Imports System.Reflection

''' <summary>
''' Provides loosely-coupled messaging between
''' various colleague objects. All references to objects
''' are stored weakly, to prevent memory leaks.
''' </summary>
Public Class Messenger

   Public Sub New()
   End Sub

   ''' <summary>
   ''' Registers a callback method to be invoked when a specific message is broadcasted.
   ''' </summary>
   ''' <param name="message">The message to register for.</param>
   ''' <param name="callback">The callback to be called when this message is broadcasted.</param>
   Public Sub Register(ByVal message As String, ByVal callback As [Delegate])

      If String.IsNullOrEmpty(message) Then
         Throw New ArgumentException("'message' cannot be null or empty.")
      End If

      If callback Is Nothing Then
         Throw New ArgumentNullException("callback")
      End If

      Dim parameters As ParameterInfo() = callback.Method.GetParameters()

      If parameters IsNot Nothing AndAlso parameters.Length > 1 Then
         Throw New InvalidOperationException("The registered delegate can have no more than one parameter.")
      End If

      Dim parameterType As Type = If((parameters Is Nothing OrElse parameters.Length = 0), Nothing, parameters(0).ParameterType)
      _messageToActionsMap.AddAction(message, callback.Target, callback.Method, parameterType)
   End Sub

   ''' <summary>
   ''' Notifies all registered parties that a message is being broadcasted.
   ''' </summary>
   ''' <param name="message">The message to broadcast.</param>
   Public Sub NotifyColleagues(ByVal message As String)

      If String.IsNullOrEmpty(message) Then
         Throw New ArgumentException("'message' cannot be null or empty.")
      End If

      Dim actions = _messageToActionsMap.GetActions(message)

      If actions IsNot Nothing Then
         actions.ForEach(Function(action) action.DynamicInvoke())
      End If

   End Sub

   ''' <summary>
   ''' Notifies all registered parties that a message is being broadcasted.
   ''' </summary>
   ''' <param name="message">The message to broadcast</param>
   ''' <param name="parameter">The parameter to pass together with the message</param>
   Public Sub NotifyColleagues(ByVal message As String, ByVal parameter As Object)

      If String.IsNullOrEmpty(message) Then
         Throw New ArgumentException("'message' cannot be null or empty.")
      End If

      Dim actions = _messageToActionsMap.GetActions(message)

      If actions IsNot Nothing Then
         actions.ForEach(Function(action) action.DynamicInvoke(parameter))
      End If

   End Sub

   ''' <summary>
   ''' This class is an implementation detail of the Messenger class.
   ''' </summary>
   Private Class MessageToActionsMap
      ' Stores a hash where the key is the message and the value is the list of callbacks to invoke.
      ReadOnly _map As New Dictionary(Of String, List(Of WeakAction))()

      Friend Sub New()
      End Sub

      ''' <summary>
      ''' Adds an action to the list.
      ''' </summary>
      ''' <param name="message">The message to register.</param>
      ''' <param name="target">The target object to invoke, or null.</param>
      ''' <param name="method">The method to invoke.</param>
      ''' <param name="actionType">The type of the Action delegate.</param>
      Friend Sub AddAction(ByVal message As String, ByVal target As Object, ByVal method As MethodInfo, ByVal actionType As Type)

         If message Is Nothing Then
            Throw New ArgumentNullException("message")
         End If

         If method Is Nothing Then
            Throw New ArgumentNullException("method")
         End If

         SyncLock _map

            If Not _map.ContainsKey(message) Then
               _map(message) = New List(Of WeakAction)()
            End If

            _map(message).Add(New WeakAction(target, method, actionType))
         End SyncLock
      End Sub

      ''' <summary>
      ''' Gets the list of actions to be invoked for the specified message
      ''' </summary>
      ''' <param name="message">The message to get the actions for</param>
      ''' <returns>Returns a list of actions that are registered to the specified message</returns>
      Friend Function GetActions(ByVal message As String) As List(Of [Delegate])

         If message Is Nothing Then
            Throw New ArgumentNullException("message")
         End If

         Dim actions As List(Of [Delegate])
         SyncLock _map

            If Not _map.ContainsKey(message) Then
               Return Nothing
            End If

            Dim weakActions As List(Of WeakAction) = _map(message)
            actions = New List(Of [Delegate])(weakActions.Count)

            For i As Integer = weakActions.Count - 1 To -1 + 1 Step -1

               Dim weakAction As WeakAction = weakActions(i)

               If weakAction Is Nothing Then
                  Continue For
               End If

               Dim action As [Delegate] = weakAction.CreateAction()

               If action IsNot Nothing Then
                  actions.Add(action)

               Else
                  ' The target object is dead, so get rid of the weak action.
                  weakActions.Remove(weakAction)
               End If

            Next

            ' Delete the list from the map if it is now empty.
            If weakActions.Count = 0 Then
               _map.Remove(message)
            End If

         End SyncLock
         Return actions
      End Function

   End Class

   ''' <summary>
   ''' This class is an implementation detail of the MessageToActionsMap class.
   ''' </summary>
   Private Class WeakAction
      ReadOnly _delegateType As Type
      ReadOnly _method As MethodInfo
      ReadOnly _targetRef As WeakReference

      ''' <summary>
      ''' Constructs a WeakAction.
      ''' </summary>
      ''' <param name="target">The object on which the target method is invoked, or null if the method is static.</param>
      ''' <param name="method">The MethodInfo used to create the Action.</param>
      ''' <param name="parameterType">The type of parameter to be passed to the action. Pass null if there is no parameter.</param>
      Friend Sub New(ByVal target As Object, ByVal method As MethodInfo, ByVal parameterType As Type)

         If target Is Nothing Then
            _targetRef = Nothing

         Else
            _targetRef = New WeakReference(target)
         End If

         _method = method

         If parameterType Is Nothing Then
            _delegateType = GetType(Action)

         Else
            _delegateType = GetType(Action(Of )).MakeGenericType(parameterType)
         End If

      End Sub

      ''' <summary>
      ''' Creates a "throw away" delegate to invoke the method on the target, or null if the target object is dead.
      ''' </summary>
      Friend Function CreateAction() As [Delegate]

         ' Rehydrate into a real Action object, so that the method can be invoked.
         If _targetRef Is Nothing Then
            Return [Delegate].CreateDelegate(_delegateType, _method)

         Else

            Try

               Dim target As Object = _targetRef.Target

               If target IsNot Nothing Then
                  Return [Delegate].CreateDelegate(_delegateType, target, _method)
               End If

            Catch
            End Try

         End If

         Return Nothing
      End Function

   End Class

   ReadOnly _messageToActionsMap As New MessageToActionsMap()

End Class