I want to call a service interface in the ViewModel constructor without using any libraries for MVVVM like MVVMLight etc like
public HomePageViewModel(IHomeService homeService)
{
}
This is how we do it using a MVVM light.
IService _tService;
public HomePageViewModel(IHomeService homeService, INavigationServiceExtended navigationService, IService tService)
{
_tService=tService;
}
How can I get the send the IHomeService instance from the view while binding the View to the ViewModel .
When using DI you either inject the dependency in the calling code, in your case you do something like this:
var homePageViewMdel = new HomePageViewModel(new HomeServiceImplementation());
Or either you would use an IoC container like (Unity, DryIoc or Autofac) or write your own IoC from scratch.
Related
I'm devolping a WPF application, using Prism 7.2. I have a module, which implements the IModule interface, where I register the views and viewmodels in the RegisterTypes method, e.g.:
containerRegistry.Register<IPanelOptimizationViewModel, PanelOptimizationViewModel>();
The problem arises when I try to resolve the implementation:
var vm = containerProvider.Resolve<IPanelOptimizationViewModel>();
whereupon I get the following Unity.ResolutionFailedException:
'Resolution failed with error: No public constructor is available for type
XXX.Infrastructure.Interfaces.IView.'
The PanelOptimizationViewModel class derives from a base class:
public class PanelOptimizationViewModel : ViewModelBase, IPanelOptimizationViewModel
{
public PanelOptimizationViewModel(IPanelOptimizationView view, IPanelOptimizationInputViewModel inpVM) : base(view)
}
and the ViewModelBase looks like this:
public class ViewModelBase : BindableBase, IViewModel
{
public IView View { get; set; }
public ViewModelBase(IView view)
{
View = view;
View.ViewModel = this;
}
}
The interfaces IView and IViewModel are defined in a common Infrastructure project. They are not registered anywhere in the container, but if I remove the IPanelOptimizationInputViewModel parameter, no runtime exception is thrown - leading me to think that I don't need to do this, either.
As far as I have been able to understand, the Unity.Container will use the "most parameterized" constructor (see Unity not using the default constructor of the class), but I cannot provide a parameter in the Register method to specify this, as one apparently could before (pre Prism 7's container abstraction), with the RegisterType method.
How to solve this? Is there an overload of the Prism.Ioc.IContainerRegistry.Register method that allows me to set up the registration for constructor injection?
Should I work directly with the Unity container?
Basically, I am trying to inject a child view's viewmodel into the constructor of my "main" viewmodel, but this does not go well as long as the wrong constructor is called on the base class, with the wrong set of parameters... (if that is what is happening).
Needless to say, all child views and viewmodels have been registered in the RegisterTypes method in the module.
Any help on this would be greatly appreciated
Should I work directly with the Unity container?
Yes, you can evade Prism's "abstraction" of the container by calling the GetContainer() extension method (for your container).
containerRegistry.GetContainer() // here you get a plain IUnityContainer
.RegisterType( ... );
I started working with the MVVM pattern in a new project.
Everything is ok, but i came to the following problem.
The implementation looks like this:
I have a MainView, the main app window. In this window i have a telerik RadGroupPanel in wich I host the rest of the app views as tabs.
The rest of the viewModels does not know about this RadGroupPanel which is hosted in MainVIew.
How should i correctly add those views to the RadGroupPanel from the commands in the viewModels?
Thanks.
Have you considered injecting your view into the ViewModel using an interface to maintain separation? I know this breaks MVVM but I've successfully used this on a number of WPF projects. I call it MiVVM or Model Interface-to-View ViewModel.
The pattern is simple. Your Usercontrol should have an interface, call it IView. Then in the ViewModel you have a property with a setter of type IMyView, say
public IMyView InjectedView { set { _injectedView = value; } }
Then in the view you create a dependency property called This
public MyUserControl : IMyView
{
public static readonly DependencyProperty ThisProperty =
DependencyProperty.Register("This", typeof(IMyView), typeof(MyUserControl));
public MyUserControl()
{
SetValue(ThisProperty, this);
}
public IMyView This { get { return GetValue(ThisProperty); } set { /* do nothing */ } }
}
finally in Xaml you can inject the view directly into the ViewModel using binding
<MyUserControl This="{Binding InjectedView, Mode=OneWayToSource}"/>
Try it out! I've used this pattern many times and you get an interface to the view injected once on startup. This means you maintain separation (Viewmodel can be tested as IView can be mocked), yet you get around the lack of binding support in many third party controls. Plus, its fast. Did you know binding uses reflection?
There's a demo project showcasing this pattern on the blog link above. I'd advocate trying out the Attached Property implementation of MiVVM if you are using a third party control.
You can have the list of viewmodels that you need to add controls for in an ObservableCollection in your main window viewmodel. You can then bind the ItemsSource of the RadGroupPanel to that collection and use the ItemTemplateSelector and ContentTemplateSelector of the RadGroupPanel to select the right template to use based on the viewmodel that is bound.
I have MVVM Project and I want to share one object( singleton ) from the model between several viewmodel what is the good practice to do that?
Thank you for the help
If the object is needed and does not provide value without it force the interface within the object via Constructor Injection; do not push a concrete type via injection always make use of an interface.
Since you are not making use of an IoC container such as Unity, you will need to create your singleton instance at the startup of your application and then make sure that the given instance is passed in via the given ViewModels constructor as needed.
A better approach would be pushing the singleton instance to a service which can provide the needed behavior and then disregard pushing the singleton into the Model. This would be a more MVVM purist approach and will separate concerns across your Models/ViewModels.
EDIT:
If you were making use of Unity you would define a Lifetime Manager at the time of registration.
// Register a type to have a singleton lifetime without mapping the type
// Uses the container only to implement singleton behavior
myContainer.RegisterType<MySingletonObject>(new ContainerControlledLifetimeManager());
// Following code will return a singleton instance of MySingletonObject
// Container will take over lifetime management of the object
myContainer.Resolve<MySingletonObject>();
Once you do this any attempt to resolve MySingletonObject via the IUnityContainer would resolve to the same instance providing the singleton behavior you so desire across the application. ViewModels themselves should not need to have the same instance returned. The data it needs should be abstracted away via a service as referenced earlier which could potentially behave like a singleton and provide a stateful implementation if needed but the ViewModel should not need to be a singleton. If you find yourself making either a Model or ViewModel a singleton; take a step back and analyze your design.
If you have control over all viewmodels then an easy approach (that I've used personally) is to just put a static variable on the base class of all viewmodels and make that accessible to all inheritors (either protected or even public if its useful outside of the viewmodels).
It's good practice anyway to have a common base class for your viewmodels since it allows you to implement property notification in one place (and other common functionality, like messaging etc.) instead of replicating it in all viewmodels.
Something like this is what I've used in my projects:
public class MyViewModelBase : INotifyPropertyChanged
{
private static MySharedSingleton _sharedObj;
static MyViewModelBase()
{
_sharedObj = new MySharedSingleton(/* initialize it here if needed */);
}
// or public
protected MySharedSingleton SharedObject { get { return _sharedObj; } }
// INotifyPropertyChanged stuff
// ...
}
public class SomeViewModel : MyViewModelBase
{
void SomeMethod()
{
SharedObject.DoStuff();
}
}
If the construction of the shared singleton object is heavy, you can of course use any of the standard techniques for lazy instantiation of it.
I would suggest that you inject the dependency into each view model (either constructor or property injection for example), and always work against abstractions in your view models, so that your dependency can easily be mocked or replaced as required. You then just need to ensure that each view model uses the same instance of your type - if you are using an IoC container, you can register a shared instance of your type easily.
I use a separate class for my global singleton with a model. This relieves me of agonizing over how to inject this model into view models and other models. E.g.
The singleton:
public class ApplicationModel
{
public string LoggedOnUser { get; set; }
// Etc.
private ApplicationModel() {
// Set things up.
}
private static ApplicationModel _active;
public static ApplicationModel Current {
get {
if (_active == null) {
_active = new ApplicationModel();
}
return _active;
}
}
}
The view model that needs to hold no reference to the singleton:
public class SomeViewModel
{
private string _user;
public SomeViewModel() {
_user = ApplicationModel.Current.LoggedOnUser;
}
}
I have an ASP.Net MVC application and I am using StructureMap within MVC to glue the whole application together. There are some model classes that have heavyweight dependencies that are not used in all public methods, so I pass in an IContainer to the model constructor and use that to create the heavyweight dependencies on demand.
My question is where does the IContainer come from that is injected? Is it a reference to the one held centrally by MVC (it's logical parent) or is it a brand new one created and configured solely for this class?
The container injected into a constructor that has an IContainer parameter is the same container that is creating the instance of the class with the constructor.
Jeremy Miller has expressed this behaviour as "IContainer is injected into itself by default" in his blog post on NHibernate with StructureMap.
Couldn't you go with a factory model for creating those dependencies when needed in order to reduce your coupling to the container?
You could make your model take a Func as parameter and use SM's ability to autoinject those:
public class MyModel
{
Func<IHeavyDep> _heavyFactory;
public MyModel(Func<IHeavyDep> dependency)
{
_heavyFactory = dependency;
}
public void UsesHeavy()
{
var heavy = _heavyFactory();
heavy.DoMyStuff();
}
}
I'm using constructor dependency injection in my WPF application and I keep running into the following pattern, so would like to get other people's opinion on it and hear about alternative solutions.
The goal is to wire up a hierarchy of ViewModels to a similar hierarchy of Models, so that the responsibility for presenting the information in each model lies with its own ViewModel implementation. (The pattern also crops up under other circumstances but MVVM should make for a good example.)
Here's a simplified example. Given that I have a model that has a collection of further models:
public interface IPerson
{
IEnumerable<IAddress> Addresses { get; }
}
public interface IAddress
{
}
I would like to mirror this hierarchy in the ViewModels so that I can bind a ListBox (or whatever) to a collection in the Person ViewModel:
public interface IPersonViewModel
{
ObservableCollection<IAddressViewModel> Addresses { get; }
void Initialize();
}
public interface IAddressViewModel
{
}
The child ViewModel needs to present the information from the child Model, so it's injected via the constructor:
public class AddressViewModel : IAddressViewModel
{
private readonly IAddress _address;
public AddressViewModel(IAddress address)
{
_address = address;
}
}
The question is, what is the best way to supply the child Model to the corresponding child ViewModel?
The example is trivial, but in a typical real case the ViewModels have more dependencies - each of which has its own dependencies (and so on). I'm using Unity 1.2 (although I think the question is relevant across the other IoC containers), and I am using Caliburn's view strategies to automatically find and wire up the appropriate View to a ViewModel.
Here is my current solution:
The parent ViewModel needs to create a child ViewModel for each child Model, so it has a factory method added to its constructor which it uses during initialization:
public class PersonViewModel : IPersonViewModel
{
private readonly Func<IAddress, IAddressViewModel> _addressViewModelFactory;
private readonly IPerson _person;
public PersonViewModel(IPerson person,
Func<IAddress, IAddressViewModel> addressViewModelFactory)
{
_addressViewModelFactory = addressViewModelFactory;
_person = person;
Addresses = new ObservableCollection<IAddressViewModel>();
}
public ObservableCollection<IAddressViewModel> Addresses { get; private set; }
public void Initialize()
{
foreach (IAddress address in _person.Addresses)
Addresses.Add(_addressViewModelFactory(address));
}
}
A factory method that satisfies the Func<IAddress, IAddressViewModel> interface is registered with the main UnityContainer. The factory method uses a child container to register the IAddress dependency that is required by the ViewModel and then resolves the child ViewModel:
public class Factory
{
private readonly IUnityContainer _container;
public Factory(IUnityContainer container)
{
_container = container;
}
public void RegisterStuff()
{
_container.RegisterInstance<Func<IAddress, IAddressViewModel>>(CreateAddressViewModel);
}
private IAddressViewModel CreateAddressViewModel(IAddress model)
{
IUnityContainer childContainer = _container.CreateChildContainer();
childContainer.RegisterInstance(model);
return childContainer.Resolve<IAddressViewModel>();
}
}
Now, when the PersonViewModel is initialized, it loops through each Address in the Model and calls CreateAddressViewModel() (which was injected via the Func<IAddress, IAddressViewModel> argument). CreateAddressViewModel() creates a temporary child container and registers the IAddress model so that when it resolves the IAddressViewModel from the child container the AddressViewModel gets the correct instance injected via its constructor.
This seems to be a good solution to me as the dependencies of the ViewModels are very clear and they are easily testable and unaware of the IoC container. On the other hand, performance is OK but not great as a lot of temporary child containers can be created. Also I end up with a lot of very similar factory methods.
Is this the best way to inject the child Models into the child ViewModels with Unity?
Is there a better (or faster) way to do it in other IoC containers, e.g. Autofac?
How would this problem be tackled with MEF, given that it is not a traditional IoC container but is still used to compose objects?
Depending on the container can you not specify a parameter (named or otherwise) in your factory's CreateAddressViewModel method?
container.Resolve<IAddressViewModel>(new NamedParameterOverloads() { { "Address", model } };
Depending on the container your factory may have to know the name of the parameter (TinyIoC and Castle afaik), or it may had to be last in the list of constructor dependencies (YMMV depending on containers), which isn't great, but it saves creating a lot of child containers in quick succession, and the GC thrashing that will follow, and you still get DI for all your other dependencies.
Of course this falls down if your VM also has a dependency that requires the same IAddress, in that case a child container is probably the way to go unless you want the VM to have knowledge of the container.
Update:
If you're using a subcontainer of a container that uses "last register wins" (which I think Unity does), then you could pass the same child container into your Factory each time, and have your factory simply register the new IAddress - that way you wouldn't be creating a new UnityContainer instance on the heap for each iteration and it should cut down on garbage collections if you're creating lots of items.
The ViewModel sample application of the WPF Application Framework (WAF) shows how you could bring the Model and the ViewModel together. The sample uses MEF as Dependency Injection Framework.