Simple MVVM Toolkit Express: Messaging Sample

Here we want to communicate between two different view-models, CustomerListViewModel and CustomerDetailViewModel. When the IncrementOrders method is called in the list VM we send a message to the detail VM. However, because the list VM does not have a direct reference to the detail VM, we can communicate indirectly via the MessageBus.

This example will show message multicasting, with a notification broadcast to multiple listeners. And it will also show two-way communication, with the listener calling back the notifier with a specified result. For two-way communication pass two type arguments to the MessageBus.Default.Register: TOutgoing for data sent from the notifier, and TIncoming for data sent from the subscriber back to the notifier.

1. Create a new WPF project using Visual Studio 2015 or higher

• Target Framework: .NET 4.6
• Add the SimpleMvvmToolkit.Express NuGet package
• Use Code Snippets Manager to add the Simple Mvvm C# code snippets

2. Open MessageTokens.cs and define a string constant that will serve as a message token for the increase orders notification.

3. Open CustomerDetailViewModel and add code in the ctor to get notified when a request is made to increase orders.

   • Call RegisterToReceiveMessages, specifying two type arguments: <IncreaseInfo, ApprovalInfo>
     • IncreaseInfo is for outgoing data from the message sender
     • ApprovalInfo is for incoming data from the receiver back to the sender

   RegisterToReceiveMessages<IncreaseInfo, ApprovalInfo>
       (MessageTokens.IncreaseOrders, OnOrdersIncrease);

4. Insert a method that will get invoked when a notification takes place using the IncreaseOrders message token, which is just a string const.

   • The method should take two parameters: one for sender (object) and another for NotificationEventArgs<IncreaseInfo, ApprovalInfo>.
   • In the method show a ApprovalIncreaseView dialog if the request is for the customer belonging to the view-model instance.

     In the Closed event of the dialog, you will call back the notifier by invoking the Completed event args property.

   void OnOrdersIncrease(object sender, NotificationEventArgs
   	<IncreaseInfo, ApprovalInfo> e)
   {
   	if (e.Data.CustomerName != Customer.CustomerName) return;
   
   	ApproveIncreaseView appoveView = new ApproveIncreaseView(e.Data);
   	appoveView.Closed += (s, ea) =>
   	{
   		// Callback notifier with result
   		ApprovalInfo approveInfo = new ApprovalInfo
   			(Customer.CustomerName, e.Data.Amount,
   				(bool)appoveView.DialogResult);
   		e.Completed(approveInfo);
   	};
   	appoveView.Show();
   }
   

5. Open CustomerListViewModel and add code to the IncreaseOrders method that sends a message to subscribers when an customer orders is increased.

   • Specify a method that the subscriber will callback with the result

   SendMessage(MessageTokens.IncreaseOrders, new NotificationEventArgs
       <IncreaseInfo, ApprovalInfo>(null, increaseInfo, OnIncreaseResponse));

6. Insert a method called OnIncreaseResponse that accepts an ApprovalInfo parameter

   • Set the MessageText property depending on information in ApprovalInfo
   • Reverse the increase if the result is false

   void OnIncreaseResponse(ApprovalInfo approve)
   {
   	// Set message text
   	string resultText = approve.Result ? "approved" : "rejected";
   	MessageText = string.Format("Order quantity of {0} {1} for {2}",
   		approve.Amount, resultText, approve.CustomerName);
   	MessageVisibility = Visibility.Visible;
   
   	// Reverse increase if rejected
   	if (!approve.Result)
   	{
   		SelectedCustomer.Orders -= 1000;
   	}
   }

When you run the app, click the Increase Orders button. This will push a message to each instance of CustomerDetailViewModel. If the name of the current customer matches that instance's customer name, the view-model will invoke the Completed property of the NotificationEventArgs, which in this case is the OnIncreaseResponse method of CustomerListViewModel.