Nick's .NET Travels

Continually looking for the yellow brick road so I can catch me a wizard....

Using the Universal Windows Platform SplitView Control with Visual States and BuildIt.Lifecycle

In this post I’m going to add a SplitView to the About page. I’ve also created two visual states that will control whether the Pane of the SplitView is visible or not (ie IsPaneOpen) and whether the display behaviour is for it to overlay or shift the content over (ie Inline). In the main content area I’ve also added two buttons which will be used to switch between each visual state. Lastly, in the SplitView.Pane has a Close button which will be used to return to the previous page in the application.

<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
       <VisualStateManager.VisualStateGroups>
           <VisualStateGroup x:Name="SplitViewStates">
               <VisualState x:Name="Minimised" />
               <VisualState x:Name="Expanded">
                   <VisualState.Setters>
                       <Setter Target="RootSplitView.(SplitView.IsPaneOpen)"
                               Value="True" />
                       <Setter Target="RootSplitView.(SplitView.DisplayMode)"
                               Value="Inline" />
                   </VisualState.Setters>
               </VisualState>
           </VisualStateGroup>
       </VisualStateManager.VisualStateGroups>

       <SplitView  x:Name="RootSplitView">
           <SplitView.Pane>
               <Grid>
                   <Border Background="Yellow" />
                   <Button Click="CloseClick">Close</Button>
               </Grid>
           </SplitView.Pane>

           <StackPanel VerticalAlignment="Center"
                       HorizontalAlignment="Center">
               <TextBlock Text="About Page"
                          FontSize="40" />
               <TextBlock Text="{Binding AboutTitle}"
                          FontSize="40" />
               <Button Click="ExpandClick">Expand</Button>
               <Button Click="CollapseClick">Collapse</Button>
           </StackPanel>
       </SplitView>
   </Grid>
And here is the code behind for the page:

private void ExpandClick(object sender, RoutedEventArgs e)
{
    (DataContext as AboutViewModel).Expand();
}
private void CollapseClick(object sender, RoutedEventArgs e)
{
    (DataContext as AboutViewModel).Collapse();
}

And then the implementation of the Expand and Collapse methods on the AboutViewModel. For this we’re defining states whose names match the visual states I declared in the XAML. The Expand and Collapse method then switch between these states. I’ve shown the whole class (excluding the AboutTitle property as I’ve changed the base class to include the StateManager and OnCompletion.

public class AboutViewModel : BaseStateManagerViewModelWithCompletion<DefaultCompletion>
{
    public AboutViewModel()
    {
        StateManager.Group<AboutExpandStates>().DefineAllStates();
    }

    public void Close()
    {
        OnComplete(DefaultCompletion.Complete);
    }

    public void Expand()
    {
        StateManager.GoToState(AboutExpandStates.Expanded);
    }

    public void Collapse()
    {
        StateManager.GoToState(AboutExpandStates.Minimised);
    }
}

Lastly, I needed to modify the About state for the PrimaryRegion so that the Completion event would return the application to the Home state:

.StateWithViewModel<AppStates, AboutViewModel>(AppStates.About)
    .OnComplete(DefaultCompletion.Complete).ChangeToPreviousState()

    .WhenChangedToWithData<AppStates, AboutViewModel, string>((vm, d) => vm.AboutTitle = d)
.EndState();

And here is the output with the state manager triggering changes that switch visual states expanding and collapsing the Pane of the SplitView.

image image

Additional Windows using Regions with BuildIt.Lifecycle

In the first post on Getting Started with BuildIt.Lifecycle I touched on the notion of an application region being a logical representation of a Window that’s part of an application. Most Windows Phone, iOS and Android applications will only have a single region but the very nature of Windows lends itself to multi-windowed applications. In this post we’ll extend the sample application to launch a new Window by creating a new region within the application. We’ll start with the declaration of the region:

public enum AdditionalStates
{
    Base,
    Landing
}

public class AdditionalRegion : StateAwareApplicationRegion
{

    public AdditionalRegion()
    {
        StateManager.GroupWithViewModels<AdditionalStates>()
            .StateWithViewModel<AdditionalStates, LandingViewModel>(AdditionalStates.Landing)
                .OnComplete(DefaultCompletion.Complete)
                .CloseRegion(this)
            .EndState();
    }

    protected override async Task CompleteStartup()
    {

        await base.CompleteStartup();

        await StateManager.GoToState(AdditionalStates.Landing, false);
    }
}

You may notice this is slightly different from the PrimaryRegion I defined in my initial post as it inherits from the StateAwareApplicationRegion instead of just ApplicationRegion, and that there’s no StateManager property. The StateAwareApplicationRegion was added as a convenience class, which contains the StateManager property and implements the IHasStates interface. Note that both regions in my sample application now inherit from this class.

There is only a single state defined for the AdditionalRegion, which will correlate to the only page that will get loaded. When this region is started, the state manager transitions to the Landing state, which will display the LandingPage. In order for this to work, we need to create and correlate a LandingPage to the Landing state. I’ve added a page to the application based on the Blank Page template, called LandingPage. In App.xaml.cs I then needed to add the association between the new page and the Landing state:

LifecycleHelper.RegisterView<LandingPage>().ForState(AdditionalStates.Landing);

On the MainPage of the application I’ve added another Button which simply calls a new OpenAnotherWindow method on the MainViewModel.

private void AnotherWindowClick(object sender, Windows.UI.Xaml.RoutedEventArgs e)
{
    ViewModel.OpenAnotherWindow();
}

The OpenAnotherWindow method simply calls OnComplete with a new enumeration value of AnotherWindow (this enumeration value is independent of the fact that I’ll be using it to open a new window, I could have called it “ShowMeTheMoney” so long as it correlates to the handler I’ll define in the PrimaryRegion).

public void OpenAnotherWindow()
{
    OnComplete(MainCompletion.AnotherWindow);
}

As I just mentioned, I need to define what happens when OnComplete is called. To do this I need to update the state declarations in the PrimaryRegion. Here you can see the additional OnComplete which handles the AnotherWindow enumeration value, and calls LaunchRegion.

StateManager.GroupWithViewModels<AppStates>()
    .StateWithViewModel<AppStates, MainViewModel>(AppStates.Home)
        .OnCompleteWithData(MainCompletion.Settings,vm=>vm.HelloWithTime)
        .ChangeState(AppStates.Settings)

        .OnCompleteWithData< AppStates, MainViewModel,MainCompletion,string>(MainCompletion.About,null)
        .ChangeState(AppStates.About)
                   
        .WhenChangedTo(async vm => await vm.LoadData())
                   
        .OnComplete(MainCompletion.AnotherWindow)
        .LaunchRegion(this,TypeHelper.Ref<AdditionalRegion>())
    .EndState()

Note: The TypeHelper is there to try to eliminate the need to specify all the Type arguments for the LaunchRegion method. I’ll look to refactor this more as we go to make it less obtuse to use.

That’s it, when you run the application, clicking the new Button will spawn of new windows (and actually will keep spawning new windows each time you click the button, even if the previous window is still open).

Refactoring Data Passing Methods in BuildIt.Lifecycle

In my earlier post, Passing Data Between States with BuildIt.Lifecycle, the state declarations for the primary application region started to look really messy. As promised I’ve tidied up the extension helper methods, making it much easier to define delegates that are invoked when changing to and from a state. The updated state declartion now looks like:

StateManager.GroupWithViewModels<AppStates>()
    .StateWithViewModel<AppStates, MainViewModel>(AppStates.Home)
        .OnCompleteWithData(MainCompletion.Settings,vm=>vm.HelloWithTime)
            .ChangeState(AppStates.Settings)
        .OnCompleteWithData< AppStates, MainViewModel,MainCompletion,string>(MainCompletion.About,null)
            .ChangeState(AppStates.About)
        .WhenChangedTo(async vm => await vm.LoadData())
    .EndState()
               
    .StateWithViewModel<AppStates, SettingsViewModel>(AppStates.Settings)
        .OnComplete(DefaultCompletion.Complete).ChangeToPreviousState()
        .WhenChangedToWithData<AppStates, SettingsViewModel, string>((vm, d) => vm.SettingsTitle = d)
    .EndState()
               
    .StateWithViewModel<AppStates, AboutViewModel>(AppStates.About)
        .WhenChangedToWithData<AppStates, AboutViewModel, string>((vm, d) => vm.AboutTitle = d)
    .EndState();

Note that if you’re attempting to use these updates they haven’t yet made their way into the NuGet packages. I’ll endeavour to update these packages later this week. In the meantime I’d recommend downloading the source from the BuildIt repository on GitHub.

Loading Data Using Services with BuildIt.Lifecycle

The BuildIt.Lifecycle automatically creates instances of ViewModels and associates them with the corresponding page/view as the user steps between application states. Behind the scenes this uses Autofac to create these instances and ensure any dependencies are resolved. ViewModels that require access to device hardware, or to services that load or save data, can simply include the interface in the constructor. Take the following example where the MainViewModel relies on an instance of the IDataService interface to populate the Contacts collection.

public interface IDataService
{
    Task<IEnumerable<string>> LoadContacts();
}

public class MainViewModel : BaseViewModelWithCompletion<MainCompletion>
{
    public ObservableCollection<string> Contacts { get; }=new ObservableCollection<string>();
    private IDataService Data { get; }
    public MainViewModel(IDataService data)
    {
        Data = data;
    }
    public async Task LoadData()
    {
        var contactList = await Data.LoadContacts();
        contactList.DoForEach(Contacts.Add);
    }

We need to make two changes to our application declaration. The first is to register the implementation of IDataService, which we’ll do in App.xaml.cs

var core = new SimpleStatesApplication();
var wm = new WindowManager(core);
await core.Startup(builder =>
{
    builder.RegisterType<UWPDataService>().As<IDataService>();
});

public class UWPDataService : IDataService
{
    public async Task<IEnumerable<string>>  LoadContacts()
    {
        return new []{"Joe", "Bob", "Frank"};
    }
}

The other change is to then invoke this method when the MainViewModel is used, this is when the Home state is changed to. We add the following to our state declarations

(group.Item2.States[AppStates.Home] as IViewModelStateDefinition<AppStates, MainViewModel>)
    .WhenChangedTo(async vm => await vm.LoadData());

Note that we can also register services as part of our application class, allowing them to be registered within the PCL, rather than the platform specific libraries. This is useful for services that you want to use the same implementation for all supported platforms.

public class SimpleStatesApplication : RegionAwareBaseApplication<PrimaryRegion>
{
    protected override void RegisterDependencies(ContainerBuilder builder)
    {
        base.RegisterDependencies(builder);
        builder.RegisterType<BasicDebugLogger>().As<ILogService>();
    }
}

The ILogService is used within BuildIt.Lifecycle, so registering this implementatino will allow you to view the diagnostic information in the Output window within Visual Studio.

Passing Data Between States with BuildIt.Lifecycle

Yesterday I discussed in my post, Navigating Back using State History with BuildIt.Lifecycle, how we needed to extend the state manager concept to understand and track state history. The next challenge that app developers face, that doesn’t fit nicely into the current state manager concept is that you often need to pass data between pages of the application. Think of a simple example where you have a list of contacts on one page; the user taps on one of those contacts and is taken to a new page that shows the details of that person. What you need to pass between pages is information about which contact the user tapped. I’ll discuss later how we can pass data between states, and thus view models, later in the post, but first I wanted to dig into the concept of passing data between pages a bit further.

When the user tapped on the contact in the list, there is a lot of information that we can record about that event:

- Mouse/Touch position (don’t forget we’re not just dealing with mobile devices, apps are a big part of the desktop experience on Mac and PC now)

- List index (the index of the item tapped in the list)

- List item (the object in the list that corresponds to the cell that was tapped)

- Item Id (some sort of unique identifier of the item that was tapped)

With all these pieces of information, the question is which one to pass to the details page. Let’s review them individually:

- Mouse/Touch position – this in itself isn’t very useful as it only has meaning on the page that’s currently in view and that point in time. For example, if the user were to scroll the list, all of a sudden this information is useless as it now points to a different cell in the list

- List index – this is only useful if the details page has a reference to exactly the same list that is used to populate the contacts list page. If the underlying data set changed (eg sync with backend caused new contacts to be downloaded), then the list index might now point to a different item

- List item – if the items in the list contain all the details necessary to populate the details page then passing the list item to the details page is ok. However, if you have a large list off contacts, chances are you only load enough data about each contact to display the item in the list (eg perhaps only photo and name). In this case, passing the whole entity isn’t that useful since the details page has to load the whole contact anyhow. The other point worth considering is about serialization of navigation parameters – depending on the platform there are mechanisms for persisting navigation stack, including data parameters; some of these have size and complexity constraints on the data passed between pages.

- Item Id – this is probably the best choice for this scenario. It contains enough information for the contact to be loaded. If performance is a concern (ie you want the contact summary information to be instantly available when the user arrives at the details page, even if details have to be progressively loaded), then you should cache the contact summary information in memory in a service which can be accessed as soon as the details page is loaded. The cache service might also cache the last X number of loaded contacts in memory to make it quick for a user to go between contacts.

Ok, so this works for this scenario but how do we summarize this into a more generic form. Essentially it comes down to passing the minimal set of information between pages that is required to completely load the next page. Ideally pages (and their corresponding ViewModel) should be self sufficient, relying only on application services to load data. Application services, most likely passed into the ViewModel using some sort of dependency injection, provide an abstraction over loading/saving data both locally (in memory and to disk) and across the network (calling services or performing data sync). To this end, by supplying the minimum set of information (eg item id in the case of the contacts list scenario), the ViewModel is only reliant on that piece of information. This reduces the necessary testing surface area as there are few permutations of input data that need to be teted.

Now that we have an understanding of how to determine what data to pass between pages, we again need to come back to thinking in terms of states and passing data during a transition between states. If you recall, our ViewModels aren’t aware of states or the ability to change between them. All they’re able to do is indicate that they’re complete. In the following code, the MainViewModel exposes a public HelloWithTime property as well as a private AboutWithTime property, the former will be passed into the Settings state, the latter into the About state. You’ll observe that the OnComplete call for settings hasn’t been changed, whilst there is now a call to OnCompleteWithData, passing in the AboutWithTime property value, for completing with the About completion value.

public class MainViewModel : BaseViewModelWithCompletion<MainCompletion>
{
    public string HelloWithTime => $"Hello World at {DateTime.Now.ToString("h:mm tt")}";
    private string AboutWithTime => $"About information {DateTime.Now.ToString("h:mm tt")}";

    public void DisplaySettings()
    {
        OnComplete(MainCompletion.Settings);
    }
    public void DisplayAbout()
    {
        OnCompleteWithData(MainCompletion.About,AboutWithTime);
    }
}

Our state declaration now looks like the following:

var group = StateManager.GroupWithViewModels<AppStates>()
    .StateWithViewModel<AppStates, MainViewModel>(AppStates.Home)
        .OnCompleteWithData(MainCompletion.Settings,vm=>vm.HelloWithTime)
            .ChangeState(AppStates.Settings)
        .OnCompleteWithData< AppStates, MainViewModel,MainCompletion,string>(MainCompletion.About,null)
            .ChangeState(AppStates.About)
        .EndState()
    .StateWithViewModel<AppStates, SettingsViewModel>(AppStates.Settings)
        .OnComplete(DefaultCompletion.Complete).ChangeToPreviousState()
        .EndState()
    .StateWithViewModel<AppStates, AboutViewModel>(AppStates.About)
        .EndState();

(group.Item2.States[AppStates.Settings] as IViewModelStateDefinition<AppStates, SettingsViewModel>)
    .WhenChangedToWithData<AppStates, SettingsViewModel, string>((vm, d) => vm.SettingsTitle = d);

(group.Item2.States[AppStates.About] as IViewModelStateDefinition<AppStates, AboutViewModel>)
    .WhenChangedToWithData<AppStates, AboutViewModel, string>((vm, d) => vm.AboutTitle = d);

What’s interesting here is that there are two parts to passing data between states. The first is how the data is exposed out of the state that’s being completed. Here I’ve shown two ways – completion with Settings uses the public HelloWithTime property to retrieve the data to pass to the next state; completion with About can’t access the AboutWithTime property, since it’s private, which is why it relies on the MainViewModel to pass out the AboutWithTime value when it invokes OnCompleteWithData. Note that currently in the state declaration you still need to call OnCompleteWithData passing in a null value for the data accessor, otherwise the data value isn’t passed – this will be fixed shortly.

The second part of passing data between states is how the data is passed into the new state. This is done by calling the WhenChangedToWithData and specifying a function that will be invoked with the corresponding data. Note this method is a generic method and you can actually call it multiple times with different data types, which means that you can handle arriving at this state with different data (perhaps from different states). One thing to be aware of is that behind the scenes the data is being serialized to json, so whatever data you pass between states need to be serializable to json. Ideally the data being passed between states should be kept to a minimum.

Navigating Back using State History with BuildIt.Lifecycle

So far I’ve only discussed transitioning between states with an assumption that every transition is forward looking and that our state manager doesn’t track any sort of history. Other than simple applications, or poorly architected applications that don’t make use of the devices back navigation feature, it is a requirement for most applications to track the page/view/state that the user was previously in, allowing them to easily navigate back to where they just came from. This doesn’t fit into the normal concept of a finite state machine, since this wouldn’t normally have any “memory”. However, our state manager can easily be extended to include a history of which states have been visited.

Again, let’s start with updating the user interface on our simple application. On the settings page we didn’t have any way to get back to the main/home page of our application. I’ll add a new button, resulting in the following XAML and codebehind:

<Page x:Class="SimpleStates.Pages.SettingsPage"
      xmlns="
http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:local="using:SimpleStates.Pages"
      xmlns:d="
http://schemas.microsoft.com/expression/blend/2008"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      mc:Ignorable="d">
    <StackPanel Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <TextBlock Text="Settings Page"
                   VerticalAlignment="Center"
                   HorizontalAlignment="Center"
                   FontSize="40" />
        <Button Click="GoBackClick">Go Back</Button>
    </StackPanel>
</Page>

public sealed partial class SettingsPage
{
    public SettingsPage()
    {
        InitializeComponent();
    }
    private void GoBackClick(object sender, RoutedEventArgs e)
    {
        (DataContext as SettingsViewModel).Complete();
    }
}

The only change I needed to make to the SettingsViewModel is to change the back class. Note that in this case since there is only one way that the state can be exited, I’m going to use the built in DefaultCompletion enumeration, which has a single Complete value.

public class SettingsViewModel : BaseViewModelWithCompletion<DefaultCompletion>
{
    public void Complete()
    {
        OnComplete(DefaultCompletion.Complete);
    }
}

The only thing left to do is to update the state declaration for my application to change to the previous state when the Settings state is Complete, as follows:

StateManager.GroupWithViewModels<AppStates>()
    .StateWithViewModel<AppStates, MainViewModel>(AppStates.Home)
        .OnComplete(MainCompletion.Settings).ChangeState(AppStates.Settings)
        .OnComplete(MainCompletion.About).ChangeState(AppStates.About)
        .EndState()
    .StateWithViewModel<AppStates, SettingsViewModel>(AppStates.Settings)
        .OnComplete(DefaultCompletion.Complete).ChangeToPreviousState()
        .EndState()
    .StateWithViewModel<AppStates, AboutViewModel>(AppStates.About);

The state history feature does allow for skipping states that are in the history, so you can easily return to the start of an application by changing to the corresponding state using ChangeBackToState instead of ChangeState.

Note: The state history feature can be turned on/off on individual state groups. By default TrackHistory is disabled for regular state groups. State groups that have corresponding view models has TrackHistory enabled by default.

Declarative Navigation using BuildIt.Lifecycle

Yesterday I showed in the post, Navigation using BuildIt.Lifecycle, that by wiring up to events in the HomeViewModel, the PrimaryRegion could then change the application state, thereby causing a navigation to the corresponding page. Since this pattern is likely to be so common I felt there was an opportunity to refactor it. In this post I’ll show a couple of ways that we can now re-write this code.

The first is just a refactoring of what we did yesterday. There are now OnEvent and ChangeState extension methods that allow for a declarative way to say that when an event occurs on the ViewModel, it should correspond to a change in state. You will notice that the method for declaring the state has changed from DefineViewModelState to StateWithViewModel, and that we now have an EndState method to conclude the state declaration. This is all in aid of trying to improve the fluid usage of this library and we’re likely to see a number of iterations of this before it settles, so apologies if this causes you to rework your code.

StateManager.GroupWithViewModels<AppStates>()
    .StateWithViewModel<AppStates, MainViewModel>(AppStates.Home)
        .OnEvent((vm, a) => vm.ShowSettings += a,
            (vm, a) => vm.ShowSettings -= a)
        .ChangeState(AppStates.Settings)
        .OnEvent((vm, a) => vm.ShowAbout += a,
            (vm, a) => vm.ShowAbout -= a)
        .ChangeState(AppStates.About)
        .EndState()
    .StateWithViewModel<AppStates, SettingsViewModel>(AppStates.Settings)
        .EndState()
    .StateWithViewModel<AppStates, AboutViewModel>(AppStates.About);

This refactoring was all very good except it still required events to be declared in the ViewModel, including invoking methods etc. I suspect that the most common use of this pattern will be for a single event to be declared which includes an enumeration indicating what type of “completion” the ViewModel. For example in this case we’d have an enumeration which includes Settings and About values, and depending on which value is included in the event, we’d choose to change to the corresponding state (in theory you could use the same enumeration that is used to define the states (ie AppStates) but I would avoid this as it could get confusing as there would be an implied link, rather than explicit declaration of intent).

Rather than every developer having to implement this pattern, we can simply change our MainViewModel to inherit from BaseViewModelWithCompletion and declaring an enumeration, MainCompletion:

public enum MainCompletion
{
    Base,
    Settings,
    About
}

public class MainViewModel:BaseViewModelWithCompletion<MainCompletion>
{
    public void DisplaySettings()
    {
        OnComplete(MainCompletion.Settings);
    }
    public void DisplayAbout()
    {
        OnComplete(MainCompletion.About);
    }
}

With this change in place, we can simplify our state declarations further to:

StateManager.GroupWithViewModels<AppStates>()
    .StateWithViewModel<AppStates, MainViewModel>(AppStates.Home)
        .OnComplete(MainCompletion.Settings).ChangeState(AppStates.Settings)
        .OnComplete(MainCompletion.About).ChangeState(AppStates.About)
        .EndState()
    .StateWithViewModel<AppStates, SettingsViewModel>(AppStates.Settings)
        .EndState()
    .StateWithViewModel<AppStates, AboutViewModel>(AppStates.About);

What’s really interesting now is that we’ve got a very declarative way to define what the navigation is within our application. We have events that trigger changes between states. This is very similar to what you’d expect if you were looking at a diagram of a finite state machine where you have states, and triggers that cause changes between states. The logical extension of this would be to have some kind of dsl that could be used to describe this in some form of XML, making it possible to build a designer around it!

Navigation using BuildIt.Lifecycle

In my post, Getting Started with BuildIt.Lifecycle, I showed how to get started with BuildIt.Lifecycle. The next step is to add basic navigation between pages/views. To do this, we need to change our mindset to again think of this as a state transition. Before I cover what I mean by this, let’s add two buttons to our Home page that we’ll use to trigger the navigation to either the Settings or About pages.

<StackPanel VerticalAlignment="Center"
            HorizontalAlignment="Center">
    <TextBlock Text="Main Page" FontSize="40" />
    <Button Click="SettingsClick">Settings</Button>
    <Button Click="AboutClick">About</Button>
</StackPanel>

As you can see from the XAML there are event handlers wired up to the Click event on each of the Buttons. In each of the event handlers we simply route a call to a corresponding method on the ViewModel.

private MainViewModel ViewModel => DataContext as MainViewModel;

private void SettingsClick(object sender, RoutedEventArgs e)
{
    ViewModel.DisplaySettings();
}

private void AboutClick(object sender, RoutedEventArgs e)
{
    ViewModel.DisplayAbout();
}

Now we need to think about what displaying either the Settings or the About page means in the context of our application state. When the application is launched the PrimaryRegion transitions to the AppStates.Home state, which correlates to the HomePage being displayed. When the user clicks on either button, what we want the application to do is to transition from the Home state to the either the Settings or About state, depending on which button the user clicks.

Unlike some other frameworks where you can navigate from one ViewModel to another ViewModel by calling a base method such as ShowViewModel or call Navigate on some sort of Navigation wrapper, the HomeViewModel doesn’t actually know anything about the states of region (this is the responsibility of the StateManager that’s part of the PrimaryRegion). Instead, what the HomeViewModel has to do is to raise an event signalling that an event has occurred that it needs assistance dealing with. So the implementation for DisplaySettings and DisplayAbout becomes:

public event EventHandler ShowSettings;
public event EventHandler ShowAbout;
public void DisplaySettings()
{
    ShowSettings.SafeRaise(this);
}
public void DisplayAbout()
{
    ShowAbout.SafeRaise(this);
}

Note: SafeRaise is a helper method defined in BuildIt.General but with the new features in C# 6 you could also write this as:

ShowAbout?.Invoke(this,EventArgs.Empty);

With the HomeViewModel raising these events, we need to subscribe to them somewhere so that the PrimaryRegion can trigger the state change on its StateManager to go to either the Settings or About states. This is done by wiring and unwiring event handlers as part of the state transitions into and from the Home state:

sm.DefineViewModelState<MainViewModel>(AppStates.Home)
    .WhenChangedTo(vm =>
    {
        vm.ShowSettings += ChangeToSettings;
        vm.ShowAbout += ChangeToAbout;
    })
    .WhenChangingFrom(vm =>
    {
        vm.ShowSettings -= ChangeToSettings;
        vm.ShowAbout -= ChangeToAbout;
    });

And then it’s just a matter of implementing the ChangeToSettings and ChangeToAbout methods which just need to invoke a state change on the StateManager.

private async void ChangeToAbout(object sender, EventArgs e)
{
    await StateManager.GoToState(AppStates.About);
}

private async void ChangeToSettings(object sender, EventArgs e)
{
    await StateManager.GoToState(AppStates.Settings);
}

And we’re done; now when the user clicks on either the Settings or the About buttons, the PrimaryRegion will trigger the state change, which will in turn navigate to the corresponding page.

Getting Started with BuildIt.Lifecycle

Towards the end of last year I started playing with the concept of using states to manage more than just visual states on a page. States are already used to manage the internal states of a control (eg Pressed state of a button). If you think about the pages that make up an application, they also represent, at some level, the states of an application. As I discussed in my post, States v’s Regions for Multi-Window Applications, there is also a need for regions to accommodate for platforms where there are multiple windows and/or secondary screens. In this post I’m going to show how to get started with an incubatory framework that uses states as the primary mechanism to determine navigation.

We’re going to start with a UWP application based on the Blank App (Universal Windows) new project template.

image

Next, we’ll add a Core library based on the Class Library (Portable for iOS, Android and Windows) new project template.

image

Before writing code, I’ll add a NuGet reference to BuildIt.Lifecycle to the both projects, and then an additional reference to BuildIt.Lifecycle.UWP package to the UWP project. I’ll also add a reference to the SimpleStates.Core PCL project to the SimpleStates UWP project.

To get started I’m just going to wire up three pages, Home, Settings and About. Note that the BuildIt.Lifecycle library uses the term View in place of Page, to be more consistent with the non-XAML platforms. For this reason I often interchange between them. More importantly though, we need to step back and put these pages into the context of states within the app. Once the app has launched it will transition to the Home state. From there the user can transition the app to either the Settings or the About state. The user must then return to the Home state before going to a different state. If you feel overly enthusiastic you could actually create a state diagram that represents this. I’ll represent this using an enum to define what the state values are.

public enum AppStates
{
    Base,
    Home,
    Settings,
    About
}

Each state is to be associated with both a Page and a corresponding ViewModel. To this end, I’ll create three pages, HomePage, SettingsPage and AboutPage in the UWP project, and I’ll create three ViewModels, HomeViewModel, SettingsViewModel and AboutViewModel in the PCL. The association between the AppStates enumeration values and their corresponding ViewModel will be done in a minute when I use an instance of the StateManager to define the states of the application. Some frameworks, like MvvmCross, rely on a convention to associate the page with the viewmodel (and implicitly the state of the application). To avoid reflection, this framework requires manual association between the page and the corresponding state. So I need to add the following calls to the beginning of the constructor in the App.xaml.cs file.

public App()
{
    LifecycleHelper.RegisterView<Pages.MainPage>().ForState(AppStates.Home);
    LifecycleHelper.RegisterView<SettingsPage>().ForState(AppStates.Settings);
    LifecycleHelper.RegisterView<AboutPage>().ForState(AppStates.About);
As I mentioned I need to create an instance of the StateManager which will track the states of the application, including what the current state is (ie what page is currently being displayed). In order to do this I need an entity that holds the instance of the StateManager, and will effectively manage the current state of the main window of the application.

public class PrimaryRegion : ApplicationRegion, IHasStates
{
    public IStateManager StateManager { get; }


    public PrimaryRegion()
    {
        var ssm = new StateManager();
        var sm = ssm.GroupWithViewModels<AppStates>().Item2;
        sm.DefineViewModelState<MainViewModel>(AppStates.Home);
        sm.DefineViewModelState<SettingsViewModel>(AppStates.Settings);
        sm.DefineViewModelState<AboutViewModel>(AppStates.About);
        StateManager = ssm;
    }
}

You’ll notice that I refer to this as a region. This is because this framework understands about application regions, such as multiple windows, and whilst it’s not a topic for this post, it allows you to be able to manage those windows. What we do need to do is create a region manager that knows about the PrimaryRegion class, and in fact will set that region to be the default region:

public class SimpleStatesApplication : RegionAwareBaseApplication<PrimaryRegion>
{
    protected override void DefineApplicationRegions()
    {
        // We don't need to do anything as the PrimaryRegion automatically gets defined
        // use "RegionManager.DefineRegion<AnotherRegion>();" to register other regions
    }
}

The primary region needs to override the CompleteStartup task to force an initial transition to the Home state.

protected override async Task CompleteStartup()
{

    await base.CompleteStartup();

    await StateManager.GoToState(AppStates.Home, false);
}

This application can now be run and the Home page will be presented, and actually will have the corresponding MainViewModel set as its DataContext. In the next post I’ll discuss navigating between pages (ie transitioning to different states).

Connecting Universal Windows Platform Visual States to the StateManager

In my post State Management using BuildIt.States I made changes to elements on the page in a Universal Windows Platform (UWP) application using a StateManager. This was done to demonstrate the StateManager and this would work on any platform. However, for XAML platforms, this would normally be done via visual states. The following would describe the same states and corresponding changes to the UI.

<VisualStateManager.VisualStateGroups>
    <VisualStateGroup x:Name="MainStatesGroup">
        <VisualState x:Name="StateOne">
            <VisualState.Setters>
                <Setter Target="HelloTextBlock.(TextBlock.Text)" Value="State One" />
            </VisualState.Setters>
        </VisualState>
        <VisualState x:Name="StateTwo">
            <VisualState.Setters>
                <Setter Target="HelloTextBlock.(TextBlock.Text)" Value="State Two" />
            </VisualState.Setters>
        </VisualState>
        <VisualState x:Name="StateThree">
            <VisualState.Setters>
                <Setter Target="HelloTextBlock.(TextBlock.Text)" Value="State Three" />
            </VisualState.Setters>
        </VisualState>
        <VisualState x:Name="StateFour">
            <VisualState.Setters>
                <Setter Target="HelloTextBlock.(TextBlock.Text)" Value="State Four" />
            </VisualState.Setters>
        </VisualState>
    </VisualStateGroup>
</VisualStateManager.VisualStateGroups>

Connecting the ViewModel StateManager (which I setup in the post BuildIt.States from a ViewModel) to the visual states can be done using the VisualStateChanger class that exists in the BuildIt.Lifecycle.UWP library (https://www.nuget.org/packages/BuildIt.Lifecycle.UWP/). Again, this code is a little unrefined – I’ll be making changes to make it easier to connect up the VisualStateChanger without having to retrieve the StageGroup instance.

private VisualStateChanger<MainStates> Changer { get; set; }
protected override void OnNavigatedTo(NavigationEventArgs e)
{
    base.OnNavigatedTo(e);

    var sm = ViewModel.StateManager;
    var grp = sm.StateGroups[typeof(MainStates)] as StateGroup<MainStates, DefaultTransition>;
    Changer = new VisualStateChanger<MainStates>(this, grp);
}

Refactoring StateManager to StateManager Binding

In my previous post, BuildIt.States from a ViewModel, I showed a rather messy way to connect on StateManager to another. I’m just in the process of refactoring BuildIt.States to include an extra method on the IStateManager interface to enable simple binding between two StateManagers, and their respective StateGroups. This would significantly simplify the code necessary, as well as ensuring all StateGroups within the two StateManagers are synchronized.

private IStateBinder Binder { get; set; }
protected override void OnNavigatedTo(NavigationEventArgs e)
{
    base.OnNavigatedTo(e);

    Binder = manager.Bind(ViewModel.StateManager);
}
protected override void OnNavigatingFrom(NavigatingCancelEventArgs e)
{
    base.OnNavigatingFrom(e);

    Binder?.Unbind();
    Binder = null;
}
I’ll be updating the BuildIt.States NuGet package in the coming days to include this additional method.

BuildIt.States from a ViewModel

In my previous post on State Management using BuildIt.States I demonstrated using the BuildIt.States library to declare states for a page. However, it’s not enough to declare states in the UI space. The management of state should be done in the ViewModel space, making it possible to test and keep the logic separate from the UI implementation.

To do this, I can create another instance of the StateManager within the ViewModel for the page. I need to declare the same states, just without any of the additional attributes required to update the user interface. The ViewModel ends up looking like the following – I’ve included a StateName property and a method, UpdateState, which can be invoked to change the state.

public class MainViewModel : NotifyBase, IHasStates
{
    public IStateManager StateManager { get; } = new StateManager();
    private string stateName;
    public string StateName
    {
        get { return stateName; }
        set
        {
            stateName = value;
            OnPropertyChanged();
        }
    }
    public MainViewModel()
    {
        StateManager.Group<MainStates>()
            .DefineState(MainStates.StateOne)
            .DefineState(MainStates.StateTwo)
            .DefineState(MainStates.StateThree)
            .DefineState(MainStates.StateFour);
    }
    public async Task UpdatState(MainStates state)
    {
        var ok = await StateManager.GoToState(state);
        if (ok)
        {
            StateName = state.ToString();
        }
    }
}

Now, in the UI space, the page needs to wire to and unwire from the StateChanged event handler that is available on each of the StateGroup entities in the StateManager.

protected override void OnNavigatedTo(NavigationEventArgs e)
{
    base.OnNavigatedTo(e);

    var sm = ViewModel.StateManager;
    var grp = sm.StateGroups[typeof(MainStates)] as StateGroup<MainStates, DefaultTransition>;
    grp.StateChanged += Grp_StateChanged;
}
protected override void OnNavigatingFrom(NavigatingCancelEventArgs e)
{
    base.OnNavigatingFrom(e);

    var sm = ViewModel.StateManager;
    var grp = sm.StateGroups[typeof(MainStates)] as StateGroup<MainStates, DefaultTransition>;
    grp.StateChanged -= Grp_StateChanged;
}

private void Grp_StateChanged(object sender, StateEventArgs<MainStates> e)
{
    manager.GoToState(e.State, e.UseTransitions);
}

I also updated each of the event handlers for the buttons to call the UpdateState method on the ViewModel. Now I can control the state of the UI by updating the state of the StateManager within the ViewModel.

State Management using BuildIt.States

Over the last month or so I’ve been refactoring some code I prototyped earlier in the year that allows for easier state management within an entity. One of the outputs of this was a portable library that’s available via NuGet, and the source is available via the BuildIt repository on GitHub (https://github.com/builttoroam/BuildIt). In my post, Cross Platform Visual States, I gave an example of how to define states for a Xamarin.Forms page. The StateManager can be used in applications for any supported platform, making it easy to define states. In this post, I’ll provide a quick walk through of using the StateManager, this time in a UWP project as an alternative to using visual states.

To get started, add a reference to the BuildIt.States NuGet plackage. Then in the codebehind file for the page, create an instance of the StateManager class.

private readonly IStateManager manager = new StateManager();

Next, we’re going to create an enumeration that lists each of the possible states. Note that we include a placeholder “Base” state which will represent the default state before any call to change state. If you don’t define a placeholder state, the first state will become the default, meaning that any initial transition to the first state won’t have any effect, since the StateManager thinks it’s already in that state.

private enum MainStates
{
    Base,
    StateOne,
    StateTwo,
    StateThree,
    StateFour
}

I’ll add some XAML to the page in order to show a label using a TextBlock and some buttons that will trigger the state changes.

<Page x:Class="StateManagerIntro.MainPage"
      xmlns="
http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      mc:Ignorable="d">

    <StackPanel VerticalAlignment="Center"
                HorizontalAlignment="Center">
        <TextBlock Text="Hello World!"
                   x:Name="HelloTextBlock" />
        <Button Click="FirstStateClick">State One</Button>
        <Button Click="SecondStateClick">State Second</Button>
        <Button Click="ThirdStateClick">State Third</Button>
        <Button Click="FourthStateClick">State Fourth</Button>
    </StackPanel>
</Page>

private void FirstStateClick(object sender, RoutedEventArgs e)
{
    manager.GoToState(MainStates.StateOne);
}

private void SecondStateClick(object sender, RoutedEventArgs e)
{
    manager.GoToState(MainStates.StateTwo);
}

private void ThirdStateClick(object sender, RoutedEventArgs e)
{
    manager.GoToState(MainStates.StateThree);
}

private void FourthStateClick(object sender, RoutedEventArgs e)
{
    manager.GoToState(MainStates.StateFour);
}

The event handlers for the buttons invoke GoToState on the StateManager in order to invoke a state change to the specified enum value. The only thing left to do is to define the different states.

manager.Group<MainStates>()
    .DefineState(MainStates.StateOne)
        .Target(HelloTextBlock)
        .Change(x=>x.Text,(x,v)=>x.Text=v)
        .ToValue("State One")

    .DefineState(MainStates.StateTwo)
        .Target(HelloTextBlock)
        .Change(x => x.Text, (x, v) => x.Text = v)
        .ToValue("State Two")

    .DefineState(MainStates.StateThree)
        .Target(HelloTextBlock)
        .Change(x => x.Text, (x, v) => x.Text = v)
        .ToValue("State Three")

    .DefineState(MainStates.StateFour)
        .Target(HelloTextBlock)
        .Change(x => x.Text, (x, v) => x.Text = v)
        .ToValue("State Four");

The fluid nature of the state declarations make it relatively straightforward to see what each state defines. In each case, the state changes the Text on the HelloTextBlock to a value that matches the state. When this application is run, preseing each of the buttons invokes the state change, updating the Text on the HelloTextBlock accordingly.