Send object to VIewModel with mvvm-light

Question

I'm pretty new to MVVM light world, and after searches I can't find what I want to do.

My WP7 application contains a pivot, each pivot item content is View1 and viewmodel is VM1.

When loading my application, I'd like to create every pivot item with the same view and view model but with different parameter.

example :

PivotItem 1 -> send param "car" to the view model PivotItem 2 -> send param "truck" to the view model, etc.

Google told me to use messaging but if I send 2 messages from my MainViewModel, both PivotItem1 and PivotItem2 ViewModel will receive these messages.

Am I wrong with this approach ? Is there another solution to succeed ?

Thank you in advance for your answer.

PS : be indulgent, english is not my native language, don't hesitate to ask for further information.

Regards,

Aymeric Lagier

Answer 1

To seperate the messages use the second constructor signature whereby you can pass a token. This token can be anything but I generally use an enum to store all my message types within the system. Create a static class in a common library and reference this in all projects where you need to send or receive messages.

The following code hopefully shows this approach, notice I am sending a string as a value within the message but this can be anything, even a complex object such as one of your business objects.

namespace MyProject.Common
{
    public static class AppMessages
    {
        enum MessageTypes
        {
             ViewmodelA,
             ViewmodelB
        }

        public static class ViewModelAUpdate
        {
            public static void Send(string value)
            {
                 Messenger.Default.Send(value, MessageTypes.ViewmodelA);
            }

            public static void Register(object recipient, Action<string> action)
            {
                Messenger.Default.Register(recipient, MessageTypes.ViewmodelA, action);
            }
        }

        public static class ViewModelBUpdate
        {
            public static void Send(string value)
            {
                Messenger.Default.Send(value, MessageTypes.ViewmodelB);
            }

            public static void Register(object recipient, Action<string> action)
            {
                Messenger.Default.Register(recipient, MessageTypes.ViewmodelB, action);
            }
        }
    }
}

Answer 2

How about using a method to set the message you want to receive. (this could be done as a parameter in the constructor or a property as well)

    public void RegisterForAppMessage(AppMessages.MessageTypes messageType)
    {
        switch (messageType)
        {
            case AppMessages.MessageTypes.PivotViewItem1Message:
                 AppMessages.PivotViewItem1Message.Register(this,DoSomethingWhenIRecievePivotViewItem1Messages)
            break;
            case AppMessages.MessageTypes.PivotViewItem2Message:
                 AppMessages.PivotViewItem2Message.Register(this,DoSomethingWhenIRecievePivotViewItem2Messages)
            break;
        }
    }

    private void DoSomethingWhenIRecievePivotViewItem2Messages(string obj)
    {
        // TODO: Implement this method
        throw new NotImplementedException();
    }

    private void DoSomethingWhenIRecievePivotViewItem1Messages(string obj)
    {
        // TODO: Implement this method
        throw new NotImplementedException();
    }

Answer 3

Messaging sounds a bit heavy for this purpose. Could you simply inject a parameter into your ViewModel. If you already have MVVMLight you also have support for SimpleIOC. Maybe let the view locate its ViewModel when the view is resolved and there decide which parameter to use on the view model? You can see an example of it here