JavaScript如何处理移动端拍摄图片旋转问题

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/**
 * 处理图片文件（处理移动端拍摄图片旋转问题）
 * fileObj.file 图片文件独享
 * fileObj.resolution 在指定图片格式为 image/jpeg 或 image/webp的情况下，可以从 0 到 1 的区间内选择图片的质量。
 * fileObj.fileType 输入的文件类型，1 file对象，2 blob对象，3 base64字符串
 * fileObj.fileName 输出的文件名称，默认为picture.jpeg
 * fileObj.callback 回调函数
 */
function handleImageFile(fileObj) {
  // 给参数附初始值
  fileObj.fileName = fileObj.hasOwnProperty("fileName") ? "images/" + fileObj.fileName : "images/picture.jpeg";
  // 获取文件类型
  var fType = fileObj.file.type;
  if (fType.indexOf("image") === -1) return fileObj.callback({
    status: 500,
    message: "文件类型不正确",
    data: null
  });
  if (!EXIF) return fileObj.callback({
    status: 500,
    message: "EXIF 不存在",
    data: null
  });
  if (fileObj.file) {
    // 获取照片方向角属性，用户旋转控制
    EXIF.getData(fileObj.file, function () {
      var orientation = EXIF.getTag(this, 'Orientation');
      var oReader = new FileReader();
      oReader.onload = function (e) {
        var image = new Image();
        image.src = e.target.result;
        image.onload = function () {
          var canvas = document.createElement("canvas");
          var ctx = canvas.getContext("2d");
          var resultFile = null;
          var ua = navigator.userAgent;
          canvas.width = this.naturalWidth;
          canvas.height = this.naturalHeight;
          ctx.drawImage(this, 0, 0, this.naturalWidth, this.naturalHeight);
          // android终端
          var isAdr = ua.indexOf("Android") > -1 || ua.indexOf("Adr") > -1;
          // ios终端
          var isIOS = ua.indexOf("iPhone") > -1 || ua.indexOf("iOS") > -1;
          // 修复ios 或 Android
          if (isIOS || isAdr) {
            // 如果方向角不为1，都需要进行旋转
            if (orientation && orientation !== "" && orientation !== 1) {
              switch (orientation) {
                case 6: // 需要顺时针（向左）90度旋转
                  rotateImg(this, "left", canvas);
                  break;
                case 8: // 需要逆时针（向右）90度旋转
                  rotateImg(this, "right90", canvas);
                  break;
                case 3: // 需要180度旋转，转两次
                  rotateImg(this, "right180", canvas);
                  break;
              }
            }
            resultFile = canvas.toDataURL("image/jpeg", fileObj.resolution);
          } else {
            resultFile = canvas.toDataURL("image/jpeg", fileObj.resolution);
          }
          switch (fileObj.fileType) {
            case 1:
            case 2:
              fileObj.callback({
                status: 200,
                message: "success",
                data: dataURLtoFile(resultFile, fileObj.fileType, fileObj.fileName)
              });
              break;
            case 3:
              fileObj.callback({
                status: 200,
                message: "success",
                data: resultFile
              });
              break;
            default:
              break;
          }
        };
      };
      oReader.readAsDataURL(fileObj.file);
    });
  } else {
    return fileObj.callback({
      status: 500,
      message: "文件不存在",
      data: null
    });
  }
  /**
   * 旋转图片
   */
  function rotateImg(img, direction, canvas) {
    if (img === null) return;
    // 最小与最大旋转方向，图片旋转4次后回到原方向
    var minStep = 0;
    var maxStep = 3;
    // img的高度和宽度不能在img元素隐藏后获取，否则会出错
    var width = img.width;
    var height = img.height;
    var step = 2;
    if (step === null) step = minStep;
    if (direction === "right90") {
      step++;
      step > maxStep && (step = minStep);
    } else if(direction === "right180") {
      step = 2;
    } else {
      step--;
      step < minStep && (step = maxStep);
    }
    // 旋转角度以弧度值为参数
    var degree = step * 90 * Math.PI / 180;
    var ctx = canvas.getContext("2d");
    switch (step) {
      case 0:
        canvas.width = width;
        canvas.height = height;
        ctx.drawImage(img, 0, 0, width, height);
        break;
      case 1:
        canvas.width = height;
        canvas.height = width;
        ctx.rotate(degree);
        ctx.drawImage(img, 0, -height, width, height);
        break;
      case 2:
        canvas.width = width;
        canvas.height = height;
        ctx.rotate(degree);
        ctx.drawImage(img, -width, -height, width, height);
        break;
      case 3:
        canvas.width = height;
        canvas.height = width;
        ctx.rotate(degree);
        ctx.drawImage(img, -width, 0, width, height);
        break;
    }
  }
  /**
   * type：1 file对象，2 blob对象
   */
  function dataURLtoFile(dataurl, type, filename) {
    var arr = dataurl.split(',');
    var mime = arr[0].match(/:(.*?);/)[1];
    var bstr = atob(arr[1]);
    var n = bstr.length;
    var u8arr = new Uint8Array(n);
    while (n--) {
      u8arr[n] = bstr.charCodeAt(n);
    }
    if (type === 1) { // 转换成file对象
      return new File([u8arr], filename, {
        type: mime
      });
    } else { // 转换成成blob对象
      return new Blob([u8arr], {
        type: mime
      });
    }
  }
}