrame>
  );
}
```

```tsx
import { Canvas } from "fabric";
import { useLayoutEffect, useRef, useState } from "react";

/**
 * The page every demo draws on. The document keeps this size; CSS scales the
 * canvas down to fit its card, so saved documents and exports never depend on
 * the screen.
 */
export const PAGE = { width: 480, height: 300 };

// Selection handles in the site's style. Fabric does not save these.
const SELECTION = {
  borderColor: "#6f00ff",
  borderScaleFactor: 1.5,
  borderOpacityWhenMoving: 0.5,
  cornerStyle: "circle",
  cornerSize: 11,
  cornerColor: "#ffffff",
  cornerStrokeColor: "#6f00ff",
  transparentCorners: false,
} as const;

export function styleSelection(canvas: Canvas) {
  canvas.set({
    selectionColor: "rgba(111, 0, 255, 0.06)",
    selectionBorderColor: "#6f00ff",
    selectionLineWidth: 1,
  });
  const style = () => canvas.getActiveObject()?.set(SELECTION);
  canvas.on("selection:created", style);
  canvas.on("selection:updated", style);
}

/**
 * Creates a Fabric.js canvas once the `<canvas>` element exists, and disposes
 * it on unmount. The engine is created from the returned `canvas`.
 */
export function useFabricCanvas({ width, height } = PAGE) {
  const elementRef = useRef<HTMLCanvasElement>(null);
  const [canvas, setCanvas] = useState<Canvas | null>(null);

  // A layout effect creates the canvas before the first paint.
  useLayoutEffect(() => {
    const created = new Canvas(elementRef.current!, {
      width,
      height,
      backgroundColor: "#ffffff",
    });
    styleSelection(created);
    setCanvas(created);

    return () => {
      // Dispose after the engine's own cleanup, so its last recovery copy
      // still sees the objects.
      setTimeout(() => created.dispose().catch(() => undefined));
    };
  }, [width, height]);

  return { elementRef, canvas };
}
```

```tsx
import { Circle, type FabricObject, Rect, Triangle } from "fabric";
import type { LucideIcon } from "lucide-react";

import { cn } from "@/lib/utils";

import { PAGE } from "./use-fabric-canvas";

export const COLORS = ["#f2836f", "#5fe6c4", "#7ad9ec", "#6f00ff", "#ffd666"];

// Demo shapes are placed by their top-left corner, like a design tool.
const CORNER = { originX: "left", originY: "top" } as const;

/** A new shape at a random spot on the page. */
export function newShape(kind: "square" | "circle", fill?: string) {
  const left = 40 + Math.random() * (PAGE.width - 160);
  const top = 30 + Math.random() * (PAGE.height - 120);
  const color = fill ?? COLORS[Math.floor(Math.random() * COLORS.length)];
  return kind === "square"
    ? new Rect({
        ...CORNER,
        left,
        top,
        width: 88,
        height: 64,
        rx: 10,
        ry: 10,
        fill: color,
      })
    : new Circle({ ...CORNER, left, top, radius: 34, fill: color });
}

/** A small composed drawing, so no demo starts on an empty page. */
export function seedShapes(): FabricObject[] {
  return [
    new Rect({
      ...CORNER,
      left: 48,
      top: 56,
      width: 176,
      height: 124,
      rx: 16,
      ry: 16,
      fill: COLORS[4],
    }),
    new Circle({ ...CORNER, left: 268, top: 44, radius: 58, fill: COLORS[0] }),
    new Rect({
      ...CORNER,
      left: 256,
      top: 196,
      width: 176,
      height: 52,
      rx: 26,
      ry: 26,
      fill: COLORS[1],
    }),
    new Triangle({
      ...CORNER,
      left: 96,
      top: 196,
      width: 72,
      height: 60,
      fill: COLORS[3],
    }),
  ];
}

export function DemoFrame({
  className,
  ...props
}: React.ComponentProps<"div">) {
  return (
    <div
      className={cn(
        "flex w-full max-w-md flex-col gap-3 text-left outline-none",
        className
      )}
      {...props}
    />
  );
}

/**
 * Scales the fixed-size Fabric canvas to the frame's width. The rules are
 * `!important` because Fabric writes its sizes as inline styles.
 */
export function DemoCanvas({
  elementRef,
  label,
  page = PAGE,
  children,
}: {
  elementRef: React.Ref<HTMLCanvasElement>;
  label: string;
  page?: { width: number; height: number };
  children?: React.ReactNode;
}) {
  return (
    <div
      className="relative overflow-hidden rounded-lg border bg-white shadow-xs [&_.canvas-container]:aspect-(--page)! [&_.canvas-container]:h-auto! [&_.canvas-container]:w-full! [&_canvas]:h-full! [&_canvas]:w-full!"
      style={
        { "--page": `${page.width} / ${page.height}` } as React.CSSProperties
      }
    >
      <div>
        <canvas ref={elementRef} aria-label={label} />
      </div>
      {children}
    </div>
  );
}

export function Toolbar({ className, ...props }: React.ComponentProps<"div">) {
  return (
    <div
      className={cn("flex flex-wrap items-center gap-1.5", className)}
      {...props}
    />
  );
}

export function ToolButton({
  icon: Icon,
  label,
  iconOnly = false,
  primary = false,
  className,
  ...props
}: React.ComponentProps<"button"> & {
  icon: LucideIcon;
  label: string;
  iconOnly?: boolean;
  primary?: boolean;
}) {
  return (
    <button
      type="button"
      title={label}
      aria-label={iconOnly ? label : undefined}
      className={cn(
        "inline-flex h-8 min-w-0 items-center justify-center gap-1.5 rounded-md border px-2.5 text-[13px] font-medium whitespace-nowrap transition-colors disabled:pointer-events-none disabled:opacity-40 [&_svg]:size-3.5 [&_svg]:shrink-0",
        primary
          ? "border-transparent bg-foreground text-background hover:bg-foreground/85"
          : "bg-background hover:bg-muted",
        iconOnly && "w-8 px-0",
        className
      )}
      {...props}
    >
      <Icon aria-hidden />
      {!iconOnly && <span className="truncate">{label}</span>}
    </button>
  );
}

const TONES = {
  idle: "bg-zinc-400",
  busy: "bg-amber-400",
  ok: "bg-emerald-500",
  error: "bg-red-500",
};

/** One line of status text. Its height never changes, so the card never jumps. */
export function Status({
  tone = "idle",
  children,
}: {
  tone?: keyof typeof TONES;
  children: React.ReactNode;
}) {
  return (
    <p
      role="status"
      className="flex h-5 min-w-0 items-center gap-2 text-[13px] text-muted-foreground"
    >
      <span
        aria-hidden
        className={cn("size-1.5 shrink-0 rounded-full", TONES[tone])}
      />
      <span className="truncate">{children}</span>
    </p>
  );
}
```

保存的 JSON 里每个对象都有稳定的 `id`。移动、编组、保存、重新打开之后它都不会变，所以你可以把对象和自己的数据关联起来。

## 顺带得到的功能

创建引擎的同时也打开了撤销重做。每个用户操作，比如添加、移动或缩放图形，都是一步撤销：

```ts
await engine.undo();
await engine.redo();
```

## 保存到存储

在真实项目里，给引擎一个存储适配器。这样 `save()` 和 `load(id)` 就能用了，自动保存也会在用户编辑时帮你保存文档：

```ts
import { createDocumentEngine } from "fabricjs-document-engine";
import { createLocalStorage } from "fabricjs-document-engine/storage";

const engine = createDocumentEngine({
  canvas,
  storage: createLocalStorage({ prefix: "my-app:" }),
  autosave: true,
});

await engine.load("project-42");
await engine.save();
```

想把 Fabric.js 保存 JSON 的结果存进你自己的数据库，也是同样的代码。详见[存储适配器](https://fabricjs-document-engine.jscrate.dev/zh/docs/storage/storage-adapters)。

## 下一步

- [编辑器教程](https://fabricjs-document-engine.jscrate.dev/zh/docs/overview/tutorial)：一步步做一个小型画布编辑器。
- [保存和加载](https://fabricjs-document-engine.jscrate.dev/zh/docs/guides/save-and-load)：自定义属性、图片和加载规则。
- [撤销和重做](https://fabricjs-document-engine.jscrate.dev/zh/docs/guides/undo-redo)：事务和键盘快捷键。
- [React](https://fabricjs-document-engine.jscrate.dev/zh/docs/frameworks/react)：给工具栏用的 hooks。