//第一個引數就是原來的字串，第二個是寬度，第三個就是回撥方法
export const  cutImageBase64=(base64, w, callback)=> {
  let newImage = new Image();
  let quality = 0.6;    //壓縮係數0-1之間
  newImage.src = base64;
  newImage.setAttribute("crossOrigin", 'Anonymous');  //url為外域時需要
  let imgWidth, imgHeight;
  newImage.onload = function () {
      imgWidth = this.width;
      imgHeight = this.height;
      let canvas = document.createElement("canvas");
      let ctx = canvas.getContext("2d");
      if (Math.max(imgWidth, imgHeight) > w) {
          if (imgWidth > imgHeight) {
              canvas.width = w;
              canvas.height = w * imgHeight / imgWidth;
          } else {
              canvas.height = w;
              canvas.width = w * imgWidth / imgHeight;
          }
      } else {
          canvas.width = imgWidth;
          canvas.height = imgHeight;
          quality = 0.6;
      }
      ctx.clearRect(0, 0, canvas.width, canvas.height);
      ctx.drawImage(this, 0, 0, canvas.width, canvas.height);
      let base64 = canvas.toDataURL("image/jpeg", quality); //壓縮語句
      // 如想確保圖片壓縮到自己想要的尺寸,如要求在50-150kb之間，請加以下語句，quality初始值根據情況自定
      while (base64.length / 1024 > 150) {
          quality -= 0.01;
          base64 = canvas.toDataURL("image/jpeg", quality);
      }
      // 防止最後一次壓縮低於最低尺寸，只要quality遞減合理，無需考慮
      while (base64.length / 1024 < 50) {
          quality += 0.001;
          base64 = canvas.toDataURL("image/jpeg", quality);
      }
      callback(base64);//必須通過回撥函式返回，否則無法及時拿到該值
  }
}