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The MVVM pattern – Design time data
Ok, I’ve broken my promise. I’ve said that the previous post would have been the last one about MVVM, but I’ve changed my mind
I realized, in fact, that I didn’t talk about one of the most interesting pros of working with the MVVM pattern: design time data.Design time data
In one of the posts of the series we’ve talked about dependency injection, which is an easy way to swap the implementation of a service used by ViewModel. There are various reasons to do that: to make refactoring easier, to replace the data source in a quick way, etc. There’s another scenario for which dependency injection can be very useful: helping designers to define the user interface of the application. If this requirement is simple to satisfy when it comes to static data (like the header of a page), which are rendered in real time by the Visual Studio designer, things are a little bit more tricky with dynamic data. It’s very likely that most of the data displayed in the application isn’t static, but it’s loaded when the application is running: from a REST service, from a local database, etc. By default, all these scenarios don’t work when the XAML page is displayed in the designer, since the application isn’t effectively running. To solve this problem, the XAML has introduced the concept of design time data: it’s a way to define a data source that is displayed only when the XAML page is displayed in Visual Studio or in Blend in design mode, even if the app isn’t effectively running.
The MVVM pattern makes easier to support this scenario, thanks to the separation of layers provided by the pattern and the dependency injection approach: it’s enough to swap in the dependency container the service which provides the real data of the app with a fake one, which creates a set of static sample data.
However, compared to the sample we’ve seen in the post about the dependency injection, there are a couple of changes to do. Specifically, we need to detect when the XAML page is being displayed in the designer rather than at runtime and load the data from the correct data source. To do this, we use again one of the features offered by the MVVM Light toolkit, which is a property offered by the ViewModelBase class that tells us if a class is being used by the designer or by the running app.
Let’s see in details the changes we need to do. We’re going to use the same sample we’ve seen in the post about dependency injection, which can be found also on my GitHub repository https://github.com/qmatteoq/UWP-MVVMSamples. The app is very simple: it displays a list of news, retrieved from a RSS feed. In the old post we implemented an interface, called IRssService, which offers a method with the following signature:
public interface IRssService { Task<List<FeedItem>> GetNews(string url); }Then, the interface is implemented by two classes: one called RssService, which provides the real data from a real RSS feed, and one called FakeRssService, which provides instead fake static data.
public class RssService : IRssService { public async Task<List<FeedItem>> GetNews(string url) { HttpClient client = new HttpClient(); string result = await client.GetStringAsync(url); var xdoc = XDocument.Parse(result); return (from item in xdoc.Descendants("item") select new FeedItem { Title = (string)item.Element("title"), Description = (string)item.Element("description"), Link = (string)item.Element("link"), PublishDate = DateTime.Parse((string)item.Element("pubDate")) }).ToList(); } } public class FakeRssService : IRssService { public Task<List<FeedItem>> GetNews(string url) { List<FeedItem> items = new List<FeedItem> { new FeedItem { PublishDate = new DateTime(2015, 9, 3), Title = "Sample news 1" }, new FeedItem { PublishDate = new DateTime(2015, 9, 4), Title = "Sample news 2" }, new FeedItem { PublishDate = new DateTime(2015, 9, 5), Title = "Sample news 3" }, new FeedItem { PublishDate = new DateTime(2015, 9, 6), Title = "Sample news 4" } }; return Task.FromResult(items); } }Before starting to explore the changes we need to do in the application to support design data, I would like to highlight a possible solution to handle asynchronous operations. One of the challenges in creating fake data comes from the fact that, typically, real services use asynchronous methods (since they retrieve data from a source which may take some time to be processed). The RssService is a good example: since the GetNews() method is asynchronous, it has to return a **Task
** object, so that it can be properly called by our ViewModel using the await keyword. However, it’s very unlikely that the fake service needs to use asynchronous methods: it just returns static data. The problem is that, since both services implement the same interface, we can’t have one service that returns **Task** operations while the other one plain objects. A workaround, as you can see from the sample code, is to use the **FromResult()** method of the **Task** class. Its purpose is to to encapsulate into a **Task** object a simple response. In this case, since the **GetNews()** method returns a **Task<List >** response, we create a fake **List ** collection and we pass it to the **Task.FromResult()** method. This way, even if the method isn’t asynchronous, it will behave like if it is, so we can keep the same signature defined by the interface. ###
The ViewModelLocator
The first change we have to do is in the ViewModelLocator. In our sample app we have the following code, which registers into the dependency container the IRssService interface with the RssService implementation:
public class ViewModelLocator { public ViewModelLocator() { ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default); SimpleIoc.Default.Register<IRssService, RssService>(); SimpleIoc.Default.Register<MainViewModel>(); } public MainViewModel Main => ServiceLocator.Current.GetInstance<MainViewModel>(); }We need to change the code so that, based on the way the app is being rendered, the proper service is used. We can ues the IsInDesignModeStatic property offered by the ViewModelBase class to detect if the app is running or if it’s being rendered by the designer:
public class ViewModelLocator { public ViewModelLocator() { ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default); if (ViewModelBase.IsInDesignModeStatic) { SimpleIoc.Default.Register<IRssService, FakeRssService>(); } else { SimpleIoc.Default.Register<IRssService, RssService>(); } SimpleIoc.Default.Register<MainViewModel>(); } public MainViewModel Main => ServiceLocator.Current.GetInstance<MainViewModel>(); }In case the app is being rendered in the designer, we connect the IRssService interface with the FakeRssService class, which returns fake data. In case the app is running, instead, we connect the IRssService interface with the RssService class.
The ViewModel
To properly support design time data we need also to change a bit the ViewModel. The reason is that, when the app is rendered by the designer, isn’t really running; the designer takes care of initializing all the required classes (like the ViewModelLocator or the different ViewModels), but it doesn’t execute all the page events. As such, since typically the application loads the data leveraging events like OnNavigatedTo() or Loaded, we will never see them in the designer. Our sample app is a good example of this scenario: in our ViewModel we have a RelayCommand called LoadCommand, which takes care of retrieving the data from the RssService:
private RelayCommand _loadCommand; public RelayCommand LoadCommand { get { if (_loadCommand == null) { _loadCommand = new RelayCommand(async () => { List<FeedItem> items = await _rssService.GetNews("http://wp.qmatteoq.com/rss"); News = new ObservableCollection<FeedItem>(items); }); } return _loadCommand; } }By using the Behaviors SDK described in this post, we have connected this command to the Loaded event of the page:
<Page x:Class="MVVMLight.Advanced.Views.MainView" xmlns:interactivity="using:Microsoft.Xaml.Interactivity" xmlns:core="using:Microsoft.Xaml.Interactions.Core" DataContext="{Binding Source={StaticResource ViewModelLocator}, Path=Main}" mc:Ignorable="d"> <interactivity:Interaction.Behaviors> <core:EventTriggerBehavior EventName="Loaded"> <core:InvokeCommandAction Command="{Binding Path=LoadCommand}" /> </core:EventTriggerBehavior> </interactivity:Interaction.Behaviors> <!-- page content here --> </Page>However, when the page is being rendered in the designer, the LoadCommand command is never invoked, since the Loaded event of the page is never launched. As such, we have to retrieve the data from our service also in the ViewModel’s constructor which, instead, is executed by the designer when it creates an instance of our ViewModel. However, we need to do it only when the ViewModel is being rendered by the designer: when the app is running normally, it’s correct to leave the data loading operation to the command. To achieve this goal we leverage the IsInDesign property, which is part of the ViewModelBase class we’re already using as base class for our ViewModel:
public MainViewModel(IRssService rssService) { _rssService = rssService; if (IsInDesignMode) { var task = _rssService.GetNews("abc"); task.Wait(); List<FeedItem> items = task.Result; News = new ObservableCollection<FeedItem>(items); } }Only if the app is running in design mode, we retrieve the data from the service and we populate the News property, which is the collection connected to the ListView control in the page. Since the GetNews() method is asynchronous and you can’t call asynchronous methods using the async / await pattern in the constructor, you need first to call the Wait() method on the Task and then access to the Result property to get the list of FeedItem objects. In a real application this approach would lead to a synchronous call, which would block the UI thread. However, since our FakeRssService isn’t really asynchronous, it won’t have any side effect.
This sample shows you also the reason why, in case you wanted to make things simpler, you can’t just call the GetNews() method in the constructor also when the application is running: since we’ can’t properly use the async / await pattern, we would end up with unpredictable behaviors. As such, it’s correct to continue calling the data loading methods in the page events that are triggered when the page is being loaded or navigated: since they’re simple methods or event handlers, they can be used with the async and await keywords.
And we’re done!
Now the job is done. If we launch the application, we should continue to normally see the data coming from the real RSS feed. However, if we open the MainPage.xaml page in the Visual Studio designer or in Blend, we should see something like this:
The designer has created an instance of our ViewModel, which received from the dependency container a FakeRssService instance. Since the ViewModel is running in design mode, it will excecute the code we wrote in the constructor, which will retrieve the fake data. Nice, isn’t it? Thanks to this implementation, we can easily see how our collection of news will look like and, if it doesn’t satisfy us, easily change the DataTemplate we have defined in the ItemTemplate property.
As usual, you can find the sample code used in this blog post on my GitHub repository: https://github.com/qmatteoq/UWP-MVVMSamples Specifically, you’ll find it in the project called MVVMLight.Advanced. Happy coding!
Introduction to MVVM – The series
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The MVVM pattern – Services, helpers and templates
In this last post of the series about MVVM we’re going to introduce some concepts and libraries that can make your life easier when you develop a Universal Windows app leveraging the MVVM pattern.
Services, services and services
In one of the previous posts we have created a sample app to display a list of news retrieved from a RSS feed. While developing the app we introduced the concept of service: a class that takes care of performing some operations and passing the results to the ViewModel. Services can be useful also to reach another important goal of the MVVM pattern: avoiding writing platform specific code directly in the ViewModel, to make it easier to share them with other platforms or applications. As usual, I prefer to explain concepts with real examples, so let’s start with a new one.
Let’s say that you’re developing an awesome application that needs to display a dialog to the user. By applying the knowledge we’ve learned in the previous posts, you’ll probably end up to create a command like the following one:
private RelayCommand _showDialogCommand; public RelayCommand ShowDialogCommand { get { if (_showDialogCommand == null) { _showDialogCommand = new RelayCommand(async () => { MessageDialog dialog = new MessageDialog("Hello world"); await dialog.ShowAsync(); }); } return _showDialogCommand; } }The command shows the dialog using a specific API of the Universal Windows Platform, which is the MessageDialog class. Now let’s say that your customer asks you to port this amazing app to another platform, like Android or iOS, using Xamarin, the cross-platform technology that allows to create apps for all the major mobile platforms using C# and the framework .NET. In this scenario, your ViewModel has a problem: you can’t reuse it as it is in Android or iOS, because they use a different API to display a message dialog. Moving platform specific code in a service is the best way to solve this problem: the goal is to change our ViewModel so that it just describes the operation to do (displaying a dialog) without actually implementing it.
Let’s start by creating an interface, which describes the operations to perform:
public interface IDialogService { Task ShowDialogAsync(string message); }This interface will be implemented by a real class, which will be different for each platform. For example, the implementation for the Universal Windows Platform will be like the following one:
public class DialogService: IDialogService { public async Task ShowDialogAsync(string message) { MessageDialog dialog = new MessageDialog(message); await dialog.ShowAsync(); } }On Xamarin Android, instead, you will leverage the AlertDialog class, which is specific from Android:
public class DialogService : IDialogService { public async Task ShowDialogAsync(string message) { AlertDialog.Builder alert = new AlertDialog.Builder(Application.Context); alert.SetTitle(message); alert.SetPositiveButton("Ok", (senderAlert, args) => { }); alert.SetNegativeButton("Cancel", (senderAlert, args) => { }); alert.Show(); await Task.Yield(); } }Now that we have moved the platform specific APIs in a service, we can leverage the dependency injection approach (which I’ve described in a previous post) to use, in the ViewModel, the interface instead of the real class. This way, our command will just describe the operation to perform, demanding to the DialogService class to effectively execute it. With this new approach, the ViewModel will add a dependency to the IDialogService class in the constructor, like in the following sample:
public class MainViewModel : ViewModelBase { private readonly IDialogService _dialogService; public MainViewModel(IDialogService dialogService) { _dialogService = dialogService; } }Then we can change our command in the following way:
private RelayCommand _showDialogCommand; public RelayCommand ShowDialogCommand { get { if (_showDialogCommand == null) { _showDialogCommand = new RelayCommand(async () => { await _dialogService.ShowDialogAsync("Hello world"); }); } return _showDialogCommand; } }By using the dependency injection approach, the Android application will register in the container the DialogService implementation which uses the Android APIs; vice versa, the Windows 10 application will register, instead, the implementation which uses the UWP APIs. However,now our ViewModel can be shared as it is between the two versions of the application, without having to change it. We can move the ViewModel, for example, in a Portable Class Library, which we can share across the Windows, Xamarin Android, Xamarin iOS, WPF, etc. versions of the application.
To help developers in moving the platform specific code into services, there are many libraries which offer a set of services ready to be used in your applications. One of the best ones, which plays well with MVVM Light, is Cimbalino Toolkit (http://cimbalino.org/), which is specific for the Windows world. Other than many converters, behaviors and helpers classes, it includes a wide set of services to handle storage, settings, network access, etc. All the services are provided with their own interfaces, so that it’s easy to use them with the dependency inection’s approach.
If you want to know more about reusing your ViewModels on different platforms, I strongly suggest you to read this article from Laurent Bugnion, the creator of MVVM Light itself. The tutorial will help you to learn how you can reuse your binding knowledge also on platforms like Android and iOS, which doesn’t support binding out of the box.
Implementing the INotifyPropertyChanged interface in an easy way
In the second post of the series we’ve learned that, to properly support the INotifyPropertyChanged interface, we need to change the way we define the properties in the ViewModel. We can’t use the standard get / set syntax, but in the setter we need to call a method that dispatches the notification to the binding channel that the value has changed. This approach makes the code more “noisy”, since the simple definition of a property requires many lines of code.
Please welcome Fody, a library that is able to change the code you wrote at build time. Fody offers many addons and one of them is called Fody.PropertyChanged. Its purpose is to automatically turn every standard property into a property that, under the hood, implements the INotifyPropertyChanged interface. All you have to do is to decorate your class (like a ViewModel) with the [ImplementPropertyChanged] attribute.
For example, the following code:
[ImplementPropertyChanged] public class MainViewModel : ViewModelBase { public string Message { get; set; } }is converted, during the compilation, into this:
public class MainViewModel: ViewModelBase, INotifyPropertyChanged { private string _message; public string Message { get { return _message; } set { _message = value; OnPropertyChanged("Message) } } }This way, you can simplify the code you need to write in your ViewModel and make it less verbose. To use this special attribute, you have to:
- Install the package called Fody.PropertyChanged from NuGet (https://www.nuget.org/packages/PropertyChanged.Fody/)
- To properly work, Fody requires a special XML file in the root of the project, which describes which is the addon to apply at compile time. The name of the file is FodyWeavers.xml and the content shoudl look like this:
<?xml version="1.0" encoding="utf-8" ?> <Weavers> <PropertyChanged /> </Weavers>
That’s all!
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MVVM and Template10
Template10 is a new template, specific for Universal Windows apps development, which has been created to make the developer’s life easier, by providing a cleaner and simpler way to handle the initialization of an app, new controls, MVVM helpers, etc. Template10 is a great starting point also for MVVM apps, since it offers a set of classes that will help you to solve some of the platform specific challenges that may arise during the development, like handling the navigation or the page’s lifecycle. I won’t dig into this topic in this post, since I’ve already talked about it in another post on this blog. If you’re planning to create a Universal Windows app with the MVVM pattern, I strongly suggest you to give it a read and evaluate it.
That’s all folks!
We’ve reached the end of our learning path. I hope you found these series useful to understand the power of the MVVM pattern and that, after reading it, you will try to develop your next application using it. As usual, remember that you can find the samples used as a reference in these posts on GitHub: https://github.com/qmatteoq/UWP-MVVMSamples Happy coding!
Introduction to MVVM – The series
- Introduction
- The practice
- Dependency Injection
- Advanced scenarios
- Services, helpers and templates
- Design time data
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The MVVM pattern – Dependency injection
In the previous posts we’ve learned the basic concepts of the MVVM pattern and how to apply them in a real application. In this post we’ll introduce some more advanced concepts, which will be helpful to implement the MVVM pattern in real and complex projects. We will do it by building a sample application slightly more complex than the one we built in the previous post.
Let’s create a feed reader
To understand what is the dependency injection mentioned in the title of the post we’re going to create an application which will show to the user a list of news retrieved from a RSS feed. As we did in the previous post, the first step is to split the application in the three main components:
- The model, which is the base entity that identifies a news from the feed.
- The view, which is the XAML page that will display the news using a ListView control.
- The ViewModel, which will take care of retrieving the news from the RSS feed and pass it to the View.
Since the goal of the MVVM pattern is to separate, as much as possible, the different layers of the application, it’s a good practice to avoid including the logic required to parse the RSS feed directly in the ViewModel. A better approach is to handle this task to a specific class (we will call it RssService), which will take care of downloading the XML of the RSS feed, turn it into a collection of objects and return it to the ViewModel.
Let’s start with the model and define a class, called FeedItem, which identifies a single news from the feed:
public class FeedItem { public string Title { get; set; } public string Description { get; set; } public string Link { get; set; } public string Content { get; set; } public DateTime PublishDate { get; set; } }Now we can create the service that, by leveraging LINQ to XML (a powerful language to manipulate XML files included into the framework .NET), is able to parse the RSS feed and convert it into a collection of FeedItem objects:
public class RssService: IRssService { public async Task<List<FeedItem>> GetNews(string url) { HttpClient client = new HttpClient(); string result = await client.GetStringAsync(url); var xdoc = XDocument.Parse(result); return (from item in xdoc.Descendants("item") select new FeedItem { Title = (string)item.Element("title"), Description = (string)item.Element("description"), Link = (string)item.Element("link"), PublishDate = DateTime.Parse((string)item.Element("pubDate")) }).ToList(); } }The RssService class has an asynchronous method which uses the HttpClient class and its GetStringAsync() method to download the content of the RSS feed. Then, by using LINQ to XML, we convert the XML into a collection of FeedItem objects: every single node of the XML file (like title, description, link, etc.) is stored into a property of the FeedItem object.
Now we have a class that accepts the URL of a RSS feed in input and returns a list of FeedItem objects, that we can manipulate inside the ViewModel. Here is how the ViewModel of the page that displays the news looks like:
public class MainViewModel: ViewModelBase { private List<FeedItem> _news; public List<FeedItem> News { get { return _news; } set { Set(ref _news, value); } } private RelayCommand _loadCommand; public RelayCommand LoadCommand { get { if (_loadCommand == null) { _loadCommand = new RelayCommand(async () => { RssService rssService = new RssService(); List<FeedItem> items = await rssService.GetNews("http://wp.qmatteoq.com/rss"); News = items; }); } return _loadCommand; } } }We have reused the knowledge acquired in the previous posts to:
- Define a property called News, which type is **List
**, where we’re going to store the list of news, that we will display in the page. - We have defined a command, which we can connect to an event (like the button’s click), that creates a new instance of the RssService class and call the GetNews() method.
In the end, we can create the XAML page:
<Page x:Class="MVVMLight.Advanced.Views.MainView" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" DataContext="{Binding Source={StaticResource ViewModelLocator}, Path=Main}" mc:Ignorable="d"> <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"> <ListView ItemsSource="{Binding Path=News}"> <ListView.ItemTemplate> <DataTemplate> <StackPanel> <TextBlock Text="{Binding Path=PublishDate}" Style="{StaticResource SubtitleTextBlockStyle}" /> <TextBlock Text="{Binding Path=Title}" Style="{StaticResource TitleTextBlockStyle}" /> </StackPanel> </DataTemplate> </ListView.ItemTemplate> </ListView> </Grid> <Page.BottomAppBar> <CommandBar> <CommandBar.PrimaryCommands> <AppBarButton Icon="Refresh" Label="Load" Command="{Binding Path=LoadCommand}" /> </CommandBar.PrimaryCommands> </CommandBar> </Page.BottomAppBar> </Page>In the page we added:
- A ListView control, which ItemsSource property is connected, using binding, to the News property of the ViewModel. When the ViewModel uses the RssService method to load into the News property the list of news, the control will update its visual layout to display them. Specifically, we have defined an ItemTemplate which shows the publishing date of the news and the title.
- An AppBarButton in the application bar, which is connected to the LoadCommand propert. When the button is pressed, the ViewModel will download the news from the RSS feed using the RssService class.
Now that we have our new sample application up & running we can introduce some new concepts that will help us to make it better.
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Dependency injection
Let’s say that, at some point, we need to replace the RssService class with another one that, instead of retrieving the data from a real RSS feed, generates some fake data to display inside the app. There are many reasons to do it: for example, a designer may have the requirement to work on the user interface and he needs to simulate some “borderline” scenarios, like a news with a very long title. Or maybe our application handles more complex data and, instead of a RSS feed, we have a database or a cloud service which can be hard to configure just for test purposes.
In our sample application, it isn’t that hard to accomplish this goal. We just need to create a new class (for example, FakeRssService) which returns a set of fake FeedItem objects, like the following one:
public class FakeRssService { public Task<List<FeedItem>> GetNews(string url) { List<FeedItem> items = new List<FeedItem> { new FeedItem { PublishDate = new DateTime(2015, 9, 3), Title = "Sample news 1" }, new FeedItem { PublishDate = new DateTime(2015, 9, 4), Title = "Sample news 2" }, new FeedItem { PublishDate = new DateTime(2015, 9, 5), Title = "Sample news 3" }, new FeedItem { PublishDate = new DateTime(2015, 9, 6), Title = "Sample news 4" } }; return Task.FromResult(items); } }Then we just need to change the code executed when the LoadCommand is execute to use this new class as replacement of the old one:
private RelayCommand _loadCommand; public RelayCommand LoadCommand { get { if (_loadCommand == null) { _loadCommand = new RelayCommand(async () => { FakeRssService rssService = new FakeRssService (); List<FeedItem> items = await rssService.GetNews("http://wp.qmatteoq.com/rss"); News = new ObservableCollection<FeedItem>(items); }); } return _loadCommand; } }Up to now, things are quite simple. However, let’s say that we’re working on a more complex application which, instead of having one single page that uses the RssService class, has 30 pages, which means that we have 30 ViewModels that are using the RssService class. Consequently, when we need to replace it with the fake one we’ve just created, we need to edit every single ViewModel and change the used class. You can easily imagine how this approach is time consuming and, most of all, makes easier to introduce errors. It’s enough to forget to replace back the original class in one of the ViewModels when we have finished the testing phase to create a big issue.
Let’s introduce the dependency injection concept, which is a way to handle this scenario in a better way. The problem I have just described raises from the fact that, in the previous approach, the RssService class (which we can consider it as a dependency of the ViewModel, since it can’t work properly without it) was created at build time. With dependency injection, we change approach and we start to solving these dependencies at runtime, when the application is running.
This is possible thanks to a special class, called container, that we can consider like a box: inside it, we’re going to register all the ViewModels and all the dependencies that are required by our application. When the application is running and we need a new instance of a ViewModel (simply because the user is navigating to a new page), we won’t create it anymore manually, but we will ask to the container to do it for us. The container will check if it’s able to solve all the dependencies (which means that all the services used by the ViewModels are available) and, if the answer is yes, it will return to us a new instance of the ViewModel with all the dependencies already solved and ready to be used. Technically speaking, we say that the dependencies are “injected” into to the ViewModel: this is why this technique is called dependency injection.
Why does this approach solve the previous problem, which is avoid to change all the ViewModels if we need to change the implementation of a service? Because the connection between the ViewModel and the service is handled by the container, which is a single instance across the whole application. Every ViewModel won’t have anymore to manually create a new instance of the RssService class, but it will be automatically included in the ViewModel’s constructor. It’s the container’s job to create a new instance of the RssService class and pass it to the ViewModel, so that it can use it. When we need to replace the RssService class with the FakeRssService one, we just need to change the class registered in the container and, automatically, all the ViewModels will start to use the new one.
To properly support this approach, there are a couple of changes we need to make to our application. Let’s see them.
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Define an interface
The first problem in our code is that RssService is a class and, as such, we don’t have a way to easily swap it with another one in the container. To do it, we need something that describes in an abstract way the RssService class and the operation that it can perform, so that we can leverage it both in the RssService and in the FakeRssService classes. This is what interfaces are for: their purpose is to describe a set of properties and methods which, in the end, will be implemented by a real class. The first step, consequently, is to create an interface which describes the operations performed by our RSS service. The same interface will be implemented both by the real service (the RssService class) and the fake one (the FakeRssService class). After we’ve done this modify, we’ll be able to:
- Reference the service in the ViewModel using the interface instead of the real class.
- Register in the container the connection between the interface and the real class that we want to use. At runtime, the container will inject into the ViewModel the implementation we have chosen. If we want to swap it, we just need to register in the container another class for the same interface.
Let’s start by creating an interface and call it IRssService:
public interface IRssService { Task<List<FeedItem>> GetNews(string url); }Our service, for the moment, exposes just one asynchronous operation: GetNews(), which takes as input the URL of the RSS feed and returns, as ouput, a collection of FeedItem objects.
Now we need to change our classes RssService and FakeRssService to implement this interface, like in the following example:
public class RssService : IRssService { public async Task<List<FeedItem>> GetNews(string url) { HttpClient client = new HttpClient(); string result = await client.GetStringAsync(url); var xdoc = XDocument.Parse(result); return (from item in xdoc.Descendants("item") select new FeedItem { Title = (string)item.Element("title"), Description = (string)item.Element("description"), Link = (string)item.Element("link"), PublishDate = DateTime.Parse((string)item.Element("pubDate")) }).ToList(); } } public class FakeRssService : IRssService { public Task<List<FeedItem>> GetNews(string url) { List<FeedItem> items = new List<FeedItem> { new FeedItem { PublishDate = new DateTime(2015, 9, 3), Title = "Sample news 1" }, new FeedItem { PublishDate = new DateTime(2015, 9, 4), Title = "Sample news 2" }, new FeedItem { PublishDate = new DateTime(2015, 9, 5), Title = "Sample news 3" }, new FeedItem { PublishDate = new DateTime(2015, 9, 6), Title = "Sample news 4" } }; return Task.FromResult(items); } }As you can see, both services are using the same interface and they are implementing the same method: the only difference is that the RssService class is returning real data, while the FakeRssService one is manually creating a set of fake FeedItem objects.
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The ViewModel
There are a couple of changes we need to apply to our ViewModel:
- We don’t need anymore to manually create a new instance of the RssService class in the command, but we need to add it as parameter in the ViewModel’s constructor. We need to reference the IRssService interface, otherwise we won’t be able to easily swap between its different implementations.
- We need to change the LoadCommand, in order to use this new instance of the service.
Here is how our updated ViewModel looks like:
public class MainViewModel : ViewModelBase { private readonly IRssService _rssService; public MainViewModel(IRssService rssService) { _rssService = rssService; } private ObservableCollection<FeedItem> _news; public ObservableCollection<FeedItem> News { get { return _news; } set { Set(ref _news, value); } } private RelayCommand _loadCommand; public RelayCommand LoadCommand { get { if (_loadCommand == null) { _loadCommand = new RelayCommand(async () => { List<FeedItem> items = await _rssService.GetNews("http://wp.qmatteoq.com/rss"); News = new ObservableCollection<FeedItem>(items); }); } return _loadCommand; } } }###
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The ViewModelLocator
The last step is also the most important one, which is creating the container and registering into it all the ViewModels with their dependencies. Typically this configuration is done when the app starts so, when we use MVVM Light and the ViewModelLocator approach, the best place where to do it is the ViewModelLocator itself, since it’s the class that takes care of dispatching all the ViewModels.
Here is our new ViewModeloLocator implementation looks like:
public class ViewModelLocator { public ViewModelLocator() { ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default); SimpleIoc.Default.Register<IRssService, RssService>(); SimpleIoc.Default.Register<MainViewModel>(); } public MainViewModel Main { get { return ServiceLocator.Current.GetInstance<MainViewModel>(); } } }First of all, we need to higlight that this code is just a sample: there are plenty of available libraries to implement the dependency injection approach, like Ninject or LightInject. However, most of the MVVM toolkits and frameworks provide also an infrastructure to handle this scenario and MVVM Light makes no exception, by providing a simple container identified by the SimpleIoc class, which is set as default container of the app by using the SetLocationProvider() of the ServiceLocator class.
The next step is to register all the ViewModels and theri dependencies in the container. The SimpleIoc class offers the Register() method to achieve this goal, which can be used in two ways:
- The **Register
()** version, which is used when we just need to a new instance of a class which isn’t described by an interface. It’s the case of our ViewModels since, in our context, we don’t need a way to completely swap them. - The Register<T, Y>() version, which is used, instead, when we need to register a class that is described by an interface. In this case, we specify as T the interfance and as Y the implementation we want to use.
In the end, we need to change the way the ViewModelLocator class defines the Main property: instead of manually creating a new MainViewModel object, we ask to the container to give us a new instance of the class, by using the **GetInstance
** method (where **T** is the type we need). After we have done these changes, here is what we’ll happen when we’ll launch our application again:
- The application will create a new instance of the ViewModelLocator class, which will start the configuration of the container. The SimpleIoc class will register the MainViewModel and the RssService classes (the second one, described by the IRssService interface).
- The application will trigger the navigation to the main page of the app. Consequently, since the DataContext property of the Page is connected to the Main property of the ViewModelLocator, the locator will try to create a new instance of the ViewModel.
- The container defined in the ViewModelLocator class will check if there’s a class which type is MainViewModel registered. The answer is yes, however the class depends from a class that implements the IRssService interface. Consequently, the container will start another search and it will check if there’s a class connected to teh IRssService interface registered. Also in this case the answer is yes: the container will return a new instance of the MainViewModel class with the RssService implementation ready to be used.
If any of the previous conditions isn’t satisfied (for example, the container doesn’t find a class registered for the IRssService interface), we will get an exception, since the container wasn’t able to resolve all the dependencies. Now that we have reached the end of the journey, you should be able to understand why the depenncy injection is extremely useful for our scenario. As soon as we need to do some tests and we want to use the FakeRssService class in replacement of the RssService one, we juest need to change one line of code in the ViewModelLocator class. Instead of:
SimpleIoc.Default.Register<IRssService, RssService>();
we would need to do :
SimpleIoc.Default.Register<IRssService, FakeRssService>();
Thanks to this new approach, it doesn’t matter if our application has just one ViewModel or 50 ViewModels which are dependening from the IRssService interface: automatically, the container will take care of injecting the proper real class to any of them.
This approach is extremely useful not just when you’re doings tests, but also if you need to perform heavy refactoring. Let’s say that, at some point, our application doesn’t have to retrieve the news from a RSS feed anymore, but from a REST service published on the cloud. As long as our new service will follow the same structure defined by the IRssService interface (in our case, a GetNews() method which returns a collection of FeedItem objects) we won’t have to change anything in the ViewModels. We just need to create a new class which implements the IRssService interface and register it into the container, in replacement of the RssService one.
In the next post
We’re reaching the end of our learning path. In the next post we’re going to see some other advances scenarios, like sending messages and handling secondary events. As usual, you can play with the sample project described in this post on GitHub: https://github.com/qmatteoq/UWP-MVVMSamples Happy coding!
Introduction to MVVM – The series
- Introduction
- The practice
- Dependency Injection
- Advanced scenarios
- Services, helpers and templates
- Design time data
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