How to call a function(which works with UIElements) in View from ViewModel using MVVM pattern

Question

I'm using MVVM Light Toolkit and in my View, I have a function that takes a screenshot and returns byte array of that screenshot. Since taking an screenshot (using UIElements) is related to view not ViewModel.

byte[] TakeScreenShot(Canvas sourceUiElement)    

I need to get the return value of the function in my ViewModel but I can't come up with a proper way of doing it.

I other hand if I wanted to do move this function to my ViewModel, I need to have access to that element in view but without referencing the View in my ViewModel (maybe as argument or something to a Command?)

Answer 1

Since this question is tagged as MvvmLight, then here is an MvvmLight Toolkit answer. Use said toolkit's Messenger class. Simply define the following message classes somewhere in your application:

public class TakeScreenshotMessage : MessageBase { }
public class ScreenshotTakenMessage : GenericMessage<byte[]>
{
    public ScreenshotTakenMessage (byte[]content) : base(content) { }
    public ScreenshotTakenMessage (object sender, byte[]content) : base(sender, content) { }
    public ScreenshotTakenMessage (object sender, object target, byte[]content) : base(sender, target, content) { }
}

In your code-behind's constructor, register for the TakeScreenshotMessage like this:

Messenger.Default.Register<TakeScreenshotMessage>(this, (msg) =>
{
    byte[] bytes = this.TakeScreenShot(someCanvas);
    Messenger.Default.Send(new ScreenshotTakenMessage(bytes));
});

And in your view model, register for the ScreenshotTakenMessage like this:

Messenger.Default.Register<ScreenshotTakenMessage>(this, (msg) =>
{
    byte[] bytes = msg.Content.
    //do something with your bytes
});

Now you can take a screen shot at any time simply by calling the following from anywhere in your application (i.e. view models, views, helper classes, etc.):

Messenger.Default.Send(new TakeScreenshotMessage());