Question

I can’t seem to find a way to detect when a panel finishes loading. I’m adding a couple features to an already existing WPF/Prism project. I need to get called back when the page finishes rendering. I tried this:

In the UserControl at the top of the XAML that I need to get called back on when the page loads:

<UserControl x:....

xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
...>

<i:Interaction.Triggers>
        <i:EventTrigger EventName="Loaded">
            <i:InvokeCommandAction Command="{Binding DoSomething}" />
        </i:EventTrigger>
    </i:Interaction.Triggers>

Then in the code behind file:

public void DoSomething(object sender, EventArgs e)
{
     System.Diagnostics.Debug.WriteLine("received callback");
}

It currently does not work and I was not sure if this is the right path or not since I’m new to WPF, prism, and .NET.

Answer 1

Well I see you already know the answer… anyway…
What you did wrong you had been binding event to command in your XAML and you didn’t have command 🙂 Basicly you messed up standard “code-behind approach” with let’s say “MVVM approach”/binding.

So when you touched the code-behind and created your DoSomethingEventHandler there you could simply

    <UserControl .. Loaded="UserControl_Loaded"> 
    .. 
    </UserControl>

with your CodeBehind:

    private void UserControl_Loaded(object sender, System.Windows.RoutedEventArgs e)
    {
        // Do Something
    }

It would be fine… but it’s not MVVM :]

Well and to your new approach (no code-behind/let’s say MVVM)…

If you use Prism v4.1 you can look here this feature most people miss in Prism 🙂 but there is solution mentioned 🙂

Well as you know you need ICommand you are binding to… It does not have to be DelegateCommand you can use what you want… (I prefer and use RelayCommand)

Well but what we learned from aforementioned link?

1. Sometimes… not this time… ICommand can change Controls enabled state and it’s not updated when you don’t use Prism's Interactivity and you simply use Blend SDK's Interactivity… You don’t need it this time… I admit… but maybe some day 🙂
2. And you don’t need those silly parameters in handler since you don’t get them anyway :D(When you need those Args You can use MVVM Light for example.)

So the best you can do is this:

<UserControl
    xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
    xmlns:prism="clr-namespace:Microsoft.Practices.Prism.Interactivity;assembly=Microsoft.Practices.Prism.Interactivity"
...    
>
...
<i:Interaction.Triggers>
    <i:EventTrigger EventName="Loaded">
        <prism:InvokeCommandAction Command="{Binding MyCommand}" />
    </i:EventTrigger>
</i:Interaction.Triggers>

and your ViewModel for example:

    public ICommand MyCommand
    {
        get
        {
            return new RelayCommand(this.MyCommandExecute);
        }
    }

    private void MyCommandExecute()
    {
        // DO Something...
    }

Sorry that I unswered already answered question, but I thought I might add some interesting info 🙂