MVVM Light messenger service not receiving message in view model - wpf

Question

Not sure if I am just doing this wrong or am misunderstanding some of the examples already on stack overflow here and here.
I am trying to take a selected item from my first view model and pass it to another view model I am navigating to. The purpose of this is so I can display the item that has been passed and allow the user to work with it.
Passing from first view model
This is just a small snippet of the first view model. Here I am first navigating to the new page/view model. Then pass the SelectedRule object using a messenger. Navigation is done using a ViewModelLocator class / navigation service provided with MVVM Light.
private ApprovedBomRule _selectedRule = new ApprovedBomRule();
public ApprovedBomRule SelectedRule
{
get { return _selectedRule;}
set { Set(ref _selectedRule, value); }
}
private void NavigateToUpdateRule()
{
//Navigate to Update Rule page
_navigationService.NavigateTo("UpdateBomRuleView");
//Pass selected rule as a parameter using messenger service
ApprovedBomRule ruleToSend = SelectedRule; // Selected by user.
Messenger.Default.Send(ruleToSend);
}
On receiving view model
Here is my second view model where I register the same type of SelectedRule from above and set it to the public variable.
public class UpdateBomRuleViewModel : ViewModelBase
{
private ApprovedBomRule _passedRule;
public ApprovedBomRule PassedRule
{
get => _passedRule;
set => Set(ref _passedRule, value);
}
//Constructor
public UpdateBomRuleViewModel()
{
//Register message type
Messenger.Default.Register<ApprovedBomRule>(this, GetMessage);
}
//Set the property to passed object
public void GetMessage(ApprovedBomRule rule)
{
PassedRule = rule;
}
}
My constructor is reached and the register method is set, but the GetMessage() function is never called. What am I missing here?
EDIT
I narrowed down the problem to the fact that the register method is being called after the message is sent. Now the second problem I am running into is how do I have my second view model register before the send? I am using a viewmodel locator in my pages to determine the view models for each page. Even though I am doing the _navigation.NavigateTo() before sending the data, the view model is not initialized until after the send.
Example of viewmodel locator in page
<local:BasePage x:Class="YAI.BomConfigurator.Desktop.Views.Rules.UpdateBomRuleView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:YAI.BomConfigurator.Desktop"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800"
Title="UpdateBomRuleView"
DataContext="{Binding UpdateBomRuleViewModel, Source={StaticResource Locator}}">
<Grid>
<TextBlock Text="{Binding PassedRule.Description}"
VerticalAlignment="Center"
HorizontalAlignment="Center">
</TextBlock>
</Grid>

Answer 1

Okay so I sort of found a solution to the problem. I used my ServiceLocator to get the instance before navigating.
var vm = ServiceLocator.Current.GetInstance<UpdateBomRuleViewModel>();
//Navigate to Update Rule page
_navigationService.NavigateTo("UpdateBomRuleView");
//Pass selected rule as a parameter using messenger service
ApprovedBomRule ruleToSend = SelectedRule; // Selected by user.
Messenger.Default.Send(ruleToSend);
This caused my register to be called before the send. I don't necessarily like this solution because the var vm is not being used for anything, but it works for now.
thank you for looking at the question.

Answer 2

You need to wait for the page show up before sending the message. It is weird that MVVMLight doesn't offer any NavigateAsync method like Prism, so you have to roll for your own.
await Application.Current.Dispatcher.Invoke(
() => _navigationService.NavigateTo("UpdateBomRuleView");
ApprovedBomRule ruleToSend = SelectedRule; // Selected by user.
Messenger.Default.Send(ruleToSend);
Slightly modified from my UWP code, but it should be fine for WPF.