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Xamarin.Forms and Prism - How to pass data and navigate to another view?

This's my first question here, so hi everybody.
I'm working on the mobile app in Xamarin.Forms with Prism. I've created ListView where shown data from the database.
When the user clicks in the selected row app should navigate to a new view and pass the selected item from ListView.
<ListView x:Name="DefectsBase"
RowHeight="65"
ItemsSource="{Binding Defects}"
ItemSelected="ShowDetailsEvent"
IsPullToRefreshEnabled="true"
RefreshCommand="{Binding Refresh}"
IsRefreshing="{Binding IsRefreshing}">
Code backend:
async void ShowDetailsEvent(object sender, EventArgs e)
{
var myListView = (ListView)sender;
var myItem = myListView.SelectedItem;
var p = new NavigationParameters();
p.Add("selectedDefect", myItem);
await _navigationService.NavigateAsync("DefectDetailsView", p);
}
Unfortunately, the app doesn't respond to pressing the selected row in ListView.

As I can see you are already using Prism and you have a List page with Items and you want to navigate to some details page based on the selected/taped/chosen item which the user taps in the ListView.
The idea is to move as much code and logic as we can to the view model and keep our code-behind. This is pretty easy to solve using Prism and EventToCommand behaviour.
In the example and answer below, I will show you how to solve this with few lines of code, with a nice code approach.
First of all, I recommend you use EventToCommand behaviour, you can include it with prism xmlns, like this: xmlns:prism="http://prismlibrary.com", later on, you can use it with ListView.
Remove ItemSelected event from your ListView and move the markup about it to the <ListView.Behaviors> part. Here is my code sample for the ListView which binds to some ObserverableCollection of the Car models:
<ListView ItemsSource="{Binding Cars}">
<ListView.ItemTemplate>
<DataTemplate>
...
</DataTemplate>
</ListView.ItemTemplate>
<ListView.Behaviors>
<prism:EventToCommandBehavior EventName="ItemTapped"
Command="{Binding SelectedCarCommand}"
EventArgsParameterPath="Item" />
</ListView.Behaviors>
The main part here is <ListView.Behaviors>, where you can see that I am binding to the SelectedCarCommand which will be invoked when the user taps on some of the items from the list. I am using the ItemTapped event for this and passing the current "taped" item from the list as a parameter.
In order to follow this XAML part in my view model of this page, I have declared the DelegateCommand and method which will be called when the command is invoked. The view model part looks like this:
This is my CarListPageViewModel, take a look at DelegateCommand and SelectedCar method.
public class CarListPageViewModel
{
private readonly INavigationService _navigationService;
public ObservableCollection<Car> Cars { get; set; }
public DelegateCommand<Car> SelectedCarCommand { get; private set; }
public CarListPageViewModel(INavigationService navigationService, IDataProvider dataProvider)
{
_navigationService = navigationService;
// Insert test data into collection of Cars
Cars = new ObservableCollection<Car>(dataProvider.GetData());
SelectedCarCommand = new DelegateCommand<Car>(SelectedCar);
}
private async void SelectedCar(Car selectedCar)
{
NavigationParameters navigationParameters = new NavigationParameters
{
{ "selectedCar", selectedCar }
};
await _navigationService.NavigateAsync(nameof(CarDetailsPage), navigationParameters);
}
}
As you can see we have DelegateCommand defined with the type of parameter which will be passed, in my case, this is the Car class, the same class as our items in the ListView.
In the constructor, I did my initialization and defined the method which will be called, that method has a parameter of the type Car.
When the user taps on one of the items in the ListView, SelectedCar (method) will be called and we can pass the data to the next view using NavigationParameters and NavigationService.
In order to retrieve the passed data we can use INavigationAware in the details view model and with the OnNavigatedTo method, access the data which is being passed.
This is my CarDetailsPageViewModel, take a look at OnNavigatedTo method.
public class CarDetailsPageViewModel : BindableBase, INavigationAware
{
private string carTitle;
public string CarTitle
{
get { return carTitle; }
set { SetProperty(ref carTitle, value); }
}
private string photoUrl;
public string PhotoUrl
{
get { return photoUrl; }
set { SetProperty(ref photoUrl, value); }
}
public CarDetailsPageViewModel() { }
public void OnNavigatedTo(INavigationParameters parameters)
{
if (parameters.ContainsKey("selectedCar"))
{
Car car = parameters.GetValue<Car>("selectedCar");
if (car != null)
{
CarTitle = $"{car.Make} {car.Model}";
PhotoUrl = car.PhotoUrl;
}
}
}
public void OnNavigatedFrom(INavigationParameters parameters) { }
}
From this answer and example, you can see:
How to, use EventToCommand behaviour with ListView
Define and use DelegateCommand with passing parameter
How to navigate to another view and pass navigation parameter and
... finally how to access the passed data.
Code and this sample you can find on my GitHub profile here.
Hope this answer was helpful for you!
Wishing you lots of luck with coding! 👋

Hybrid MVVM implementation for a PropertyGrid (in DevExpress)

I need your help! Following is basically what I have in my main XAML view :
<Button x:Name="button1" Content= "{Binding Customer1, Mode=TwoWay}" Margin="271,52,103,106" Click="button1_Click" />
The code-behind of the main XAML (Code-behind, since it's not a 100% pure MVVM, and a rather hybrid one) goes like this :
public MainWindow()
{
InitializeComponent();
this.DataContext = new MyViewModel();
}
private void button1_Click(object sender, RoutedEventArgs e)
{
DXDialog d = new DXDialog("Information", DialogButtons.OkCancel,true);
d.Content = new PropertyGrid();
d.SizeToContent = System.Windows.SizeToContent.WidthAndHeight;
d.Owner = this;
d.WindowStartupLocation = System.Windows.WindowStartupLocation.CenterOwner;
var result = d.ShowDialog();
if (result == true)
{
}
}
As you can see, I have a Button whose content is bound to a String property in the ViewModel Class. Upon Clicking the button, I'm opening a DXDialog which contains a PropertyGrid with the Properties of the ViewModel class. Let me show you my ViewModel Class below :
public class MyViewModel : ViewModelBase, INotifyPropertyChanged
{
Customer currentCustomer;
protected string _customer1;
public string Customer1 {
get { return this._customer1; }
set { this.SetProperty(ref this._customer1, value, "Customer1"); }
}
public MyViewModel()
{
//Customers = new ObservableCollection<Customer>();
//Customers.Add(new Customer() { Name = "Name1" });
Customer1 = "ABC";
}
}
In the Dialog I'm being able to edit the value of the property but don't yet know how I can save it in a way that it immediately reflects even on the button of the main View {Reflects everywhere it must be bound to, I mean}. I can see the execution coming to the following line in the main code behind
if (result == true)
{
}
But I don't know how to get the edited values and plug them into the right place.
Basically, My requirement is to have multiple controls (Buttons, in this case) bound to multiple instances of a ViewModel class, and then, upon clicking the buttons, I should be able to edit those specific ViewModel instances inside the PropertyGrid of the DXDialogue, and after clicking "Ok", the changes should reflect on the relevant buttons as well.
-Ron

To display ViewModel's properties in the PropertyGrid, assign the ViewModel to its SelectedObject property,and make sure that the ShowProperties option is set to All.
Changes will be reflected in buttons bound to the ViewModel only of you use one and the same ViewModel instance in the main and the dialog windows.
var grid = new PropertyGrid();
grid.SelectedObject = this.DataContext;
grid.ShowProperties = ShowPropertiesMode.All;
d.Content = grid;

Prism: View First with multiple ViewModels

I'm using Prism with Unity IOC-Container in a WPF-Project. For all my other Views I'm using only one ViewModel per View. Because this View should be a Mask for both Input and Output of Data, I'd like to use two ViewModels.
For the current navigation to the View i use this Code:
_regionManager.RequestNavigate(RegionNames.ContentRegionName, typeof(Events).ToString());`
The Code Behind of my View:
public partial class Events : UserControl
{
public Events(EventsViewModel viewModel)
{
InitializeComponent();
}
}
One of the ViewModels:
public class EventsViewModel : BindableBase
{
public EventsViewModel()
{
// Some Code
}
// Some other Code
}
I heard about ViewModel Discovery, where you give the Constructor of the View an Interface instead of an actual ViewModel. But i could only find exacly this much information.
// Example of such a Method
public Events(IViewModel viewModel)
{
InitializeComponent();
DataContext = viewModel
}
public Interface IViewModel
{
}
My question is now: How do I navigate to the View and tell it wich ViewModel it should get as DataContext? I'm relatively new to programming and the MVVM-Pattern and english is not my native language so maybe I missed some Information. I would be glad if someone had an answer for this. Thanks in advance.
Edit: Workaround
I came up with a workaround wich works for me. I used the method SetDefaultViewTypeToViewModelTypeResolver() from the ViewModelLocationProvider and customized it.
// Bootstrapper.cs
protected override void InitializeShell()
{
var window = (MainWindow)this.Shell;
Application.Current.MainWindow = window;
// Calling the method
ViewModelLocationProvider.SetDefaultViewTypeToViewModelTypeResolver(ResolveViewModel);
var regionManager = Container.Resolve<IRegionManager>();
window._regionManager = regionManager;
globalRegionManager = regionManager;
regionManager.RegisterViewWithRegion(RegionNames.ContentRegionName, typeof(StartScreen));
regionManager.RegisterViewWithRegion(RegionNames.ContentRegionName, typeof(Stock));
window.Show();
}
// Property for handing over the desired ViewModel
public static Type DynamicViewModel { private get; set; }
private Type ResolveViewModel (Type viewType)
{
string _viewModel = null;
var name = viewType.FullName.Replace(".Views.", ".ViewModels.");
if (DynamicViewModel != null)
_viewModel = DynamicViewModel.ToString();
else
_viewModel = $"{name}ViewModel";
var fullName = IntrospectionExtensions.GetTypeInfo(viewType).Assembly.FullName;
var typeString = string.Format(CultureInfo.InvariantCulture, $"{_viewModel}, {fullName}");
DynamicViewModel = null;
return Type.GetType(typeString);
}
Then when I want to navigate, I hand over the ViewModel beforehand.
private void Button_Click(object sender, System.Windows.RoutedEventArgs e)
{
Bootstrapper.DynamicViewModel = typeof(EventsViewModel);
_regionManager.RequestNavigate(RegionNames.ContentRegionName, typeof(Events).ToString());
}
A little tricky but it seems to work without any Exceptions.
If there is a cleaner way I'm alway happy to here it. :)

Here are some techniques for getting the view model for a view (view first).
View discovery in views code behind constructor
public EventsView(EventsViewModel view_model)
{
InitializeComponent();
DataContext = view_model;
}
Explicitly newing up the view model in code behind constructor
public EventsView()
{
InitializeComponent();
DataContext = new EventsViewModel();
}
View model locator in the XAML for the view
<UserControl x:Class="EventsModule.Views.EventsView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:prism="http://prismlibrary.com/"
prism:ViewModelLocator.AutoWireViewModel="True">
<Grid>
<TextBlock Text="{Binding EventName}"></TextBlock>
</Grid>
</UserControl>
As long as you register your view with the region manager using any one of the techniques below, any one of the above will work. Then navigate like you are doing in your question.
RegionManager.RegisterViewWithRegion(RegionNames.ContentRegionName, typeof(EventsView));
UnityContainer.RegisterType(typeof(object), typeof(EventsView), typeof(EventsView).FullName);
UnityContainer.RegisterTypeForNavigation<EventsView>(typeof(EventsView).FullName);
The first will activate the view in the region and is usually seen in the module Initialize method. Those last 2 are for registering a view for later navigation. The last one requires the Prism.Unity namespace.
I don't believe that a view can have more the one view model since the view object only has one DataContext property on it. You may have to extend one view model to include everything you need. Someone may come along and prove me wrong on this. I have seen where a view model is shared with more than one view, but never a view having more than one view model.

Databinding with unnamed textboxes and listbox

I have been taught lately when using WPF and databinding it is good practice to not name any of the fields but only to associate them with the properties in the other classes. My problem right now is how do I add the data from 3 textboxes (the user enters), save the binded information to the model which then posts the account information into the listbox on the side. I need to add the data to my model. My code from main.xaml is below:
<ListBox ItemsSource="{Binding Path=Files}" SelectedItem="{BindingPath=CurrentItem}" />
<TextBox Text="{Binding Path=bankaccount}"/>
<TextBox Text="{Binding Path=accountnumber}"/>
<TextBox Text="{Binding Path=accounttype}"/>
<Button Content="Save Data To Listbox" Click="Save_Click"/>
Now I will show my FileModel class which holds all of my properties which will be from the textboxes
private short _BankAccount;
private long _AccountNumber;
private char _AccountType;
public short bankaccount{ get { return _BankAccount;} set {_BankAccount= value; Notify("bankaccount"); } }
public long accountnumber{ get { return _AccountNumber;} set {_AccountNumber= value; Notify("accountnumber"); } }
public char accounttype{ get { return _AccountType;} set{_AccountType= value; Notify("accounttype"); } }
I use a class called ProgramModel As my middle point between the Mainpage and my FileModel page and here is that code:
public class ProgramModel : INotifyPropertyChanged
{
public ObservableCollection<FileModel> Files { get; set; }
private FileModel _currentItem;
public FileModel CurrentItem { get { return _currentItem; } set { _currentItem = value; Notify("CurrentItem"); } }
public ProgramModel()
{
Files = new ObservableCollection<FileModel>();
}
And to finish it off I have my mainpage:
internal partial class MainWindow
{
public ProgramModel Model { get; set; }
private ViewSettings _viewSettings = new ViewSettings();
public MainWindow()
{
InitializeComponent();
this.DataContext = Model = new ProgramModel();
}
private void Save_Click(object sender, RoutedEventArgs e)
{
FileModel filemodel = new FileModel();
Model.Files.Add(new FileModel( filemodel.bankaccount, filemodel.accountnumber, filemodel.accounttype));
}
I feel like I am adding to the Files Collection incorrectly from the save button event. If you guys can help me out that would be great! All 3 textboxes and the listbox are on the Main page. Let me know if you have any questions. Also, this is a learning experience so let me know if I posted too much code or not enough. Thanks!

You read the values from a new FileModel instance instead of from what is bound to the view. Code should be this:
Model.Files.Add(new FileModel
(
Model.CurrentItem.bankaccount,
Model.CurrentItem.accountnumber,
Model.CurrentItem.accounttype
));
Make sure CurrentItem is actually initialized with an instance, don't see that in your code. Also, you could use a command here and have all the relevant logic in your bound view model without the need for the event.
Also, right now you bind the current item to the selected item in the ListBox, this will modify an existing instance instead. Not sure if this is intended. If you want those fields to be for input of new instances don't bind the ListBox to it.

I'm not going to answer your question directly because implementing proper data binding will take a bit of code to do so.
Using proper data binding, it is possible to have almost no code behind on your view.cs! (Specially if you start using frameworks)
Please take a look on A Simple MVVM Example for you to follow good practice.
By following this example, you will see that you can also use data binding on buttons and other controls.
Your View Model which is ProgramModel : INotifyPropertyChanged should handle all the work (data processing).
Your model should not handle the UI update notifications thus,
public short bankaccount{ get { return _BankAccount;} set {_BankAccount= value; Notify("bankaccount"); } }
will be moved to the ProgramModel (View Model).
Save_Click method will also be converted into an ICommand and be binded to the button in view like <Button Content="Save Data To Listbox" Command="{Binding SaveExec}"/>
The point is, if you are studying data binding, you should implement it right. Hope you understand...
In the end, it is possible for your Main.cs to only be..
internal partial class MainWindow
{
public MainWindow()
{
InitializeComponent();
this.DataContext = new ProgramModel();
}
}

Just a small change and it should work . Change Your bindings as shown below for the TextBoxes.
<TextBox Text="{Binding Path=CurrentItem.bankaccount}"/>
<TextBox Text="{Binding Path=CurrentItem.accountnumber}"/>
<TextBox Text="{Binding Path=CurrentItem.accounttype}"/>

View Model not updating UI when inside constructor

I have a feeling that my view isn't being updated because the NotifyPropertyChanged event is firing prior to the UI being constructed but I don't know how to overcome this.
I am not really posting code for analysis because I know that the databindings work. They just fail during the construction of the page.
I am strictly posting it so you can get an idea of what I am talking about.
public Obj1 SelectedObj1
{
get { return _SelectedObj1; }
set { _SelectedObj1 = value; NotifyPropertyChanged("SelectedObj1"); }
}
public Obj2 SelectedObj2
{
get { return _SelectedObj2; }
set { _SelectedObj2= value; NotifyPropertyChanged("SelectedObj2"); }
}
public Obj3 SelectedObj3
{
get { return _SelectedObj3; }
set { _SelectedObj3 = value; NotifyPropertyChanged("SelectedObj3"); }
}
Inside my constructor
public constructor(){
BuildFakeData();
SelectedObj1 = observableCollection[0];
SelectedObj2 = SelectedObj1.obj2s.Count > 0 ? SelectedObj1.obj2s[0] : null;
SelectedObj3 = SelectedObj2.obj3s.Count > 0 ? SelectedObj2.obj3s[0] : null;
}
My question is, when you are doing MVVM, if you set bound properties in the constructor, say for a DataGrid selected Row, will it populate or is it failing because the XAML isn't built yet?
Here is where the datacontext is created in the view
<Window.Resources>
<vm:ViewModel x:Key="viewModel"/>
</Window.Resources>
<Grid
DataContext="{StaticResource viewModel}">
Here is where I am setting the selected item for the grid
<igWPF:XamDataGrid
ActiveDataItem="{Binding SelectedObj1}"
DataSource="{Binding observableCollection}"

If your view doesn’t exist yet when the view model is created, then of course, the view isn’t listening yet when your properties update. However, when the view is then created and the view model is assigned as its data context, then the view will automatically load the values from the view model (its data context).
So, INPC shoulnd’t be an issue there at all. You could create properties without INPC in your example and have it work (since the values are already set in the constructor).