Develop Reference

Prism/WPF views/viewmodels being created twice

I have a Prism 7 / WPF / MVVM app that is configured with AutowireViewModel="True" in the views that have a viewmodel. Most of my view models have dependencies that I have configured in the Prism Unity container and these dependencies are injected into the viewmodel contructor. I do not explicitly create instances of views/viewmodels in the code behind anywhere. I also do not set the data context in XAML i.e. using DataContext element or d:DataContext). The only references in the XAML are to views (i.e. part of the HamburgerMenu control).
All is working fine except each view/viewmodel is ALWAYS constructed twice for some reason, whether this is the main window, or views within modules. I have placed breakpoints in the module managers (only gets hit once) - and also in the view constructors and viewmodel constructors which are both hit twice.
The following is some code for App class and also some code/view/viewmodel for a module named Messaging that is loaded at runtime not on demand. Due to wishing to use the MVVM pattern as much as possible, I have very little code in the code behind of the views. NOTE: I tried to see if this is just an issue with views for modules that are loaded on start up, but the same fate is present for the modules loaded on demand.
The App/PrismApplication class in full:
public partial class App : PrismApplication
{
private ILoggerFactory loggerFactory;
private TaskbarIcon _taskbarIcon;
protected override Window CreateShell()
{
// Register an instance of the Window (used by the dialog service to get a handle on the window when displaying dialogs)
Container.GetContainer().RegisterInstance(typeof(Window), "MainWindow", Container.Resolve<MainWindow>(), new ContainerControlledLifetimeManager());
return Container.Resolve<MainWindow>();
}
protected override void OnInitialized()
{
base.OnInitialized();
// Create the links to each module for the banner view to display
var menuService = Container.Resolve<IMenuService>();
var regionManager = Container.Resolve<IRegionManager>();
menuService.AddItem("Home", "Home", () => regionManager.RequestNavigate("MainRegion", "HomeMainView"));
menuService.AddItem("Messaging", "Messaging", () => regionManager.RequestNavigate("MainRegion", "MessagingMainView"));
menuService.AddItem("Charts", "Charts", () => regionManager.RequestNavigate("MainRegion", "ChartsMainView"));
menuService.AddItem("Admin", "Admin", () => regionManager.RequestNavigate("MainRegion", "AdminMainView"));
// Register banner view with region manager and display it
regionManager.RegisterViewWithRegion("BannerRegion", typeof(BannerView));
// Load the desired module into the main window on start up
Container.Resolve<IRegionManager>().RequestNavigate("MainRegion", "HomeMainView");
}
protected override void RegisterTypes(IContainerRegistry containerRegistry)
{
var container = this.ConfigureLogging(containerRegistry);
// Register types
container.RegisterInstance<IDbConnectionFactory>(
new SqlConnectionFactory(
ConfigurationManager.ConnectionStrings["Messaging"].ConnectionString,
loggerFactory));
/************************************************************/
// TODO: Not sure if need singletons or not - to test......
/************************************************************/
containerRegistry.RegisterSingleton<IMetroMessageDisplayService, MetroMessageDisplayService>();
containerRegistry.RegisterSingleton<IUserService, UserService>();
containerRegistry.RegisterSingleton<IUserStore, UserStore>();
containerRegistry.RegisterSingleton<IMenuService, MenuService>();
containerRegistry.Register<ICustomerService, CustomerService>();
containerRegistry.Register<INotifyIconService, NotifyIconService>();
containerRegistry.RegisterDialog<DefaultDialog, DefaultDialogViewModel>("Default");
containerRegistry.RegisterDialog<HtmlDialog, HtmlDialogViewModel>("Html");
// Get the current user's details - prevent a deadlock in the way we use Task.Run(...).Result
// Then add to container as we will be injecting into VMs
var clientUser = Task.Run(GetCurrentUserDetails).Result;
containerRegistry.RegisterInstance(clientUser);
containerRegistry.RegisterSingleton<IClientUser, ClientUser>();
// Add the task bar icon
_taskbarIcon = (TaskbarIcon)FindResource("NotifyIcon");
containerRegistry.RegisterInstance(_taskbarIcon);
// Create a logger instance
var logger = loggerFactory.CreateLogger<App>();
logger.LogDebug("Finished registering types in App.xaml.cs");
}
private async Task<ClientUser> GetCurrentUserDetails()
{
var userService = Container.Resolve<IUserService>();
var data = await userService.GetClientUsersAsync(ConfigurationManager.AppSettings.Get("TempUserName")).ConfigureAwait(true);
if (!data.Any())
{
// log unable to load user from database then return as no point in loading messages for a user that cannot be found!!
return null;
}
return data.FirstOrDefault();
}
protected override IModuleCatalog CreateModuleCatalog()
{
// We are returning a type that reads the modules from the config file.
return new ConfigurationModuleCatalog();
}
protected override void OnExit(ExitEventArgs e)
{
// The icon would clean up automatically, but this is cleaner
_taskbarIcon.Dispose();
base.OnExit(e);
}
private IUnityContainer ConfigureLogging(IContainerRegistry containerRegistry)
{
// Configure logging - Needed Unity.Microsoft.Logging package
// see https://github.com/unitycontainer/microsoft-logging and https://github.com/unitycontainer/examples/tree/master/src/Logging/Microsoft.Logging
var serilogLogger = new LoggerConfiguration()
.ReadFrom.AppSettings()
.CreateLogger();
this.loggerFactory = new LoggerFactory().AddSerilog(serilogLogger);
var container = containerRegistry.GetContainer();
container.AddExtension(new LoggingExtension(loggerFactory));
return container;
}
}
The messaging module:
public class MessagingModule : IModule
{
public void OnInitialized(IContainerProvider containerProvider)
{
// Register main view with region manager
var regionManager = containerProvider.Resolve<IRegionManager>();
regionManager.RegisterViewWithRegion("MainRegion", typeof(MessagingMainView));
}
public void RegisterTypes(IContainerRegistry containerRegistry)
{
containerRegistry.RegisterSingleton<IMessageStore, MessageStore>();
containerRegistry.RegisterSingleton<IMessageService, MessageService>();
containerRegistry.RegisterSingleton<IChatService, ChatService>();
containerRegistry.RegisterSingleton<IFileDialogService, FileDialogService>();
containerRegistry.RegisterDialog<InboxMessageView, InboxMessageViewModel>("HtmlMessage");
// Here we are loading the config/settings from the app.config for this module so we
// can register a ChatServiceConfiguration type as it is injected into ChatService
var filename = Assembly.GetExecutingAssembly().Location;
var configuration = ConfigurationManager.OpenExeConfiguration(filename);
if (configuration != null)
{
var hubUrl = configuration.AppSettings.Settings["HubUrl"].Value;
if (string.IsNullOrEmpty(hubUrl))
{
throw new ArgumentException("The HubUrl app setting cannot ne null or whitespace.");
}
containerRegistry.RegisterInstance(new ChatServiceConfiguration(hubUrl));
containerRegistry.RegisterSingleton<IChatServiceConfiguration, ChatServiceConfiguration>();
}
}
}
The MessagingMainView:
<UserControl x:Class="Ascensos.Wpf.Modules.Messaging.Views.MessagingMainView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:Controls="http://metro.mahapps.com/winfx/xaml/controls"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:iconPacks="http://metro.mahapps.com/winfx/xaml/iconpacks"
xmlns:materialDesign="http://materialdesigninxaml.net/winfx/xaml/themes"
xmlns:materialDesignConverters="clr-namespace:MaterialDesignThemes.Wpf.Converters;assembly=MaterialDesignThemes.Wpf"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:views="clr-namespace:Ascensos.Wpf.Modules.Messaging.Views"
xmlns:helpers="clr-namespace:Ascensos.Wpf.Modules.Messaging.Helpers"
xmlns:prism="http://prismlibrary.com/"
prism:ViewModelLocator.AutoWireViewModel="True"
d:DesignHeight="300"
d:DesignWidth="400"
mc:Ignorable="d">
</UserControl>
The code behind:
public sealed partial class MessagingMainView : UserControl
{
public MessagingMainView()
{
this.InitializeComponent();
}
}
The viewmodel (calls a base message view model class):
public sealed class MessagingMainViewModel
{
public MessagingMainViewModel()
{
}
}
UPDATE: I have removed code from my app, so that only the above code is within the view and viewmodel. The view constructor and viewmodel constructor is still being hit twice during the module initialization.

I do not explicitly create instances of views/viewmodels in the code behind anywhere (i.e. only in XAML).
This is a contradiction - creating a view model in xaml means creating one in the same way as in code behind or anywhere else.
As said before, just don't do it. Remove all references to your view models' constructors from xaml, too (like <DataContext><MyViewViewModel/></DataContext>). To get intellisense in xaml, use d:DataContext.

I have found the issue. It is the following line in App CreateShell() method:
Container.GetContainer().RegisterInstance(typeof(Window), "MainWindow", Container.Resolve<MainWindow>(), new ContainerControlledLifetimeManager());
Commenting this out has sorted the problem. Looking at it again I have been rather silly to not spot this, like the novice I am. This code was added so I could get access to the MainWindow/MetroWindow in a service class - but I will need to try this another way. Thanks for your time #Haukinger.

How is Setup class instantiated in MVVMCross in Xamarin?

I'm starting learning MVVM cross, In the android app, I have a splash screen class:
[Activity(MainLauncher = true,
Label = "#string/app_name",
Theme = "#style/Theme.Splash",
NoHistory = true,
ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation,
ScreenOrientation = ScreenOrientation.Portrait)]
public class SplashScreen : MvxSplashScreenActivity
{
public SplashScreen() : base(Resource.Layout.SplashScreen)
{
}
}
and this is the Setup class:
public class Setup : MvxAndroidSetup
{
protected Setup(Context applicationContext) : base(applicationContext)
{
}
protected override IMvxApplication CreateApp()
{
return null;
}
}
the problem is that the debugger doesn't hit the constructor of the Setup Class, instead I get "An unhandled exception" after the constructor of the splash screen
EDIT
I've already defined the App class in the PCL project:
public class App : MvxApplication
{
public override void Initialize()
{
base.Initialize();
}
also defined the AppStart:
public class AppStart : MvxNavigatingObject, IMvxAppStart
{
public async void Start(object hint = null)
{
//hardcoded login for this demo
//var userService = Mvx.Resolve<IUserDataService>();
//await userService.Login("gillcleeren", "123456");
ShowViewModel<MainViewModel>();
}
}
The main reason behind this project is to understand the sequence of code required and executed by MVVM Cross, so I provide the minimum code till it runs successfully without runtime errors.

Update
I have read your code again more thoroughly and I can see the issue now. You defined the constructor of the Setup class as protected, which makes it invisible for activation.
On MvvmCross for Android the magic happens inside MvxAndroidSetupSingleton class (see the source code here) which searches for the Setup type you defined. The FindSetupType method looks for your defined Setup class first and then inside the CreateSetup method Activator.CreateInstance is used to build the Setup instance. The CreateInstance method variant used however searches only for public constructors, which means it doesn't find your protected one. The result is that it cannot build the Setup class and crashes.
Original answer
The reason this happens is that you have no Core libary that would define the MvvmCross App class and would initialize other required setup. I suggest you to start with a simple tutorial or to look into the official sample projects to see what is necessary to make MvvmCross work in a Xamarin.Android app.

Xamarin iOS Prism (Unity) - Keyboard Extension

I am trying to build a Xamarin Forms Mobile Application using Prism (Unity) Framework - and add a iOS keyboard extension to the Mobile App.
I added the Keyboard Extension Project and added the reference to the iOS application.
I added a ViewController in the Keyboard Extension Project (which created an XIB file as well) as below:
public partial class ViewController1 : UIViewController
{
public ViewController1() : base("ViewController1", null)
{
}
public override void DidReceiveMemoryWarning()
{
base.DidReceiveMemoryWarning();
// Release any cached data, images, etc that aren't in use.
}
public override void ViewDidLoad()
{
base.ViewDidLoad();
// Perform any additional setup after loading the view, typically from a nib.
}
}
When I rebuild and run, and install the keyboard and click on the keyboard icon to switch, the view of the keyboard is not returned.
Here is the code for the App.xaml.cs in main application
public partial class App : PrismApplication
{
public App(IPlatformInitializer initializer = null) : base(initializer)
{
}
protected override void OnInitialized()
{
InitializeComponent();
NavigationService.NavigateAsync("NavigationPage/MainPage?title=Hello%20from%20Xamarin.Forms");
}
protected override void RegisterTypes()
{
Container.RegisterTypeForNavigation<NavigationPage>();
Container.RegisterTypeForNavigation<MainPage>();
}
}
I don't know what's missing...?
I can't find any answers on how to bind the view of the Keyboard by code because I am building this using a Windows PC .. any help would be appreciated.

You have to create an instance of your keyboard view and assign that object to your view controller View property:
public override void ViewDidLoad()
{
base.ViewDidLoad();
//bind view to controller
var viewNib = UINib.FromName("KeyboardView", null);
View = viewNib.Instantiate(this, null)[0] as UIView;
}
Replace "KeyboardView" with name of your .xib file

Error: No matching constructor found on type

I have a WPF application where I am using multiple forms. There is one main form which gets opened when we start the application which is know as MainWindow.xaml. This form then have multiple forms which gets opened depending on the user option. There is a form StartClassWindow.xaml. Currently I am working on this form so I want it to start directly instead of MainWindow.xaml. So to do this I changed the app.xaml startupuri:
<Application x:Class="Class.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
DispatcherUnhandledException="Application_DispatcherUnhandledException"
StartupUri="StartClassWindow.xaml">
<Application.Resources>
</Application.Resources>
</Application>
But then it started giving error like below:
No matching constructor found on type 'Class.StartClassWindow'. You
can use the Arguments or FactoryMethod directives to construct this
type.' Line number '3' and line position '9'.
Here is the StartClassWindow.xaml.cs:
namespace Class
{
public partial class StartClassWindow : System.Windows.Window
{
public StartClassWindow(string classData)
{
InitializeComponent();
className = classData;
function();
}
//rest of the code.
}
}

You need to add a parameter-less constructor to your StartClassWindow like this:
public StartClassWindow(string classData)
{
InitializeComponent();
className = classData;
function();
}
public StartClassWindow()
{
}
Or if you don't want to have another constructor you can override the OnStartup method in the App.xaml.cs but you should remove the StartupUri="StartClassWindow.xaml" in your App.xaml first. Like below:
protected override void OnStartup(StartupEventArgs e)
{
StartClassWindow st = new StartClassWindow("");
st.Show();
}

"Normally", your constructor must be parameterless:
public Login()
But, since you are using dependency injection, like this:
public Login(IUserService userService)
The constructor isn't parameterless, and the framework cannot instantiate the page if it's not told how to.
For this there are a couple of options:
Remove service from constructor
Just like this, but you'll need to access the userservice differently:
public Login()
pre .net 4.8, 4.7, using Unity (or Prism)
You can use a dependency injection framework like Unity to register the components.
It is described here:
https://www.wpftutorial.net/ReferenceArchitecture.html
public class App : Application
{
protected override void OnStartup(StartupEventArgs e)
{
IUnityContainer container = new UnityContainer();
container.RegisterType<IUserService, UserService>();
MainWindow mainWindow = container.Resolve<MainWindow>();
mainWindow.Show();
}
}
Manual using Navigation Service
Manually navigate, and do your own construction:
NavigationService.Navigate(new LoginPage(new UserService);
As described here: https://learn.microsoft.com/en-us/dotnet/desktop/wpf/app-development/navigation-overview?view=netframeworkdesktop-4.8
.net 5 WPF, using built in DI
If you are using .net 5, here is a tutorial. Make sure to register both the window and the service:
https://executecommands.com/dependency-injection-in-wpf-net-core-csharp/
Here's an example:
private void ConfigureServices(ServiceCollection services)
{
services.AddScoped<IUserService,UserService>();
services.AddSingleton<MainWindow>();
}
private void OnStartup(object sender, StartupEventArgs e)
{
var mainWindow = serviceProvider.GetService<MainWindow>();
mainWindow.Show();
}

Prevent view caching in Nancy when using the Spark template engine

I'm using a self-hosted Nancy with Spark templates. I've disabled the cache specifically (though in DEBUG is should be disabled by default).
protected override void ApplicationStartup(Nancy.TinyIoc.TinyIoCContainer container, IPipelines pipelines)
{
base.ApplicationStartup(container, pipelines);
...
StaticConfiguration.Caching.EnableRuntimeViewDiscovery = true;
StaticConfiguration.Caching.EnableRuntimeViewUpdates = true;
}
However, making changes to the templates while the app is running doesn't seem to work, as the template changes are not picked up.
Is there anything else required to disable views caching?

Since you're application is self hosted, I'm guessing you've either overriden the view location convention to find views as embedded resources in your assembly or have configured your Visual Studio project to copy views into the output directory at compile time. In both cases your application is not running off the view files you have in the Visial Studio project, but rather off copies of them. In that case caching is not the issue.

Ok, managed to get this to work by adding a custom ViewCache in the bootstrapper:
public class MyBootstrapper : DefaultNancyBootstrapper
{
#if DEBUG
protected override IRootPathProvider RootPathProvider
{
get
{
// this sets the root folder to the VS project directory
// so that any template updates in VS will be picked up
return new MyPathProvider();
}
}
protected override NancyInternalConfiguration InternalConfiguration
{
get
{
return NancyInternalConfiguration.WithOverrides(
x =>
{ x.ViewCache = typeof(MyViewCache); });
}
}
#endif
The new ViewCache just reloads the template on every request:
public class MyViewCache : IViewCache
{
...
public TCompiledView GetOrAdd<TCompiledView>(
ViewLocationResult viewLocationResult, Func<ViewLocationResult, TCompiledView> valueFactory)
{
//if (viewLocationResult.IsStale())
// {
object old;
this.cache.TryRemove(viewLocationResult, out old);
// }
return (TCompiledView)this.cache.GetOrAdd(viewLocationResult, x => valueFactory(x));
}
}
Somehow the viewLocationResult.IsStale() was always returning false.
By default, this is an instance of FileSystemViewLocationResult which just compares the last update time of the view, but the timestamp this.lastUpdated was being updated before calling IsStale() from the DefaultViewCache, so the template was never removed from the cache
public override bool IsStale()
{
return this.lastUpdated != this.fileSystem.GetLastModified(this.fileName);
}