vue实现压缩图片预览并上传功能（promise封装）

本文实例为大家分享了vue实现压缩图片预览并上传的具体代码，供大家参考，具体内容如下

主要用到filereader、canvas 以及 formdata 这三个h5的api

过程大致分为三步：

用户使用input file上传图片的时候，用filereader读取用户上传的图片数据（base64格式）
把图片数据传入img对象，然后将img绘制到canvas上，再调用canvas.toDataURL对图片进行压缩
获取到压缩后的base64格式图片数据，转成二进制塞入formdata，再通过XmlHttpRequest提交formdata。

模板：

<template>
 <div class="image-box">
  <input type="file" accept="image/*" @change="imageHandle">
  <img ref="upImg"/>
 </div>
</template>

获取图片数据

methods: {
   //监听input file的change事件
  imageHandle(e) {
   //**这个是必不可少的，在下面的reader.onload中this就不再指vm了**
   let that = this;
   let maxSize = 100 * 1024;
   let files = e.srcElement.files;
   if (!files.length) return; //文件长度大于0
   if (!/^image\//.test(files[0].type)) return; //必须是图片才处理
   if (!window.FileReader) return; //支持FileReader
   //创建filereader对象
   let reader = new FileReader();
   reader.readAsDataURL(files[0]); //将图片转成base64格式
   reader.onload = function() {
    let result = this.result;
    let img = new Image();
    img.src = result;
    let formdata = new FormData();
    if (this.result.length <= maxSize) {
     that.$refs.upImg.src = result; //预览图片
     img = null;
     //上传图片
     formdata.append("image", that._upload(result, files[0].name, files[0].type));
     that.$store.dispatch("uploadImage", formdata)
        .then(data => {
          if (data === 1) {
          that.$toast("上传成功", "success");
          } else if (data === -1) {
          that.$toast("图片为空", "error");
          } else {
          that.$toast("上传失败", "error");
          }
        })
        .catch(error => that.$toast("上传失败", "error"));
    } else {
     img.onload = function() {
      //压缩图片
      let data = that._compress(img);
      //图片预览
      that.$refs.upImg.src = data;
      //上传图片
      formdata.append("image", that._upload(data, files[0].name, files[0].type));
      that.$store.dispatch("uploadImage", formdata)
          .then(data => {
            if (data === 1) {
            that.$toast("上传成功", "success");
            } else if (data === -1) {
            that.$toast("图片为空", "error");
            } else {
            that.$toast("上传失败", "error");
            }
          })
          .catch(error => that.$toast("上传失败", "error"));
     };
    }
   };
  },

压缩图片

在IOS中，canvas绘制图片是有两个限制的：

首先是图片的大小，如果图片的大小超过两百万像素，图片也是无法绘制到canvas上的，调用drawImage的时候不会报错，但是你用toDataURL获取图片数据的时候获取到的是空的图片数据。

再者就是canvas的大小有限制，如果canvas的大小大于大概五百万像素（即宽高乘积）的时候，不仅图片画不出来，其他什么东西也都是画不出来的。

应对第一种限制，处理办法就是瓦片绘制了。瓦片绘制，也就是将图片分割成多块绘制到canvas上，我代码里的做法是把图片分割成100万像素一块的大小，再绘制到canvas上。

而应对第二种限制，我的处理办法是对图片的宽高进行适当压缩，我代码里为了保险起见，设的上限是四百万像素，如果图片大于四百万像素就压缩到小于四百万像素。四百万像素的图片应该够了，算起来宽高都有2000X2000了。

如此一来就解决了IOS上的两种限制了。

除了上面所述的限制，还有两个坑，一个就是canvas的toDataURL是只能压缩jpg的，当用户上传的图片是png的话，就需要转成jpg，也就是统一用canvas.toDataURL(‘image/jpeg', 0.1) ， 类型统一设成jpeg，而压缩比就自己控制了。

另一个就是如果是png转jpg，绘制到canvas上的时候，canvas存在透明区域的话，当转成jpg的时候透明区域会变成黑色，因为canvas的透明像素默认为rgba(0,0,0,0)，所以转成jpg就变成rgba(0,0,0,1)了，也就是透明背景会变成了黑色。解决办法就是绘制之前在canvas上铺一层白色的底色。

_compress(img) {
   let canvas = document.createElement("canvas");
   let ctx = canvas.getContext("2d");
   //瓦片
   let tCanvas = document.createElement("canvas");
   let tctx = tCanvas.getContext("2d");
   let initSize = img.src.length;
   let width = img.width;
   let height = img.height;
   //如果图片大于四百万像素，计算压缩比并将大小压至400万以下
   let ratio;
   if ((ratio = (width * height) / 4000000) > 1) {
    ratio = Math.sqrt(ratio);
    widht /= ratio;
    height /= ratio;
   } else {
    ratio = 1;
   }
   canvas.width = width;
   canvas.height = height;
   //铺底色
   ctx.fillStyle = "#fff";
   ctx.fillRect(0, 0, canvas.width, canvas.height);
   //如果图片像素大于100万则使用瓦片绘制
   let count;
   if ((count = (width * height) / 1000000) > 1) {
    count = ~~(Math.sqrt(count) + 1); //计算要分成多少瓦片，~~在这里表示取整
    //计算每块瓦片的宽高
    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, canvas.width, canvas.height);
   }
   //进行压缩
   let ndata = canvas.toDataURL("image/jpeg", 0.3);
   tCanvas.width = tCanvas.height = canvas.width = canvas.height = 0;
   return ndata;
  },

上传

完成图片压缩后，就可以塞进formdata里进行上传了，先将base64数据转成字符串，再实例化一个ArrayBuffer，然后将字符串以8位整型的格式传入ArrayBuffer，再通过BlobBuilder或者Blob对象，将8位整型的ArrayBuffer转成二进制对象blob，再将blob转为File对象

_upload(data, name, type) {
   let text = window.atob(data.split(",")[1]);
   let buffer = new ArrayBuffer(text.length);
   let ubuffer = new Uint8Array(buffer);
   let pecent = 0,
    loop = null;

   for (var i = 0; i < text.length; i++) {
    ubuffer[i] = text.charCodeAt(i);
   }

   let Builder =
    window.BlobBuilder ||
    window.WebKitBlobBuilder ||
    window.MozBlobBuilder ||
    window.MSBlobBuilder;
   let blob;
   if (Builder) {
    var builder = new Builder();
    builder.append(buffer);
    blob = builder.getBlob(type);
   } else {
    blob = new window.Blob([ubuffer], { type: type });
   }
   // blob 转file
   var fileOfBlob = new File([blob], name, { type: type });
   return fileOfBlob;
  }
 }

将图片压缩上传封装到一个js文件里

const UploadImg = {
  imageHandle(files, maxSize, imgDom) {
    let that = this;
    let formdata = new FormData();
    let reader = new FileReader();
    reader.readAsDataURL(files[0]); //将图片转成base64格式
    //reader.onload是异步，要用到Promise对象将值返回出去
    return new Promise((resolved, rejected) => {
      reader.onload = function () {
        let result = this.result;
        let img = new Image();
        img.src = result;
        if (this.result.length <= maxSize) {
          imgDom.src = result;
          img = null;
          formdata.append("image", that._upload(result, files[0].name, files[0].type));
          resolved(formdata);
        } else {
          img.onload = function () {
            let data = that._compress(img);
            imgDom.src = data;
            formdata.append("image", that._upload(data, files[0].name, files[0].type));
            resolved(formdata);
          };
        }
      };
    })

  },
  _compress(img) {
    let canvas = document.createElement("canvas");
    let ctx = canvas.getContext("2d");
    //瓦片
    let tCanvas = document.createElement("canvas");
    let tctx = tCanvas.getContext("2d");
    let width = img.width;
    let height = img.height;
    //如果图片大于四百万像素，计算压缩比并将大小压至400万以下
    let ratio;
    if ((ratio = (width * height) / 4000000) > 1) {
      ratio = Math.sqrt(ratio);
      widht /= ratio;
      height /= ratio;
    } else {
      ratio = 1;
    }
    canvas.width = width;
    canvas.height = height;
    //铺底色
    ctx.fillStyle = "#fff";
    ctx.fillRect(0, 0, canvas.width, canvas.height);
    //如果图片像素大于100万则使用瓦片绘制
    let count;
    if ((count = (width * height) / 1000000) > 1) {
      count = ~~(Math.sqrt(count) + 1); //计算要分成多少瓦片
      //计算每块瓦片的宽高
      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, canvas.width, canvas.height);
    }
    //进行最小压缩
    let ndata = canvas.toDataURL("image/jpeg", 0.3);
    tCanvas.width = tCanvas.height = canvas.width = canvas.height = 0;
    return ndata;
  },
  _upload(data, name, type) {
    let text = window.atob(data.split(",")[1]);
    let buffer = new ArrayBuffer(text.length);
    let ubuffer = new Uint8Array(buffer);

    for (var i = 0; i < text.length; i++) {
      ubuffer[i] = text.charCodeAt(i);
    }

    let Builder =
      window.BlobBuilder ||
      window.WebKitBlobBuilder ||
      window.MozBlobBuilder ||
      window.MSBlobBuilder;
    let blob;
    if (Builder) {
      var builder = new Builder();
      builder.append(buffer);
      blob = builder.getBlob(type);
    } else {
      blob = new window.Blob([ubuffer], { type: type });
    }
    // blob 转file
    var fileOfBlob = new File([blob], name, { type: type });
    return fileOfBlob;
  }
}

export default UploadImg

调用代码

import UploadImg from "../../util/uploadImg";

methods: {
  imageHandle(e) {
   let maxSize = 100 * 1024;
   let imgDom = this.$refs.upImg;
   let files = e.srcElement.files;
   if (!files.length) return; //文件长度大于0
   if (!/^image\//.test(files[0].type)) return; //必须是图片才处理
   if (!window.FileReader) return; //支持FileReader

   if (this.docEntry === "" || this.lineId === "") {
    this.$toast("请填写完整信息", "error");
    return;
   }
   // let formdata = new FormData();
   UploadImg.imageHandle(files, maxSize, imgDom).then(formdata => {
    formdata.append("docEntry", this.docEntry);
    formdata.append("lineId", this.lineId);
    formdata.append("action", "ProductionListImage");
    this.$store
     .dispatch("uploadImage", formdata)
     .then(data => {
      if (data === 1) {
       this.$toast("上传成功", "success");
      } else if (data === -1) {
       this.$toast("图片为空", "error");
      } else {
       this.$toast("上传失败", "error");
      }
     })
     .catch(error => this.$toast("上传失败", "error"));
   });
  }
 }

参考链接：移动端利用H5实现压缩图片上传功能

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