很多网页会加上水印，用于版权标识、防止盗用等。

比如这样：

ant design 和 arco design 都提供了 Watermark 水印组件。

这种水印是咋实现的呢？

调试下就知道了：

arco design 的：

ant desigin 的：

首先，有一个 div 覆盖在需要加水印的区域，宽高 100%，绝对定位，设置 pointer-events:none 也就是不响应鼠标事件。

然后 background 设置 repeat，用 background image 平铺。

那这个 image 是什么呢？

点击这个 data url：

是个包含文字的图片。

而我们并没有传入图片作为参数：

所以说要用 canvas 画出来，做一些旋转，并导出 base64 的图片，作为这个 div 的背景就好了。

当然，也可以传入图片作为水印：

同样是用 canvas 画出来。

那怎么画呢？

根据传入的参数来画：

上面是 antd 的 Watermark 组件的参数。

可以传入宽高、旋转角度、字体样式、水印间距、水印偏移等。

虽然参数很多，但只是一些细节。

arco design 的 Watermark 组件画出的图片是上面的样子，所以 repeat 之后是这样的：

如果仔细看你会发现 ant design 的 Watermark 组件是这样的：

交错排列的。

这是因为它 canvas 画的内容就是交错的 2 个：

整体思路是很清晰的：用 canvas 把文字或者图片画出来，导出 base64 的 data url 设置为 div 的重复背景，这个 div 整个覆盖在需要加水印的元素上，设置 pointer-events 是 none。

此外，上节还讲过通过 MutationObserver 监听 dom 的修改，改了之后重新添加水印。

antd 就是这么做的：

思路理清了，我们来写下代码：

npx create-vite

去掉 index.css 和 StrictMode：

然后写下 Watermark/index.tsx

import { useRef, PropsWithChildren, CSSProperties, FC } from "react";

export interface WatermarkProps extends PropsWithChildren {
    style?: CSSProperties;
    className?: string;
    zIndex?: string | number;
    width?: number;
    height?: number;
    rotate?: number;
    image?: string;
    content?: string | string[];
    fontStyle?: {
        color?: string;
        fontFamily?: string;
        fontSize?: number | string;
        fontWeight?: number | string;
    };
    gap?: [number, number];
    offset?: [number, number];
    getContainer?: () => HTMLElement;
}

const Watermark: FC<WatermarkProps> = (props) => {
    const {
        className,
        style,
        zIndex,
        width,
        height,
        rotate,
        image,
        content,
        fontStyle,
        gap,
        offset,
    } = props;

    const containerRef = useRef<HTMLDivElement>(null);

    return props.children ? (
        <div className={className} style={style} ref={containerRef}>
            {props.children}
        </div>
    ) : null;
};

export default Watermark;

style、className 就不用解释了。

width、height、rotate、offset、gap 等都是水印的参数：

gap 是两个水印之间的空白距离。

offset 是水印相对于 container 容器的偏移量，也就是左上角的空白距离。

然后我们封装个 useWatermark 的自定义 hook 来绘制水印：

import {
    useRef,
    PropsWithChildren,
    CSSProperties,
    FC,
    useCallback,
    useEffect,
} from "react";
import useWatermark from "./useWatermark";

export interface WatermarkProps extends PropsWithChildren {
    style?: CSSProperties;
    className?: string;
    zIndex?: string | number;
    width?: number;
    height?: number;
    rotate?: number;
    image?: string;
    content?: string | string[];
    fontStyle?: {
        color?: string;
        fontFamily?: string;
        fontSize?: number | string;
        fontWeight?: number | string;
    };
    gap?: [number, number];
    offset?: [number, number];
    getContainer?: () => HTMLElement;
}

const Watermark: FC<WatermarkProps> = (props) => {
    const {
        className,
        style,
        zIndex,
        width,
        height,
        rotate,
        image,
        content,
        fontStyle,
        gap,
        offset,
    } = props;

    const containerRef = useRef<HTMLDivElement>(null);

    const getContainer = useCallback(() => {
        return props.getContainer
            ? props.getContainer()
            : containerRef.current!;
    }, [containerRef.current, props.getContainer]);

    const { generateWatermark } = useWatermark({
        zIndex,
        width,
        height,
        rotate,
        image,
        content,
        fontStyle,
        gap,
        offset,
        getContainer,
    });

    useEffect(() => {
        generateWatermark({
            zIndex,
            width,
            height,
            rotate,
            image,
            content,
            fontStyle,
            gap,
            offset,
            getContainer,
        });
    }, [
        zIndex,
        width,
        height,
        rotate,
        image,
        content,
        JSON.stringify(props.fontStyle),
        JSON.stringify(props.gap),
        JSON.stringify(props.offset),
        getContainer,
    ]);

    return props.children ? (
        <div className={className} style={style} ref={containerRef}>
            {props.children}
        </div>
    ) : null;
};

export default Watermark;

getContainer 默认用 containerRef.current，或者传入的 props.getContainer。

调用 useWatermark，返回 generateWatermark 方法。

然后当参数变化的时候，重新调用 generateWatermark 绘制水印。

getContainer 我们加了 useCallback 避免每次都变，对象参数（fontSize）、数组参数（gap、offset）用 JSON.stringify 序列化后再放到 deps 数组里：

然后来实现 useWatermark 的 hook。

新建 Watermark/useWatermark.ts

import { useEffect, useState } from "react";
import { WatermarkProps } from ".";
import { merge } from "lodash-es";

export type WatermarkOptions = Omit<
    WatermarkProps,
    "className" | "style" | "children"
>;

export default function useWatermark(params: WatermarkOptions) {
    const [options, setOptions] = useState(params || {});

    function drawWatermark() {}

    useEffect(() => {
        drawWatermark();
    }, [options]);

    return {
        generateWatermark: (newOptions: Partial<WatermarkOptions>) => {
            setOptions(merge({}, options, newOptions));
        },
        destroy: () => {},
    };
}

参数就是 WatermarkProps 去了 style、className、children。

把传入的参数保存到 options 的 state，根据它渲染。

调用返回的 generateWatermark 的时候设置 options 触发重绘。

这里用到了 lodash-es 包的 merge 来合并参数。

安装下：

npm install --save lodash-es

npm i --save-dev @types/lodash-es

然后来处理下 options，和默认 options 做下合并：

这里的 toNumber 会把第一个参数转为 number，如果不是数字的话就返回第二个参数的默认值：

具体的合并逻辑是这样的：

先合并传入的 options

然后如果没有传入的会用默认值。

fontStyle 是默认 fontStyle 和传入的 fontStyle 的合并

width 的默认值，如果是图片就用默认 width，否则 undefined，因为后面文字宽度是动态算的。

offset 的默认值是 0。

因为处理完之后肯定是有值的，所以断言为 Required<WatermarkOptions> 类型。

这个 Required 是去掉可选用的，相对的，Partial 是给属性添加可选修饰。

合并完之后，就拿到绘制的 options 了。

import { useEffect, useRef, useState } from "react";
import { WatermarkProps } from ".";
import { merge } from "lodash-es";

export type WatermarkOptions = Omit<
    WatermarkProps,
    "className" | "style" | "children"
>;

export function isNumber(obj: any): obj is number {
    return (
        Object.prototype.toString.call(obj) === "[object Number]" && obj === obj
    );
}

const toNumber = (value?: string | number, defaultValue?: number) => {
    if (value === undefined) {
        return defaultValue;
    }
    if (isNumber(value)) {
        return value;
    }
    const numberVal = parseFloat(value);
    return isNumber(numberVal) ? numberVal : defaultValue;
};

const defaultOptions = {
    rotate: -20,
    zIndex: 1,
    width: 100,
    gap: [100, 100],
    fontStyle: {
        fontSize: "16px",
        color: "rgba(0, 0, 0, 0.15)",
        fontFamily: "sans-serif",
        fontWeight: "normal",
    },
    getContainer: () => document.body,
};

const getMergedOptions = (o: Partial<WatermarkOptions>) => {
    const options = o || {};

    const mergedOptions = {
        ...options,
        rotate: options.rotate || defaultOptions.rotate,
        zIndex: options.zIndex || defaultOptions.zIndex,
        fontStyle: { ...defaultOptions.fontStyle, ...options.fontStyle },
        width: toNumber(
            options.width,
            options.image ? defaultOptions.width : undefined
        ),
        height: toNumber(options.height, undefined)!,
        getContainer: options.getContainer!,
        gap: [
            toNumber(options.gap?.[0], defaultOptions.gap[0]),
            toNumber(
                options.gap?.[1] || options.gap?.[0],
                defaultOptions.gap[1]
            ),
        ],
    } as Required<WatermarkOptions>;

    const mergedOffsetX = toNumber(mergedOptions.offset?.[0], 0)!;
    const mergedOffsetY = toNumber(
        mergedOptions.offset?.[1] || mergedOptions.offset?.[0],
        0
    )!;
    mergedOptions.offset = [mergedOffsetX, mergedOffsetY];

    return mergedOptions;
};

export default function useWatermark(params: WatermarkOptions) {
    const [options, setOptions] = useState(params || {});

    const mergedOptions = getMergedOptions(options);

    function drawWatermark() {}

    useEffect(() => {
        drawWatermark();
    }, [options]);

    return {
        generateWatermark: (newOptions: Partial<WatermarkOptions>) => {
            setOptions(merge({}, options, newOptions));
        },
        destroy: () => {},
    };
}

有了 options，接下来创建 dom，开始绘制：

用 useRef 保存水印元素的 dom。

调用 getCanvasData 方法来绘制，返回 base64Url、width、height 这些信息。

生成水印的 dom 元素，挂载到 container 下，设置 style。

注意 background-size 是 gap + width、gap + height 算出的。

接下来只要实现 getCanvasData 方法，用 cavas 画出水印就好了。

import { useEffect, useRef, useState } from "react";
import { WatermarkProps } from ".";
import { merge } from "lodash-es";

export type WatermarkOptions = Omit<
    WatermarkProps,
    "className" | "style" | "children"
>;

export function isNumber(obj: any): obj is number {
    return (
        Object.prototype.toString.call(obj) === "[object Number]" && obj === obj
    );
}

const toNumber = (value?: string | number, defaultValue?: number) => {
    if (!value) {
        return defaultValue;
    }
    if (isNumber(value)) {
        return value;
    }
    const numberVal = parseFloat(value);
    return isNumber(numberVal) ? numberVal : defaultValue;
};

const defaultOptions = {
    rotate: -20,
    zIndex: 1,
    width: 100,
    gap: [100, 100],
    fontStyle: {
        fontSize: "16px",
        color: "rgba(0, 0, 0, 0.15)",
        fontFamily: "sans-serif",
        fontWeight: "normal",
    },
    getContainer: () => document.body,
};

const getMergedOptions = (o: Partial<WatermarkOptions>) => {
    const options = o || {};

    const mergedOptions = {
        ...options,
        rotate: options.rotate || defaultOptions.rotate,
        zIndex: options.zIndex || defaultOptions.zIndex,
        fontStyle: { ...defaultOptions.fontStyle, ...options.fontStyle },
        width: toNumber(
            options.width,
            options.image ? defaultOptions.width : undefined
        ),
        height: toNumber(options.height, undefined)!,
        getContainer: options.getContainer!,
        gap: [
            toNumber(options.gap?.[0], defaultOptions.gap[0]),
            toNumber(
                options.gap?.[1] || options.gap?.[0],
                defaultOptions.gap[1]
            ),
        ],
    } as Required<WatermarkOptions>;

    const mergedOffsetX = toNumber(mergedOptions.offset?.[0], 0)!;
    const mergedOffsetY = toNumber(
        mergedOptions.offset?.[1] || mergedOptions.offset?.[0],
        0
    )!;
    mergedOptions.offset = [mergedOffsetX, mergedOffsetY];

    return mergedOptions;
};

const getCanvasData = async (
    options: Required<WatermarkOptions>
): Promise<{ width: number; height: number; base64Url: string }> => {};

export default function useWatermark(params: WatermarkOptions) {
    const [options, setOptions] = useState(params || {});

    const mergedOptions = getMergedOptions(options);
    const watermarkDiv = useRef<HTMLDivElement>();

    const container = mergedOptions.getContainer();
    const { zIndex, gap } = mergedOptions;

    function drawWatermark() {
        if (!container) {
            return;
        }

        getCanvasData(mergedOptions).then(({ base64Url, width, height }) => {
            const wmStyle = `
      width:100%;
      height:100%;
      position:absolute;
      top:0;
      left:0;
      bottom:0;
      right:0;
      pointer-events: none;
      z-index:${zIndex};
      background-position: 0 0;
      background-size:${gap[0] + width}px ${gap[1] + height}px;
      background-repeat: repeat;
      background-image:url(${base64Url})`;

            if (!watermarkDiv.current) {
                const div = document.createElement("div");
                watermarkDiv.current = div;
                container.append(div);
                container.style.position = "relative";
            }

            watermarkDiv.current?.setAttribute("style", wmStyle.trim());
        });
    }

    useEffect(() => {
        drawWatermark();
    }, [options]);

    return {
        generateWatermark: (newOptions: Partial<WatermarkOptions>) => {
            setOptions(merge({}, options, newOptions));
        },
        destroy: () => {},
    };
}

接下来实现 getCanvasData 方法。

创建个 canvas 元素，拿到画图用的 context。

封装 drawText、drawImage 两个方法，优先绘制 image。

然后封装个 configCanvas 方法，用来设置 canvas 的宽高、rotate、scale：

宽高同样是 gap + width、gap + height。

用 tanslate 移动中心点到宽高的一半的位置再 schale、rotate。

因为不同屏幕的设备像素比不一样，也就是 1px 对应的物理像素不一样，所以要在单位后面乘以 devicePixelRatio。

我们设置了 scale 放大 devicePixelRatio 倍，这样接下来绘制尺寸就不用乘以设备像素比了。

const getCanvasData = async (
    options: Required<WatermarkOptions>
): Promise<{ width: number; height: number; base64Url: string }> => {
    const { rotate, image, content, fontStyle, gap } = options;

    const canvas = document.createElement("canvas");
    const ctx = canvas.getContext("2d")!;

    const ratio = window.devicePixelRatio;

    const configCanvas = (size: { width: number; height: number }) => {
        const canvasWidth = gap[0] + size.width;
        const canvasHeight = gap[1] + size.height;

        canvas.setAttribute("width", `${canvasWidth * ratio}px`);
        canvas.setAttribute("height", `${canvasHeight * ratio}px`);
        canvas.style.width = `${canvasWidth}px`;
        canvas.style.height = `${canvasHeight}px`;

        ctx.translate((canvasWidth * ratio) / 2, (canvasHeight * ratio) / 2);
        ctx.scale(ratio, ratio);

        const RotateAngle = (rotate * Math.PI) / 180;
        ctx.rotate(RotateAngle);
    };

    const drawText = () => {};

    function drawImage() {}

    return image ? drawImage() : drawText();
};

先来实现 drawImage：

function drawImage() {
    return new Promise<{ width: number; height: number; base64Url: string }>(
        (resolve) => {
            const img = new Image();
            img.crossOrigin = "anonymous";
            img.referrerPolicy = "no-referrer";

            img.src = image;
            img.onload = () => {
                let { width, height } = options;
                if (!width || !height) {
                    if (width) {
                        height = (img.height / img.width) * +width;
                    } else {
                        width = (img.width / img.height) * +height;
                    }
                }
                configCanvas({ width, height });

                ctx.drawImage(img, -width / 2, -height / 2, width, height);
                return resolve({
                    base64Url: canvas.toDataURL(),
                    width,
                    height,
                });
            };
            img.onerror = () => {
                return drawText();
            };
        }
    );
}

new Image 指定 src 加载图片。

onload 的时候，对于没有设置 width 或 height 的时候，根据图片宽高比算出另一个值。

然后调用 configCanvas 修改 canvas 的宽高、缩放、旋转。

之后在中心点绘制一张图片，返回 base64 的结果。

当加载失败时，onerror 里绘制文本。

这里的 crssOrign 设置 anonymous 是跨域的时候不携带 cookie，而 refererPolicy 设置 no-referrer 是不携带 referer，都是安全相关的。

然后实现 drawText：

const drawText = () => {
    const { fontSize, color, fontWeight, fontFamily } = fontStyle;
    const realFontSize = toNumber(fontSize, 0) || fontStyle.fontSize;

    ctx.font = `${fontWeight} ${realFontSize}px ${fontFamily}`;
    const measureSize = measureTextSize(ctx, [...content], rotate);

    const width = options.width || measureSize.width;
    const height = options.height || measureSize.height;

    configCanvas({ width, height });

    ctx.fillStyle = color!;
    ctx.font = `${fontWeight} ${realFontSize}px ${fontFamily}`;
    ctx.textBaseline = "top";

    [...content].forEach((item, index) => {
        const { height: lineHeight, width: lineWidth } =
            measureSize.lineSize[index];

        const xStartPoint = -lineWidth / 2;
        const yStartPoint =
            -(options.height || measureSize.originHeight) / 2 +
            lineHeight * index;

        ctx.fillText(
            item,
            xStartPoint,
            yStartPoint,
            options.width || measureSize.originWidth
        );
    });
    return Promise.resolve({ base64Url: canvas.toDataURL(), height, width });
};

fontSize 转为 number。

如果没有传入 width、height 就自己计算，这个 measureTextSize 待会实现。

设置 textBaseline 为 top，顶部对齐。

然后依次绘制文字。

绘制文字要按照坐标来，在 measureTextSize 里计算出每一行的 lineSize，也就是行高、行宽。

在行宽的一半的地方开始绘制文字，行内每个文字的位置是行高的一半 * index。

然后实现 measureTextSize 方法：

const measureTextSize = (
    ctx: CanvasRenderingContext2D,
    content: string[],
    rotate: number
) => {
    let width = 0;
    let height = 0;
    const lineSize: Array<{ width: number; height: number }> = [];

    content.forEach((item) => {
        const {
            width: textWidth,
            fontBoundingBoxAscent,
            fontBoundingBoxDescent,
        } = ctx.measureText(item);

        const textHeight = fontBoundingBoxAscent + fontBoundingBoxDescent;

        if (textWidth > width) {
            width = textWidth;
        }

        height += textHeight;
        lineSize.push({ height: textHeight, width: textWidth });
    });

    const angle = (rotate * Math.PI) / 180;

    return {
        originWidth: width,
        originHeight: height,
        width: Math.ceil(
            Math.abs(Math.sin(angle) * height) +
                Math.abs(Math.cos(angle) * width)
        ),
        height: Math.ceil(
            Math.abs(Math.sin(angle) * width) +
                Math.abs(height * Math.cos(angle))
        ),
        lineSize,
    };
};

ctx.measureText 是用来测量文字尺寸的。

fontBoudingAscent 是 baseline 到顶部的距离，而 fontBoundingBoxDescent 是到底部的距离：

加起来就是行高。

然后如果有旋转的话，要用 sin、cos 函数算出旋转后的宽高。

这样经过计算和绘制，文字和图片的水印就都完成了。

我们测试下：

改下 App.tsx

import Watermark from "./Watermark";

const App = () => {
    return (
        <Watermark content={["测试水印", "神说要有光"]}>
            <div style={{ height: 800 }}>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
            </div>
        </Watermark>
    );
};

export default App;

把 gap 设为 0：

import Watermark from "./Watermark";

const App = () => {
    return (
        <Watermark
            content={["测试水印", "神说要有光"]}
            gap={[0, 0]}
            fontStyle={{
                color: "green",
            }}>
            <div style={{ height: 800 }}>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
            </div>
        </Watermark>
    );
};

export default App;

也没问题：

只是现在 offset 还没有支持，也就是左上角的空白距离。

这个就是改下 left、top 的值就好了，当然，width、height 也要从 100% 减去这块距离。

const offsetLeft = mergedOptions.offset[0] + "px";
const offsetTop = mergedOptions.offset[1] + "px";

const wmStyle = `
width:calc(100% - ${offsetLeft});
height:calc(100% - ${offsetTop});
position:absolute;
top:${offsetTop};
left:${offsetLeft};
bottom:0;
right:0;
pointer-events: none;
z-index:${zIndex};
background-position: 0 0;
background-size:${gap[0] + width}px ${gap[1] + height}px;
background-repeat: repeat;
background-image:url(${base64Url})`;

测试下：

import Watermark from "./Watermark";

const App = () => {
    return (
        <Watermark
            content={["测试水印", "神说要有光"]}
            gap={[0, 0]}
            offset={[50, 100]}
            fontStyle={{
                color: "green",
            }}>
            <div style={{ height: 800 }}>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
                <p>
                    Lorem ipsum dolor, sit amet consectetur adipisicing elit.
                    Quos quod deserunt quidem quas in rem ipsam ut nesciunt
                    asperiores dignissimos recusandae minus, eaque, harum
                    exercitationem esse sapiente? Eveniet, id provident!
                </p>
            </div>
        </Watermark>
    );
};

export default App;

这样水印组件就完成了。

但现在的水印组件作用并不大，因为只要打开 devtools 就能轻易删掉。

我们要加上防删功能，前面讲过，用 MutationObserver：

创建完水印节点后，首先 disnonnect 去掉之前的 MutationObserver 的监听，然后创建新的 MutationObserver 监听 container 的变动。

export default function useWatermark(params: WatermarkOptions) {
    const [options, setOptions] = useState(params || {});

    const mergedOptions = getMergedOptions(options);
    const watermarkDiv = useRef<HTMLDivElement>();
    const mutationObserver = useRef<MutationObserver>();

    const container = mergedOptions.getContainer();
    const { zIndex, gap } = mergedOptions;

    function drawWatermark() {
        if (!container) {
            return;
        }

        getCanvasData(mergedOptions).then(({ base64Url, width, height }) => {
            const offsetLeft = mergedOptions.offset[0] + "px";
            const offsetTop = mergedOptions.offset[1] + "px";

            const wmStyle = `
      width:calc(100% - ${offsetLeft});
      height:calc(100% - ${offsetTop});
      position:absolute;
      top:${offsetTop};
      left:${offsetLeft};
      bottom:0;
      right:0;
      pointer-events: none;
      z-index:${zIndex};
      background-position: 0 0;
      background-size:${gap[0] + width}px ${gap[1] + height}px;
      background-repeat: repeat;
      background-image:url(${base64Url})`;

            if (!watermarkDiv.current) {
                const div = document.createElement("div");
                watermarkDiv.current = div;
                container.append(div);
                container.style.position = "relative";
            }

            watermarkDiv.current?.setAttribute("style", wmStyle.trim());

            if (container) {
                mutationObserver.current?.disconnect();

                mutationObserver.current = new MutationObserver((mutations) => {
                    const isChanged = mutations.some((mutation) => {
                        let flag = false;
                        if (mutation.removedNodes.length) {
                            flag = Array.from(mutation.removedNodes).some(
                                (node) => node === watermarkDiv.current
                            );
                        }
                        if (
                            mutation.type === "attributes" &&
                            mutation.target === watermarkDiv.current
                        ) {
                            flag = true;
                        }
                        return flag;
                    });
                    if (isChanged) {
                        watermarkDiv.current = undefined;
                        drawWatermark();
                    }
                });

                mutationObserver.current.observe(container, {
                    attributes: true,
                    subtree: true,
                    childList: true,
                });
            }
        });
    }

    useEffect(() => {
        drawWatermark();
    }, [options]);

    return {
        generateWatermark: (newOptions: Partial<WatermarkOptions>) => {
            setOptions(merge({}, options, newOptions));
        },
        destroy: () => {},
    };
}