handleBeforeUpload(file) {
      let _this = this
      return new Promise((resolve, reject) => {
        const check = _this.uploadList.length < _this.mutileUploadData.maxNum
        if (!check) {
          this.$Notice.warning({
            title: '最多上傳' + this.mutileUploadData.maxNum + '
 
張'
          })
          reject(false)//重點
        }
        debugger
        let image = new Image()
        image.src = URL.createObjectURL(file)
        image.onload = () => {
          let width = _this.mutileUploadData.maxWidth
          let height = _this.mutileUploadData.maxHeight
          
 
if (
            (width && image.width > width) ||
            (height && image.height > height)
          ) {
            this.$Modal.warning({
              title: '圖片尺寸',
              content:
                '圖片尺寸 ' +
                file.name +
                ' 太大，不能超過' +
                this
 
.mutileUploadData.maxWidth +
                '*' +
                this.mutileUploadData.maxHeight +
                '。',
              onOk() {}
            })
            reject(false)//重點
          }
          resolve(true)//重點
        }
        image.onerror = () => {
          reject()
        }
      })
    }

為什麼要用Promise呢，因為image.onload()方法非同步，