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

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持三水点靠木。