Happiness in the Universe

Peter, his boss, and the universe were finally satisfied. Peter's boss and the universe were allowed to be notified of the events that interested them, reducing the burden of implementation and the cost of unnecessary round-trips. Peter could notify each of them, ignoring how long it took them to return from their target methods while still getting his results asynchronously. The result was the following complete solution:

 Delegate Sub WorkStarted() Delegate Sub WorkProgressing() Delegate Function WorkCompleted() As Integer Class Worker     Public Event started As WorkStarted     Public Event progressing As WorkProgressing     Public completed As WorkCompleted     Public Sub DoWork()         Console.WriteLine("Worker: work started")         RaiseEvent started()         Console.WriteLine("Worker: work progressing")         RaiseEvent progressing()         Console.WriteLine("Worker: work completed")         If Not (completed Is Nothing) Then             Dim wc As WorkCompleted             For Each wc In completed.GetInvocationList()                 Dim res As IAsyncResult = wc.BeginInvoke( _              New AsyncCallback(AddressOf WorkGraded), wc)             Next         End If     End Sub     Public Sub WorkGraded(ByVal res As IAsyncResult)         Dim wc As WorkCompleted = CType(res.AsyncState, WorkCompleted)         Dim grade As Integer = wc.EndInvoke(res)         Console.WriteLine("Worker grade= " & grade.ToString())     End Sub End Class Class Boss     Public Function WorkCompleted() As Integer         System.Threading.Thread.Sleep(3000)         Console.WriteLine("Better...")         Return 4 ' out of 10     End Function End Class Class Universe     Shared Sub WorkerStartedWork()         Console.WriteLine("Universe notices worker starting work")     End Sub     Shared Function WorkerCompletedWork() As Integer         System.Threading.Thread.Sleep(4000)         Console.WriteLine("Universe pleased with worker's work")         Return 7     End Function     Shared Sub Main()         Dim peter As Worker = New Worker()         Dim theboss As Boss = New Boss()         Dim d1, d2 As WorkCompleted         d1 = AddressOf theboss.WorkCompleted         d2 = AddressOf Universe.WorkerCompletedWork         peter.completed = CType(System.Delegate.Combine(d1, d2), _              WorkCompleted)         AddHandler peter.started, AddressOf Universe.WorkerStartedWork         peter.DoWork()         Console.WriteLine("Main: worker completed work")         Console.ReadLine()     End Sub End Class 

Peter knew that getting results asynchronously came with issues, because as soon as he fired events asynchronously, the target methods were likely to be executed on another thread, as was Peter's notification of when the target method has completed. However, Peter was familiar with Chapter 14: Multithreaded User Interfaces, so he understood how to manage such issues when building WinForms applications.

And they all lived happily ever after. The end.