创建预先计算的任务

项目
03/28/2023

本文介绍如何使用 Task.FromResult 方法检索缓存中包含的异步下载操作的结果。 FromResult 方法返回一个将所提供的值作为其 Task<TResult> 属性的已完成的 Result 对象。执行返回 Task<TResult> 对象的异步运算，且已计算该 Task<TResult> 对象的结果时，此方法将十分有用。

示例

下面的示例从 Web 下载字符串。它定义了 DownloadStringAsync 方法。此方法从 Web 异步下载字符串。此示例还使用 ConcurrentDictionary<TKey,TValue> 对象来缓存先前操作的结果。如果此缓存中包含输入的地址，DownloadStringAsync 会使用 FromResult 方法来生成包含位于该地址的内容的 Task<TResult> 对象。否则，DownloadStringAsync 从 Web 下载文件并将结果添加到缓存中。

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Net.Http;
using System.Threading.Tasks;

public static class DownloadCache
{
    private static readonly ConcurrentDictionary<string, string> s_cachedDownloads = new();
    private static readonly HttpClient s_httpClient = new();

    public static Task<string> DownloadStringAsync(string address)
    {
        if (s_cachedDownloads.TryGetValue(address, out string? content))
        {
            return Task.FromResult(content);
        }

        return Task.Run(async () =>
        {
            content = await s_httpClient.GetStringAsync(address);
            s_cachedDownloads.TryAdd(address, content);

            return content;
        });
    }

    public static async Task Main()
    {
        string[] urls = new[]
        {
            "https://learn.microsoft.com/aspnet/core",
            "https://learn.microsoft.com/dotnet",
            "https://learn.microsoft.com/dotnet/architecture/dapr-for-net-developers",
            "https://learn.microsoft.com/dotnet/azure",
            "https://learn.microsoft.com/dotnet/desktop/wpf",
            "https://learn.microsoft.com/dotnet/devops/create-dotnet-github-action",
            "https://learn.microsoft.com/dotnet/machine-learning",
            "https://learn.microsoft.com/xamarin",
            "https://dotnet.microsoft.com/",
            "https://www.microsoft.com"
        };

        Stopwatch stopwatch = Stopwatch.StartNew();
        IEnumerable<Task<string>> downloads = urls.Select(DownloadStringAsync);

        static void StopAndLogElapsedTime(
            int attemptNumber, Stopwatch stopwatch, Task<string[]> downloadTasks)
        {
            stopwatch.Stop();

            int charCount = downloadTasks.Result.Sum(result => result.Length);
            long elapsedMs = stopwatch.ElapsedMilliseconds;

            Console.WriteLine(
                $"Attempt number: {attemptNumber}\n" +
                $"Retrieved characters: {charCount:#,0}\n" +
                $"Elapsed retrieval time: {elapsedMs:#,0} milliseconds.\n");
        }

        await Task.WhenAll(downloads).ContinueWith(
            downloadTasks => StopAndLogElapsedTime(1, stopwatch, downloadTasks));

        // Perform the same operation a second time. The time required
        // should be shorter because the results are held in the cache.
        stopwatch.Restart();

        downloads = urls.Select(DownloadStringAsync);

        await Task.WhenAll(downloads).ContinueWith(
            downloadTasks => StopAndLogElapsedTime(2, stopwatch, downloadTasks));
    }
    // Sample output:
    //     Attempt number: 1
    //     Retrieved characters: 754,585
    //     Elapsed retrieval time: 2,857 milliseconds.

    //     Attempt number: 2
    //     Retrieved characters: 754,585
    //     Elapsed retrieval time: 1 milliseconds.
}

Imports System.Collections.Concurrent
Imports System.Net.Http

Module Snippets
    Class DownloadCache
        Private Shared ReadOnly s_cachedDownloads As ConcurrentDictionary(Of String, String) =
            New ConcurrentDictionary(Of String, String)()
        Private Shared ReadOnly s_httpClient As HttpClient = New HttpClient()

        Public Function DownloadStringAsync(address As String) As Task(Of String)
            Dim content As String = Nothing

            If s_cachedDownloads.TryGetValue(address, content) Then
                Return Task.FromResult(Of String)(content)
            End If

            Return Task.Run(
                Async Function()
                    content = Await s_httpClient.GetStringAsync(address)
                    s_cachedDownloads.TryAdd(address, content)
                    Return content
                End Function)
        End Function
    End Class

    Public Sub StopAndLogElapsedTime(
            attemptNumber As Integer,
            stopwatch As Stopwatch,
            downloadTasks As Task(Of String()))

        stopwatch.Stop()

        Dim charCount As Integer = downloadTasks.Result.Sum(Function(result) result.Length)
        Dim elapsedMs As Long = stopwatch.ElapsedMilliseconds

        Console.WriteLine(
            $"Attempt number: {attemptNumber}{vbCrLf}" &
            $"Retrieved characters: {charCount:#,0}{vbCrLf}" &
            $"Elapsed retrieval time: {elapsedMs:#,0} milliseconds.{vbCrLf}")
    End Sub

    Sub Main()
        Dim cache As DownloadCache = New DownloadCache()
        Dim urls As String() = {
                "https://learn.microsoft.com/aspnet/core",
                "https://learn.microsoft.com/dotnet",
                "https://learn.microsoft.com/dotnet/architecture/dapr-for-net-developers",
                "https://learn.microsoft.com/dotnet/azure",
                "https://learn.microsoft.com/dotnet/desktop/wpf",
                "https://learn.microsoft.com/dotnet/devops/create-dotnet-github-action",
                "https://learn.microsoft.com/dotnet/machine-learning",
                "https://learn.microsoft.com/xamarin",
                "https://dotnet.microsoft.com/",
                "https://www.microsoft.com"
            }
        Dim stopwatch As Stopwatch = Stopwatch.StartNew()
        Dim downloads As IEnumerable(Of Task(Of String)) =
                urls.Select(AddressOf cache.DownloadStringAsync)
        Task.WhenAll(downloads).ContinueWith(
                Sub(downloadTasks)
                    StopAndLogElapsedTime(1, stopwatch, downloadTasks)
                End Sub).Wait()

        stopwatch.Restart()
        downloads = urls.Select(AddressOf cache.DownloadStringAsync)
        Task.WhenAll(downloads).ContinueWith(
                Sub(downloadTasks)
                    StopAndLogElapsedTime(2, stopwatch, downloadTasks)
                End Sub).Wait()
    End Sub
    ' Sample output:
    '     Attempt number 1
    '     Retrieved characters: 754,585
    '     Elapsed retrieval time: 2,857 milliseconds.
    '
    '     Attempt number 2
    '     Retrieved characters: 754,585
    '     Elapsed retrieval time: 1 milliseconds.
End Module

在上一示例中，首次下载每个 URL 时，其值存储在缓存中。 FromResult 方法可以使 DownloadStringAsync 方法创建包含这些预计算结果的 Task<TResult> 对象。下载字符串的后续调用将返回缓存的值，并且速度要快得多。

另请参阅

基于任务的异步编程

创建预先计算的任务

示例

另请参阅

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