Reactive Extensions with #uwpdev Part 3

The way I understood Rx, it was designed to do away with how we (developers) handle events.

A ListView control provides a few ways of handling how developers deal with user interaction. You can have ItemClick and disable Item Selection. You could have Item Selection with ability to select more than one Item.
My personal favourite is to allow ItemClick and to bind a Command property in ListViewExtensions in UWP Toolkit.

There are times when you really need to handle the event in code behind – I know purists would have a day here but say you had a video previews running in a list view. You don’t want all of them running all the time – you want to ensure that only those rendered on screen are running the video.
How would one do that ?

The usual route would be to find the ScrollViewer in the ListView control and hook into the ViewChanged event.
Every time ViewChanged event is raised, OnScrollViewerViewChanged method will be called.

By every time I mean every time.. could be maybe times every second. You then start have to add hacks like lastChecked time and if Time Elapsed is greater than 100ms etc etc.

private void WireUpListViewScroll(ListView listView)
{
    if (listView == null)
    {
        return;
    }

    if (scrollViewer != null)
    {
        scrollViewer.ViewChanged -= OnScrollViewerViewChanged;
    }

    var scrollViewer = listView.FindDescendant<ScrollViewer>();

    if (scrollViewer == null)
    {
        return;
    }

    scrollViewer.ViewChanged += OnScrollViewerViewChanged;
    
    ProcessChannelUserControls(listView, true);
}

private void OnScrollViewerViewChanged(object sender, ScrollViewerViewChangedEventArgs e)
{
    this.ProcessChannelUserControls(listView);
}

So lets look up what Reactive extensions has for us. For starters, Observable.FromEventPattern allows you to create an observable from event and to subscribe to it. No need to unsubscribe from event handers etc.. just dispose the subscription.

Notice I also use Throttle with a TimeSpan.. I don’t want too many hits – once every 0.35 seconds seemed sufficiently responsive for my use and what’s what I used. the scrollViewerSub I use is a SerialDisposable – every time I add a new subscription, it disposes the old one.

private void WireUpListViewScroll(ListView listView)
{
    if (listView == null)
    {
        return;
    }

    var scrollViewer = listView.FindDescendant<ScrollViewer>();

    if (scrollViewer == null)
    {
        return;
    }

    this.scrollViewerSub.Disposable = Observable.FromEventPattern(scrollViewer, "ViewChanged")
        .Throttle(TimeSpan.FromSeconds(0.35))
        .ObserveOn(CoreDispatcherScheduler.Current)
        .SubscribeOn(TaskPoolScheduler.Default)
        .Subscribe(x =>
        {
            this.ProcessChannelUserControls(listView);
        });

    ProcessChannelUserControls(listView, true);
}

Now that the ViewChanged logic is in place, let’s look at the rest of the code for toggling video previews.

private async void ProcessChannelUserControls(ListView listview, bool initial = false)
{
    try
    {
        var isp = listview.ItemsPanelRoot as ItemsStackPanel;

        if (isp == null)
        {
            return;
        }

        await ToggleAnimatedPreviews(listview, isp, initial);
    }
    catch { }
}

private async Task ToggleAnimatedPreviews(ListView listView, ItemsStackPanel isp, bool initial)
{
    var firstGroupPos = isp.FirstVisibleIndex;
    var lastGroupPos = isp.LastVisibleIndex;

    if (firstGroupPos == -1 && lastGroupPos == -1)
    {
        firstGroupPos = 0;
        lastGroupPos = 0;
    }

    if (initial)
    {
        DependencyObject obj = null;
        while (true)
        {
            obj = listView.ContainerFromIndex(0);

            if (obj != null)
            {
                break;
            }

            await Task.Delay(100);
        }
    }

    for (int i = 0; i < listView.Items.Count; i++)
    {
        var container = listView.ContainerFromIndex(i);

        var vp = container?.FindDescendant<AnimatedPreviewPlayerUC>();

        if (i >= firstGroupPos && i <= lastGroupPos)
        {
            vp?.Play();
        }
        else
        {
            vp?.Pause();
        }
    }
}

These are some of the places I have used Reactive Extensions whilst doing #uwpdev with MVVM. I am still learning.

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Reactive Extensions with #uwpdev Part 2

In my post yesterday I talked about adding a few reactive bits inside MVVM using ReactiveUI.

Jamie Mutton mentioned that writing async task code inside a subscription can lead to side effects and he posted a gist on various options. I haven’t tried those yet – I will do so soon. I have mentioned something about not being a purist and I found it easier to inject Rx elements into MVVM whilst following known (to myself) programming style.
Being a slow learner, I prefer to inject things slowly. Add reactive bits at your own pace. You can go the whole 9 yards or go a step at a time.

Today let’s see how to add Reactive Extensions to Service layer. MVVM should really be MVVMS but its a mouthful and not a palindrome.

The Daily Mail Online app has many channels that group content as desired by the editors. Within the app, there’s a Channel list and the content is displayed within FlipView.

Channel list is bound to a RelayCommand (MVVMLight) – setting the correct ChannelIndex. The FlipView is bound to a list of ChannelViewModels with ChannelIndex as the SelectedIndex. When the channel changes, the Data Service is notified and new data is requested.

The sequence is as follows

  1. When Channel Index changes, it pushes a new Tuple through the Active Channel Subject.
  2. When Active Channel Subject publishes a new item:
    • the subscriber in the service, if data is out of data, it downloads new data set and saves it to database, publishing the tuple through to Channel Data Subject
    • the subscriber in the view model, pulls and renders data from database
  3. When Channel Data Subject publishes a new item, the view model pulls it from database and renders it
public int ChannelIndex
{
    get
    {
        return this.channelIndex;
    }

    set
    {
        try
        {
            this.RaiseAndSetIfChanged(ref this.channelIndex, value);
            this.RaisePropertyChanged(nameof(this.Channel));
            this.RaisePropertyChanged(nameof(this.ChannelViewModel));

        }
        catch { }

        var channel = this.Channel;

        SubChannel selectedSub = null;

        if (channel.HasSubChannels)
        {
            selectedSub = channel.GetDefaultSubChannel();
        }

        this.ChannelDataService.SetActiveSource(new Tuple(channel, selectedSub));
    }
}

The Channel Data Service is a monstrosity in itself but that’s another issue. What I have in the service are Reactive Subject instances to publish – in this instance a Tuple for both ActiveChannel and when a new data set is available.

Subjects are considered to be educational material – however in absence of any alternative, I decided to start with those.

public BehaviorSubject ActivateChannelSubject { get; private set; }

public BehaviorSubject ChannelDataSubject { get; private set; }

As you saw above, the ChannelIndex calls a method SetActiveSource. This merely pushes item to the Subject.

public void SetActiveSource(Tuple tuple)
{
    this.SelectedChannelTuple = tuple;

    this.ActivateChannelSubject.OnNext(tuple);
}

Within the service itself, I have a subscription to this subject. Other subscriber includes the MainViewModel for the app

this.ActivateChannelSubject.Subscribe(async tuple =>
{
    try
    {
        // checks to see if existing data is up to date.
        // If it isn't, it pulls necessary content
        await this.ProcessChannel(tuple, true);
    }
    catch { }
});
public async Task ProcessChannel(
    Tuple channelTuple,
    bool notifySubscribers,
    bool forceRefresh = false)
{
    if (!NetworkHelper.Instance.ConnectionInformation.IsInternetAvailable)
    {
        return;
    }

    try
    {
        // check if data is up to date

        // download content as json

        // inject new data into the local db

        if (notifySubscribers)
        {
            this.ChannelDataSubject.OnNext(channelTuple);
        }
    }
}

I remember the warning Jamie gave about the side effects when using async task in subscription. I promise to take a look at the alternative but this is what it is right now..
The try catch around ProcessChannel are just to make sure any exceptions do not take the app down as async lambda is async void and it can burn the house down if not very careful.

Lets took at the subscriptions in the view model’s constructor

this.ChannelDataService.ActivateChannelSubject
    .ObserveOn(RxApp.TaskpoolScheduler)
    .SubscribeOn(RxApp.TaskpoolScheduler)
    .Subscribe(async tuple =>
{
    if (tuple != null)
    {
        await this.ProcessChannelData();
    }
});

this.ChannelDataService.ChannelDataSubject
    .ObserveOn(RxApp.TaskpoolScheduler)
    .SubscribeOn(RxApp.TaskpoolScheduler)
    .Subscribe(async tuple =>
{
    if (tuple != null)
    {
        await this.ProcessChannelData();
    }
});

The View Model itself pulls the data, groups it and renders it on UI thread

private async Task ProcessChannelData()
{
    try
    {
        await semaphore.WaitAsync();

        await DispatcherHelper.ExecuteOnUIThreadAsync(() => this.IsBusy = true);

        var channelData = this.ChannelDataService.GetChannelData();
        await this.RenderChannelData(channelData);
    }
    finally
    {
        semaphore.Release();

        await DispatcherHelper.ExecuteOnUIThreadAsync(() => this.IsBusy = false);
    }
}

private async Task RenderChannelData(ChannelData channelData)
{
    if (channelData == null)
    {
        await DispatcherHelper.ExecuteOnUIThreadAsync(() => this.ResetData());

        return;
    }

    var configSettings = this.ChannelDataService.ConfigurationSettings;

    var itemData = await channelData?.ProcessItems(ChannelService.ChannelDictionary, ChannelService.ChannelList[0], this.AppSettings.Country);

    if (itemData == null)
    {
        return;
    }

    var puffData = channelData.ProcessPuffs(ChannelService.ChannelDictionary, ChannelService.ChannelList[0]);

    System.Diagnostics.Debug.WriteLine($"Item count is {itemData.Item1.Count}, Group count is {itemData.Item2.Count}");

    await DispatcherHelper.ExecuteOnUIThreadAsync(() =>
    {
        try
        {
            ViewHelper.SetStatusBar(this.Channel.StatusColor, Colors.White, this.AppSettings.ElementTheme);

            if (this.Items.Count > 0)
            {
                this.Items.Clear();
            }

            this.Items.AddRange(itemData.Item1);

            if (this.Content.Count > 0)
            {
                this.Content.Clear();
            }

            this.Content.AddRange(itemData.Item2);
        }
        catch { }

        try
        {
            if (PuffContent.Count > 0)
            {
                this.PuffContent.Clear();
            }

            this.PuffContent.AddRange(puffData);
        }
        catch { }
    });
}

This I believe is like 11/2 steps at max.. There a long way to go.. much to refine but things work. I force use of TaskPool unless I need a UI.. in that case, queue it up on the Dispatcher.

I have done it the non reactive way before and whilst it all works, it feels better with reactive model. Things split into independently observables

Reactive Extensions with #uwpdev

Traditionally modern Windows application development (something done in the last 10 years) would involve using XAML. XAML is a great way to define a UI – it addition to it being declarative, it also supports Binding and Commanding.
MVVM – Model View View-Model has become the defacto pattern when developing XAML based applications… even a slow learner like myself eventually graduated to MVVM.

Anything that uses Binding needs to implement INotifyPropertyChanged interface by means on invoking PropertyChanged event to notify the UI of change.

MvvmLight toolkit which provides a large collection of helpful plumbing (RelayCommand, SimpleIoC, NavigationService and more) also provides an object called ObservableObject. This implements the INotifyPropertyChanged and devs just have to inherit from it and introduce their own properties ready for binding.
ReactiveUI a Reactive Extensions MVVM toolkit provides a similar class called ReactiveObject.

namespace ReactiveUI
{
    //
    // Summary:
    //     ReactiveObject is the base object for ViewModel classes, and it implements INotifyPropertyChanged.
    //     In addition, ReactiveObject provides Changing and Changed Observables to monitor
    //     object changes.
    [DataContract]
    public class ReactiveObject : IReactiveNotifyPropertyChanged, IHandleObservableErrors, IReactiveObject, INotifyPropertyChanged, INotifyPropertyChanging, IEnableLogger
    {
        protected ReactiveObject();

        //
        // Summary:
        //     Represents an Observable that fires *before* a property is about to be changed.
        [IgnoreDataMember]
        public IObservable Changing { get; }
        //
        // Summary:
        //     Represents an Observable that fires *after* a property has changed.
        [IgnoreDataMember]
        public IObservable Changed { get; }
        //
        [IgnoreDataMember]
        public IObservable ThrownExceptions { get; }

        public event PropertyChangingEventHandler PropertyChanging;
        public event PropertyChangedEventHandler PropertyChanged;

        //
        public bool AreChangeNotificationsEnabled();
        public IDisposable DelayChangeNotifications();
        //
        public IDisposable SuppressChangeNotifications();
    }
}

When creating observable objects, even ViewModel I tend to derive from ReactiveObject. Any derived class has access to tons of ReactiveExtensions.

For example, I mentioned that my work app Daily Mail Online is a SPA UWP app. I use a property called ViewMode to identify what View is loaded / shown to the user and what isn’t.

public ViewModes ViewMode
{
    get
    {
        return this.viewMode;
    }

    private set
    {
        this.RaiseAndSetIfChanged(ref this.viewMode, value);
    }
}

The method RaiseAndSetIfChanged is another extension method available to a reactive object. This raises PropertyChanged event if value has changed and UI needs to be informed. This would be great if ViewMode was bound to XAML but I wanted to observe any changes from different places. What I created was an observable using another extension method

public IObservable ViewModeObservable { get; private set; }
this.ViewModeObservable = this.WhenAnyValue(x => x.ViewMode)
    .Where(x => x != ViewModes.NotSet)
    .ObserveOn(CoreDispatcherScheduler.Current)
    .SubscribeOn(TaskPoolScheduler.Default);

I used an extension method called WhenAnyValue. It allows one to convert any property the raises PropertyChanged event as Observable.

Notice that I have set ObserveOn and SubscribeOn methods – those are not necessary here rather a subscriber can choose to set those. I set those here because I know this needs to be run on UI thread and any subscribers would need to do the same.
Now lets see how we can observe changes raised by this observable.

I hook into the ViewModel’s observable property in the View’s code behind. I am not an MVVM purist and I am happy to get into code behind if needed

this.MainViewModel.ViewModeObservable.Subscribe(async viewMode =>
{
    await _semaphoreViewMode.WaitAsync();

    try
    {
        switch (viewMode)
        {
            case ViewModes.Article:

                // do something
                break;

            case ViewModes.RelatedArticle:

                // do something
                break;

            case ViewModes.Channel:

                // do something
                break;

            case ViewModes.ReadLater:

                // do something
                break;

            case ViewModes.Comments:

                // do something
                break;

            case ViewModes.Search:

                // do something
                break;

            case ViewModes.Topic:

                // do something
                break;

            case ViewModes.Profile:

                // do something
                break;
        }
    }
    finally
    {
        _semaphoreViewMode.Release();
    }
});

The Subscribe extension method returns an instance of IDisposable. One can use that or use SerialDisposable to auto dispose previous subscription when a new subscription happens.

One can Subscribe to an Observable as many places as one needs.

Note: Its very likely that I am doing Reactive Extensions all wrong. Feel free to suggest better ways of using Rx

More on #UWPCommunityToolkit CacheBase #uwpdev

In the first version of UWP Community Toolkit, we only had ImageCache which had its origin in Windows App Studio. A few issues were raised to optimise it and one mentioned extensible cache that can be used to create any case.

FileCache, ImageCache, VideoCache, JsonCache.. you name it.. Yesterday I mentioned CacheBase. FileCache and ImageCache that ship with UWP Community Toolkit are implementations of CacheBase by giving it a specific type.

Today I had to implement ability to pull configuration settings from our server. I tried using current prod version of FileCache but my implementation was somewhat wrong there. It would try to create File from Stream and return null and then fail internally (fixed in current dev branch) however I needed something today. Enter ConfigCache.. well JsonCache really

public class ConfigCache : CacheBase<ConfigurationSetting>
{
    JsonSerializer jsonSerializer = new JsonSerializer();

    /// <summary>
    /// Private singleton field.
    /// </summary>
    private static ConfigCache _instance;

    /// <summary>
    /// Gets public singleton property.
    /// </summary>
    public static ConfigCache Instance => _instance ?? (_instance = new ConfigCache() { MaintainContext = false });

    protected override async Task<ConfigurationSetting> InitializeTypeAsync(StorageFile baseFile)
    {
        using (var stream = await baseFile.OpenStreamForReadAsync())
        {
            return InitializeTypeAsync(stream);
        }
    }

    protected override Task<ConfigurationSetting> InitializeTypeAsync(IRandomAccessStream stream)
    {
        var config = InitializeTypeAsync(stream.AsStream());
        return Task.FromResult<ConfigurationSetting>(config);
    }

    private ConfigurationSetting InitializeTypeAsync(Stream stream)
    {
        var reader = new StreamReader(stream);

        using (var jsonReader = new JsonTextReader(reader))
        {
            return jsonSerializer.Deserialize<ConfigurationSetting>(jsonReader);
        }
    }
}

In this case I am using a specific type to deserialise json to. How do I use it ?

ConfigCache.Instance.CacheDuration = TimeSpan.FromDays(1);

this.ConfigurationSettings = await ConfigCache.Instance.GetFromCacheAsync(new Uri(urlPath));

This would ensure that if configuration is older than a day, it will be downloaded again. Either way the caller will get deserialised data.

Customise Scroll Bar in #uwp

Update (20th April 2017):

Sample app with horizontal scrollbar customisation can be found at https://github.com/hermitdave/ScrollBarCustomisation

 

Controls like ListView / GridView contain a ScrollViewer which hosts scrollable content. The ScrollViewer contains two ScrollBar controls one for each scroll type (horizontal and vertical).

Getting to a control’s template can be painful.. why not just head to
Default control styles and templates and download the templates you need

If you want to do it the hard way..
You can get to control’s ScrollViewer by right clicking on it and click Edit Template > Edit a Copy
Screenshot (39)

Looking at the template you can see the ScrollViewer but there’s little you can do right now.

<ControlTemplate TargetType="ListView">
    <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}">
        <ScrollViewer x:Name="ScrollViewer" AutomationProperties.AccessibilityView="Raw" BringIntoViewOnFocusChange="{TemplateBinding ScrollViewer.BringIntoViewOnFocusChange}" HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}" IsHorizontalScrollChainingEnabled="{TemplateBinding ScrollViewer.IsHorizontalScrollChainingEnabled}" IsVerticalScrollChainingEnabled="{TemplateBinding ScrollViewer.IsVerticalScrollChainingEnabled}" IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}" IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}" TabNavigation="{TemplateBinding TabNavigation}" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}" VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}" ZoomMode="{TemplateBinding ScrollViewer.ZoomMode}">
            <ItemsPresenter FooterTransitions="{TemplateBinding FooterTransitions}" FooterTemplate="{TemplateBinding FooterTemplate}" Footer="{TemplateBinding Footer}" HeaderTemplate="{TemplateBinding HeaderTemplate}" Header="{TemplateBinding Header}" HeaderTransitions="{TemplateBinding HeaderTransitions}" Padding="{TemplateBinding Padding}"/>
        </ScrollViewer>
    </Border>
</ControlTemplate>

You need to go and Edit the template for ScrollViewer next.
You can right click on the ListView, tap Esc key which then moves focus on ScrollViewer. Click control’s Border and right click > Edit Template > Edit a copy

<ControlTemplate TargetType="ScrollViewer">
    <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}">
        <Grid Background="{TemplateBinding Background}">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="Auto"/>
            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
                <RowDefinition Height="*"/>
                <RowDefinition Height="Auto"/>
            </Grid.RowDefinitions>
            <ScrollContentPresenter x:Name="ScrollContentPresenter" Grid.ColumnSpan="2" ContentTemplate="{TemplateBinding ContentTemplate}" Margin="{TemplateBinding Padding}" Grid.RowSpan="2"/>
            <ScrollBar x:Name="VerticalScrollBar" Grid.Column="1" HorizontalAlignment="Right" IsTabStop="False" Maximum="{TemplateBinding ScrollableHeight}" Orientation="Vertical" Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}" Value="{TemplateBinding VerticalOffset}" ViewportSize="{TemplateBinding ViewportHeight}"/>
            <ScrollBar x:Name="HorizontalScrollBar" IsTabStop="False" Maximum="{TemplateBinding ScrollableWidth}" Orientation="Horizontal" Grid.Row="1" Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}" Value="{TemplateBinding HorizontalOffset}" ViewportSize="{TemplateBinding ViewportWidth}"/>
            <Border x:Name="ScrollBarSeparator" Background="{ThemeResource SystemControlPageBackgroundChromeLowBrush}" Grid.Column="1" Grid.Row="1"/>
        </Grid>
    </Border>
</ControlTemplate>

In one instance I needed the ScrollBar to not render over the content. You can see that the ScrollViewer ScrollViewerPresenter has both ColSpan and RowSpan set. Just get rid of it so there is no overlay.
In Another instance I needed to customise it further. I needed Horizontal ScrollBar (to be of same height as the ListView) and I needed only the Left and Right buttons. So start by Setting ColSpan / RowSpan on ScrollBar as needed
Now lets get to ScrollBar template to customise it further.

<ControlTemplate x:Key="HorizontalIncrementTemplate" TargetType="RepeatButton">
    <Grid x:Name="Root">
        <Grid x:Name="HorizontalRoot" IsHitTestVisible="False">
            <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="Auto" />
            </Grid.ColumnDefinitions>
            <Rectangle x:Name="HorizontalTrackRect"
                        Grid.ColumnSpan="5"
                        Margin="0"
                        StrokeThickness="{ThemeResource ScrollBarTrackBorderThemeThickness}"
                        Fill="{ThemeResource SystemControlPageBackgroundChromeLowBrush}"
                        Stroke="{ThemeResource SystemControlForegroundTransparentBrush}" />
            <RepeatButton x:Name="HorizontalSmallDecrease"
                        Grid.Column="0"
                        MinHeight="12"
                        IsTabStop="False"
                        Interval="50"
                        Margin="0"
                        Template="{StaticResource HorizontalDecrementTemplate}"
                        Width="12"
                        VerticalAlignment="Center" />
            <RepeatButton x:Name="HorizontalLargeDecrease"
                        Grid.Column="1"
                        HorizontalAlignment="Stretch"
                        VerticalAlignment="Stretch"
                        IsTabStop="False"
                        Interval="50"
                        Template="{StaticResource RepeatButtonTemplate}"
                        Width="0" />
            <Thumb x:Name="HorizontalThumb"
                        Grid.Column="2"
                        Background="{ThemeResource SystemControlForegroundChromeHighBrush}"
                        Template="{StaticResource HorizontalThumbTemplate}"
                        Height="12"
                        MinWidth="12"
                        AutomationProperties.AccessibilityView="Raw" />
            <RepeatButton x:Name="HorizontalLargeIncrease"
                        Grid.Column="3"
                        HorizontalAlignment="Stretch"
                        VerticalAlignment="Stretch"
                        IsTabStop="False"
                        Interval="50"
                        Template="{StaticResource RepeatButtonTemplate}" />
            <RepeatButton x:Name="HorizontalSmallIncrease"
                        Grid.Column="4"
                        MinHeight="12"
                        IsTabStop="False"
                        Interval="50"
                        Margin="0"
                        Template="{StaticResource HorizontalIncrementTemplate}"
                        Width="12"
                        VerticalAlignment="Center" />
        </Grid>
        <Grid x:Name="HorizontalPanningRoot" MinWidth="24">
            <Border x:Name="HorizontalPanningThumb"
                    VerticalAlignment="Bottom"
                    HorizontalAlignment="Left"
                    Background="{ThemeResource SystemControlForegroundChromeDisabledLowBrush}"
                    BorderThickness="0"
                    Height="2"
                    MinWidth="32"
                    Margin="0,2,0,2"/>
        </Grid>
        <Grid x:Name="VerticalRoot" IsHitTestVisible="False">
            <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
            <RowDefinition Height="Auto" />
            </Grid.RowDefinitions>
            <Rectangle x:Name="VerticalTrackRect"
                        Grid.RowSpan="5"
                        Margin="0"
                        StrokeThickness="{ThemeResource ScrollBarTrackBorderThemeThickness}"
                        Fill="{ThemeResource SystemControlPageBackgroundChromeLowBrush}"
                        Stroke="{ThemeResource SystemControlForegroundTransparentBrush}" />
            <RepeatButton x:Name="VerticalSmallDecrease"
                        Height="12"
                        MinWidth="12"
                        IsTabStop="False"
                        Interval="50"
                        Margin="0"
                        Grid.Row="0"
                        Template="{StaticResource VerticalDecrementTemplate}"
                        HorizontalAlignment="Center" />
            <RepeatButton x:Name="VerticalLargeDecrease"
                        HorizontalAlignment="Stretch"
                        VerticalAlignment="Stretch"
                        Height="0"
                        IsTabStop="False"
                        Interval="50"
                        Grid.Row="1"
                        Template="{StaticResource RepeatButtonTemplate}" />
            <Thumb x:Name="VerticalThumb"
                        Grid.Row="2"
                        Background="{ThemeResource SystemControlForegroundChromeHighBrush}"
                        Template="{StaticResource VerticalThumbTemplate}"
                        Width="12"
                        MinHeight="12"
                        AutomationProperties.AccessibilityView="Raw" />
            <RepeatButton x:Name="VerticalLargeIncrease"
                        HorizontalAlignment="Stretch"
                        VerticalAlignment="Stretch"
                        IsTabStop="False"
                        Interval="50"
                        Grid.Row="3"
                        Template="{StaticResource RepeatButtonTemplate}" />
            <RepeatButton x:Name="VerticalSmallIncrease"
                        Height="12"
                        MinWidth="12"
                        IsTabStop="False"
                        Interval="50"
                        Margin="0"
                        Grid.Row="4"
                        Template="{StaticResource VerticalIncrementTemplate}"
                        HorizontalAlignment="Center" />
        </Grid>
        <Grid x:Name="VerticalPanningRoot" MinHeight="24">
            <Border x:Name="VerticalPanningThumb"
                    VerticalAlignment="Top"
                    HorizontalAlignment="Right"
                    Background="{ThemeResource SystemControlForegroundChromeDisabledLowBrush}"
                    BorderThickness="0"
                    Width="2"
                    MinHeight="32"
                    Margin="2,0,2,0"/>
        </Grid>
    </Grid>
</ControlTemplate>

In my case I only needed the two RepeatButton named HorizontalSmallDecrease and HorizontalSmallIncrease. Comment out the remaining controls (VerticalTrackRect, VerticalLargeDecrease, VerticalThumb, VerticalLargeIncrease)
For smoother scroll, modify RepeatButton’s Interval property, I set it to 5 rather than 50.

For further customisation I even created Attached Property that resizes the RepeatButton (the visibility is controlled by the ScrollBar so setting width while a hack works well.

public static class ScrollBarHelper
{
    static double InitialWidth = Double.MinValue;

    public static readonly DependencyProperty DCustomiseScrollBehaviourProperty =
    DependencyProperty.RegisterAttached("CustomiseScrollBehaviour", typeof(bool),
    typeof(ScrollBarHelper), new PropertyMetadata(false, OnCustomiseScrollBehaviourPropertyChanged));

    public static void SetCustomiseScrollBehaviour(DependencyObject d, bool value)
    {
        d.SetValue(DCustomiseScrollBehaviourProperty, value);
    }

    public static bool GetCustomiseScrollBehaviour(DependencyObject d)
    {
        return (bool)d.GetValue(DCustomiseScrollBehaviourProperty);
    }

    private static void OnCustomiseScrollBehaviourPropertyChanged(DependencyObject d,
        DependencyPropertyChangedEventArgs e)
    {
        if (!(bool)e.NewValue)
            return;

        var control = d as ScrollBar;
        if (control != null)
        {
            Observable.FromEventPattern<RoutedEventArgs>(control, "Loaded")
                .SubscribeOn(TaskPoolScheduler.Default)
                .ObserveOn(CoreDispatcherScheduler.Current)
                .Subscribe(args =>
                {
                    ProcessPosition(args.Sender);
                });

            Observable.FromEventPattern<ScrollEventArgs>(control, "Scroll")
                .Throttle(TimeSpan.FromMilliseconds(50))
                .SubscribeOn(TaskPoolScheduler.Default)
                .ObserveOn(CoreDispatcherScheduler.Current)
                .Subscribe(args =>
                {
                    ProcessPosition(args.Sender);
                });
        }
    }

    private static void ProcessPosition(object sender)
    {
        ScrollBar sb = (sender as ScrollBar);

        RepeatButton rbLeft = (RepeatButton)sb.FindDescendantByName("HorizontalSmallDecrease");
        RepeatButton rbRight = (RepeatButton)sb.FindDescendantByName("HorizontalSmallIncrease");

        if (rbLeft == null || rbRight == null)
            return;

        double pos = sb.Value;

        if (pos == 0 && InitialWidth == Double.MinValue)
        {
            InitialWidth = rbLeft.Width;
        }

        if (Math.Abs(pos - sb.Minimum) <= 15)
        {
            // hide the left scroll button
            rbLeft.Width = 0;
        }
        else if (Math.Abs(pos - sb.Maximum) <= 15)
        {
            // hide right scroll button
            rbRight.Width = 0;
        }
        else
        {
            rbLeft.Width = rbRight.Width = InitialWidth;
        }
    }
}

You could directly attached to ScrollBar’s Loaded / Scroll events but I used Reactive Extensions to throttle scroll changes as I wanted scroll experience to not degrade. The result.. not perfect but close to what I needed