Prism - Using the Event Aggregator 

Wow, thank you so much

Thank you for the kind words

@@BrianLagunas Very true, I learned a lot from him. Because of Brian, I became confident to work with WPF. Now I really embrace all cool things you can do with both of these technologies. Flicker Free Rendering, Custom Control Development is childs play and fantastic clean scalable architecture which Prism provides.

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Thank you so much for the kind words. I will try not to disappoint. Thanks for watching.

Thank you very much for the kind words. You're awesome!

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Thanks for watching. It is part of the official documentation. Take a look here prismlibrary.github.io/docs/event-aggregator.html

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You can find docs here: prismlibrary.github.io/ and you can see samples here: github.com/PrismLibrary/Prism-Samples-Wpf

Yes, of course. It just cannot be in a Shared Library because each project referencing the shared library will compile it into it's own assembly creating multiple definitions of the same class. So just make sure you define it in any other project type.

There are two primary differences between these two. CanExecute for Commands. A Command can say whether or not it is valid for execution by calling Command.RaiseCanExecuteChanged() and having its CanExecute delegate return false. If you consider the case of a "Save All" CompositeCommand compositing several "Save" commands, but one of the commands saying that it can't execute, the Save All button will automatically disable (nice!). EventAggregator is a Messaging pattern and Commands are a Commanding pattern. Although CompositeCommands are not explicitly a UI pattern, it is implicitly so (generally they are hooked up to an input action, like a Button click). EventAggregator is not this way - any part of the application effectively raise an EventAggregator event: background processes, ViewModels, etc. It is a brokered avenue for messaging across your application with support for things like filtering, background thread execution, etc. Hope this helps explain the differences. It's more difficult to say when to use each, but generally I use the rule of thumb that if it's user interaction that raises the event use a command, for anything else use EventAggregator.

I was using the wrong package of Prism. I was using PrismCore and not Prism for wpf. Hope this is useful for someone

Hard to say without seeing your code, but if you are getting a build error with the InitialComponent, make sure the x:class in the xaml matches the namespace of the code behind file. This can get messed up if you move the MainWindow file to another folder and changed the namespace of the c# file

You can find all Prism WPF samples here: github.com/PrismLibrary/Prism-Samples-Wpf

If that class is a service, then it will automatically be injected by the container when the class is resolved. If you're talking about a business model class, that's code smell however still can be done with the ServiceLocator pattern.

@@BrianLagunas Wow, that was an incredibly quick response. I did it in the service class. Thanks, you are the best.

So what's the problem exactly?

I have two windows services and I need to send a message from one to another (.Net Framework 4.7.2)

Hard to say what your problem is without seeing the code. Put a reproduction app up on github, give me the link, and I'll take a look for you

@@BrianLagunas Ok, please check this out: github.com/Astralius/PrismEventAggregationProblem. I stripped it down to the parts that may matter.

@Brian Lagunas: Have you cloned it yet? :)

@@AzgariusPWI It worked for me 🤷‍♀️

Issue is now resolved on the linked github, with SO reference (for anyone who might have similar problem).

I am actually a Pluralsight author :). The EventAggregator is a much better implementation of the MessagingCenter. From the API usage, to being able to filter events, and controlling which thread the events are invoked on. Simply put, the IEventAggregator is much better than the MessagingCenter

The view you navigate into a region will contain all the functionality of your camera. Nothing changes. Just add your camera code and then navigate to that view.

If events need to be shared, placed them in an assembly that is shared with all projects. Usually this is in some type of Core/Shared project.

If you are asking for the EventAggregator in the View's ctor, then make sure the container is creating your view.

I have no idea. All I know is that you're doing something wrong :)

Yes

It's possible, but not recommended.

@@BrianLagunas Thanks

If it's not working you definitely missed something.

Hard to say what your issue is. What is null? Seems that is the source of your problem.

yes, there is a NullReference exception at ea.GetEvent.Subscribe(Method) .FYI, my MVVM structure is a bit different from you. I have defined a IEventAggregator in main window model and passed it into the subscriber and publisher view models. One of other warnings for me shown as IEventAggregator defined in the mainwindow model will be always null as it has been never set or initialized. It would be nice if you could tell me how to set it or initialize as other variables or classes. Apart from that, I am beginner started learning events recently and came across Prism which I found reduces programming effort and also makes program easy.

@@badboy13592 You need to find out why your EventAggregator (EA) instance is null. Are you creating an instance of it anywhere. Doesn't sound like you are using Dependency Injection, so you'll need to create the instance yourself and make sure you are using the same instance everywhere. static EA ea = new EA() would do it.

@@BrianLagunas Thanks for reply. It worked but the constructor shall be IEA=EA()

There is only one IEventAggregator in Prism. You'll always get the right one.

@@BrianLagunas Oh okay, so if I instantiate a new EventAggregator, I still get the same instance?

@@themrfj No, you can't instantiate it. It must come from the container. So inject the EA into your View's ctor, and use that one.

@@BrianLagunas Hmm, I think I might just stick to events through a shared library. My project doesn't really allow for the viewmodels in my shell project to inherit from a Prism class.

@@themrfj your VMs don't have to inherit from any Prism class.

Can you elaborate on what you mean?

@@BrianLagunas How to call a button out in viewModel? for example : Button button = .... ( taken from View ) ; button.background = Brushes.Red; => i using prism MVVM

You would create properties in your VM and bind those to the button properties.

you mean:

Yes like that. For colors you may need to use a converter on your bidding to convert your property into a brush type

Yeah I learned my lesson when I made these videos, and if you check out my latest videos, you'll see that I don't do this anymore. I was basically copying my Pluralsight course introduction styles. I learn with each video I do. Thanks for the feedback though.

@@BrianLagunas For sure! Lovin' the modern videos :)