Como: criar tarefas pré-computadasHow to: Create Pre-Computed Tasks

Esse documento descreve como usar o método Task.FromResult para recuperar os resultados das operações de download assíncronas armazenados em um cache.This document describes how to use the Task.FromResult method to retrieve the results of asynchronous download operations that are held in a cache. O método FromResult retorna um objeto Task<TResult> que contém o valor fornecido como sua propriedade Result.The FromResult method returns a finished Task<TResult> object that holds the provided value as its Result property. Este método é útil quando você executa uma operação assíncrona que retorna um objeto Task<TResult> e o resultado do objeto Task<TResult> já está calculado.This method is useful when you perform an asynchronous operation that returns a Task<TResult> object, and the result of that Task<TResult> object is already computed.

ExemploExample

O exemplo a seguir faz o download de cadeias de caracteres da Web.The following example downloads strings from the web. Ele define o método DownloadStringAsync.It defines the DownloadStringAsync method. Esse método faz o download das cadeias de caracteres da Web de forma assíncrona.This method downloads strings from the web asynchronously. Este exemplo também usa um objeto ConcurrentDictionary<TKey,TValue> para armazenar em cache os resultados de operações anteriores.This example also uses a ConcurrentDictionary<TKey,TValue> object to cache the results of previous operations. Se o endereço de entrada for mantido no cache, o DownloadStringAsync usará o método FromResult para produzir um objeto Task<TResult> que manterá o conteúdo nesse endereço.If the input address is held in this cache, DownloadStringAsync uses the FromResult method to produce a Task<TResult> object that holds the content at that address. Caso contrário, o DownloadStringAsync baixará o arquivo da Web e adicionará o resultado ao cache.Otherwise, DownloadStringAsync downloads the file from the web and adds the result to the cache.

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

// Demonstrates how to use Task<TResult>.FromResult to create a task
// that holds a pre-computed result.
class CachedDownloads
{
   // Holds the results of download operations.
   static ConcurrentDictionary<string, string> cachedDownloads =
      new ConcurrentDictionary<string, string>();

   // Asynchronously downloads the requested resource as a string.
   public static Task<string> DownloadStringAsync(string address)
   {
      // First try to retrieve the content from cache.
      string content;
      if (cachedDownloads.TryGetValue(address, out content))
      {
         return Task.FromResult<string>(content);
      }

      // If the result was not in the cache, download the
      // string and add it to the cache.
      return Task.Run(async () =>
      {
         content = await new WebClient().DownloadStringTaskAsync(address);
         cachedDownloads.TryAdd(address, content);
         return content;
      });
   }

   static void Main(string[] args)
   {
      // The URLs to download.
      string[] urls = new string[]
      {
         "http://msdn.microsoft.com",
         "http://www.contoso.com",
         "http://www.microsoft.com"
      };

      // Used to time download operations.
      Stopwatch stopwatch = new Stopwatch();

      // Compute the time required to download the URLs.
      stopwatch.Start();
      var downloads = from url in urls
                      select DownloadStringAsync(url);
      Task.WhenAll(downloads).ContinueWith(results =>
      {
         stopwatch.Stop();

         // Print the number of characters download and the elapsed time.
         Console.WriteLine("Retrieved {0} characters. Elapsed time was {1} ms.",
            results.Result.Sum(result => result.Length),
            stopwatch.ElapsedMilliseconds);
      })
      .Wait();

      // Perform the same operation a second time. The time required
      // should be shorter because the results are held in the cache.
      stopwatch.Restart();
      downloads = from url in urls
                  select DownloadStringAsync(url);
      Task.WhenAll(downloads).ContinueWith(results =>
      {
         stopwatch.Stop();

         // Print the number of characters download and the elapsed time.
         Console.WriteLine("Retrieved {0} characters. Elapsed time was {1} ms.",
            results.Result.Sum(result => result.Length),
            stopwatch.ElapsedMilliseconds);
      })
      .Wait();
   }
}

/* Sample output:
Retrieved 27798 characters. Elapsed time was 1045 ms.
Retrieved 27798 characters. Elapsed time was 0 ms.
*/
Imports System.Collections.Concurrent
Imports System.Diagnostics
Imports System.Linq
Imports System.Net
Imports System.Threading.Tasks

' Demonstrates how to use Task<TResult>.FromResult to create a task 
' that holds a pre-computed result.
Friend Class CachedDownloads
    ' Holds the results of download operations.
    Private Shared cachedDownloads As New ConcurrentDictionary(Of String, String)()

    ' Asynchronously downloads the requested resource as a string.
    Public Shared Function DownloadStringAsync(ByVal address As String) As Task(Of String)
        ' First try to retrieve the content from cache.
        Dim content As String
        If cachedDownloads.TryGetValue(address, content) Then
            Return Task.FromResult(Of String)(content)
        End If

        ' If the result was not in the cache, download the 
        ' string and add it to the cache.
        Return Task.Run(async Function()
                            content = await New WebClient().DownloadStringTaskAsync(address)
                            cachedDownloads.TryAdd(address, content)
                            Return content
                        End Function)
    End Function

    Shared Sub Main(ByVal args() As String)
        ' The URLs to download.
        Dim urls() As String = {"http://msdn.microsoft.com", "http://www.contoso.com", "http://www.microsoft.com"}

        ' Used to time download operations.
        Dim stopwatch As New Stopwatch()

        ' Compute the time required to download the URLs.
        stopwatch.Start()
        Dim downloads = From url In urls _
                        Select DownloadStringAsync(url)
        Task.WhenAll(downloads).ContinueWith(Sub(results)
                                                 ' Print the number of characters download and the elapsed time.
                                                 stopwatch.Stop()
                                                 Console.WriteLine("Retrieved {0} characters. Elapsed time was {1} ms.", results.Result.Sum(Function(result) result.Length), stopwatch.ElapsedMilliseconds)
                                             End Sub).Wait()

        ' Perform the same operation a second time. The time required
        ' should be shorter because the results are held in the cache.
        stopwatch.Restart()
        downloads = From url In urls _
                    Select DownloadStringAsync(url)
        Task.WhenAll(downloads).ContinueWith(Sub(results)
                                                 ' Print the number of characters download and the elapsed time.
                                                 stopwatch.Stop()
                                                 Console.WriteLine("Retrieved {0} characters. Elapsed time was {1} ms.", results.Result.Sum(Function(result) result.Length), stopwatch.ElapsedMilliseconds)
                                             End Sub).Wait()
    End Sub
End Class

' Sample output:
'Retrieved 27798 characters. Elapsed time was 1045 ms.
'Retrieved 27798 characters. Elapsed time was 0 ms.
'

Esse exemplo calcula o tempo necessário para baixar várias cadeias de caracteres duas vezes.This example computes the time that is required to download multiple strings two times. O segundo conjunto de operações de download deve levar menos tempo do que o primeiro conjunto porque os resultados são mantidos no cache.The second set of download operations should take less time than the first set because the results are held in the cache. O método FromResult permite que o método DownloadStringAsync crie objetos Task<TResult> que contêm esses resultados pré-computados.The FromResult method enables the DownloadStringAsync method to create Task<TResult> objects that hold these pre-computed results.

Consulte tambémSee also