Microsoft Translator Service on ioProgrammo

16/02/2017 2 comments

I have written a new article for the n°211 of ioProgrammo (February/March 2017). This time I introduce my Translator Service Library and show how to use it to build a chat with real-time translation capabilities.

ioProgrammo February/March 2017

ioProgrammo February/March 2017

Here it is a short video showing the project that comes with the article:

How to shutdown a Windows 10 IoT Core device from a UWP app

13/12/2016 1 comment

When dealing with an embedded device, we may have the need to turn it off. The same is true for Windows 10 IoT Core devices: in fact, the DefaultApp and the Device Portal have an option that allows to shutdown or restart the board. So, if we deploy our app, how can we implement this feature?

First of all, we need to add the Windows IoT Extensions for the UWP to our project:

Windows IoT Extensions for the UWP

Windows IoT Extensions for the UWP

In this way, we have access to the Windows.System.ShutdownManager class, that we can use to manage the shutdown of devices. It exposes two static methods:

So, for example:

// Shutdowns the device immediately:
ShutdownManager.BeginShutdown(ShutdownKind.Shutdown, TimeSpan.FromSeconds(0));

// Restarts the device within 5 seconds:
ShutdownManager.BeginShutdown(ShutdownKind.Restart, TimeSpan.FromSeconds(5));

In order for this code to work properly, we need also to give an extra capability to our app. Let’s open the Package.appxmanifest file with the Visual Studio XML Editor and add or update the following values:

<Package
         ...
         xmlns:iot="http://schemas.microsoft.com/appx/manifest/iot/windows10"
         IgnorableNamespaces="uap mp iot">
  
  <Capabilities>
    ...
    <iot:Capability Name="systemManagement" />
  </Capabilities>
</Package>

If we don’t set this capability, we’ll get an UnauthorizedAccessException error when calling ShutdownManager methods.Beg

Xamarin.Forms and Azure Mobile apps on ioProgrammo

08/09/2016 1 comment

I have written a new article for the n°206 of ioProgrammo (September 2016). This time I talk about Xamarin.Forms and how to use it together with Azure Mobile apps in order to create an app that is strictly integrated with Facebook.

ioProgrammo September 2016

ioProgrammo September 2016

Categories: .NET, Azure, C#, General, Xamarin

A simple NavigationService for Xamarin.Forms

11/07/2016 1 comment

If we do a search on the Internet, we’ll find a lot of implementations of a NavigationService for Xamarin.Forms. While all of them are surely valid, there are two things that I don’t like much:

  • if we want to pass arguments to the new page, we need to use the constructor of the page itself (i.e., adding an object parameter to it);
  • they don’t provide a convenient way to clear navigation history, that is often a requirement (i.e., after login/registration).

So, I ended up writing my own implementation. It is very simple and doesn’t cover all the scenarios, but at this moment it completely satifies my need:

public class NavigationService
{
    private Dictionary<string, Type> pages { get; }
        = new Dictionary<string, Type>();

    public Page MainPage => Application.Current.MainPage;

    public void Configure(string key, Type pageType) => pages[key] = pageType;

    public void GoBack() => MainPage.Navigation.PopAsync();

    public void NavigateTo(string pageKey, object parameter = null,
        HistoryBehavior historyBehavior = HistoryBehavior.Default)
    {
        Type pageType;
        if (pages.TryGetValue(pageKey, out pageType))
        {
            var displayPage = (Page)Activator.CreateInstance(pageType);
            displayPage.SetNavigationArgs(parameter);

            if (historyBehavior == HistoryBehavior.ClearHistory)
            {
                MainPage.Navigation.InsertPageBefore(displayPage,
                    MainPage.Navigation.NavigationStack[0]);

                var existingPages = MainPage.Navigation.NavigationStack.ToList();
                for (int i = 1; i < existingPages.Count; i++)
                    MainPage.Navigation.RemovePage(existingPages[i]);
            }
            else
            {
                MainPage.Navigation.PushAsync(displayPage);
            }
        }
        else
        {
            throw new ArgumentException($"No such page: {pageKey}.",
                nameof(pageKey));
        }
    }
}

public enum HistoryBehavior
{
    Default,
    ClearHistory
}

public static class NavigationExtensions
{
    private static ConditionalWeakTable<Page, object> arguments
        = new ConditionalWeakTable<Page, object>();

    public static object GetNavigationArgs(this Page page)
    {
        object argument = null;
        arguments.TryGetValue(page, out argument);

        return argument;
    }

    public static void SetNavigationArgs(this Page page, object args)
        => arguments.Add(page, args);
}

The idea of this NavigationService is to first register all the pages using the Configure method (line 8), just like the MVVM Light approach. The core of the implementation is the NavigateTo method (lines 12-41). It tries to get the page type corresponding the passed key (line 16) and then instantiate it using Activator.CreateInstance.

This is the first difference with other solutions: we create the page using its default parameterless constructor. The actual parameter, if any, is stored in a kind of Dictionary using the SetNavigationArgs extension method at line 19 (its implementation is at lines 62-63).

Then, we check whether we want to clear history after navigation: if this case, we insert the new page before the current one (lines 23-24) and then, at lines 26-28, we remove all the other pages from the stack. Otherwise, at line 32 we just perform a normal PushAsync of the page.

Let’s see how to use it. First of all, we configure the NavigationService in the App class:

public partial class App : Application
{
    public static NavigationService NavigationService { get; }
        = new NavigationService();

    public App()
    {
        InitializeComponent();

        NavigationService.Configure("MainPage", typeof(MainPage));
        NavigationService.Configure("SecondPage", typeof(SecondPage));
        NavigationService.Configure("ThirdPage", typeof(ThirdPage));

        MainPage = new NavigationPage(new MainPage());
    }
}

Then, in MainPage.xaml we write something like this:

<StackLayout Margin="12" HorizontalOptions="Center">
  <Entry x:Name="arguments" Placeholder="Arguments..." />
  <StackLayout Orientation="Horizontal" Spacing="0">
    <Label Text="Clear History" VerticalOptions="Center" Margin="0,0,10,0" />
    <Switch x:Name="clearHistory" VerticalOptions="Center" />
  </StackLayout>
  <Button x:Name="navigate" Text="Go to second page" Clicked="navigate_Clicked" />
</StackLayout>

So we have an Entry that allows to specify arguments to be passed to the target (line 2), a Switch to clear history after navigation (line 5) and a Button to actually go to the second page (line 7). The corresponding code-behind follows:

private void navigate_Clicked(object sender, EventArgs e)
{
    var historyBehavior = clearHistory.IsToggled
        ? HistoryBehavior.ClearHistory : HistoryBehavior.Default;

    App.NavigationService.NavigateTo("SecondPage", arguments.Text, 
        historyBehavior);
}

We simply get the HistoryBehavior based on Switch value and then we pass it along with the target page and the arguments to the NavigateTo method. Finally, in the Second Page we override the OnAppearing method:

protected override void OnAppearing()
{
    var args = this.GetNavigationArgs();
    passedArguments.Text = $"Passed arguments: {args}";

    base.OnAppearing();
}

Using the GetNavigationArgs extension method, at line 3 we retrieve the arguments we have specified with NavigateTo.

Another interesting point about this solution is that it can easily be integrated with an MVVM approach, but we’ll go deeper in this argument in a future post.

You can download the complete example using the link below:
A simple NavigationService for Xamarin.Forms

Categories: C#, Xamarin

A Behavior for CameraPressed event in Universal Windows apps

30/05/2016 1 comment

If we’re developing a Universal app that uses the camera, on Windows Mobile we may want to use the hardware camera button for example to take a photo. Of course, we need to take into account that on desktops and tablets we don’t have this feature. To add a bit more complexity, suppose we’re following the MVVM pattern. So, how can we manage this scenario?

In order to handle the hardware camera button, we just need a reference to the Windows Mobile Extensions for the UWP. In this way, we obtain access to Mobile specific features:

Windows Mobile Extensions for the UWP

Windows Mobile Extensions for the UWP

As we’re working with MVVM, the best solution is to create a custom Behavior. First of all, let’s add its reference through NuGet, searching for the package Microsoft.Xaml.Behaviors.Uwp.Managed:

UWP Behavoirs package on NuGet

UWP Behavoirs package on NuGet

Now we have all the prerequisites to create the CameraPressedBehavior:

[ContentProperty(Name = "Actions")]
public sealed class CameraPressedBehavior : Behavior
{
    public static readonly DependencyProperty ActionsProperty = 
        DependencyProperty.Register(
        "Actions",
        typeof(ActionCollection),
        typeof(CameraPressedBehavior),
        new PropertyMetadata(null));

    public ActionCollection Actions
    {
        get
        {
            var actionCollection = (ActionCollection)
                this.GetValue(ActionsProperty);

            if (actionCollection == null)
            {
                actionCollection = new ActionCollection();
                this.SetValue(ActionsProperty, actionCollection);
            }

            return actionCollection;
        }
    }

    [System.Runtime.CompilerServices.PlatformSpecific]
    private readonly bool isTypePresent;

    public CameraPressedBehavior()
    {
        isTypePresent = 
            Windows.Foundation.Metadata.ApiInformation.IsTypePresent(
                "Windows.Phone.UI.Input.HardwareButtons");
    }

    protected override void OnAttached()
    {
        base.OnAttached();

        if (isTypePresent)
            HardwareButtons.CameraPressed += HardwareButtons_CameraPressed;
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();

        if (isTypePresent)
            HardwareButtons.CameraPressed -= HardwareButtons_CameraPressed;
    }

    private void HardwareButtons_CameraPressed(object sender, CameraEventArgs e)
    {
        Interaction.ExecuteActions(AssociatedObject, Actions, EventArgs.Empty);
    }       
}

At lines 4-26 we define the Actions dependency property that stores the action to be executed when then button is pressed. This part of code is usually present in every behavior.

Then, in the Behavior costructor (lines 31-36), we use the ApiInformation.IsTypePresent method to check whether the Windows.Phone.UI.Input.HardwareButtons type is actually present, i.e. if the platform provides access to hardware buttons. If this is the case, in the OnAttached method (lines 38-44) we register for the CameraPressed event, otherwise we do nothing. In the same way, in the OnDetaching method we remove the event registration, if necessary. Thanks to these checks, the code can safely be executed also on platforms that don’t have hardware buttons.

Finally, in the CameraPressed event handler (lines 54-57), we call the Interaction.ExecuteActions static method, that in turn executes all the registered actions.

Now we can use the Behavior in our XAML page. For example:

<Page
    ...
    xmlns:Interactivity="using:Microsoft.Xaml.Interactivity" 
    xmlns:Interactions="using:Microsoft.Xaml.Interactions.Core"
    xmlns:behaviors="using:See4Me.Behaviors"
    ...>

    <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        ...
        <Interactivity:Interaction.Behaviors>
            <behaviors:CameraPressedBehavior>
                <Interactions:InvokeCommandAction 
                              Command="{Binding TakePhotoCommand}"  />
            </behaviors:CameraPressedBehavior>            
        </Interactivity:Interaction.Behaviors>
    </Grid>
</Page>

We add the CameraPressedBehavoir to the page (lines 10-15) so that, when the button is pressed, the InvokeCommandAction is executed, causing the TakePhotoCommand to be run. As said before, this behavior on desktops and tables has no effects, so it actually works only on phones.

Clear history in Xamarin Android with MVVM Light NavigationService

14/03/2016 Comments off

If we’re working with Xamarin, MVVM Light provides quite useful helpers (derived from the Windows implementation) that simplify a lot our work. Among the others, we have a INavigationService class that abstracts the concept of starting a new Activity (Android) rather than activating a certain Controller (iOS), providing a unique approach to page navigation.

This approach is general and the interface is the same on all the platforms. While this helps having a common navigation system, it can leads to platform specific problem. Imagine we are on Android and we need to delete history (for example, after a login). On this platform, navigation history can be deleted only in conjunction with the start of an Activity (i.e., we need to tell Android to delete any existing task before starting the Activity). But if we’re using INavigationService, we have a NavigateTo method to which we can pass only the pageKey and an optional object that will be received by the new activity. We haven’t control on how the new activity is started. So, how can we delete history?

We can tweak the original NavigationService implementation for Android to handle this scenario. First of all, we need to define a NavigationParameter class:

public class NavigationParameter
{
    public ActivityFlags? Flags { get; }

    public object Content { get; }

    public NavigationParameter(ActivityFlags? flags = null)
    {
        Flags = flags;
    }

    public NavigationParameter(object content, ActivityFlags? flags = null)
    {
        Content = content;
        Flags = flags;
    }
}

It allows us to pass custom activation flags rather the a single object to the NavigateTo method. To use this class, a little modification to NavigationService.cs file is requested. Let’s get the original implementation from CodePlex. Then, locate the following code in the NavigateTo(string pageKey, object parameter) method:

if (parameter != null)
{
    lock (_parametersByKey)
    {
        // ...
    }
}

We need to replace it with the following code:

if (parameter != null)
{
    lock (_parametersByKey)
    {
        var guid = Guid.NewGuid().ToString();

        var flags = (parameter as NavigationParameter)?.Flags;
        var actualParameter = (parameter as NavigationParameter)?.Content 
            ?? parameter;

        _parametersByKey.Add(guid, actualParameter);
        intent.PutExtra(ParameterKeyName, guid);

        if (flags.HasValue)
            intent.SetFlags(flags.Value);
    }
}

The core of the change is at lines 7-9. We check if parameter is a NavigationParameter: in this case, we extract the activation flags and the actual parameter; otherwise, we assume we don’t have any activation flag and that the actual parameter is the parameter argument itself. So, we can continue to use the NavigateTo method in the usual way (no breaking changes). Then, at lines 14-15, if flags has value, we pass it to the Intent.SetFlags method.

So, when we need to clear history, we just need to create a NavigationParameter with the correct activation flags, like in the following example:

private NavigationParameter CreateParameter<T>(T parameter, bool clearHistory)
{
    var flags = clearHistory ? (ActivityFlags.NewTask | ActivityFlags.ClearTask) 
        : default(ActivityFlags);
    var args = new NavigationParameter(parameter, flags);

    return args;
}
        
private void NavigateToHomePage<T>(T parameter)
{
    var args = this.CreateParameter(parameter, clearHistory: true);
    navigationService.NavigateTo(Constants.HomePage, args);
}

Note that, as we use NavigationParamenter only to pass activation flags and the new activity receives always the actual parameter, the GetAndRemoveParameter(Intent) method will keep working in the usual way. Moreover, as already said, we can continue to use NavigateTo method as before if we don’t need to clear history.

Categories: C#, MVVM, Xamarin

Pay attention to .NET Native when using Reflection (MVVM scenario)

22/02/2016 Comments off

Among the other things, .NET Native tries to reduce the app size by analyzing the code and striping out unused .NET objects. This approach works very well, but we need to pay attention when we you work with Reflection, because in this case we must explicitly tell to .NET Native which elements are available for it.

A particular scenario occurs if we’re following the MVVM pattern and we have a classic app with a GridView or a ListView and we want to invoke a command when the user clicks an item, passing the item itself to the command. With the Universal Windows Platform, this task can be accomplished using an EventTriggerBehavoir form XamlBehavoirs along with an InvokeCommandAction and an InputConverter:

<GridView IsItemClickEnabled="True" ItemsSource="{Binding People}">
    <GridView.ItemTemplate>
        <DataTemplate>
            ...
        </DataTemplate>
    </GridView.ItemTemplate>

    <Interactivity:Interaction.Behaviors>
        <Interactions:EventTriggerBehavior EventName="ItemClick">
            <Interactions:InvokeCommandAction 
                Command="{Binding ItemSelectedCommand}" 
                InputConverter="{StaticResource EventArgsConverter}" 
                InputConverterParameter="ClickedItem" />
        </Interactions:EventTriggerBehavior>
    </Interactivity:Interaction.Behaviors>
</GridView>

At lines 9-14 we use an EventTriggerBehavior to catch the ItemClick event of the GridView and execute in response the ItemSelectedCommand action of the ViewModel. Thanks to InputConverter and InputConverterParamter (lines 12-13), this action automatically receives the clicked item as argument:

public sealed class EventArgsConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, 
        string language)
    {
        if (value != null)
        {
            var propertyPath = parameter as string;
            if (!string.IsNullOrWhiteSpace(propertyPath))
            {
                var propertyPathParts = propertyPath.Split('.');
                object propertyValue = value;
                foreach (var propertyPathPart in propertyPathParts)
                {
                    var propInfo = propertyValue.GetType().GetTypeInfo().
                        GetDeclaredProperty(propertyPathPart);

                    propertyValue = propInfo.GetValue(propertyValue);
                }

                return propertyValue;
            }
        }

        return value;
    }

    public object ConvertBack(object value, Type targetType, object parameter, 
        string language)
    {
        throw new NotImplementedException();
    }
}

This Converter extracts the property name of the event argument that is passed as parameter (line 8, in this case ClickedItem) and then uses Reflection (lines 15-18) to get the actual value that will be received by the command.

This code works without problem in Debug mode, but when we switch to Release, enabling the .NET Native toolchain (that is required for app submission), if we click an item on the grid we’ll get a runtime error like the following one:

Unhandled exception at 0x77026D7E (combase.dll)

This occurs because the .NET Native toolchain has removed the ItemClickEventArgs class from the compiled code, as it hasn’t found any reference to it in the code, because it is accessed only at runtime through Reflection.

So, as said at the beginning we need to tell to .NET Native to keep this type during compilation. Let’s open the file Default.rd.xml inside the project Properties folder: it contains the Runtime Directives for .NET Native. All that we need is to add a declaration for the ItemClickEventArgs type:

<Directives xmlns="http://schemas.microsoft.com/netfx/2013/01/metadata">
    <Application>
        <!--
          An Assembly element with Name="*Application*" applies to all 
          assemblies in the application package. The asterisks are not 
          wildcards.
        -->
        
        <Assembly Name="*Application*" Dynamic="Required All" />

        <!-- Add your application specific runtime directives here. -->
        <Type Name="Windows.UI.Xaml.Controls.ItemClickEventArgs" 
              Dynamic="Required Public" />

    </Application>
</Directives>

At lines 12-13 we have specified that we want to keep the ItemClickEventArgs type in compilation, even if it isn’t statically referenced in the code. Now we can compile the app in Release mode again: this time everything will work as expected.

We can refer to MSDN documentation to get more information about the Runtime Directives Configuration File.