問題描述

我目前正在為網(wǎng)絡(luò)連接較差的環(huán)境進(jìn)行開發(fā).我的應(yīng)用程序有助于自動為用戶下載所需的 Google Drive 文件.它對于小文件(從 40KB 到 2MB)工作得相當(dāng)好，但對于大文件(9MB)卻經(jīng)常失敗.我知道這些文件大小可能看起來很小，但就我客戶的網(wǎng)絡(luò)環(huán)境而言，Google Drive API 經(jīng)常出現(xiàn) 9MB 文件失敗.

I'm currently developing for an environment that has poor network connectivity. My application helps to automatically download required Google Drive files for users. It works reasonably well for small files (ranging from 40KB to 2MB), but fails far too often for larger files (9MB). I know these file sizes might seem small, but in terms of my client's network environment, Google Drive API constantly fails with the 9MB file.

我已經(jīng)得出結(jié)論，我需要以較小的字節(jié)塊下載文件，但我不知道如何使用 Google Drive API 做到這一點.我一遍又一遍地閱讀了 this，并嘗試了以下代碼:

I've concluded that I need to download files in smaller byte chunks, but I don't see how I can do that with Google Drive API. I've read this over and over again, and I've tried the following code:

// with the Drive File ID, and the appropriate export MIME type, I create the export request
var request = DriveService.Files.Export(fileId, exportMimeType);

// take the message so I can modify it by hand
var message = request.CreateRequest();
var client = request.Service.HttpClient;

// I change the Range headers of both the client, and message
client.DefaultRequestHeaders.Range =
    message.Headers.Range =
    new System.Net.Http.Headers.RangeHeaderValue(100, 200);
var response = await request.Service.HttpClient.SendAsync(message);

// if status code = 200, copy to local file
if (response.IsSuccessStatusCode)
{
    using (var fileStream = new FileStream(downloadFileName, FileMode.CreateNew, FileAccess.ReadWrite))
    {
        await response.Content.CopyToAsync(fileStream);
    }
}

但是，生成的本地文件(來自 fileStream)仍然是全長的(即 ??40KB 驅(qū)動器文件為 40KB 文件，9MB 文件為 500 內(nèi)部服務(wù)器錯誤).在旁注中，我還嘗試了 ExportRequest.MediaDownloader.ChunkSize，但據(jù)我觀察，它只會改變 ExportRequest.MediaDownloader.ProgressChanged 回調(diào)的頻率調(diào)用(即如果 ChunkSize 設(shè)置為 256 * 1024，回調(diào)將每 256KB 觸發(fā)一次).

The resultant local file (from fileStream) however, is still full-length (i.e. 40KB file for the 40KB Drive file, and a 500 Internal Server Error for the 9MB file). On a sidenote, I've also experimented with ExportRequest.MediaDownloader.ChunkSize, but from what I observe it only changes the frequency at which the ExportRequest.MediaDownloader.ProgressChanged callback is called (i.e. callback will trigger every 256KB if ChunkSize is set to 256 * 1024).

我該如何繼續(xù)?

推薦答案

您似乎正朝著正確的方向前進(jìn).從您上次的評論來看，請求將根據(jù)塊大小更新進(jìn)度，因此您的觀察是準(zhǔn)確的.

You seemed to be heading in the right direction. From your last comment, the request will update progress based on the chunk size, so your observation was accurate.

查看 SDK 中 MediaDownloader 的源代碼找到了以下(強調(diào)我的)

核心下載邏輯.我們下載媒體并將其寫入一次輸出流 ChunkSize 個字節(jié)，提高 ProgressChanged每個塊之后的事件.分塊行為在很大程度上是歷史性的工件:先前的實現(xiàn)發(fā)出多個 Web 請求，每個請求對于 ChunkSize 字節(jié).現(xiàn)在我們在一個請求中完成所有事情，但是 API為了兼容性，保留了客戶端可見的行為.

The core download logic. We download the media and write it to an output stream ChunkSize bytes at a time, raising the ProgressChanged event after each chunk. The chunking behavior is largely a historical artifact: a previous implementation issued multiple web requests, each for ChunkSize bytes. Now we do everything in one request, but the API and client-visible behavior are retained for compatibility.

您的示例代碼只會下載一個從 100 到 200 的塊.使用這種方法，您必須跟蹤索引并手動下載每個塊，將它們復(fù)制到每個部分下載的文件流中

Your example code will only download one chunk from 100 to 200. Using that approach you would have to keep track of an index and download each chunk manually, copying them to the file stream for each partial download

const int KB = 0x400;
int ChunkSize = 256 * KB; // 256KB;
public async Task ExportFileAsync(string downloadFileName, string fileId, string exportMimeType) {

    var exportRequest = driveService.Files.Export(fileId, exportMimeType);
    var client = exportRequest.Service.HttpClient;

    //you would need to know the file size
    var size = await GetFileSize(fileId);

    using (var file = new FileStream(downloadFileName, FileMode.CreateNew, FileAccess.ReadWrite)) {

        file.SetLength(size);

        var chunks = (size / ChunkSize) + 1;
        for (long index = 0; index < chunks; index++) {

            var request = exportRequest.CreateRequest();

            var from = index * ChunkSize;
            var to = from + ChunkSize - 1;

            request.Headers.Range = new RangeHeaderValue(from, to);

            var response = await client.SendAsync(request);

            if (response.StatusCode == HttpStatusCode.PartialContent || response.IsSuccessStatusCode) {
                using (var stream = await response.Content.ReadAsStreamAsync()) {
                    file.Seek(from, SeekOrigin.Begin);
                    await stream.CopyToAsync(file);
                }
            }
        }
    }
}

private async Task<long> GetFileSize(string fileId) {
    var file = await driveService.Files.Get(fileId).ExecuteAsync();
    var size = file.size;
    return size;
}

這段代碼對驅(qū)動 api/server 做了一些假設(shè).

This code makes some assumptions about the drive api/server.

• 服務(wù)器將允許分塊下載文件所需的多個請求.不知道請求是否受到限制.
• 服務(wù)器仍然接受 Range 標(biāo)頭，如開發(fā)人員文檔中所述

• That the server will allow the multiple requests needed to download the file in chunks. Don't know if requests are throttled.
• That the server still accepts the Range header like stated in the developer documenation

這篇關(guān)于C# - 以字節(jié)塊從 Google Drive 下載的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網(wǎng)！