My Code behind looks like this...
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
DataContext = new MainWindowViewModel();
}
}
My ViewModel looks like this...
class MainWindowViewModel : INotifyPropertyChanged
{
public MainWindowViewModel()
{
bool flag = Application.Current.MainWindow.IsInitialized;
if (flag)
{
// Do something...
}
}
I guess my question is....Does this conform to the MVVM design pattern? The only other way to do this is How to fire a Command when a window is loaded in wpf
I don't know why, but I don't want to use mvvm-light or any other boilerplate code..
Accessing UI component from ViewModel is violation of MVVM pattern.
Application.Current.MainWindow.IsInitialized is breaking that pattern.
Custom behaviours is more in accordance with MVVM. So, i would suggest to go with the approach you mentioned as link in your question.
Accessing UI component breaks the testability of your ViewModel. How would you write testcase for your ViewModel class? Application.Current will be null when you try to test it via unit test and it will throw null reference exception.
One of the main motive of MVVM was to seperate UI logic from business
logic so that business logic can be tested separately without worrying
about its consumer which is view.
There is no "pure" way to do this in MVVM without boilerplate code. In general, you shouldn't need to do work in response to the VIew within your VM - just the concept is a violation of MVVM, since your ViewModel is trying to do something in response the View, and things should always flow the other way.
The ViewModel shouldn't, in a real scenario, care about the View's state at all - it should be doing nothing but presenting data for data binding by the View.
Most of the time when people are attempting this, it's to try to avoid loading data up front. This is something that's typically handled better by pushing the data to load and starting it directly on a background thread within the ViewModel, then updating the property within the VM when it completes. C# 5's async/await language features can be used to simplify this quite a bit.
While it is generally believed that having some load/unload logic is a pattern violation, there is a set of use cases, where it's necessary. E.g. a view model may need to be subscribe to some events. If it didn't unsubscribe when unloaded, it might not be garbage collected, depending on the nature of the subscription.
What would break the pattern is accessing view state from within the view model, e.g. manipulating controls. The role of the view model is to expose data to the view and managing load/unload behaviour is part of this contract. Knowing when a view model is loaded means knowing when to expose that data.
While it is true the view model should not care about state of the view, it must know how to prepare data for presentation in the view. More importantly the view model is a layer between the model and the view that makes them separate. Yet in other words: since 'model' means logic, then 'view model' means logic of getting data to display. And it is also about knowing when to fetch it/make it available/etc.
You may want to take a look at this blog post, which provides a convenient way of making a view model aware of being loaded. It is not 100% correct in terms of MVVM purity, because it passes FrameworkElement back into the view model, but imagine we ignore this parameter.
The sample code below is based on the above blog post, but with purer signatures. You could implement IViewModel interface on your classes:
public interface IViewModel : INotifyPropertyChanged
{
void Load();
void Unload();
}
Then instruct the view to call adequate methods when loaded or unloaded by using an attached property:
ViewModelBehavior.LoadUnload="True"
Notice the last line has its place in XAML - the view is the one that enforces a certain behaviour, not vice-versa.
What you are currently doing is correct and that is how it is really done with other frameworks behind the scenes.
Since you mentioned MVVM-Light, I suggest you can take a look at caliburn micro. It has very nice framework to conform the MVVM Pattern. Caliburn micro makes it easy to hook up bindings with events on the controls. Just check out its documentation and it is still considered as MVVMy..
in particular because MVVM is mainly used to guarantee easy to maintain and testable code, you should bear in mind that Application.Current will be null if you use the MainViewModel in UnitTests. Therefore this code will end in a NullPointerException in your tests.
You should consider using the Initialized event if you want to ensure that something is initialized already. But you create the ViewModel after you called InitializeComponent - I simply would leave the check out.
Related
I heard its the next best thing in building WPF UIs, but all existing
examples have dozens of lines of code - can I get a Hello World
for MVVM that explains in no uncertain terms what its all about?
I'm fairly new to C#/.net as well, so maybe point me to some
resources that could help too?
Much appreciated!
One sentence explanation:
MVVM is a reimagining of the well loved Model-View-Presenter (MVP) pattern that is designed to work especially well with databinding facilities supplied with WPF to separate application logic from UI design.
Longer, more useful, explanation:
The basic concept of MVVM is the break apart a WPF application into separate components each of which has one responsibility in the process of getting information on screen.
Firstly you have the model. This is a class with very limited functionality that is generally populated from some outside source such as a database or webservice. For example:
public class MessageModel
{
public string Message { get; set; }
}
On top of that you layer the ViewModel, this is where the logic of the application sits, it notifies the view of changes to the model and ensures data consistency. By implementing the INotifyPropertyChanged interface two way databinding between the ViewModel and the view is given for free by WPF:
public class MessageViewModel : INotifyPropertyChanged
{
private MessageModel _model;
public string Message
{
get { return _model.Message; }
set
{
if (_model.Message != value)
{
_model.Message = value;
OnPropertyChanged("Message");
}
}
}
}
Finally you have the View. This is a xaml file that describes the layout of the controls used to display and edit the data in the ViewModel:
<Canvas>
<TextBox Text={"Binding Message"} />
</Canvas>
The reason that you go to all this effort is that the Model is very lightweight and easily passed across domain boundaries. It is simple to send or receive it from a webservice or map it to a database table. The ViewModel, on the other hand is complex, but has few dependencies - it doesn't care where the model gets it's data from, only that it is there and it has no concept of a view at all which makes it very testable (the logic of your application doesn't rely on a UI to test). Finally the xaml is well compartmentalised and can be handed off to a designer who needs to know nothing about the logic of the application, only that the ViewModel will present certain data under certain names. This encapsulation makes it very easy to define roles in large projects, or put together a limited UI to test logic against while the real one is being polished.
MVVM is a star-fan relationship. The fan knows the star but the star does not know the fan. The fan loves his star so much that if the star changes himself ( I mean his dressing style ), the fan changes himself accordingly.
Now replace "star" with "ViewModel" and "fan" with "View" and read it once again.
One sentence? Here goes.
MVVM is a UI segregation pattern where the Xaml (View) binds to a facade (View Model) allowing the guts of your program (Model) to avoid having UI concerns leak down a layer.
The simple statement that helped me get my head around it best was "Could I unit test my business logic without the user interface?" I think this should be the question you ask while learning and designing using MVVM concepts.
This site has a great diagram that explains it.
Basically you have 3 components:
1) Model - The data model of your application. this is pretty standard and the same as any MVP or MVC app.
2) View - The XAML that defines the view/layout of your app.
3) View Model - Because WPF demands that the view attach to things in certain ways (like requires that collections implement INotifyCollectionChanged and stuff like that) it normally requires that you massage your data a little bit to get it in a form suitable for the view to display. This is where the view model codes in. It packages up the data into view models that the view can easily display. This is what your view XAML will bind to. It must respond to events from the model layer and update itself.
(Then your controllers hang on the side somewhere - ideally using WPF commands - and make changes to the model, which fires events to update the view model)
a pattern where the frontend(view) and backend(modal) communicates (back and forth) using a common mediator(view-modal).
The MVVM pattern is when the UI interfaces with an xaml-friendly intermediate object to get at your xaml-unfriendly actual data.
I would say something like: "Presentation pattern for separation of concern between user interface and it's logic"
An improved answer:
MVVM is all about the future; you want to separate your application logic from the framework so that either the framework can evolve, and your app may not have to change, or your app can evolve and you won't have to worry so much about changing the actual UI aspects.
Actually, MVVM is a refinement of a pattern that has been around for some time. I can remember evolving the pattern when I was working in MFC. There are at least two reasons for doing so. MFC or <> is fairly complex, and mixing MFC constructs in with your application logic makes the application kind of brittle. Example: replacing a listbox with a combobox (or selector in modern terms) is much more difficult if the logic for supporting the content of the list/selector is combined with the logic to manipulate list/selector itself.
Some really good one-sentence (or close to it) explanations already. I'll just add to it by saying that if you're past the basic level and you want an example of how to use MVVM in a real-world app that has menus, tool bar, options dialogs, docking tools windows, etc., take a look at SoapBox Core and this demo using SoapBox Core. It is open sourced so you can get lots of ideas.
Because you can't data-bind to your own codebehind
(only half joking here)
In my WPF application, my Model class inherits from a class ParentClass as such:
Model : ParentClass, INotifyPropertyChanged
My application is simply meant to provide a UI to test and validate ParentClass. So Model really is not much more than an INotifyPropertyChanged implementation layed over ParentClass
However I still need to access the data from that parent class and have it be responsive to the UI.
Assuming Slicing is a property in ParentClass, and WPFSlicing is a property in Model, this is how I am currently doing it. I am wrapping every parent property in a WPF-compatible property.
public Thickness WPFSlicing
{
get { return Slicing; }
set
{
Slicing = value;
OnPropertyChanged("Slicing");
}
}
Is there a smarter way to do this? I have many properties in my parent class and this seems like a very uninspired method.
This is a very interesting question.
Typically, I've found that in MVVM applications, you're trying to isolate a model as an end-result -- a product/piece of data -- of the View Model. If you were to have a Bicycle shop, for example, you would see the Storefront as the View, the Sales Person as the ViewModel (assuming this is a customizable Bicycle that is built to order), and the Model as the finished Bicycle object.
In the case of building the Bicycle, while it is in a "prototyping" stage that needs to be represented, I tend to wrap that up in a ViewModel -- because there is logic in it. It seems like an extra step, but in the end, you can have validations at that ViewModel while constructing it. If your model is inflexible to have the INotifyPropertyChanged added to it (or if it was generated from a service), you'll have issues if you have "0" tires on the Bicycle -- that should cause problems!
A lot of folks tend to get a little lazier and see MVVM as a pattern that abstracts out prototyped models (where data input is going back/forth, updated) to models -- when they should in fact be ViewModels.
Per the example, I would have an MVVM Directory that looks like:
Models
-Bicycle (an object that can be passed across a service, etc -- data)
Views
-BicycleCreatorView (the view or data template of the model)
-StoreFrontView (the view of the entire store/app)
ViewModels
-BicycleCreatorViewModel (the view model which CONSTRUCTS a Bicycle model as the end result)
-StoreFrontViewModel (the view model for the entire store)
Now, you could very easily ALSO have the BicycleCreatorViewModel have a constructor which takes in a Bicycle model and pre-populates. That's not uncommon. Someone might come into a store and say, "Hey, can you make this similar to this other one? Here's what it looks like." While the end-result is to have another property (probably just a get {}) which actually renders a Bicycle object, IF validation is good and we don't have something unusual with 0 tires, no seat (maybe that's a feature?), etc.
So, in short -- I would always have your Model (if you cannot extend it in ANY way) be wrapped up into its OWN ViewModel, for this purpose. That would be the true MVVM pattern. You can always use stuff like ReactiveUI or other toolkits that can wrap the properties up. You may spend a little more time doing this, but the end-product will be far more flexible and less error-prone than one otherwise. Essentially you are doing this already, but you could probably rewrite it so it seems "cleaner" and has the line's drawn.
In theory, you might also inspect whether you could approach it in a method like this:
Is there an aspect-oriented toolkit you could use, potentially? Maybe make your classes partial to include INotifyPropertyChanged on the extension and then XmlIgnore certain pieces, if serialization is an issue/at question?
Problem is we have very little knowledge on where the model comes from and how you're using it. Hope that helps or gives you an interesting idea. Would love to see if you come up with a solution that is more "inspired" than the standard.
I offer an alternate view. This all can be extremly simply solved with AOP framework, such as PostSharp. All you do, is attach [NotifyPropertyChanged] attribute to your class, and your model has everything wired up.
http://www.postsharp.net/aspects/examples/inotifypropertychanged
Be warned though, it costs, but it can be good investment, imo.
Ps, it's matter of view, but I don't see anything wrong with having all the DOMAIN model classes implement INotifyPropertyChanged. It doesn't kill performance, the only downside is that it clutters the code a little. That's what I have done, my domain model entities implement from CommonEntity, and INotifyPropertyChanged. INotifyPropertyChanged is not part of WPF!
It works, and is definitely nicer than wrapping your models inside viewmodels.
I'm writing a ViewModel-first MVVM application using Caliburn.Micro
My View contains a 3rd party UserControl that implements a Method I want/need to call from the associated ViewModel. How do I do that while still upholding the MVVM principles?
There exists an old thread here on SO where a similar question is asked in a more specific context. I would appreciate it if someone could flesh-out the approaches suggested there a bit.
Approach one suggests that the View could subscribe to an IEventAggregator message. But wouldn't I have to use the code behind file to do that? (I thought that was a big no no in MVVM)
Regarding approach two, I have no idea how to do that. And regarding approach three, thats what I tried first but somehow I didn't quite get it to work.
Let me clarify your understanding:
Yes code in the code-behind is generally avoided, but only because MVVM makes it so easy to bind to viewmodel properties and commands in order to wire up your visual element with the functionality behind the scenes
Code that is view-specific in the code-behind of the view is perfectly acceptable assuming it doesn't cross the boundary of concern. For instance, I have a view in my application that does some visual processing of the page, and to do so I require that there is code in the view. This code may also interact with the viewmodel layer, but it will not directly reference the viewmodel, therefore keeping my components loosely coupled
If you have controls that need particular methods calling, then creating an event aggregator message to propagate the notification to the view is perfectly fine since you are still maintaining the separation of concern between the viewmodel and view (and the application components remain encapsulated and testable)
Example View (I've left all event aggregator wire up code and potential dependency injection stuff out for clarity):
public class MyView : IHandle<SomeNotificationMessageType>
{
// Handler for event aggregator messages of type SomeNotificationMessageType
public void Handle(SomeNotificationMessageType message)
{
// Call a method on one of the page controls
SomePageControl.SomeMethod();
}
}
Obviously, what you wouldn't do is something like this in the ViewModel:
public class MyViewModel : IViewAware
{
public void DoSomethingThatAffectsView()
{
var view = this.GetView() as MyView;
view.SomePageControl.SomeMethod();
}
}
Which violates the MVVM principles since you are tightly coupling MyViewModel and MyView.
What if you wanted to use the Context property in caliburn micro which allows multiple views over the same view model? The code above would break - even if you checked the View type, you would still end up with spaghetti code e.g.
public class MyViewModel : IViewAware
{
public void DoSomethingThatAffectsView()
{
var myview = this.GetView() as MyView;
if(myview != null)
myview.SomePageControl.SomeMethod();
var myotherview = this.GetView() as MyOtherView;
if(myotherview != null)
myotherview.SomePageControl.SomeMethod();
// ad infinitum...
}
}
Of course this is subjective: it may be that your usercontrol affects the viewmodel and the view in a complex way, in which case you might want to consider looking at the architecture and working out how that usercontrol can better fit
Have you got any background on what the UC is and what the method on it does?
Greetings my smart programming friends!
I have created an OnResponseEvent for an object that is injected into my viewmodel via UnityContainer.
From my injected object class:
public delegate void ResponseEventHandler(AbstractResponse response);
public event ResponseEventHandler OnResponseEvent;
Constructor for MainWindowViewModel:
public MainWindowViewModel(ITrack track)
{
this._track = Track;
track.OnResponseEvent += UpdateTrackResponseWindow;
}
Created delegate to handle the OnResponseEvent in my MainWindowViewModel:
private delegate void HandleTrackResponseCallback(AbstractResponse message);
Since the OnResponseEvent is sending a message, I need to interpret the message in a separate thread and display in a listBox.
Here is where I get confused. If I were using WinForms to write this application, I could use the following code because WinForms knows about my listBox:
if (ListBox.InvokeRequired)
{
var d = new HandleTrackResponseCallback(UpdateTrackResponseWindow);
Invoke(d, new object[] { message });
}
else
lstTrackResponse.Text = Interpret(message); //Interpret is a separate method
However, I am using WPF, and my MainWindowViewModel class does not know anything about my listBox which is located in a view.
Can anyone provide example code how I might handle the OnResponseEvent on a different thread in my MainWindowViewModel class?
Thanks so much, any help is greatly appreciated.
Manipulating the view from the view-model goes against the basic architectural principles of MVVM. Even if it weren't a being called from a different thread it would still not be a wise thing to do.
In MVVM, you would handle this like you do any other data that needs to be displayed in the view:
process and store the data in the view-model itself (using your event handler)
expose it as property including change notification
access that property from the view using traditional data binding
Something as simple as:
<TextBox Text="{Binding Response}"/>
This approach even takes care of the threading problem because now you handle the concurrency issues in the view-model, instead of in the view. Just lock access to the property that TextBox.Text is bound to while you are modifying it.
In general, MVVM avoids ever having a direct dependency of the view-model on the view for several reasons:
the big picture goal of loose-coupling between the view-model and the view
the ability to unit test the view-model without a view
At first this separation might seem clumsy and requires extra work (see above) but it really is worth it and it's what we need to do to get the benefits of the MVVM approach.
I've never used MVVM before, so I'm probably missing something obvious. When I create a new Panorama application, there's already a ViewModel folder containing ItemViewModel and MainViewModel.
I thought "MainViewModel.cs" is the file that organizes the panorama. However, within MainViewModel, it has this line:
public MainViewModel()
{
this.Items = new ObservableCollection<ItemViewModel>();
}
The ItemViewModel has no interaction with the panorama. These are the then instantiated like this:
this.Items.Add(new ItemViewModel()
{
LineOne = "first line",
LineTwo = "second line",
LineThree = "third line"
});
Why isn't ItemViewModel just a 'Model'? It implements INotifyPropertyChanged, but for what purpose? I would've thought that the ObservableCollection in MainViewModel would be enough to notify any changes, as demonstrated here
Difference is quite simple.
Model holds business logic.
View model contains presentation logic and is additionally shaped to fit views.
In Your case - view model implements INotifyPropertyChanged. That's pure presentation logic.
Model is not responsible for notifying one particular UI that something has changed, it's responsible for transferring invoices, calculating wages etc.
Sometimes (when model is simple) this abstraction is not necessary though.
Some wiki quotes:
Model: as in the classic MVC pattern, the model refers to either
(a) an object model that represents the real state content (an object-oriented approach), or
(b) the data access layer that represents that content (a data-centric approach).
ViewModel: the ViewModel is a “Model of the View” meaning it is an abstraction of the View that also serves in data binding between the View and the Model. It could be seen as a specialized aspect of what would be a Controller (in the MVC pattern) that acts as a data binder/converter that changes Model information into View information and passes commands from the View into the Model. The ViewModel exposes public properties, commands, and abstractions. The ViewModel has been likened to a conceptual state of the data as opposed to the real state of the data in the Model.
It's the same general concept behind all MV[x] architectures, albeit MVC, MVP or MVVM:
You have the model on the one side, which is basically a software abstraction of your business domain. It does not know and does not care about any UI-related stuff (like e.g. in your case 'notifying the UI about changes'). It implements business logic and that's it.
On the other side you have the UI, with specific needs both in technical terms (e.g. 'WPF wants to bind to an ObservableCollection') and also in terms of user presentation (think e.g. about different date orderings or different decimal signs in different languages).
To cope with this and to be able to clearly separate these requirements in a clean architecture, you need the [x], in your case the ViewModel. It's the only layer within the software that knows both about the UI and the Model. Otherwise, there should be no connection whatsoever between the two.
In your simple example, this might look like overkill, but a normal business software will have dozens or even hundreds of such MV[x] triplets, and you would have no way to maintain a clean architecture without this.
To answer your question: What you have in your example is just a bit of wiring code to set up the described architecture.
HTH!
Thomas
The ObservableCollection will notify when items are added or deleted from the list, but the INotifyPropertyChanged on the ItemViewModel is needed if you want notifications to happen when those properties change.