在Web开发中，您可能需要旋转图像，这在CSS中很容易做到。像这样的简单代码变换：rotate(90deg);。但是如果我们想用 JS 来做呢？

TLDR

将图像绘制到浏览器环境中的画布上并旋转它。但在此之前，我们需要做一些数学运算来保持原始图像的长宽比。

核

假设我们已经加载了图像，计算旋转后的图像可以如下完成：

const { PI, sin, cos, abs } = Math;
const angle = (degree * PI) / 180;
const sinAngle = sin(angle);
const cosAngle = cos(angle);

const rotatedWidth = abs(imageWidth * cosAngle)   abs(imageHeight * sinAngle);

const rotatedHeight = abs(imageWidth * sinAngle)   abs(imageHeight * cosAngle);

接下来，我们使用一些画布 API 来进行实际的旋转：

const canvas = document.createElement('canvas');

const { width: canvasWidth, height: canvasHeight } = canvas;
const canvasCtx2D = canvas.getContext('2d');

canvasCtx2D.clearRect(0, 0, canvasWidth, canvasHeight);
canvasCtx2D.translate(canvasWidth / 2, canvasHeight / 2);
canvasCtx2D.rotate(angle);

canvasCtx2D.drawImage(
  image,
  -imageWidth / 2,
  -imageHeight / 2,
  imageWidth,
  imageHeight,
);

return canvas.toDataURL('image/png');

包起来

核心代码到位后，我们可以进行一些优化并编写专用的 React hook 来使用它：

import { useEffect, useRef, useState } from 'react';

type RotatedImage = {
  src: string;
  width: number;
  height: number;
} | null;

let canvas: HTMLCanvasElement | null = null;
let canvasCtx2D: CanvasRenderingContext2D | null = null;

const getRotatedImage = (
  image: HTMLImageElement | null,
  rotation: number,
): RotatedImage => {
  canvas ??= document.createElement('canvas');
  canvasCtx2D ??= canvas.getContext('2d');

  if (!image || !canvasCtx2D) return null;

  const { width: imageWidth, height: imageHeight, currentSrc } = image;
  const degree = rotation % 360;
  if (!degree) {
    return {
      src: currentSrc,
      width: imageWidth,
      height: imageHeight,
    };
  }

  const { PI, sin, cos, abs } = Math;
  const angle = (degree * PI) / 180;
  const sinAngle = sin(angle);
  const cosAngle = cos(angle);

  canvas.width = abs(imageWidth * cosAngle)   abs(imageHeight * sinAngle);
  canvas.height = abs(imageWidth * sinAngle)   abs(imageHeight * cosAngle);

  // The width and height of the canvas will be automatically rounded.
  const { width: canvasWidth, height: canvasHeight } = canvas;

  canvasCtx2D.clearRect(0, 0, canvasWidth, canvasHeight);
  canvasCtx2D.translate(canvasWidth / 2, canvasHeight / 2);
  canvasCtx2D.rotate(angle);

  canvasCtx2D.drawImage(
    image,
    -imageWidth / 2,
    -imageHeight / 2,
    imageWidth,
    imageHeight,
  );

  const src = canvas.toDataURL('image/png');
  canvas.width = 0;
  canvas.height = 0;

  return {
    src,
    width: canvasWidth,
    height: canvasHeight,
  };
};

const useRotateImage = (imageSrc: string, rotation?: number): RotatedImage => {
  const imageEle = useRef(null);
  const [rotatedImage, setRotatedImage] = useState(null);

  useEffect(() => {
    if (typeof rotation === 'number') {
      let currImage = imageEle.current;

      if (currImage?.currentSrc !== imageSrc) {
        currImage = new Image();
        imageEle.current = currImage;

        currImage.src = imageSrc;
      }

      currImage.decode().then(
        () => setRotatedImage(getRotatedImage(currImage, rotation)),
        () => setRotatedImage(null),
      );
    }
  }, [imageSrc, rotation]);

  return rotatedImage;
};

export default useRotateImage;

这里我复用了同一个canvas元素，以减少重复创建。其次，需要注意的是，我在每次旋转后将其宽度和高度设置为0，以减少内存使用。对了，我还做了清理画布的操作。这是因为在HTML规范中当你修改画布的宽度和高度时（无论是否与之前相同）都会清除画布，这与canvasCtx2D.clearRect(0, 0, canvasWidth, canvasHeight)相同，这现代浏览器支持。

在useRotateImage中，我保留对图像元素的引用，并在image.decode()之后设置旋转图像状态，该状态在图像数据准备好后解析。

以下是一个在线用例：

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