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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