Vue+Element ui上传图片限制图片尺寸

1.引入element-ui upload组件

<el-upload class="avatar-uploader" :headers="myHeaders" :action="上传图片的地址" :show-file-list="false" :on-success="handleAvatarSuccess" :before-upload="beforeAvatarUpload">
    <img v-if="imageUrl" :src="imageUrl" class="avatar" />
    <i v-else class="el-icon-plus avatar-uploader-icon"></i>
</el-upload>


2.在style中 定义el-upload的样式

<style>
  .avatar-uploader .el-upload {
    border: 1px dashed #d9d9d9;
    border-radius: 6px;
    cursor: pointer;
    position: relative;
    overflow: hidden;
  }
  .avatar-uploader .el-upload:hover {
    border-color: #409EFF;
  }
  .avatar-uploader-icon {
    font-size: 28px;
    color: #8c939d;
    width: 178px;
    height: 178px;
    line-height: 178px;
    text-align: center;
  }
  .avatar {
    width: 178px;
    height: 178px;
    display: block;
  }
</style>


3.在methods里 定义限制图片的方法

methods:{
    //图片上传成功的回调函数
    handleAvatarSuccess(res, file) {
        if (file.raw.isFlag && res.code == 0) {
            this.imageUrl = URL.createObjectURL(file.raw);
            this.newBanner_Form.imgUrl = res.data.url;
        }
    },
    //图片上传前的回调函数
    beforeAvatarUpload(file) {
        const isJPG = file.type === "image/jpeg" || file.type === "image/png";
            if (!isJPG) {
                this.$message.error("上传头像图片只能是 JPG和PNG 格式!");
            }
            //调用[限制图片尺寸]函数
            this.limitFileWH(702, 285, file).then((res) => {
                file.isFlag = res
            })
            return isJPG && file.isFlag;
    },
    //限制图片尺寸
    limitFileWH(E_width, E_height, file) {
        let _this = this;
        let imgWidth = "";
        let imgHight = "";
        const isSize = new Promise(function(resolve, reject) {
            let width = E_width;
            let height = E_height;
            let _URL = window.URL || window.webkitURL;
            let img = new Image();
            img.onload = function() {
                imgWidth = img.width;
                imgHight = img.height;
                let valid = img.width == width && img.height == height;
                valid ? resolve() : reject();
            }
            img.src = _URL.createObjectURL(file);
        }).then(() => {
            return true;
        }, () => {
            _this.$message.warning({
            message: '上传文件的图片大小不合符标准,宽需要为' + E_width + 'px，高需要为' + E_height + 'px。当前上传图片的宽高分别为：' + imgWidth + 'px和' +
                            imgHight + 'px',
            btn: false
        })
            return false;
        });
            return isSize
    },
}


4.解决before-upload钩子返回false时，文件仍然上传成功的问题

如果我们在before-upload中直接返回true或者是false，那么它其实也是会上传文件的，因为它也会触发on-change函数。
我这里是采用在对应的函数中返回一个promise来解决的，就像下面这样：

beforeAvatarUpload(file) {
      return new Promise((resolve,reject) => {
        const isJPG = file.type === 'image/jpeg' || file.type === 'image/png';
        if (!isJPG) {
          this.$message.error('上传头像图片只能是 JPG和PNG 格式!');
        }
        //调用[限制图片尺寸]函数
        this.limitFileWH(702, 285, file).then((res) => {
          file.isFlag = res;
        });
        if (file.isFlag) {
          return resolve(true);
        } else {
          return reject(false);
        }
      })
      
    },