How to pass a parameter from a viewmodel to its parent viewmodel - mvvmcross

Question

First View is a listview including some items.
eg. item0, item1, item2
When click a "new" button, the second view will be shown.
Then click a "save" button and input a name(eg. item3). The "item3" will be save in seconde viewmodel
After saving success. The first view will refresh and show "item3"
How to pass name "item3" from second viewmodel to first viewmodel for showing in first view?

Answer 1

Use the Messenger Plugin in MVVMCross. The First VM registers for a message. The 2nd VM Publishes(Notifies) the 1st one by publishing a Message of the Message type(class) registered. For example:
1st VM
In order to use the messenger plugin you need to register it first, using Injection as follows:
private readonly IMvxMessenger _messenger; // VM Instance var
private MvxSubscriptionToken myToken;
public FirstViewModel(IDataService dataService, IMvxMessenger messenger)
{
// Registers the DataService(SQLite Plugin) and the Messenger Service Plugin
_dataService = dataService;
_messenger = messenger;
// Suscribe to the Meessage and pass it the name of a method to be called when the 2nd VM(the Publisher), publishes(notifies) a MyMessage to all Subscribers...
myToken = _messenger.Subscribe<MyMessage>( OnMyMessageArrived );
}
private void OnMyMessageArrived( MyMessage p_myMessage )
{
// Set the 1st VM's property from p_myMessage
MyProperty = p_myMessage.Item3
}
And the MyMessage class should look like this:
public class MyMessage : MvxMessage
{
public MyMessage( object sender, string p_item3 ): base( sender )
{
Item3 = p_item3;
}
public string Item3 { private set; get; }
}
And the 2nd VM, when ready to send the value back, it should Publish the MyMessage with the value to be passed back to 1st VM(and any subscribers) as follows:
_messenger.Publish( new MyMessage( this, item3 ) );
It's a good practice to have the Subscribers to unsubscribe from any messages they subscribe to, so back in the 1st VM, perhaps just before you close it you should do:
_messenger.Unsubscribe<MyMessage>( myToken );
For a more complete example checkout Stuart's N=9 video and associated sample. Here's the link to both:
http://slodge.blogspot.com/2013/05/n9-getting-message-n1-days-of-mvvmcross.html
R,
Pap

Answer 2

I'm not familiar with mvvmcross, so I'll answer your question using general M/VM/V/C architecture patterns:
Views and ViewModels shouldn't be aware of each other, that's the Controller's job. Your FirstView should alert its controller that the user clicked New and it is the controller's responsibility to then create and manage your SecondView.
The process would be like this (abstract, psuedocode-ish):
void ControllerAction() {
using(View listView = new MyListView() ) {
ListViewViewModel listViewVM = CreateListViewVM();
listViewVM.NewClicked = () => this.NewClicked(listViewVM); // wiring-up ViewModel events via anonymous methods
listView.SetViewModel( listViewVM );
listView.ShowModal();
}
}
void NewClicked(ListViewViewModel parentViewViewModel) {
using(View newEntryView = new NewEntryView()) {
if( newEntryView.ShowDialog() == DialogResult.OK ) {
parentViewViewModel.Refresh();
parentViewViewModel.SelectedItem = newEntryView.NewItemText; // passing the new item's name
}
}
}