# 自动保存

> 给画布编辑器加上 Fabric.js 自动保存：追踪未保存的改动，显示保存状态，失败自动重试，旧保存不会覆盖新内容。

Source: https://fabricjs-document-engine.jscrate.dev/zh/docs/guides/autosave
Last updated: 2026-09-28

Fabric.js 自动保存需要两样东西：一个存储适配器和 `autosave` 配置项。之后引擎会追踪未保存的改动，并在用户编辑时保存。

```tsx
"use client";

import { isDocumentEngineError } from "fabricjs-document-engine";
import {
  useDocumentEngine,
  useDocumentState,
} from "fabricjs-document-engine/react";
import { createLocalStorage } from "fabricjs-document-engine/storage";
import { CircleIcon, SquareIcon, Trash2Icon } from "lucide-react";
import { useTranslations } from "next-intl";
import { useEffect } from "react";

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

const storage = createLocalStorage({ prefix: "docs-demo-v2:" });
const DOCUMENT_ID = "autosave-demo";
const TONE = {
  saved: "ok",
  saving: "busy",
  unsaved: "idle",
  error: "error",
} as const;

export default function AutosaveDemo() {
  const t = useTranslations("demos");
  const { elementRef, canvas } = useFabricCanvas();
  const engine = useDocumentEngine(canvas, {
    storage,
    autosave: { delay: 600 },
    document: { id: DOCUMENT_ID },
  });
  const state = useDocumentState(engine);

  // Open the saved drawing. On a first visit, start from a small one.
  useEffect(() => {
    engine?.load(DOCUMENT_ID).catch((error) => {
      if (
        !isDocumentEngineError(error) ||
        error.code !== "DOCUMENT_NOT_FOUND"
      ) {
        console.error(error);
        return;
      }
      engine.canvas.add(...seedShapes());
      engine.clearHistory();
    });
  }, [engine]);

  // "conflict" cannot happen with one tab, so it shares the error line.
  const status =
    state?.saveStatus === "conflict" ? "error" : (state?.saveStatus ?? "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"))}
        />
        <ToolButton
          icon={Trash2Icon}
          label={t("clear")}
          onClick={() => canvas?.remove(...canvas.getObjects())}
        />
      </Toolbar>

      <Status tone={TONE[status]}>
        {t(`autosave.${status}`, { revision: state?.revision ?? 0 })}
      </Status>
    </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>
  );
}
```

## 开启

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

const engine = createDocumentEngine({
  canvas,
  storage: createLocalStorage(),
  autosave: { delay: 1000, maxWait: 10000 },
});
```

- `delay`：没有编辑多少毫秒后保存。
- `maxWait`：第一次出现未保存改动后，最多等这么久就保存，即使用户还在编辑。

`autosave: true` 使用上面的默认值。

## 什么算未保存的改动

每一步记录下来的撤销步骤、每次撤销和重做，以及每次调用 `updateMetadata()`，都会把文档标记为已改动。`engine.isDirty()` 告诉你是否有未保存的内容。保存进行中的编辑，会等到下一次保存时才算保存。

你的代码用 `object.set()` 做的改动不会触发 Fabric.js 事件。用 `engine.transaction()` 包起来才会被记录，详见[撤销和重做](https://fabricjs-document-engine.jscrate.dev/zh/docs/guides/undo-redo)。

## 显示保存状态

`save:status` 事件带着完整的状态：

```ts
engine.on("save:status", ({ status, lastSavedAt, error }) => {
  statusLabel.textContent = status;
});
```

`status` 的值是 `saved`、`unsaved`、`saving`、`error` 或 `conflict` 之一。在 React 里，`useDocumentState(engine).saveStatus` 给你同样的值。按[无障碍指南](https://fabricjs-document-engine.jscrate.dev/zh/docs/guides/accessibility)的做法，把状态文字放在 `aria-live` 区域里。

## 同一时间只保存一次

保存进行中再调用 `save()`，会排队一次用最新内容的后续保存。响应不可能乱序到达，所以又慢又旧的保存永远不会覆盖新内容。如果保存期间用户打开了另一份文档，这次保存的响应会被忽略。

## 失败自动重试

临时性的失败，比如网络断开，会按指数退避重试：

```ts
createDocumentEngine({
  canvas,
  storage,
  autosave: true,
  saveRetry: { attempts: 3, baseDelay: 500, maxDelay: 8000 },
});

engine.on("save:retry", ({ attempt, delay }) => {
  console.info(`Retry ${attempt} in ${delay} ms`);
});
```

如果重试也没用，比如没有权限，就在适配器里抛出带 `retryable: false` 的错误。冲突永远不会重试。

## 离开前提醒

Fabric.js 自动保存会等用户停下来，所以用户可能在编辑后一秒就关掉标签页。先让浏览器弹出确认：

```ts
import { bindUnsavedChangesWarning } from "fabricjs-document-engine";

const unbind = bindUnsavedChangesWarning(engine);
```

崩溃之类仍会丢内容的情况，请加上[恢复](https://fabricjs-document-engine.jscrate.dev/zh/docs/guides/recovery)。