使用canvas进行图片压缩(前端图片压缩核心处理)

一、原理：实际上就是利用canvas进行重新绘制

1、先将图片的file文件转成baseURL
2、创建一个image标签去接收文件获取图片的宽高和比例。
3、创建canvas画布设置画布的大小。
4、将图片绘制到canvas上面。
5、对canvas进行压缩处理，获得新的baseURL
6、将baseURL转化回文件。

二、核心代码

/**
 * 压缩图片
 * @param file 压缩文件
 * @param compressThreshold 开启压缩的阈值，默认 5M
 * @param openCompress 是否开启压缩，默认 true
 * @param pictureQuality 压缩比，默认 0.92
 * @returns
 */
export const compressFile = (
  file: File,
  compressThreshold = 5,
  openCompress = true,
  pictureQuality = 0.92
) => {
  console.log('========== Entry Compress File Function ==========');
  console.log('before compress file: ', file);
  //判断是否是图片类型
  const isAccept = ['image/png', 'image/jpeg', 'image/jpg', 'image/gif'].includes(file.type);
  if (isAccept) {
    const fileSize = file.size / 1024 / 1024;
    console.log('before compress, the file size is : ', fileSize.toFixed(2) + ' M');
    //当开启图片压缩且图片大小大于等于压缩阈值,进行压缩
    if (fileSize >= compressThreshold && openCompress) {
      //判断浏览器内核是否支持base64图片压缩
      if (typeof FileReader === 'undefined') {
        return file;
      } else {
        try {
          return new Promise(resolve => {
            //声明FileReader文件读取对象
            const reader = new FileReader();
            reader.readAsDataURL(file);
            reader.onload = () => {
              // 生成canvas画布
              const canvas = document.createElement('canvas');
              // 生成img
              // const img = document.createElement('img');
              const img = new Image();
              img.src = reader.result as any;
              img.onload = () => {
                const ctx = canvas.getContext('2d');
                //原始图片宽度、高度
                const originImageWidth = img.width,
                  originImageHeight = img.height;
                //默认最大尺度的尺寸限制在（1920 * 1080）
                const maxWidth = 1920,
                  maxHeight = 1080,
                  ratio = maxWidth / maxHeight;
                //目标尺寸
                let targetWidth = originImageWidth,
                  targetHeight = originImageHeight;
                //当图片的宽度或者高度大于指定的最大宽度或者最大高度时,进行缩放图片
                if (originImageWidth > maxWidth || originImageHeight > maxHeight) {
                  //超过最大宽高比例
                  if (originImageWidth / originImageHeight > ratio) {
                    //宽度取最大宽度值maxWidth,缩放高度
                    targetWidth = maxWidth;
                    targetHeight = Math.round(maxWidth * (originImageHeight / originImageWidth));
                  } else {
                    //高度取最大高度值maxHeight,缩放宽度
                    targetHeight = maxHeight;
                    targetWidth = Math.round(maxHeight * (originImageWidth / originImageHeight));
                  }
                }
                // canvas对图片进行缩放
                canvas.width = targetWidth;
                canvas.height = targetHeight;
                // 清除画布
                ctx?.clearRect(0, 0, targetWidth, targetHeight);
                // 绘制图片
                ctx?.drawImage(img, 0, 0, targetWidth, targetHeight);
                // quality值越小,图像越模糊,默认图片质量为0.92
                const imageDataURL = canvas.toDataURL(file.type || 'image/jpeg', pictureQuality);
                // 去掉URL的头,并转换为byte
                const imageBytes = window.atob(imageDataURL.split(',')[1]);
                // 处理异常,将ascii码小于0的转换为大于0
                const arrayBuffer = new ArrayBuffer(imageBytes.length);
                const uint8Array = new Uint8Array(arrayBuffer);
                for (let i = 0; i < imageBytes.length; i++) {
                  uint8Array[i] = imageBytes.charCodeAt(i);
                }
                const mimeType = imageDataURL?.split(',')?.[0]?.match(/:(.*?);/)?.[1];
                const newFile = new File([uint8Array], file.name, {
                  type: mimeType || 'image/jpeg'
                });
                console.log(
                  'after compress, the file size is : ',
                  (newFile.size / 1024 / 1024).toFixed(2) + 'M'
                );
                console.log('after compress file: ', newFile);
                resolve(newFile);
              };
            };
            reader.onerror = () => {
              return file;
            };
          })
            .then(res => {
              return res;
            })
            .catch(() => {
              return file;
            });
        } catch (e) {
          //压缩出错,直接返回原file对象
          return file;
        }
      }
    } else {
      //不需要压缩，直接返回原file对象
      return file;
    }
  } else {
    //非图片文件,不进行压缩,直接返回原file对象
    return file;
  }
};