因為最近遇到個移動端上傳頭像的需求，上傳到後臺的資料是base64位，其中為了提高使用者體驗，把比較大的圖片用canvas進行壓縮之後再進行上傳。在移動端呼叫拍照功能時，會發生圖片旋轉，為了解決這個問題引入了exif去判斷拍照時的資訊再去處理圖片，這是個很好的外掛。關於exif.js可以去他的GitHub上了解，這邊直接npm install exif-js --save   安裝，然後import一下就可以使用了。以下就是原始碼，可以直接使用。

<template>
  <div>
    <div style="padding:20px;">
      <div class="show">
        <div class="picture" :style="'backgroundImage:url('+headerImage+')'"></div>
      </div>
      <div style="margin-top:20px;">
        <input type="file" id="upload" accept="image" @change="upload">
        <label for="upload"></label>
      </div>
    </div>
  </div>
</template>

<script>
import Exif from 'exif-js'

export default {
  data () {
    return {
      headerImage:'',picValue:''
    }
  },
  mounted () {
  },
  methods: {
    upload (e) {
      let files = e.target.files || e.dataTransfer.files;
      if (!files.length) return;
      this.picValue = files[0];
      this.imgPreview(this.picValue);
    },
    imgPreview (file) {
      let self = this;
      let Orientation;
      //去獲取拍照時的資訊，解決拍出來的照片旋轉問題
      Exif.getData(file, function(){
          Orientation = Exif.getTag(this, 'Orientation');
      });
      // 看支援不支援FileReader
      if (!file || !window.FileReader) return;

      if (/^image/.test(file.type)) {
          // 建立一個reader
          let reader = new FileReader();
          // 將圖片2將轉成 base64 格式
          reader.readAsDataURL(file);
          // 讀取成功後的回撥
          reader.onloadend = function () {
            let result = this.result;
            let img = new Image();
            img.src = result;
            //判斷圖片是否大於100K,是就直接上傳，反之壓縮圖片
            if (this.result.length <= (100 * 1024)) {
              self.headerImage = this.result;
              self.postImg();
            }else {
              img.onload = function () {
                let data = self.compress(img,Orientation);
                self.headerImage = data;
                self.postImg();
              }
            }
          } 
        }
      },
      postImg () {
        //這裡寫介面
      },
      rotateImg (img, direction,canvas) {
        //最小與最大旋轉方向，圖片旋轉4次後回到原方向    
        const min_step = 0;    
        const max_step = 3;      
        if (img == null)return;    
        //img的高度和寬度不能在img元素隱藏後獲取，否則會出錯    
        let height = img.height;    
        let width = img.width;      
        let step = 2;    
        if (step == null) {    
            step = min_step;    
        }    
        if (direction == 'right') {    
            step++;    
            //旋轉到原位置，即超過最大值    
            step > max_step && (step = min_step);    
        } else {    
            step--;    
            step < min_step && (step = max_step);    
        }     
        //旋轉角度以弧度值為引數    
        let degree = step * 90 * Math.PI / 180;    
        let ctx = canvas.getContext('2d');    
        switch (step) {    
          case 0:    
              canvas.width = width;    
              canvas.height = height;    
              ctx.drawImage(img, 0, 0);    
              break;    
          case 1:    
              canvas.width = height;    
              canvas.height = width;    
              ctx.rotate(degree);    
              ctx.drawImage(img, 0, -height);    
              break;    
          case 2:    
              canvas.width = width;    
              canvas.height = height;    
              ctx.rotate(degree);    
              ctx.drawImage(img, -width, -height);    
              break;    
          case 3:    
              canvas.width = height;    
              canvas.height = width;    
              ctx.rotate(degree);    
              ctx.drawImage(img, -width, 0);    
              break;
        }    
    },
    compress(img,Orientation) {
      let canvas = document.createElement("canvas");
      let ctx = canvas.getContext('2d');
        //瓦片canvas
      let tCanvas = document.createElement("canvas");
      let tctx = tCanvas.getContext("2d");
      let initSize = img.src.length;
      let width = img.width;
      let height = img.height;
      //如果圖片大於四百萬畫素，計算壓縮比並將大小壓至400萬以下
      let ratio;
      if ((ratio = width * height / 4000000) > 1) {
        console.log("大於400萬畫素")
        ratio = Math.sqrt(ratio);
        width /= ratio;
        height /= ratio;
      } else {
        ratio = 1;
      }
      canvas.width = width;
      canvas.height = height;
  //        鋪底色
      ctx.fillStyle = "#fff";
      ctx.fillRect(0, 0, canvas.width, canvas.height);
      //如果圖片畫素大於100萬則使用瓦片繪製
      let count;
      if ((count = width * height / 1000000) > 1) {
        console.log("超過100W畫素");
        count = ~~(Math.sqrt(count) + 1); //計算要分成多少塊瓦片
  //            計算每塊瓦片的寬和高
        let nw = ~~(width / count);
        let nh = ~~(height / count);
        tCanvas.width = nw;
        tCanvas.height = nh;
        for (let i = 0; i < count; i++) {
          for (let j = 0; j < count; j++) {
            tctx.drawImage(img, i * nw * ratio, j * nh * ratio, nw * ratio, nh * ratio, 0, 0, nw, nh);
            ctx.drawImage(tCanvas, i * nw, j * nh, nw, nh);
          }
        }
      } else {
        ctx.drawImage(img, 0, 0, width, height);
      }
      //修復ios上傳圖片的時候 被旋轉的問題
      if(Orientation != "" && Orientation != 1){
        switch(Orientation){
          case 6://需要順時針（向左）90度旋轉
              this.rotateImg(img,'left',canvas);
              break;
          case 8://需要逆時針（向右）90度旋轉
              this.rotateImg(img,'right',canvas);
              break;
          case 3://需要180度旋轉
              this.rotateImg(img,'right',canvas);//轉兩次
              this.rotateImg(img,'right',canvas);
              break;
        }
      }
      //進行最小壓縮
      let ndata = canvas.toDataURL('image/jpeg', 0.1);
      console.log('壓縮前：' + initSize);
      console.log('壓縮後：' + ndata.length);
      console.log('壓縮率：' + ~~(100 * (initSize - ndata.length) / initSize) + "%");
      tCanvas.width = tCanvas.height = canvas.width = canvas.height = 0;
      return ndata;
    },
  }
}
</script>

<style>
*{
  margin: 0;
  padding: 0;
}
.show {
  width: 100px;
  height: 100px;
  overflow: hidden;
  position: relative;
  border-radius: 50%;
  border: 1px solid #d5d5d5;
}
.picture {
  width: 100%;
  height: 100%;
  overflow: hidden;
  background-position: center center;
  background-repeat: no-repeat;
  background-size: cover; 
}
</style>