请看我的代码

为什么for历遍完了,writeFile 才执行成功呢？这异步的吗？我需要的是每一个历遍都执行完成再执行下一个，谢谢。

for (var i = 0; i <= listSize; i++) {
          console.log("i:" + i)
          let start = i * 526095
          let end = 526095
          if (i == listSize) {//最后一个
            end = res.size - (i * 526095)
          }

          var filestr = f.readFileSync(filePath, 'utf-8', start, end)
          let path = wx.env.USER_DATA_PATH + "/" + i + ".mp4"
          f.writeFile({
            filePath: path,
            encoding: 'binary',
            data: filestr,
            success() {
              console.log("writeFile:" + path)
              console.log("writeFile:" + i)
              console.log("writeFile:" + listSize)
              fileList.push(path)
              if (i == listSize) {
                console.log(fileList)
                console.log(fileList.length)
                wx.hideLoading()
                if (fileList.length != 0) that.FileUpload(0)
              }
            }, fail(res) {
              console.log("writeFile:" + res.errMsg)
            }
          })
        }