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Getting a value from a child WPF window that has a seperate viewmodel using the MVVM pattern

I have made quite a bit of progress on my first MVVM WPF application, the issue I am now having is I have a Window that has a viewmodel. This window has a button which opens another window that has another viewmodel.
Imagine a textbox on the first window. Once the second is opened the user will select a value and click save, this window will close and update the first window with its value.
When pushing save I have an ICommand on the childwindows Viewmodel that calls the SaveMethod. I have the selected value stored in a property on the Child windows viewmodel. But how do I update the Main Windows textbox with this value? I imagine I bind a property on the main windows view model, but unsure on how to continue.
Please advise, I can provide code examples if needed, but I think I may have explained it well enough, oh and thanks to everyone at StackOverflow for the help on my questions I have learnt a lot.

This is pretty straightforward using the MVVM Light framework. For the purposes of demonstration I'm going to use a string as the value you're passing, but it's easy to construct a different message type for whatever you need to pass.
In the constructor of your first Window's ViewModel you register to receive NotificationMessages. NotificationMessages are used to send string messages:
public MyFirstViewModel()
{
Messenger.Default.Register<NotificationMessage>(this, NotificationMessageReceived);
}
In the SaveMethod in your second Window's ViewModel you send a message with the value you want to pass. I'm using MyStringValue as the name of the property that stores the value chosen by the user in your second Window:
private void SaveMethod()
{
MessengerInstance.Send(new NotificationMessage(MyStringValue));
}
When that message is received by the ViewModel of the first Window the NoitificationMessageReceived method is called. I'm going to put that value in a string property on the first ViewModel called MySavedValue:
private void NotificationMessageReceived(NotificationMessage msg)
{
MySavedValue = msg.Notification;
}
In your View for the first Window you have a TextBox with its Text property bound to MySavedValue. This updates whenever MySavedValue is updated.

In the parent viewmodel, you'll need a reference to the child viewmodel. When the child window is closed, you'll want to get the value of the secondviewmodel's property and set it to a appropriate property of the first parent viewmodel.

One of the posible (and simple) solutions is to keep one ViewModel for both windows
<Grid>
<StackPanel>
<TextBox Text="{Binding TheText}" />
<Button Command="{Binding ShowOptionsCommand}" Content="..."/>
</StackPanel>
<Popup IsOpen="{Binding IsShowingOptions}">
<StackPanel>
<ListBox ItemsSource="{Binding Options}"
SelectedItem="{Binding SelectedOption,Mode=TwoWay}"/>
<Button Command="{Binding SaveOption}">Save</Button>
</StackPanel>
</Popup>
</Grid>
//ShowOptionsCommand handler
void ShowOptions()
{
IsShowingOptions = true;
}
//SaveOptionCommand handler
void SaveOption()
{
TheText = SelectedOption;
IsShowingOptions = false;
}
I'm using the Popup to simplify the example.

Personally I'd go with the mvvm light framework already mentioned, but another option is to leverage IOC, also included with the above framework.
With this pattern view models have interfaces and are bound as properties from a view model locator data source. Within that, the child view model can be injected to the parent view model. Because IOC can create singleton instances of objects, the same instance gets given to the parent as is bound to the child window. That way you get a reference to the view model but through an interface thus preserving the separation.
Just offering this as an alternative technical solution beyond those already offered.

navigation/load different views on WPF/MVVM

I am quite new to WPF development, and currently I am trying to use the MVVM on my application development. I have read a lot about MVVM navigation and switching views, but I can't find a solution for my current situation. Let's explain what it is:
First of all, I have my main View element, a Dockpanel, with some fixed areas, and a main "dynamic" area where the content should change, depending on actions:
<DockPanel>
<Label Content="Top Fixed element"/>
<StackPanel Orientation="Vertical" Height="auto" Width="150" DockPanel.Dock="Left">
<Label Content="SomeOptions"/>
<!-- some more elements -->
</StackPanel>
<Label DockPanel.Dock="Bottom" Content="Foot"/>
<ContentControl Content="{Binding CurrentMainViewElementViewModel}"/>
</DockPanel>
I have defined some DataTemplates that I would like to load in this ContentControl, here there is one of the Data Templates as example:
<Window.Resources>
<DataTemplate DataType="{x:Type ViewModel:FileLoaderVM}">
<View:FileLoaderView/>
</DataTemplate>
</Window.Resources>
This FileLoader (View and View Model are implemented, using the RelayCommand and the INotifyPropertyChanged) opens a dialog box after clicking a button, where after selecting a file it is opened and parsed, and show all the found elements inside a ListView with multiple selection(in this case, persons with their data).
What I want to do now is to load another user control in this ContentControl, when I click a button. This button is defined in my view model like this:
public ICommand LoadPersons
{
get { return new RelayCommand(param => this.loadSelectedPersons(), param => (SelectedPersons!=null && SelectedPersons.Any()));}
}
My question comes at this point, how can I modify the content of the ContentControl, loading another User Control instead of the current one directly from my view model (in this "this.loadSelectedPersons()")?
If this is not possible, how should I approach to solve this problem?
Next to this action, I want to show all the previously selected elements and manipulate in different possible ways (inserting in a DB, saving in another file and so on), and I have already for that the appropriate User Control, that I would like to show in my main view element in the ContentControl section, keeping the other elements as they are originally.

lets see if i get you right.
you have a mainviewmodel with a property (CurrentMainViewElementViewModel) bound to the ContentControl. your MainViewmodel set the FileLoaderVM to this Property. now you wanna show a "new/other" Viewmodel when a File is seleted in your FileLoaderVM?
why dont you simply expose a event from your FileLoaderVM and subscribe to this event in your MainViewModel? if you do so your MainViewModel can then set the "new/other" Viewmodel to the ContentControl

To change content of ContentControl you do not load another user control, but change value of CurrentMainViewElementViewModel (to which ContentControl.Content is bound) to a new ViewModel, which will load another UserControl (defined in DataTemplate same way as FileLoaderVM is).
This looks like a job for main ViewModel (where CurrentMainViewElementViewModel is located).
Easiest solution is to provide a method in that ViewModel
public Switch()
{
CurrentMainViewElementViewModel = SomeViewModel;
}
and call this method from FileLoaderVM.

WPF and MVVM: bind UserControl to XAML dynamically

seems like a trivial task: i am building a wpf application, using MVVM pattern. what i want is dynamically change part of a view, using different UserControls, dependent on user input.
let's say, i have got 2 UserControls, one with a button, and another with a label.
in main view i have a container for that. following XAML "works":
<GroupBox Header="container" >
<local:UserControlButton />
</GroupBox>
and a UserControl element with buttons pops up. if i change it to another one, it works too.
question is how to feed that groupbox dynamically. if i put something like that in my model view:
private UserControl _myControl;
public UserControl MyControl
{
get
{
return _myControl;
}
set
{
_myControl= value;
InvokePropertyChanged("MyControl");
}
}
and change my view XAML to something like:
<GroupBox Header="container" >
<ItemsControl ItemsSource="{Binding MyControl}" />
</GroupBox>
and feed it from command with usercontrol for button or for label: nothing happens, although "MyControl" variable is set and is "invoke property changed"..

Obviously there are many ways to skin this particular cat - but to answer the question of why it doesn't work you need to look into the ItemsSource property of ItemsControl on MSDN.
The items control is designed to show multiple items, provided through an IEnumerable passed to the ItemsSource property. You are passing a UserControl, so the binding will fail.
For your example, I would change the ItemsControl to a ContentControl and bind the content to your MyControl property. This should then work.
<GroupBox Header="container" >
<ContentControl Content="{Binding MyControl}" />
</GroupBox>
However, I would strongly recommend looking into other ways of doing this - having a control in your VM breaks MVVM to my mind. Depending on what you are doing look at data templates - #Sheridan's link in the comments provides an great description of a way to do it.

Couldn't post this as a comment so adding as answer..
Have a look at this:
Implementing an own "Factory" for reusing Views in WPF
It uses DataTemplates but doesn't require the DataTemplate section for each view. If you potentially have a lot of user controls/views you wish to display or you are reusing through multiple views or you are intending to actually dynamically generate a view (versus just loading an existing user control) then this might suite your needs.

Change a section of a window upon click event

I have a treeview at the left side of the screen, and when I click on any of the TreeViewItem, I want the right side of the screen to change accordingly.
For example, clicking on 'Project' would display on the right half of the screen, a label for project name along with the project name in a text box, and a similar label-textbox pair for some other fields. Clicking on a sub-option of 'Project' such as 'Task 1' should change the right half of the screen such that instead of labels and textboxes for project name and details, it should now be for task name/details. Atm, I only care about label-textbox pairs but in the future I'll need some more sophisticated options, maybe buttons and tables.
What I thought of was to have a grid premade for each option, when I clicked on 'Project' there would be a grid which displays all the info for a Project. And when I then clicked on 'Task 1', the Project grid should be hidden and the Task grid should be displayed with the fields filled out.
Is this possible? What should I be using to create templates that I can then choose from?

Firoz already mentioned the important bit. A rough guess is that you're not using MVVM pattern, so to minimize the adaption effort, you could add a Content Control to your window and set the content of this control whenever a selection is made. You can put any User Control in there.
Using MVVM would mean you bind that Content Control to a property on your ViewModel (of type UIElement or UserControl) and set an instance whenever a bound selected values changes. Speaking of selected Value, I think the default TreeView is not really Binding-friendly, so you might end up with behaviours that do the binding for you.

What you are asking to do is quite easy and possible, but I don't think you are thinking quite big enough.
As your project grows and the number of different things that you want to show expands, then you are going to need to show and hide more and more controls. This is quite quickly going to get unmanageable. Instead think about some other controls deal with this, in some ways you are doing something very like a tabbed dialog, just with a hierarchical set of tabs.
A tabbed dialog has a panel and a set of tabs, when you click on each tab, the content of the panel changes. In fact you can create UserControls one for each specialised set of UI that you want to display, e.g. you could have a ProjectControl that displays all of your project textboxes, labels, buttons etc.
In addition WPF has this neat feature called DataTemplates, these define how a type of data should look when it is displayed. So if you where to have a
public class MyProject
{
public string Name {get;set;}
}
Then you could define
<DataTemplate DataType="{x:Type MyProject}>
<TextBox Text="{Binding Name}"/>
</DataTemplate>
And WPF will automatically convert the data into to its visual form if you set it as the content of the tab panel.
However this type of displaying content in a panel is not the only WPF control that does this. There is also something called a NavigationFrame, which also can be used wrapped into a Window as a NavigationWindow. This control provides you ways to navigate to the next Page to display. Pages can be just like the UserControls in a tabbed dialog, but can also be URIs, enabling you to link in content from the web if you wish. In addition you can call NavigateTo from other controls enabling you build much more usable interfaces.
I worked through the process of building a full windows control panel style interface in
http://alski.net/post/2012/01/11/WPF-Wizards.aspx
and http://alski.net/post/2012/01/13/WPF-Wizards-part-2-Glass.aspx
I've added later VS2012 style glows in
http://alski.net/post/2013/09/14/WPF-Re-creating-VS2012Office-2013-window-glow.aspx
And then released the entire source code as open source at
http://winchrome.codeplex.com/
This comes with support for embedding Navigation panels with
<WinChrome:SearchableNavigationWindow
x:Class="WinChrome.Win7Demo.MainWindow"
...
xmlns:WinChrome="clr-namespace:WinChrome;assembly=WinChrome"
Style="{StaticResource Win7NavigationWindow}">
<WinChrome:SearchableNavigationWindow.Navigation>
<view:Navigation x:Name="navigationTree"/>
</WinChrome:SearchableNavigationWindow.Navigation>
(Full source code)
Where the navigation window is embedded as, but can also be a TreeView.
<UserControl x:Class="WinChrome.View.Navigation" ...>
<ScrollViewer HorizontalScrollBarVisibility="Disabled" Padding="12,0"
VerticalScrollBarVisibility="Auto" >
<StackPanel>
<Button
Margin="0,12,0,0" Style="{StaticResource LinkNavigatorButtonStyle}"
Content="Home"
Command="{Binding
RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Win7Demo:MainWindow}, AncestorLevel=1},
Path=GoHomeCommand}" />
</StackPanel>
</ScrollViewer>
(Full source code)

MVVM Light - Multiple ViewModels (and connecting them up)

I am trying to learn the MVVM pattern (C#), having come from a Windows Forms background. I am using the MVVM Light toolkit, and so far I think it is brilliant.
I have made several small applications, however one thing I am struggling with is introducing a second view.
I want to (for example), have a button on my MainViewModel, which via a RelayCommand, opens up a new Window - let's say an "About" window. I have done hours of research on the web for this however it seems I can't get my AboutViewModel to communicate with/show my AboutView.
I have placed a receiving messenger in the code-behind constructor of the AboutView.xaml - however I can't get it to receive any messages from the AboutViewModel, and thus can't make it 'Show()'.
If anyone has an example of an Mvvm Light WPF app using multiple views that would be great :)

There are two ways I can think to do this easily
The first would be to use a Popup instead of a new Window. For example, I often put properties in my ViewModel for PopupContent and IsPopupVisible, and set those values anytime I want to display my Popup control. For example, a ShowAboutPopup relay command might run something like this:
void ShowAboutPopup()
{
PopupContent = new AboutViewModel();
IsPopupVisible = true;
}
You can display it using a Popup object, or a custom UserControl. I prefer to use my own custom Popup UserControl, which will usually end up looking like this:
<Window>
<Canvas x:Name="RootPanel">
<SomePanel>
<!-- Regular content goes here -->
</SomePanel>
<local:PopupPanel Content="{Binding PopupContent}"
local:PopupPanel.IsPopupVisible="{Binding IsPopupVisible}"
local:PopupPanel.PopupParent="{Binding ElementName=RootPanel}" />
</Canvas>
</Window>
The PopupContent property is a ViewModel (such as an AboutViewModel), and DataTemplates are used to tell WPF to draw specific ViewModels with specific Views
<Window.Resources>
<DataTemplate DataType="{x:Type local:AboutViewModel}">
<local:AboutView />
</DataTemplate>
</Window.Resources>
The other method is to have some kind of ApplicationViewModel that runs on startup, and is responsible for the overall application state, which includes which window(s) are open.
Typically I prefer to have a single ApplicationView that contains a ContentControl to display the current page
<Window>
<ContentControl Content="{Binding CurrentViewModel}" />
</Window>
however it can also be used to manage multiple windows. If you do use it to manage multiple Window objects, be warned that this will not be a pure ViewModel because it will need to access some View-specific objects, and referencing UI objects it not something a ViewModel should do. For example, it may subscribe to receive ShowWindow messages, and upon receiving those messages it would create the specified View and show it, and possibly hide the current window as well.
Personally, I try to avoid multiple windows as much as possible. My usual method is to have a single View that contains consistent application objects for any page, and a ContentControl containing dynamic content that changes. I have an example using this navigation style on my blog if you're interested

As i can see you want a navigation in your MVVM app?
Word goes to the creator of MVVM Light - Laurent Bugnion - with his post about using Navigation Service for switching Views. It's actually about Windows Phone & Silverlight but same should apply to WPF.
Also this answer in related question uses this approach.