// 壓縮圖片
/*
config= {
  width: 100, // 壓縮後圖片的寬
  height: 100, // 壓縮後圖片的高
  quality: 1 // 圖片品質（清晰度），取值0-1，
}
*/
export function compressImage(file, config) {
  let files
  const read = new FileReader()
  read.readAsDataURL(file)
  return new Promise(function (resolve) {
    read.onload = function (e) {
      let img 
 
= new Image()
      img.src = e.target.result
      img.onload = function () {
        // 預設按比例壓縮
        let w = config.width || this.width
        let h = config.height || this.height
        // 生成canvas
        let canvas = document.createElement('canvas')
        let ctx = canvas.getContext('2d')
        let base64
        
 
// 建立屬性節點
        canvas.setAttribute('width', w)
        canvas.setAttribute('height', h)
        ctx.drawImage(this, 0, 0, w, h)

        base64 = canvas.toDataURL(file['type'], config.quality || 1)
        // 回撥函式返回file的值（將base64編碼轉成file）
        files = dataURLtoFile(base64, file['name'])
        resolve(files)
      }
    }
  })
};

 
// base64轉碼（將base64編碼轉回file檔案）  
function dataURLtoFile(dataurl, fileName) {
  let arr = dataurl.split(',')
  let mime = arr[0].match(/:(.*?);/)[1]
  let bstr = atob(arr[1])
  let n = bstr.length
  let u8arr = new Uint8Array(n)
  while (n--) {
    u8arr[n] = bstr.charCodeAt(n)
  }
  return new File([u8arr], fileName, { type: mime })
}