I was looking for best approach to set data context property. I found three ways
Assign data context on View (either on XAML or code behind).
Assign data context on ViewModel through constructor.
Use some configuration that takes view and view model reference and bind data context on run time through some infrastructure classes.
Which is the best option among this in terms of loosely coupled, maintainable? Or Is there any best approach?
I personally like this approach because it makes me have to write less code :). It basically uses an IValueConverter to lookup which view to use whenever a wpf control needs to present a ViewModel visually and the IValueConverter sets the datacontext for you. It also shows you how to create a datatemplate that allows you to require WPF to utilize this converter by default, something like this:
<DataTemplate DataType="{x:Type ViewModels:ViewModelBase}">
<ContentControl Content="{Binding Converter={StaticResource MyConverter}}"/>
</DataTemplate>
the 4. way would be a DataTemplate.
i think the best approach is the one which fits best in your current situation.
if i have viewmodel first dynamic scenarios i use datatemplate/contentpresenter or with view first i take your way one and so on...
Related
Is it generally desirable to implement INotifyPropertyChanged on Model classes, ViewModel classes, or both? Is it possible to implement on Model only ,not on Viewmodel? If not possible model then why
Basic rule is - There is no hard and fast of any architecture, you can modify things to suit your needs that's why some architectures are more desirable.
for your exact needs go through this
and in this discussion, there are enough points to cover both arguments, see which one matches your project..
this might help you to implement..
You must understand the meaning of INotifyPropertyChanged. It's purpose is to raise notification from target to source when you define Binding in WPF. DependencyProperty and INotifyPropertyChanged are related for autoupdation for the concept of binding. If you need to bind the property on a viewmodel to view you have to implement a notification mechanism to notify to UI if there is change in Viewmodel. Same rule is applied for model to View.
Suppose you want to code in .CS file like Viewmodel.Name =" my new name" and expect that TextBox should display the changed name.
<TextBox Text="{Binding ViewModel.Name} "/>
Here view model need to implement INotifyPropertyChanged.
<TextBox Text="{Binding Model.Name} "/>
here model need to implement INotifyPropertyChanged
Hope it clarifies.
It might be worth noting in this discussion that Microsoft themselves add INPC in their own models e.g. the classes automatically generated when you susbscribe to a web service. I've seen it various other places as well, including a vague recollection of it somewhere in their EF stuff. I myself add it automatically to my DAL entities using Castle Proxy so that I don't have to duplicate everything in the view model, although you can also use things like Fody etc to add it to the IL in a post-processing step.
I'm trying to wrap my head around WPF and MVVM.
In my test application I have a MainViewModel with a ChildViewModel property.
Similarly I have a Window that instantiates a MainViewModel and has a child control that should receive MainViewModel.ChildViewModel
For my current application I have a Window with (snipped for brevity)
<Window.DataContext>
<vm:MainWindowViewModel />
</Window.DataContext>
<Window.Content>
<view:ChildView DataContext="ChildViewModel"/>
</Window.Content>
How do I have my ChildView usercontrol define that it requires a datacontext of the type ChildViewModel and also receive it?
Currently I create it by setting it like so:
<UserControl.DataContext>
<vm:ChildViewModel>
</UserControl.DataContext>
but this creates a new instance of the ChildViewModel.
I tried to make the question as bare boned as possible. Hope it's still clear
Googling turned up a LOT of (contesting) approaches, but I can't see the forest for the trees anymore.
Your UserControl doesn't need to specify it's own ViewModel - you're already creating one and binding it.
That being said, your binding should be specified as:
<view:ChildView DataContext="{Binding ChildViewModel}"/>
The confusion likely stems from the fact that there are two distinctly different approaches to MVVM. Creating the VM within the Xaml's <DataContext> tag is a "view-first" approach - the View is creating and instantiating the ViewModel.
You're currently working in more of a ViewModel-First approach (similar to my series on MVVM), where the ViewModel's create the other ViewModels, and the VIew just binds to them.
Personally, I find VM-first easier to maintain from a code standpoint, and often prefer it. View-first has the advantage of (potentially) providing a better design time experience, which is why many MVVM advocates use that approach.
I want to know why we should use MVVM to implement Silverlight app. What're it's advantages?
We don't do Unit Test for ViewModel, so I want other reasons.
Below are my questions about some advantages people usually say:
1.Loosely Coupled : When we use MVVM , a view rely on ViewModel but not a view, why it's loosely coupled?
2.If I provide public methods in a code-behind, they can also provide reusability.
Well, the unit-testability of the view-model is a significant advantage, so you'll miss out on that benefit. Regarding the other two:
Loosely coupled
Yes, the view does rely on the view-model. They have to be connected in some way to accomplish the function of the application. As a result, they cannot be uncoupled. The only choices are tightly-coupled or loosely-coupled or somewhere in between. With MVVM your view-model interacts with your view in a very limited way: basically just objects, properties and commands. Compare this to doing everything in the code-behind where the view and its control are essentially inseparable.
Re-use
If any code in your code-behind is re-usable enough to merit being public, it can be taken out of the code-behind and put into a general-purpose class. The problem is that what's left after that is not reusable.
If you don't want to go into the MVVM deep dive, then you can get some of the benefits of MVVM by focusing on databinding. After you learn the benefits of databinding, you can reconsider the other benefits of MVVM.
We don't do Unit Test for ViewModel,
With MVVM, it isn't just about unit testing ViewModel. Ideally, your VM should be very thin and only have properties needed by view. So, it isn't really necessary to test the VM.
But, without a VM, how do you do your your feature/functional testing across layers? In Silverlight, to facilitate testing you should use commands, instead of writing code in code-behind files. This allows you to simulate button click and other GUI events while unit testing. Using MVVM pattern along with commands, you can test all of C# code (not xaml), right up to UI (Converter, VMs, etc).
If I provide public methods in a
code-behind, they can also provide
reusability.
Not going into details of how that is a bad design, I want to ask you, How does that provide reusablity? If you create a user control, then the code-behind class is a control? You want to create instances of your control and use them? This is like saying that why do we need member methods, we can just create public static methods and access them. I have a strong opinion that if we don't want to use the automatic binding provided by WPF/Silverlight, then it is better NOT to use these technologies. And to exploit the full capabilities of binding, MVVM is essential.
why it's loosely coupled?
VM is very much part of your view. It is not decoupled from the view. As I have said, your VM should be as thin as possible with only public properties needed by your view. Your business logic will be decoupled from your view (and VM).
I think this is one of the best resources available, in case you want to use and contrast the usage of MVVM vs. ANY other pattern or no pattern.
http://karlshifflett.wordpress.com/2010/11/07/in-the-box-ndash-mvvm-training/
I can answer how I use MVVM pattern.
MVVM is better in the following scenarios:
1 If several controls are bound with a single property.
MVVM:
<TextBlock x:Name="text1" Visibility="{Binding IsSomePropertyTrue, Converter={StaticResource VisibilityConverter}"/>
<TextBlock x:Name="text2" Visibility="{Binding IsSomePropertyTrue, Converter={StaticResource VisibilityConverter}"/>
I can quickly add a similar control or remove an existing control.
Compare with code-behind:
public string IsSomePropertyTrue
{
set
{
//...
text1.Visibility = value;
text2.Visibility = value;
}
}
2 Instead of a multi-converter
public Brush StateColor
{
get
{
if (this.State == State.Edited && this.IsPriority)
return new SolidColorBrush(Color.FromArgb(255, 0, 255, 0));
//...
}
}
<sdk:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<TextBlock Background="{Binding StateColor}" Text="{Binding State}"/>
</DataTemplate>
</sdk:DataGridTemplateColumn.CellTemplate>
3 As an item model in controls like ListBox or DataGrid. For example, if I want to create a list of items with a remove button near each item, I will create a ItemView control and a ItemViewModel class.
<ItemsControl ItemsSource="{Binding SomeItems}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<view:ItemView DataContext="{Binding}"/>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
4 Copying a data from one view to another:
public JournalEntryViewModel(SalesOrderViewModel vm) {}
5 ViewModel can inherit CLR-classes and implement interfaces (INotifyPropertyChanged or INotifyDataErrorInfo).
Also I use MVVM for replacing events with commands or properties. And use of ViewModels forces to call properties by intelligible names.
I was an early adopter for WPF and I can tell you what made me choose MVVM (and this more or less applies to Silverlight as well). For the project I was working on, I had to create a screen that allowed users to subscribe to notifications within the system. This was a 3 step process:
The user had to search for the item they wanted to be notified about
They had to select the item and fill out additional options regarding the subscription
The system had to provide a summary and allow the user to confirm or edit the subscription.
After implementing the functionality the first time (without MVVM), I was told that we need to exclude from the search items that were already subscribed to by the user.
After making that fix, I was informed that we needed to give the user a live preview of the subscription based on options.
By then I started noticing that some of these changes could be extracted and made easier if I didn't have to deal with manipulating the UI as I changed the logic. I had never intentionally followed MVVM but I realized that the abstraction that I made closely matched the MVVM pattern.
So this is why I recommend the pattern. It simplifies the task of changing the logic that drives the UI by separating it from the UI itself. I also would recommend that you hold off implementing it until you need it. There is a cost to using MVVM but it is amortized over the cost of changing the UI logic.
Without MVVM, your Silverlight app code very soon will turn into unmanageable mess
What is also interesting in MVVM is dynamic automatic binding.
Imagine, that your view model has properties like this: bool IsFirstNameVisible, bool IsFirstNameEnabled, string FirstName, double FirstNameWidth etc.
In your XAML file, you define TextBox with x:Name = "FirstName" and call your dynamic MVVM-binder. It inspects your view model class via reflection, looks what properties you have defined and binds them dynamically to similar properties of control with the same name, applying value converters if needed.
In this case, your get very clean XAML, without kilometer-long data-binding expressions and converters.
That is what my MVVM library does.
Separation of Conerns people. Separation of Concerns.
Forget unit testing (I love it; but that's not the thing here). Forget reusability (do you really re-use view models? No, let's be real here).
It's about Separation of Concerns and putting code and logic where it belongs. The whole idea is maintainability; being able to make changes to the code as it evolves over time without breaking other stuff and without turning the whole thing into a big pile of spaghetti.
MVVM, done properly, allows your code to be separated into logical portions that make sense and allow for reasy refactoring and change as the app's requirements change. It's easier to find where something is when you need to make a change. Trying writing any halfway complex application and then leaving it alone for a month, then coming back to it and trying to make significant changes. A properly structured application with judicious use of MVVM is way easier to grok after a layoff, and it's way easier to make changes.
I have a View that displays a DataGrid which is bound to an ObservableCollection in the ViewModel. For the sake of discussion, let's say we have a Team View containing a Team DataGrid, in which each row represents a Player.
My question is about what data type I should use to represent the players in my Team collection. Is it a good idea for the items in the collection to be ViewModels themselves? In this case, my Team View would be associated with a single Team ViewModel as well as any number of Player ViewModels (in the Team collection).
Does having multiple ViewModels associated with a single View violate any design guidelines for MVVM , and is there a preferred way of implementing this scenario?
Thanks!
No that is fine; each object should be a ViewModel in its own right. It makes for cleaner code, nicer interactions, and remember, if it works well then it's correct (even if it violates guidelines).
I would do it exactly the way you are prescribing. I'd bind my grid to a Team, which would have an ObservableCollection<Player>, where Player is another ViewModel-type class. Each row item would get the Player as its DataContext and so you're still binding to ViewModel properties as you'd expect: and Player can still have public properties for ICommands (likely RelayCommands) for manipulation!
Hope that helps!
Far from violating guidelines, I think this is recommendated design. At least in my projects you will see this pattern repeatedly.
This pattern comes in particularly useful in conjunction with DataTemplates. For example you could define a DataTemplate in your Application.Resources for your PlayerViewModel like so:
<DataTemplate DataType="viewModels:PlayerViewModel">
<StackPanel Orientation="Vertical">
<Image Source="/Images/Player.png"/>
<TextBlock Text="{Binding Name}"/>
</StackPanel>
</DataTemplate>
And then if you wanted to display a list of players you simply bind a ListBox etc to your TeamViewModel.Players ObservableCollection and you automatically get the above DataTemplate displayed for each player:
<ListBox ItemsSource="{Binding Players}"/>
I agree with both of the other answers (the ones by Kieren and Groky) but feel they fail to mention a very important consideration in this decision.
You should only create a view model if there is something view-specific about what you are doing. If all you are doing is binding to data and invoking commands which naturally belong on your model, there is no reason to create a view model.
For example, suppose:
Your Player object has a Name property, a Rank property, a Promote() method, and a Delete() method.
Your view is a simple one that allows you to edit the Name and Rank of any player, and also has buttons to promote and delete players.
In this case adding a view model between your view and your model is pointless. Such a view can bind directly to the model:
Bind TextBox.Text to the Name property
Bind Slider.Value to the Rank property
Bind the Promote button to the Promote() method
Bind the Delete button to the Delete() method
Note that instead of binding the Delete button to the Delete() method you may want to set its Command to ApplicationCommands.Delete and use a CommandBinding to invoke the Delete() method.
My point here is that in most cases if your models are well-designed there will be no need to insert a view model object. A view model is only really necessary when view-specific state needs to be tracked (such as "current Player"), conversions are too complex to be handled by simple binding, or you need commands that affect several different model objects and/or view model properties at the same time.
In my experience, if the model is correctly designed only about 50% or so of all views actually need a view model, and in the case of items in a list this is more like 20%.
An example of a time when you might use a view model for an item in a list is when you need to keep a separate "selected" flag that is part of your view but not of your model, and the basic functionality in ListBox is not enough.
By CLEAN COD SOLID principles, it is nice to associate one view model to one view.
Separation of concerns should be separated for each view and it's much easier to maintain the codebase in the future.
You can do it but it's not recommended.
I'm starting to use Binding in my WPF project and I'm actually confused about few things on the presentation side (XAML).
So I want to populate a TreeView with a List of Categories. I know how to write the right HierarchicalDataTemplate for my List of Category instances.
<HierarchicalDataTemplate ItemsSource="{Binding Path=ChildrenCategories}" DataType="{x:Type src:Category}">
<TextBlock Text="{Binding Path=Name}"></TextBlock>
</HierarchicalDataTemplate>
But what now I don't know is from where to get the list. I have here 2 solutions :
I got a Library Singleton class
which return me the right
arborescence, then I need to use an
ObjectDataProvider in my xaml which
would call the
Library.Instance.Categories method. (Which means that the controller has to be completely separated from the UI).
I got a Property ListCategories
in my page interactionLogic
(OpenUnit.xaml.cs), and bind the
tree with it.
I'm not sure about the purpose of the xaml.cs files, what are they made for? Is it normally used to store the properties (and act as a controller) or simply to have a back-end for the UI (for example get values from the UI?)?
In case the xaml.cs file is used as a controller, how do I bind my data to it, I've tried many solutions without success,my only success was with the use of static binding.
I would appreciate any comment or recommandation about UI and Logic Binding in WPF, hopefully I will get less confused.
Thanks in advance,
Boris
After reading this great article, I got a little bit less confused :
http://msdn.microsoft.com/en-us/magazine/dd419663.aspx
The article is about the Model View ViewController pattern, and how WPF integrates it. So it seems that xaml.cs files should be used as the ViewController here, and should hold the properties.
It actually make sense since it's not a good practice to mix the View and the Data, we want the designers should have a completely independant work to do.
Also for the solution 2) it is possible if you set the data context to the current file.