MVVM Light - Passing Params to Target ViewModel Before Navigating - Part II

After a lot more thinking about this, I've come up with a really neat solution, needs more testing, but here it is in current form. There's a MessageSender and MessageReceiver class which between them handle sending InitMessageBase type objects and allow buffering and requesting of a message if target is not constructed.


Example code here


Part 1 here


MessageSender

    public class MessageSender<T> where T : InitMessageBase

    {

        private T _message = default(T);

        public MessageSender()

        {

            // Send back item when a constructor asks for it

            Messenger.Default.Register<T>(this, message =>

            {

                if (this._message != null && this._message != message && !this._message.Received)

                {                     

                    // Send back original message

                    Messenger.Default.Send(this._message);

                }

            });

        }

        public void SendMessage(T message)

        {

            // Store value

            this._message = message;

            // Try and send message

            Messenger.Default.Send(message);

        }

    }

MessageReceiver

    public class MessageReceiver<T> where T : InitMessageBase

    {

        private Action<T> _handler = null;

        private T _message = null;

        public MessageReceiver(Action<T> handler)

        {

            this._handler = handler;

            // Wait for messages

            Messenger.Default.Register<T>(this, message =>

            {

                if (this._message != null && this._message != message && !message.Received)

                {

                    message.Received = true;

                    this.OnMessageReceived(message);

                }

            });

        }

        /// <summary>

        /// Send empty message on instantiation

        /// </summary>

        /// <param name="message"></param>

        public MessageReceiver(Action<T> handler, bool sendInitMessage)

            : this(handler)

        {

            if (sendInitMessage)

            {

                this._message = Activator.CreateInstance(typeof(T)) as T;

                // Send empty message

                Messenger.Default.Send<T>(this._message);

            }

        }

        private void OnMessageReceived(T message)

        {

            if (this._handler != null)

                this._handler( message );

        }

    }

InitMessageBase

    public abstract class InitMessageBase : MessageBase

    {

        // Used for a send

        public bool Received { get; set; }

        public InitMessageBase()

        {

        }

    }

An Implementation of InitMessageBase

    public class ArticleInitMessage : InitMessageBase

    {

        public  Article Value { get; set; }

        public ExampleInitMessage()

        {

        }

        public ExampleInitMessage( Article value)

        {

          this.Value = value;

        }

    }

Sending a Message

Simply create a global instance of the MessageSender and send a message or value:

if (this._articleSender == null)

this._articleSender = new MessageSender<ArticleInitMessage>();

this._articleSender.SendMessage(new ArticleInitMessage(article));

this._navService.NavigateTo(ViewModelLocator.ArticleUri);

Receiving a Message

Simply create a global instance of MessageReceiver at VM construct time and pass it an action to do:

this._articleReceiver = new MessageReceiver<ArticleInitMessage>((message) =>

{

this.SelectedItem = message.Value;

}, true);