This is a question that extends from the originally posted here:
Link to loading-xaml through runtime
I'm working on a WPF MVVM application that loads XAML content dynamically from an external source, very similar as the answer in the post above.
Here is what I got so far:
My View declares an instance of the ViewModel as a resource and creates an instance of that ViewModel
In my ViewModel constructor I'm loading a XamlString property coming from an external source (file or db..)
In my view I have a button that user clicks after ViewModel finishes loading and in the click-event code-behind I'm deserializing the dynamically loaded XAML and add it to my grid.
My question is, how can I eliminate code-behind and automate the logic so the View can render the new xaml section dynamically right after the ViewModel is done getting the XAML content and initializing the string property?
Should I use some kind of Messaging Bus so the ViewModel notifies once the property has been set so the View can add the new content?
What troubles me is the fact that ViewModels do have a reference to Views and should not be in charge of generating UI elements.
Thanks in advance!
Edit:
Just to clarify: in my particular case I am not trying to bind a Business Object or Collection (Model) to a UI element (e.g. Grid) which obviously could be accomplished through templates and binding. My ViewModel is retrieving a whole XAML Form from an external source and setting it as a string property available to the View. My question is: Who should be in charge of deserializing this XAML string property into a UI element and add it programmatically to the my grid once my Xaml string property in the VM is set?
This sounds to me more of like a View responsibility, not ViewModel. But the pattern as i understand it enforces to replace any code-behind logic with V-VM bindings.
I have a working solution now and I'd like to share it. Unfortunately I did not get rid of code-behind completely but it works as I expect it to. Here is how it works(simplified):
I have my simplified ViewModel:
public class MyViewModel : ViewModelBase
{
//This property implements INPC and triggers notification on Set
public string XamlViewData {get;set;}
public ViewModel()
{
GetXamlFormData();
}
//Gets the XAML Form from an external source (e.g. Database, File System)
public void GetXamlFormData()
{
//Set the Xaml String property
XamlViewData = //Logic to get XAML string from external source
}
}
Now my View:
<UserControl.Resources>
<ViewModel:MyViewModel x:Key="Model"></ViewModel:MyViewModel>
</UserControl.Resources>
<Grid DataContext="{StaticResource Model}">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition/>
</Grid.RowDefinitions>
<StackPanel>
<!-- This is the Grid used as a Place Holder to populate the dynamic content!-->
<Grid x:Name="content" Grid.Row="1" Margin="2"/>
<!-- Then create a Hidden TextBlock bound to my XamlString property. Right after binding happens I will trigger an event handled in the code-behind -->
<TextBlock Name="tb_XamlString" Text="{Binding Path=XamlViewData, Mode=TwoWay, UpdateSourceTrigger=LostFocus, NotifyOnValidationError=True, ValidatesOnDataErrors=True, ValidatesOnExceptions=True}" Visibility="Hidden" Loaded="tb_XamlString_Loaded" />
</StackPanel>
</Grid>
Basically I created a hidden TextBlock bound to my XAML String property in the ViewModel and I hooked its Loaded event to an event handler in the code behind of the View:
private void tb_XamlString_Loaded(object sender, RoutedEventArgs routedEventArgs)
{
//First get the ViewModel from DataContext
MyViewModel vm = content.DataContext as MyViewModel;
FrameworkElement rootObject = XamlReader.Parse(vm.XamlViewData) as FrameworkElement;
//Add the XAML portion to the Grid content to render the XAML form dynamically!
content.Children.Add(rootObject);
}
This may not be the most elegant but gets the job done. Like some people say, in MVVM there are some cases like this where little code-behind code is needed. It doesn't hurt and also part of this solution still uses the V-VM Binding principles when using the VM to retrieve and populate the XamlString property and exposing it to the View. If we would like to Unit Test the XAML parsing and loading functionality we could delegate it to a separate class.
I hope someone finds this useful!
I'm having trouble understanding what you're saying, so my answer will be based on my interpretation. You should consider posting a sample (simplified) of what you're trying to do.
1) I think you're misunderstanding what MVVM does. MVVM is mostly a binding-based pattern. Your view model should be exposing properties containing business objects and your view should just be binding to those properties. If I am misunderstanding you, and that's what you are doing, then your problem is that your view needs to be aware of when the properties get updated (after you deserialize your xaml, etc). There are two ways to do this: INotifyPropertyChanged interface on your viewmodel, or make your view model inherit from DependencyObject, and make the properties dependency properties. I won't go into details here, because this is a large subject that you should research on Google before making a decision.
2) Generally speaking, you shouldn't use click events inside your view if you're using MVVM. Instead, create properties on the view model of type ICommand (and create ICommand implementations to match, or use an implementation of DelegateCommand (google it) which will allow you to use delegates to implement the interface. The idea is, your view binds to the property and executes the handler directly inside the viewmodel.
3) If you want to push information from the viewmodel to the view, then you should create an event on the viewmodel and subscribe to it in the view, but this is a last resort, only to be used in cases like displaying a new window, etc. Generally, you should be using binding.
4) To be more specific about what you're doing, you should be binding your Grid's ItemsSource property to some property on the view model. Note, the property on the view model should be of type ObservableCollection<T> if you want to be able to add items and get instant updates.
Hope this helps.
Related
I have a view PeopleView that displays a list of People bound to a collection of People in PeopleViewModel.
In a panel within PeopleView I have a form (just a collection of text boxes and a button) which allows users to add a new person (People).
Initially I had this collection of controls as part of PeopleView but I want to separate it out into a separate UserControl (AddPerson) as it doesn't necessarily depend on the existing list of people and this AddPerson control could exist anywhere else in the application. I might want to reuse it elsewhere.
However, I'm struggling to work out where I should now be putting my button command and the associated CanExecute logic now.
It would have been in the PeopleViewModel but my AddPerson UserControl doesn't have a ViewModel. I've read that typically you wouldn't have a ViewModel for a UserControl, but I don't know how else I can make a reusable control that also contains its own business logic?
Should my AddPerson UserControl have a ViewModel, or should I be doing the business logic in the codebehind (seems very unlikely), or something else?
Commands traditionally go in the ViewModel but there's no hard fast rule on this. If you'd like to make a more reusable control, you can create a command dependency property on the control itself and then bind it to a command in your view model when the control is implemented.
This is identical to how button command is implemented
For example
MyUserControl.Xaml
<UserControl x:name="self">
<Grid>
<Button Command="{Binding ElementName=self, Path=MyDPCommand}"/>
</Grid>
</UserControl>
MyUserControl.Xaml.cs
public ICommand MyDPCommand
{
get { return (ICommand)GetValue(MyDPCommandProperty); }
set { SetValue(MyDPCommandProperty, value); }
}
public static readonly DependencyProperty MyDPCommandProperty =
DependencyProperty.Register(nameof(MyDPCommand), typeof(ICommand), typeof(MyUserControl), new PropertyMetadata(null));
And then in your implementation xaml you can bind it back to your viewmodel
MyView.Xaml
<MyUserControl MyDpCommand="{Binding MyViewModelCommand}"/>
In the end I've gone the route of implementing a ViewModel for my AddPerson view because, although it is technically a UserControl, I really am using it like any other view rather than a generic control.
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.
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.
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.
I am new to MVVM and WPF so this might be a broad or a dumb question, but:
I am using the MVVM pattern and have 1 Viewmodel, several views and a couple of models.
All of the views are just Usercontrols which are put on my mainwindow.xaml.
The view in question is bound to a model wich have several properties, one of which i want to use to dynamically change a picture in the usercontrol.
I am having a very difficult time trying to acces this property and my question is how i do this the "right" MVVM way.
My mainwindow.xaml:
<Window.Resources>
<DataTemplate DataType="{x:Type Model:Device}">
<Canvas>
<View:DeviceUserControl/>
</Canvas>
</DataTemplate>
</Window.Resources>
//---- SNIP----
<Grid Name="grid1">
<ItemsControl ItemsSource="{Binding Devices}" />
</Grid>
DeviceUserControl.xaml
//--- SNIP ---
Image Name="DeviceImage" Source="{StaticResource IconAdd}"/>
DeviceModel
//--- SNIP ---
public enum Typeenum
{
FrequenceGenerator,
Oscilloscope,
Test1,
Test2
};
public Typeenum Type { get { return type; } set { type = value; NotifyPropertyChanged("Type"); } }
I want to change the DeviceImage based on the type of the object. I have tried dependencyproperties, but it didnt work as expected (It returned the same type everytime).
I dont really need the notifyPropertyChanged as i am only interested in changing the image source when the Usercontrol is instantiated.
First of all, you should bind Views to ViewModels, not Models. At least that's what MVVM is all about. Also, if you want something to happen when a property changes, then one way is to subscribe to the PropertyChanged event in your ViewModel (which I assume you know should implement the INotifyPropertyChanged interface) then put your logic on what should happen on the property change there.
Code Sample
this.PropertyChanged += (s,e)=>{
// Your code here.
// e.g. this.MyImageSource = "http://img.com/image.jpg"
}
The code sample assumes that your event for the property changes is called PropertyChanged and that the image control's source is data bound to the MyImageSource property in the ViewModel.
Hope this helps.