Use HttpClient to implement progress bar in .NET

The IProgress<T> interface introduced in .NET 4.5 allows handling of asynchronous progress reporting, thus implementing progress bar functionality for downloading files using HttpClient.

Extension methods using IProgress<T>

Creates an extension method for HttpClient's DownloadAsync operation that accepts a IProgress<float> implementation as a parameter. This implementation will update the progress bar or UI with the download status.

public static async Task DownloadAsync(this HttpClient client, string requestUri, Stream destination, IProgress<float> progress = null, CancellationToken cancellationToken = default)
{
    // 首先獲取 http 頭信息以檢查內(nèi)容長度
    using (var response = await client.GetAsync(requestUri, HttpCompletionOption.ResponseHeadersRead))
    {
        var contentLength = response.Content.Headers.ContentLength;

        using (var download = await response.Content.ReadAsStreamAsync(cancellationToken))
        {
            // 未傳遞進度報告器或內(nèi)容長度未知時忽略進度報告
            if (progress == null || !contentLength.HasValue)
            {
                await download.CopyToAsync(destination);
                return;
            }

            // 將絕對進度（已下載字節(jié)數(shù)）轉(zhuǎn)換為相對進度（0% - 100%）
            var relativeProgress = new Progress<long>(totalBytes => progress.Report((float)totalBytes / contentLength.Value));
            // 使用擴展方法在下載時報告進度
            await download.CopyToAsync(destination, 81920, relativeProgress, cancellationToken);
            progress.Report(1); // 報告100%完成
        }
    }
}

In this method, the progress.Report() method will pass the current download progress (in percent) to your IProgress<float> implementation, allowing it to update the progress bar or other UI elements accordingly.

Stream extension for progress reporting

To handle actual progress reporting as data is written to the target stream, consider creating an extension method for the Stream class.

public static async Task CopyToAsync(this Stream source, Stream destination, int bufferSize, IProgress<long> progress = null, CancellationToken cancellationToken = default)
{
    if (source == null)
        throw new ArgumentNullException(nameof(source));
    if (!source.CanRead)
        throw new ArgumentException("必須可讀", nameof(source));
    if (destination == null)
        throw new ArgumentNullException(nameof(destination));
    if (!destination.CanWrite)
        throw new ArgumentException("必須可寫", nameof(destination));
    if (bufferSize < 0)
        throw new ArgumentOutOfRangeException(nameof(bufferSize));

    var buffer = new byte[bufferSize];
    long totalBytesRead = 0;
    int bytesRead;
    while ((bytesRead = await source.ReadAsync(buffer, 0, buffer.Length, cancellationToken).ConfigureAwait(false)) != 0)
    {
        await destination.WriteAsync(buffer, 0, bytesRead, cancellationToken).ConfigureAwait(false);
        totalBytesRead += bytesRead;
        progress?.Report(totalBytesRead);
    }
}

This extension method monitors the number of bytes written to the target stream and reports progress using the IProgress<long> implementation passed in.

By combining these extension methods, you can easily implement progress bar functionality for file download operations using HttpClient.

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