C# HttpClient.GetAsyncCancellationSafe方法代码示例

		/// <summary>
		/// Downloads the contents of the item to the local filesystem. This operation is only valid for files.
		/// </summary>
		/// <param name="destinationPath">Path to the destination file that will contain the contents of this item.</param>
		/// <param name="feedbackChannel">Allows you to receive feedback about the operation while it is running.</param>
		/// <param name="cancellationToken">Allows you to cancel the operation.</param>
		public async Task DownloadContentsAsync(string destinationPath, IFeedbackChannel feedbackChannel = null, CancellationToken cancellationToken = default(CancellationToken))
		{
			Argument.ValidateIsNotNullOrWhitespace(destinationPath, "destinationPath");

			if (Type != ItemType.File)
				throw new InvalidOperationException("You can only download files.");

			PatternHelper.LogMethodCall("DownloadContentsAsync", feedbackChannel, cancellationToken);
			PatternHelper.EnsureFeedbackChannel(ref feedbackChannel);

			using (var file = File.Create(destinationPath))
			{
				// If this file does not have any contents, there is nothing to download :)
				if (!HasContents)
					return;

				using (await _client.AcquireLock(feedbackChannel, cancellationToken))
				{
					feedbackChannel.Status = "Requesting download URL";

					var result = await _client.ExecuteCommandInternalAsync<GetDownloadLinkResult>(feedbackChannel, cancellationToken, new GetDownloadCommand
					{
						ItemID = ID
					});

					feedbackChannel.Status = "Preparing for download";

					var itemKey = _client.DecryptItemKey(EncryptedKeys);
					var dataKey = Algorithms.DeriveNodeDataKey(itemKey);
					var nonce = itemKey.Skip(16).Take(8).ToArray();
					var metaMac = itemKey.Skip(24).Take(8).ToArray();

					feedbackChannel.Status = "Pre-allocating file";

					file.SetLength(result.Size);

					feedbackChannel.Status = "Downloading";

					var chunkSizes = Algorithms.MeasureChunks(result.Size);
					var chunkCount = chunkSizes.Length;

					var chunkMacs = new byte[chunkCount][];

					// Limit number of chunks in flight at the same time.
					var concurrentDownloadSemaphore = new SemaphoreSlim(4);

					// Only one file write operation can take place at a time.
					var concurrentWriteSemaphore = new SemaphoreSlim(1);

					// For progress calculations.
					long completedBytes = 0;

					CancellationTokenSource chunkDownloadsCancellationSource = new CancellationTokenSource();

					// Get chunks in parallel.
					List<Task> chunkDownloads = new List<Task>();
					for (int i = 0; i < chunkCount; i++)
					{
						int chunkIndex = i;
						long startOffset = chunkSizes.Take(i).Select(size => (long)size).Sum();
						long endOffset = startOffset + chunkSizes[i];

						// Each chunk is downloaded and processed by this separately.
						chunkDownloads.Add(Task.Run(async delegate
						{
							var operationName = string.Format("Downloading chunk {0} of {1}", chunkIndex + 1, chunkSizes.Length);
							using (var chunkFeedbackChannel = feedbackChannel.BeginSubOperation(operationName))
							{
								byte[] bytes = null;

								using (await SemaphoreLock.TakeAsync(concurrentDownloadSemaphore))
								{
									await RetryHelper.ExecuteWithRetryAsync(async delegate
									{
										chunkDownloadsCancellationSource.Token.ThrowIfCancellationRequested();

										chunkFeedbackChannel.Status = string.Format("Downloading {0} bytes", chunkSizes[chunkIndex]);

										// Range is inclusive, so do -1 for the end offset.
										var url = result.DownloadUrl + "/" + startOffset + "-" + (endOffset - 1);

										HttpResponseMessage response;
										using (var client = new HttpClient())
											response = await client.GetAsyncCancellationSafe(url, chunkDownloadsCancellationSource.Token);

										response.EnsureSuccessStatusCode();

										bytes = await response.Content.ReadAsByteArrayAsync();

										if (bytes.Length != chunkSizes[chunkIndex])
											throw new MegaException(string.Format("Expected {0} bytes in chunk but got {1}.", chunkSizes[chunkIndex], bytes.Length));
									}, ChunkDownloadRetryPolicy, chunkFeedbackChannel, chunkDownloadsCancellationSource.Token);
								}

//.........这里部分代码省略.........