Let’s start with a diagram.
In MVVM there are three main parts, the Views, the Model and the ViewModels. The Model are just your basic POCO classes, nothing to fancy in there. The Views are your XAML files, the visual aspect of the application. The ViewModels are classes that get data from your data service classes and shape it into something the view is expecting.
Why these three parts? it’s a matter of seperation of concerns, the View doesn’t need to know where the data is coming from or in what format it’s delivered by the service. The only thing the View cares about is where it can get its data and it assumes that the data will be in the correct format. It gets that data from the ViewModel (or VM in short) through databinding. The View can make the VM do stuff by sending Commands its way, so View and ViewModel can interact with each other. The ViewModel reacts on those commands and requests data from the services, where that data comes from (in-memory, API, whatever, …) doesn’t matter for the VM, as long as it gets what it’s expecting.
Now, why would you use this? As mentioned before, it makes your code much cleaner by seperating the logic out of the View into the VM. It also makes the entire application much easier to unit test. You can just write tests against the ViewModels, since they are just normal .NET classes.
There’s a small war among developers going on about MVVM. MVVM states that no code should exist in the code-behind of a view (for example MainPage.xaml.cs). Some people cling onto this statement, others take it as a “if you can”.
Here’s my opinion on the subject: MVVM is a design pattern, meaning that it’s a set of guidelines. Guidelines, as in “not set in stone”. Sometimes you need to jump through a lot of hoops to get something done in the VM while it’s much easier in the View. If you run into such a situation: decide if it’s worth spending time and research into the subject (and blog about it after you find the solution ). Another situation is things that have to do with the view. Things like changing a state in the Visual State Manager when switching from landscape to portrait, this is something that you can take to the ViewModel but in my opinion the VSM is pure View business, it has nothing to do with the ViewModel so that’s something I typically put into code behind.
As you can tell, the discussion isn’t really worth it. Just do whatever feels right for you (if you’re working in a team, make sure everyone is on the same page about this).
Enough chit-chat, let’s get to the practical side of things. For this article I’m going to go with a Windows Phone 8.1 Silverlight project but a lot of the stuff here is applicable to any XAML technology, even ranging back to Windows Phone 7.
We’ll start with a blank slate, a brand new project started from the Blank App template. First thing you need to do when developing an MVVM application is add your MVVM stuff, either by adding a framework like Caliburn or by adding a toolkit with helper classes like MVVM Light (or by building your own of course). My weapon of choice in MVVM is always MVVM Light. Why? Because it’s a toolkit rather than a framework, it leaves a lot of the responsibilities in the hand of the developer. Frameworks like Caliburn rely more on conventions and that’s something I personally do not like. Other people have other preferences over this, so choice whatever feels right for you. For this article I’m using MVVM Light.
Let’s start by adding MVVM Light to the project. As with any good library, it’s on NuGet. If you search for MVVM Light on NuGet you’ll get some results, so what to pick? (I’ve marked the official MVVM Light NuGet packages in the screenshot).
Let’s go over them, top to bottom. The first one is the one I usually use. It adds the MVVM Light libraries to your project, creates a folder for the viewmodels, creates the MainViewModel and ViewModelLocator (more on this in a bit) and instantiates the ViewModelLocator in App.xaml. Quite a lot of work all done for you by the power of NuGet and PowerShell. The second NuGet package just adds the libraries to your project but it doesn’t create any folders or classes. The third package is the same as the second but as a Portable Class Library, use this one if you’re adding ViewModels in a PCL.
I usually go with the first one because of all the initial setup that occurs. Let’s add that package and have a look at the Solution Explorer.
For starters, NuGet added three libraries to the project. Why three? The first one is the actual MVVM Light library, the second one contains a bunch of extras like EventToCommand and SimpleIOC. In case you’re wondering why these are in a separate library, and where that Microsoft.Practices.ServiceLocation lib comes from, here’s Laurent’s answer to that:
“The Extras assembly exists because EventToCommand requires a reference to System.Windows.Interactivity, while ButtonBaseExtensions, RelayCommand, Messenger etc do not need it. Some people are reluctant to add references to assemblies if they can avoid it. So for those people who don't need EventtoCommand, they onlu use the base assembly, and the others who want the whole program can add Extras.
Let’s take a look at the generated code files, starting with MainViewModel (I removed some of the comments for brevity)
Not much in there, but there doesn’t need to be. That’s what I like about MVVM Light, it keeps things simple and, well Light . ViewModels are defined by creating a class that inherits from ViewModelBase, the name of a ViewModel (or VM in short) doesn’t need to end in “ViewModel” but I tend to do this to make them recognizable. We’ll be discussing the piece of code that’s commented out in a bit.
Next up is the ViewModelLocator class (again, removed some comments for brevity).
So what is the ViewModelLocator? It’s the class that registers all the dataservices and viewmodels in the IOC container (if you don’t know what IOC is, read the next alinea, if you do know what it is you can skip the next part). The ViewModelLocator also provides properties for every ViewModel so we can easily bind a view to a viewmodel. The get part of these properties take the VM instance out of the IOC (again, read the next part for info on IOC) and returns it to the caller. Finally there’s a Cleanup method that you can use to cleanup viewmodels if you need to.
Read this part if you’re not familiar with IOC, feel free to skip this part if you’ve already used IOC.
IOC, or Inversion Of Control, is often used in conjunction with DI or dependency injection. IOC is a technique used to make applications easily extensible and to increase the modularity. These goals can be achieved by using techniques like the Factory Pattern or Dependency Injection. With IOC and DI we can register classes in a so called container. Then, whenever we need one of those registered classes we can fetch the instance from the container instead of instantiating a new one, basically creating a Singleton effect. The big difference with a Singleton is that we can have dependency injection. If one of the classes contains a constructor that takes, for example, a dataservice as parameter and that dataservice is also registered in the container, the DI will inject the dataservice instance into the class with the constructor that takes in a dataservice. More over, we can use interfaces to register classes into the container. If that same class from before takes IDataService as a parameter, the registered instance of a class that implements IDataService will get injected into the constructor. This allows for a more abstract way of working.
Note: this was a basic explanation for IOC / DI. If you really want to get the hang of it, research it and use it, you’ll get it in no time. Some quick links to get you started (with thanks to Glenn Versweyveld for the links)
MVVM Light has a built in IOC/DI framework called SimpleIoc. It’s exactly as the name implies, a very simple, basic framework but it gets the job done. In case that it wouldn’t fulfill your needs or you feel more comfortable with, for example, AutoFac, it’s really easy to swap SimpleIoc out and another framework in. See my “10 things you might have missed about MVVM Light” article from last year for a quick sample.
Okay, final part of the MVVM Light Powershell magic is something that happened inside the App.xaml. A resource was added to the Application.Resources
The ViewModelLocator is added as an application resource, so it will get instantiated as soon as the application starts up. That means that all viewmodels and services are registered in the DI container right away. The resource is given a key so we can reference it when we set our datacontext in xaml.
MVVM Light puts the VM in the VM folder by default but the views are just hanging around in the project. If you don’t like this (as I do ) you can add a View folder and move the MainPage into that folder. Once it’s moved I have the habit of changing the namespace of MainPage to reflect the folder structure. This needs to happen in two places, xaml and code behind. In xaml find this line (it should be at the top)
and change it to
In code behind change the namespace to
Right now, your application will compile perfectly but you will get a NavigationFailedException when launching. Open up the WMAppManifest.xml. On the Application UI tab is a textbox that says Navigation Page: MainPage.xaml. Navigating in Silverlight apps is still done using strings, so the compiler is happy here but at runtime the navigation target isn’t found at this location. Change it from MainPage.xaml to /View/MainPage.xaml and see if it works. If you entered the correct folder path the app should start.
Let’s get to the interesting part. We’re now ready to hook our view up on the viewmodel. This can be done from code behind or from XAML. I prefer the XAML way because it gives you some intellisense when binding to properties on the viewmodel, since the XAML designer knows of the datacontext if it’s defined in XAML.
This is what we need to add to the opening tag of a page to set the datacontext
This is the full tag for reference:
If you don’t want to do it from XAML, it’s very easy to set it via Blend as well.
Open MainPage.xaml in Blend. Select PhoneApplicationPage in the Objects & Timeline pane.
In the properties pane, search for the DataContext Property and click on the white square next to it, select “Create databinding”
In the dialog that will popup, you’ll see the Locator key that we declared in App.xaml, Blend can interpret those resources and help you with binding to them. Select the Locator and you’ll see the Main property that is defined inside the ViewModelLocator (remember, the property that gets the MainViewModel instance from the container and returns it to the caller). Select that property and click OK, your binding is set!
Small thing about Blend: It’s AWESOME! period. If you don’t know how to use it, learn it. Learn to use it and you’ll automatically love it. Your apps will look much better and once you get the hang of using design time data you’ll get your design done much faster.
Let’s get some binding done! We’ll bind the title of the page to a property on the ViewModel just to get started. In the MainViewModel, Create a simple string autoproperty
We’ll use the MainViewModel’s constructor to give this property some data. (line 3)
Now onto the view, find the TextBlock for the page title and add the binding statement to the Text attribute (this can be done from Blend, just like the datacontext property, select the textblock, find the text property and add data binding)
The Text property now contains a Binding statement. This is always set between curly braces, followed by the word Binding and the name of the property we’re binding to. At runtime the CLR will try to find a property with that name in the datacontext of the TextBlock, since we’re not specifying a DataContext on the TextBlock it will take that of its parent control, that way we’ll finally reach the MainViewModel where the Title property is waiting for us. Run the app and behold the wonders of databinding in an MVVM scenario.
Now this works but if we were to change the Title property as it is right now, it wouldn’t reflect on the page. A bound property isn’t monitored or polled for changes, we need to trigger some sort of event to tell the subscribers that a property has changed and that they need to refresh their binding. Usually this is done by implementing INotifyPropertyChanged and calling the OnPropertyChanged event in the property’s setter. In the case of MVVM the INotifyPropertyChanged interface is already implemented in the ViewModelBase class, so all we have left to do is raise the event (or call a method that raises the event, that method is called RaisePropertyChanged and sits in the ViewModelBase) from the property’s setter.
We check if the property has really changed to avoid refreshing bindings when it’s not needed, if it does change we raise the event. According to the INotifyPropertyChanged interface we need to pass the property name as a string. MVVM Light’s ViewModelBase has an overload that takes a Func<string> so we can rely on intellisense to get the name of the property right. And with this we have a real databinding that will update the view when changed. Also, if you open MainPage in Blend now, you’ll notice that the binding is executed in design time as well. That’s one of Blend’s biggest strengths.
Okay, so we now have databinding all set up, our MVVM structure is in place. Let’s build something that resembles an actual app. Create a Model folder in your project and add a Person class to it.
Now to get some data I’ve used a website called json-generator to generate 15 instances of this class in Json format. You can download the JSON file I’ve used here
I placed the json in a static field in a static public class so my dataservice can easily get to it. I use this to simulate what could be a REST call in a real application. (if you want to know more about creating and calling REST services, read my article)
Add a folder called Services to the project and add an interface called IDataService.
The interface defines methods to fetch all the persons, to get a filtered set of persons by passing a predicate into a Where clause or fetch a specific person by name.
We’ll end up with two implementations of this interface. Let’s start with the first one, this one will be used at runtime and will query the actual service (the hardcoded JSON in this case, just assume it’s a call to a REST service). The first implementation is easy, the constructor fetches and deserializes the json. The methods then query that resultset.
Very basic data service, but enough to get my point across. Now it’s time to revisit the ViewModelLocator. When you look at the constructor of the ViewModelLocator you’ll notice that it contains a block of code that’s commented out.
That piece of code is exactly what we need. Why? Because it checks if the code is being executed in runtime or in design time. As I’ve mentioned before, the Blend and Visual Studio designers execute the code whenever a page is loaded into the designer. We can use that to hook up dummy data. The ViewModelBase class from MVVM Light contains a static boolean that tells us if we’re in design mode. We can use this to either register DataService in the container or a design time version of DataService (which we’ll build in a minute).
Uncomment the code so it looks like this (don’t worry about the DesignDataService error, the class doesn’t exist yet but it’s the next step.
In the Services folder, add a class called DesignDataService and make it implement IDataService.
I use a for loop to create 10 design time persons. Note that the GetByName method just returns the first person in the list. For design time data it doesn’t matter if the correct data is returned, we’re only using this data to get the visual aspect of the application right. Verify that the app still compiles and let’s get this thing injected.
Open the MainViewModel. Add a field for the IDataService and add IDataService as a parameter for the constructor.
This is where constructor injection will come into play. SimpleIoc will inject the registered implementation if IDataService right here in the constructor, if you want to see this set a breakpoint in the constructor, open the app and inspect the parameter.
Now I want a property that contains the persons and bind this to some sort of list element so we get a nice, scrollable overview of persons. The perfect collection for bindable properties is an ObservableCollection. It notifies subscribers whenever an item is added so that those newly added items are shown immediately. Add the property to the MainViewModel
We’re going to use the constructor of the MainViewModel to get some data in.
The constructor of ObservableCollection<T> has an overload that takes in an IEnumerable<T> that can be used to seed the collection.
Let’s dive into Blend and get that design time data to work. Open MainPage.xaml in Blend. In the Data tab in Blend you’ll notice that MainViewModel shows up as Data Context, if you expand the tree structure you’ll find your ObservableCollection (if you don’t see it, switch back to Visual Studio and Build the solution, Blend doesn’t always update unless you build the project). Take the Persons property there and drag/drop it onto the design view (make sure MainPage.xaml is opened in design). When dragging over the design you should see this message appear.
Release the mouse button and a ListBox will be created and its ItemsSource wil lbe bound to that list on our MainViewModel. But it doesn’t look quite right.
This is common in Blend and is easily fixed, right-click the ListBox, go to Layout and select “Reset all”
Note: this is currently a ListBox, we are advised by Microsoft to use LongListSelector instead of ListBox for performance reasons. To change this, go into the xaml and change <ListBox… to <phone:LongListSelector.. Blend will keep working just like it’s doing now. For this article I’m just going to stick to ListBox but the way of working is the same.
We have a list of design time items now. Let’s run the app and see how it looks.
Notice how the data is different? The ViewModelLocator has registered the real DataService into the container so we’re now getting our real data instead of the design time data.
This doesn’t look very well, let’s see what we can do to change that. Blend makes changing item templates really easy, right-click the Listbox > “Edit Additional Templates” > “Edit Generated items (ItemTemplate)” > “Edit Current”
Have a look at the Objects & Timeline pane, you’ll see that it’s changed and shows the itemTemplate
From here you can create your entire layout, drag and drop properties from the Data tab onto the elements to create databindings. After about a minute I came up with this result.
The XAML for the itemtemplate:
Result in runtime:
I’m going to split this up in two posts as this one’s long enough already and my keyboard is getting tired
Part two will focus on selecting a person from the list, navigating to a detail page, fetching and showing the details. Expect it to show up some time next week (I’ll update this post as well as soon is its ready).
In this first part I’ve discussed what MVVM was and why you should do it. I’ve shown you how I use MVVM Light and walked through the setup. We’ve also discussed design-time data and how Blend combined with this designtime data can help you getting a nice UI in a fast way.
The code for this first part can be found on my OneDrive.
This is an imported post. It was imported from my old blog using an automated tool and may contain formatting errors and/or broken images.