Purpose

ASYNC await are strictly speaking not statements, but modifiers.

 

ASYNC is a method modifier used to indicate that a method / function contains asynchronous code

 

AWAIT is used before an expression to indicate that an operation may take a while to process.

 

The compiler will (behind the scenes) construct a complicated mechanism in which the body of the method is split into a part before and after the await. When the expression returns then the code jumps to the point where it has to continue. As such this belongs to the JUMP statements.

Example

//
// This example shows that you can call an async task and wait for it to finish
// The result of the async task (in this case the size of the file that has been downloaded)
// will be come available when the task has finished
// The calling code (The Start()) function will not have to wait until the async task has
// finished. That is why the line "2....." will be printed before the results from TestClass.DoTest()
// The sample also shows an event and displays the thread id's. You can see that the DownloadFileTaskAsync() method
// starts multiple threads to download the web document in multiple pieces.
 
USING System
USING System.Threading.Tasks
 
FUNCTION Start() AS VOID
   ? "1. calling long process"
   TestClass.DoTest()
   ? "2. this should be printed while processing"
   Console.ReadKey()
 
CLASS TestClass  
  STATIC PROTECT oLock AS OBJECT     // To make sure we synchronize the writing to the screen
  STATIC CONSTRUCTOR
     oLock := OBJECT{}
 
  ASYNC STATIC METHOD DoTest() AS VOID
      LOCAL Size AS INT64
       Size := AWAIT LoooongProcess()
       ? "3. returned from long process"
       ? Size, " Bytes downloaded"
 
  ASYNC STATIC METHOD LoooongProcess() AS Task<INT64>
      VAR WebClient := System.Net.WebClient{}
      VAR FileName := System.IO.Path.GetTempPath()+"temp.txt"
       webClient:DownloadProgressChanged += OnDownloadProgress
       webClient:Credentials := System.Net.CredentialCache.DefaultNetworkCredentials
      AWAIT webClient:DownloadFileTaskAsync("http://www.xsharp.info/index.php", FileName)
      VAR dirInfo      := System.IO.DirectoryInfo{System.IO.Path.GetTempPath()}
      VAR Files        := dirInfo:GetFiles("temp.txt")
      IF Files:Length > 0
           System.IO.File.Delete(FileName)
          RETURN Files[1]:Length
      ENDIF
      RETURN 0
 
  STATIC METHOD OnDownloadProgress (sender AS OBJECT, e AS System.Net.DownloadProgressChangedEventArgs) AS VOID
    BEGIN LOCK oLock
        ? String.Format("{0,3} % Size: {1,8:N0} Thread {2}", 100*e:BytesReceived / e:TotalBytesToReceive , e:BytesReceived, ;
          System.Threading.Thread.CurrentThread:ManagedThreadId)
    END LOCK
    RETURN
 
END CLASS