我正在使用Xamarin PCL为Android和iOS创建文件上传应用程序,我已设法实现文件上传和某种进度条,但它无法正常工作.

我看到堆栈溢出的一些答案用于显示下载进度,但我想通知我的用户有关上传进度的信息,但没有找到任何解决方案.

这是我的代码：

public static async Task<string> PostFileAsync (Stream filestream, string filename, int filesize) {
        var progress = new System.Net.Http.Handlers.ProgressMessageHandler ();

        //Progress tracking
        progress.HttpSendProgress += (object sender, System.Net.Http.Handlers.HttpProgressEventArgs e) => {
            int progressPercentage = (int)(e.BytesTransferred*100/filesize);
            //Raise an event that is used to update the UI
            UploadProgressMade(sender, new System.Net.Http.Handlers.HttpProgressEventArgs(progressPercentage, null, e.BytesTransferred, null));
        };

        using (var client = HttpClientFactory.Create(progress)) {
            using (var content = new MultipartFormDataContent ("------" + DateTime.Now.Ticks.ToString ("x"))) {
                content.Add (new StreamContent (filestream), "Filedata", filename);
                using (var message = await client.PostAsync ("http://MyUrl.example", content)) {
                    var result = await message.Content.ReadAsStringAsync ();
                    System.Diagnostics.Debug.WriteLine ("Upload done");
                    return result;
                }
            }
        }
    }

显示某种进度,但当进度达到100％时,文件尚未上载.在收到最后一条进度消息后的某个时间也会打印消息“上传已完成”.

也许进展是显示从设备发出的字节而不是已经上传的字节,所以当它说是100％时,所有字节都只是发出但服务器尚未收到？

编辑：试过这个解决方案：https://forums.xamarin.com/discussion/56716/plans-to-add-webclient-to-pcl,它的效果更好一点.

解决方法:

尝试这样的事情：

我遇到了同样的问题.我通过实现自定义HttpContent来修复它.我使用此对象来跟踪上传进度的百分比,您可以添加事件并收听它.您应该自定义SerializeToStreamAsync方法.

internal class ProgressableStreamContent : HttpContent
{
    private const int defaultBufferSize = 4096;

    private Stream content;
    private int bufferSize;
    private bool contentConsumed;
    private Download downloader;

    public ProgressableStreamContent(Stream content, Download downloader) : this(content, defaultBufferSize, downloader) {}

    public ProgressableStreamContent(Stream content, int bufferSize, Download downloader)
    {
        if(content == null)
        {
            throw new ArgumentNullException("content");
        }
        if(bufferSize <= 0)
        {
            throw new ArgumentOutOfRangeException("bufferSize");
        }

        this.content = content;
        this.bufferSize = bufferSize;
        this.downloader = downloader;
    }

    protected override Task SerializeToStreamAsync(Stream stream, TransportContext context)
    {
        Contract.Assert(stream != null);

        PrepareContent();

        return Task.Run(() =>
        {
            var buffer = new Byte[this.bufferSize];
            var size = content.Length;
            var uploaded = 0;

            downloader.ChangeState(DownloadState.PendingUpload);

            using(content) while(true)
            {
                var length = content.Read(buffer, 0, buffer.Length);
                if(length <= 0) break;

                downloader.Uploaded = uploaded += length;

                stream.Write(buffer, 0, length);

                downloader.ChangeState(DownloadState.Uploading);
            }

            downloader.ChangeState(DownloadState.PendingResponse);
        });
    }

    protected override bool TryComputeLength(out long length)
    {
        length = content.Length;
        return true;
    }

    protected override void Dispose(bool disposing)
    {
        if(disposing)
        {
            content.Dispose();
        }
        base.Dispose(disposing);
    }


    private void PrepareContent()
    {
        if(contentConsumed)
        {
            // If the content needs to be written to a target stream a 2nd time, then the stream must support
            // seeking (e.g. a FileStream), otherwise the stream can't be copied a second time to a target 
            // stream (e.g. a NetworkStream).
            if(content.CanSeek)
            {
                content.Position = 0;
            }
            else
            {
                throw new InvalidOperationException("SR.net_http_content_stream_already_read");
            }
        }

        contentConsumed = true;
    }
}

参考：

> https://github.com/paulcbetts/ModernHttpClient/issues/80
> Progress bar for HttpClient uploading
> https://forums.xamarin.com/discussion/18649/best-practice-to-upload-image-selected-to-a-web-api