Invoking Events from Effects

Implement low-level multi-touch finger tracking in Xamarin.Forms controls

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last updated: 2017-03

An effect can define and invoke an event, signaling changes in the underlying native view. This article shows how to implement low-level multi-touch finger tracking, and how to generate events that signal touch activity.

The effect described in this article provides access to low-level touch events. These low-level events are not available through the existing GestureRecognizer classes, but they are vital to some types of applications. For example, a finger-paint application needs to track individual fingers as they move on the screen. A music keyboard needs to detect taps and releases on the individual keys, as well as a finger gliding from one key to another in a glissando.

An effect is ideal for multi-touch finger tracking because it can be attached to any Xamarin.Forms element.

Platform Touch Events

The iOS, Android, and Universal Windows Platform all include a low-level API that allows applications to detect touch activity. These platforms all distinguish between three basic types of touch events:

  • Pressed, when a finger touches the screen
  • Moved, when a finger touching the screen moves
  • Released, when the finger is released from the screen

In a multi-touch environment, multiple fingers can touch the screen at the same time. The various platforms include an identification (ID) number that applications can use to distinguish between multiple fingers.

In iOS, the UIView class defines three overridable methods, TouchesBegan, TouchesMoved, and TouchesEnded corresponding to these three basic events. The article Multi-Touch Finger Tracking describes how to use these methods. However, an iOS program does not need to override a class that derives from UIView to use these methods. The iOS UIGestureRecognizer also defines these same three methods, and you can attach an instance of a class that derives from UIGestureRecognizer to any UIView object.

In Android, the View class defines an overridable method named OnTouchEvent to process all the touch activity. The type of the touch activity is defined by enumeration members Down, PointerDown, Move, Up, and PointerUp as described in the article Multi-Touch Finger Tracking. The Android View also defines an event named Touch that allows an event handler to be attached to any View object.

In the Universal Windows Platform (UWP), the UIElement class defines events named PointerPressed, PointerMoved, and PointerReleased. These are described in the article Handle Pointer Input article on MSDN and the API documentation for the UIElement class.

The Pointer API in the Universal Windows Platform is intended to unify mouse, touch, and pen input. For that reason, the PointerMoved event is invoked when the mouse moves across an element even when a mouse button is not depressed. The PointerRoutedEventArgs object that accompanies these events has a property named Pointer that has a property named IsInContact which indicates if a mouse button is pressed or a finger is in contact with the screen.

In addition, the UWP defines two more events named PointerEntered and PointerExited. These indicate when a mouse or finger moves from one element to another. For example, consider two adjacent elements named A and B. Both elements have installed handlers for the pointer events. When a finger presses on A, the PointerPressed event is invoked. As the finger moves, A invokes PointerMoved events. If the finger moves from A to B, A invokes a PointerExited event and B invokes a PointerEntered event. If the finger is then released, B invokes a PointerReleased event.

The iOS and Android platforms are different from the UWP: The view that first gets the call to TouchesBegan or OnTouchEvent when a finger touches the view continues to get all the touch activity even if if the finger moves to different views. The UWP can behave similarly if the application captures the pointer: In the PointerEntered event handler, the element calls CapturePointer and then gets all touch activity from that finger.

The UWP approach proves to be very useful for some types of applications, for example, a music keyboard. Each key can handle the touch events for that key and detect when a finger has slid from one key to another using the PointerEntered and PointerExited events.

For that reason, the touch-tracking effect described in this article implements the UWP approach.

The Touch-Tracking Effect API

The Touch Tracking Effect Demos sample contains the classes (and an enumeration) that implement the low-level touch-tracking. These types belong to the namespace TouchTracking and begin with the word Touch. The TouchTrackingEffectDemos Portable Class Library project includes the TouchActionType enumeration for the type of touch events:

public enum TouchActionType
{
    Entered,
    Pressed,
    Moved,
    Released,
    Exited,
    Cancelled
}

All the platforms also include an event that indicates that the touch event has been cancelled.

The TouchEffect class in the PCL derives from RoutingEffect and defines an event named TouchAction and a method named OnTouchAction that invokes the TouchAction event:

public class TouchEffect : RoutingEffect
{
    public event TouchActionEventHandler TouchAction;

    public TouchEffect() : base("XamarinDocs.TouchEffect")
    {
    }

    public bool Capture { set; get; }

    public void OnTouchAction(Element element, TouchActionEventArgs args)
    {
        TouchAction?.Invoke(element, args);
    }
}

Also notice the Capture property. To capture touch events, an application must set this property to true prior to a Pressed event. Otherwise, the touch events behave like those in the Universal Windows Platform.

The TouchActionEventArgs class in the PCL contains all the information that accompanies each event:

public class TouchActionEventArgs : EventArgs
{
    public TouchActionEventArgs(long id, TouchActionType type, Point location, bool isInContact)
    {
        Id = id;
        Type = type;
        Location = location;
        IsInContact = isInContact;
    }

    public long Id { private set; get; }

    public TouchActionType Type { private set; get; }

    public Point Location { private set; get; }

    public bool IsInContact { private set; get; }
}

An application can use the Id property for tracking individual fingers. Notice the IsInContact property. This property is always true for Pressed events and false for Released events. It's also always true for Moved events on iOS and Android. The IsInContact property might be false for Moved events on the Universal Windows Platform when the program is running on the desktop and the mouse pointer moves without a button pressed.

You can use the TouchEffect class in your own applications by including the file in the solution's PCL project, and by adding an instance to the Effects collection of any Xamarin.Forms element. Attach a handler to the TouchAction event to obtain the touch events.

To use TouchEffect in your own application, you'll also need the platform implementations included in TouchTrackingEffectDemos solution.

The Touch-Tracking Effect Implementations

The iOS, Android, and UWP implementations of the TouchEffect are described below beginning with the simplest implementation (UWP) and ending with the iOS implementation because it is more structurally complex than the others.

The UWP Implementation

The UWP implementation of TouchEffect is the simplest. As usual, the class derives from PlatformEffect and includes two assembly attributes:

[assembly: ResolutionGroupName("XamarinDocs")]
[assembly: ExportEffect(typeof(TouchTracking.UWP.TouchEffect), "TouchEffect")]

namespace TouchTracking.UWP
{
    public class TouchEffect : PlatformEffect
    {
        ...
    }
}

The OnAttached override saves some information as fields and attaches handlers to all the pointer events:

public class TouchEffect : PlatformEffect
{
    FrameworkElement frameworkElement;
    TouchTracking.TouchEffect effect;
    Action<Element, TouchActionEventArgs> onTouchAction;

    protected override void OnAttached()
    {
        // Get the Windows FrameworkElement corresponding to the Element that the effect is attached to
        frameworkElement = Control == null ? Container : Control;

        // Get access to the TouchEffect class in the PCL
        effect = (TouchTracking.TouchEffect)Element.Effects.
                    FirstOrDefault(e => e is TouchTracking.TouchEffect);

        if (effect != null && frameworkElement != null)
        {
            // Save the method to call on touch events
            onTouchAction = effect.OnTouchAction;

            // Set event handlers on FrameworkElement
            frameworkElement.PointerEntered += OnPointerEntered;
            frameworkElement.PointerPressed += OnPointerPressed;
            frameworkElement.PointerMoved += OnPointerMoved;
            frameworkElement.PointerReleased += OnPointerReleased;
            frameworkElement.PointerExited += OnPointerExited;
            frameworkElement.PointerCanceled += OnPointerCancelled;
        }
    }
    ...
}

The OnPointerPressed handler invokes the effect event by calling the onTouchAction field in the CommonHandler method:

public class TouchEffect : PlatformEffect
{
    ...
    void OnPointerPressed(object sender, PointerRoutedEventArgs args)
    {
        CommonHandler(sender, TouchActionType.Pressed, args);

        // Check setting of Capture property
        if (effect.Capture)
        {
            (sender as FrameworkElement).CapturePointer(args.Pointer);
        }
    }
    ...
    void CommonHandler(object sender, TouchActionType touchActionType, PointerRoutedEventArgs args)
    {
        PointerPoint pointerPoint = args.GetCurrentPoint(sender as UIElement);
        Windows.Foundation.Point windowsPoint = pointerPoint.Position;  

        onTouchAction(Element, new TouchActionEventArgs(args.Pointer.PointerId,
                                                        touchActionType,
                                                        new Point(windowsPoint.X, windowsPoint.Y),
                                                        args.Pointer.IsInContact));
    }
}

OnPointerPressed also checks the value of the Capture property in the effect class in the PCL and calls CapturePointer if it is true.

The other UWP event handlers are even simpler:

public class TouchEffect : PlatformEffect
{
    ...
    void OnPointerEntered(object sender, PointerRoutedEventArgs args)
    {
        CommonHandler(sender, TouchActionType.Entered, args);
    }
    ...
}

The Android Implementation

The Android and iOS implementations are necessarily more complex because they must implement the Exited and Entered events when a finger moves from one element to another. Both implementations are structured similarly.

The Android TouchEffect class installs a handler for the Touch event:

view = Control == null ? Container : Control;
...
view.Touch += OnTouch;

The class also defines two static dictionaries:

public class TouchEffect : PlatformEffect
{
    ...
    static Dictionary<Android.Views.View, TouchEffect> viewDictionary =
        new Dictionary<Android.Views.View, TouchEffect>();

    static Dictionary<int, TouchEffect> idToEffectDictionary =
        new Dictionary<int, TouchEffect>();
    ...

The viewDictionary gets a new entry every time the OnAttached override is called:

viewDictionary.Add(view, this);

The entry is removed from the dictionary in OnDetached. Every instance of TouchEffect is associated with a particular view that the effect is attached to. The static dictionary allows any TouchEffect instance to enumerate through all the other views and their corresponding TouchEffect instances. This is necessary to allow for transferring the events from one view to another.

Android assigns an ID code to touch events that allows an application to track individual fingers. The idToEffectDictionary associates this ID code with a TouchEffect instance. An item is added to this dictionary when the Touch handler is called for a finger press:

void OnTouch(object sender, Android.Views.View.TouchEventArgs args)
{
    ...
    switch (args.Event.ActionMasked)
    {
        case MotionEventActions.Down:
        case MotionEventActions.PointerDown:
            FireEvent(this, id, TouchActionType.Pressed, screenPointerCoords, true);

            idToEffectDictionary.Add(id, this);

            capture = pclTouchEffect.Capture;
            break;

The item is removed from the idToEffectDictionary when the finger is released from the screen. The FireEvent method simply accumulates all the information necessary to call the OnTouchAction method:

void FireEvent(TouchEffect touchEffect, int id, TouchActionType actionType, Point pointerLocation, bool isInContact)
{
    // Get the method to call for firing events
    Action<Element, TouchActionEventArgs> onTouchAction = touchEffect.pclTouchEffect.OnTouchAction;

    // Get the location of the pointer within the view
    touchEffect.view.GetLocationOnScreen(twoIntArray);
    double x = pointerLocation.X - twoIntArray[0];
    double y = pointerLocation.Y - twoIntArray[1];
    Point point = new Point(fromPixels(x), fromPixels(y));

    // Call the method
    onTouchAction(touchEffect.formsElement,
        new TouchActionEventArgs(id, actionType, point, isInContact));
}

All the other touch types are processed in two different ways: If the Capture property is true, the touch event is a fairly simple translation to the TouchEffect information. It gets more complicated when Capture is false because the touch events might need to be moved from one view to another. This is the responsibility of the CheckForBoundaryHop method, which is called during move events. This method makes use of both static dictionaries. It enumerates through the viewDictionary to determine the view that the finger is currently touching, and it uses idToEffectDictionary to store the current TouchEffect instance (and hence, the current view) associated with a particular ID:

void CheckForBoundaryHop(int id, Point pointerLocation)
{
    TouchEffect touchEffectHit = null;

    foreach (Android.Views.View view in viewDictionary.Keys)
    {
        // Get the view rectangle
        try
        {
            view.GetLocationOnScreen(twoIntArray);
        }
        catch // System.ObjectDisposedException: Cannot access a disposed object.
        {
            continue;
        }
        Rectangle viewRect = new Rectangle(twoIntArray[0], twoIntArray[1], view.Width, view.Height);

        if (viewRect.Contains(pointerLocation))
        {
            touchEffectHit = viewDictionary[view];
        }
    }

    if (touchEffectHit != idToEffectDictionary[id])
    {
        if (idToEffectDictionary[id] != null)
        {
            FireEvent(idToEffectDictionary[id], id, TouchActionType.Exited, pointerLocation, true);
        }
        if (touchEffectHit != null)
        {
            FireEvent(touchEffectHit, id, TouchActionType.Entered, pointerLocation, true);
        }
        idToEffectDictionary[id] = touchEffectHit;
    }
}

If there's been a change in the idToEffectionDictionary, the method potentially calls FireEvent for Exited and Entered to transfer from one view to another. However, the finger might have been moved to an area occupied by a view without an attached TouchEffect, or from that area to a view with the effect attached.

Notice the try and catch block when the view is accessed. In a page that is navigated to that then navigates back to the home page, the OnDetached method is not called and items remain in the viewDictionary but Android considers them disposed.

The iOS Implementation

The iOS implementation is similar to the Android implementation except that the iOS TouchEffect class must instantiate a derivative of UIGestureRecognizer. This is a class in the iOS project named TouchRecognizer. This class maintains two static dictionaries that store TouchRecognizer instances:

static Dictionary<UIView, TouchRecognizer> viewDictionary =
    new Dictionary<UIView, TouchRecognizer>();

static Dictionary<long, TouchRecognizer> idToTouchDictionary =
    new Dictionary<long, TouchRecognizer>();

Much of the structure of this TouchRecognizer class is similar to the Android TouchEffect class.

Putting the Touch Effect to Work

The TouchTrackingEffectDemos program contains five pages that test the touch-tracking effect for common tasks.

The BoxView Dragging page allows you to add BoxView elements to an AbsoluteLayout and then drag them around the screen. The XAML file instantiates two Button views for adding BoxView elements to the AbsoluteLayout and clearing the AbsoluteLayout.

The method in the code-behind file that adds a new BoxView to the AbsoluteLayout also adds a TouchEffect object to the BoxView and attaches an event handler to the effect:

void AddBoxViewToLayout()
{
    BoxView boxView = new BoxView
    {
        WidthRequest = 100,
        HeightRequest = 100,
        Color = new Color(random.NextDouble(),
                          random.NextDouble(),
                          random.NextDouble())
    };

    TouchEffect touchEffect = new TouchEffect();
    touchEffect.TouchAction += OnTouchEffectAction;
    boxView.Effects.Add(touchEffect);
    absoluteLayout.Children.Add(boxView);
}

The TouchAction event handler processes all the touch events for all the BoxView elements, but it needs to exercise some caution: It can't allow two fingers on a single BoxView because the program only implements dragging, and the two fingers would interfere with each other. For this reason, the page defines an embedded class for each finger currently being tracked:

class DragInfo
{
    public DragInfo(long id, Point pressPoint)
    {
        Id = id;
        PressPoint = pressPoint;
    }

    public long Id { private set; get; }

    public Point PressPoint { private set; get; }
}

Dictionary<BoxView, DragInfo> dragDictionary = new Dictionary<BoxView, DragInfo>();

The dragDictionary contains an entry for every BoxView currently being dragged.

The Pressed touch action adds an item to this dictionary, and the Released action removes it. The Pressed logic must check if there's already an item in the dictionary for that BoxView. If so, the BoxView is already being dragged and the new event is a second finger on that same BoxView. For the Moved and Released actions, the event handler must check if the dictionary has an entry for that BoxView and that the touch Id property for that dragged BoxView matches the one in the dictionary entry:

void OnTouchEffectAction(object sender, TouchActionEventArgs args)
{
    BoxView boxView = sender as BoxView;

    switch (args.Type)
    {
        case TouchActionType.Pressed:
            // Don't allow a second touch on an already touched BoxView
            if (!dragDictionary.ContainsKey(boxView))
            {
                dragDictionary.Add(boxView, new DragInfo(args.Id, args.Location));

                // Set Capture property to true
                TouchEffect touchEffect = (TouchEffect)boxView.Effects.FirstOrDefault(e => e is TouchEffect);
                touchEffect.Capture = true;
            }
            break;

        case TouchActionType.Moved:
            if (dragDictionary.ContainsKey(boxView) && dragDictionary[boxView].Id == args.Id)
            {
                Rectangle rect = AbsoluteLayout.GetLayoutBounds(boxView);
                Point initialLocation = dragDictionary[boxView].PressPoint;
                rect.X += args.Location.X - initialLocation.X;
                rect.Y += args.Location.Y - initialLocation.Y;
                AbsoluteLayout.SetLayoutBounds(boxView, rect);
            }
            break;

        case TouchActionType.Released:
            if (dragDictionary.ContainsKey(boxView) && dragDictionary[boxView].Id == args.Id)
            {
                dragDictionary.Remove(boxView);
            }
            break;
    }
}

The Pressed logic sets the Capture property of the TouchEffect object to true. This has the effect of delivering all subsequent events for that finger to the same event handler.

The Moved logic moves the BoxView by altering the LayoutBounds attached property. The Location property of the event arguments is always relative to the BoxView being dragged, and if the BoxView is being dragged at a constant rate, the Location properties of the consecutive events will be approximately the same. For example, if a finger presses the BoxView in its center, the Pressed action stores a PressPoint property of (50, 50), which remains the same for subsequent events. If the BoxView is dragged diagonally at a constant rate, the subsequent Location properties during the Moved action might be values of (55, 55), in which case the Moved logic adds 5 to the horizontal and vertical position of the BoxView. This moves the BoxView so that its center is again directly under the finger.

You can move multiple BoxView elements simultaneously using different fingers.

Subclassing the View

Often, it's easier for a Xamarin.Forms element to handle its own touch events. The Draggable BoxView Dragging page functions the same as the BoxView Dragging page, but the elements that the user drags are instances of a DraggableBoxView class that derives from BoxView:

class DraggableBoxView : BoxView
{
    bool isBeingDragged;
    long touchId;
    Point pressPoint;

    public DraggableBoxView()
    {
        TouchEffect touchEffect = new TouchEffect
        {
            Capture = true
        };
        touchEffect.TouchAction += OnTouchEffectAction;
        Effects.Add(touchEffect);
    }

    void OnTouchEffectAction(object sender, TouchActionEventArgs args)
    {
        switch (args.Type)
        {
            case TouchActionType.Pressed:
                if (!isBeingDragged)
                {
                    isBeingDragged = true;
                    touchId = args.Id;
                    pressPoint = args.Location;
                }
                break;

            case TouchActionType.Moved:
                if (isBeingDragged && touchId == args.Id)
                {
                    TranslationX += args.Location.X - pressPoint.X;
                    TranslationY += args.Location.Y - pressPoint.Y;
                }
                break;

            case TouchActionType.Released:
                if (isBeingDragged && touchId == args.Id)
                {
                    isBeingDragged = false;
                }
                break;
        }
    }
}

The constructor creates and attaches the TouchEffect, and sets the Capture property when that object is first instantiated. No dictionary is required because the class itself stores isBeingDragged, pressPoint, and touchId values associated with each finger. The Moved handling alters the TranslationX and TranslationY properties so the logic will work even if the parent of the DraggableBoxView is not an AbsoluteLayout.

Integrating with SkiaSharp

The next two demonstrations require graphics, and they use SkiaSharp for this purpose. You might want to learn about Using SkiaSharp in Xamarin.Forms before you study these examples. The first two articles ("SkiaSharp Drawing Basics" and "SkiaSharp Lines and Paths") cover everything that you'll need here.

The Ellipse Drawing page allows you to draw an ellipse by swiping your finger on the screen. Depending how you move your finger, you can draw the ellipse from the upper-left to the lower-right, or from any other corner to the opposite corner. The ellipse is drawn with a random color and opacity.

If you then touch one of the ellipses, you can drag it to another location. This requires a technique known as "hit-testing," which involves searching for the graphical object at a particular point. The SkiaSharp ellipses are not Xamarin.Forms elements, so they cannot perform their own TouchEffect processing. The TouchEffect must apply to the entire SKCanvasView object.

The EllipseDrawPage.xaml file instantiates the SKCanvasView in a single-cell Grid. The TouchEffect object is attached to that Grid:

<Grid x:Name="canvasViewGrid"
        Grid.Row="1"
        BackgroundColor="White">

    <skia:SKCanvasView x:Name="canvasView"
                        PaintSurface="OnCanvasViewPaintSurface" />
    <Grid.Effects>
        <tt:TouchEffect Capture="True"
                        TouchAction="OnTouchEffectAction" />
    </Grid.Effects>
</Grid>

In Android and the Universal Windows Platform, the TouchEffect can be attached directly to the SKCanvasView, but on iOS that doesn't work. Notice that the Capture property is set to true.

Each ellipse that SkiaSharp renders is represented by an object of type EllipseDrawingFigure:

class EllipseDrawingFigure
{
    SKPoint pt1, pt2;

    public EllipseDrawingFigure()
    {
    }

    public SKColor Color { set; get; }

    public SKPoint StartPoint
    {
        set
        {
            pt1 = value;
            MakeRectangle();
        }
    }

    public SKPoint EndPoint
    {
        set
        {
            pt2 = value;
            MakeRectangle();
        }
    }

    void MakeRectangle()
    {
        Rectangle = new SKRect(pt1.X, pt1.Y, pt2.X, pt2.Y).Standardized;
    }

    public SKRect Rectangle { set; get; }

    // For dragging operations
    public Point LastFingerLocation { set; get; }

    // For the dragging hit-test
    public bool IsInEllipse(SKPoint pt)
    {
        SKRect rect = Rectangle;

        return (Math.Pow(pt.X - rect.MidX, 2) / Math.Pow(rect.Width / 2, 2) +
                Math.Pow(pt.Y - rect.MidY, 2) / Math.Pow(rect.Height / 2, 2)) < 1;
    }
}

The StartPoint and EndPoint properties are used when the program is processing touch input; the Rectangle property is used for drawing the ellipse. The LastFingerLocation property comes into play when the ellipse is being dragged, and the IsInEllipse method aids in hit-testing. The method returns true if the point is inside the ellipse.

The code-behind file maintains three collections:

Dictionary<long, EllipseDrawingFigure> inProgressFigures = new Dictionary<long, EllipseDrawingFigure>();
List<EllipseDrawingFigure> completedFigures = new List<EllipseDrawingFigure>();
Dictionary<long, EllipseDrawingFigure> draggingFigures = new Dictionary<long, EllipseDrawingFigure>();

The draggingFigure dictionary contains a subset of the completedFigures collection. The SkiaSharp PaintSurface event handler simply renders the objects in these the completedFigures and inProgressFigures collections:

SKPaint paint = new SKPaint
{
    Style = SKPaintStyle.Fill
};
...
void OnCanvasViewPaintSurface(object sender, SKPaintSurfaceEventArgs args)
{
    SKCanvas canvas = args.Surface.Canvas;
    canvas.Clear();

    foreach (EllipseDrawingFigure figure in completedFigures)
    {
        paint.Color = figure.Color;
        canvas.DrawOval(figure.Rectangle, paint);
    }
    foreach (EllipseDrawingFigure figure in inProgressFigures.Values)
    {
        paint.Color = figure.Color;
        canvas.DrawOval(figure.Rectangle, paint);
    }
}

The trickiest part of the touch processing is the Pressed handling. This is where the hit-testing is performed, but if the code detects an ellipse under the user's finger, that ellipse can only be dragged if it's not currently being dragged by another finger. If there is no ellipse under the user's finger, then the code begins the process of drawing a new ellipse:

case TouchActionType.Pressed:
    bool isDragOperation = false;

    // Loop through the completed figures
    foreach (EllipseDrawingFigure fig in completedFigures.Reverse<EllipseDrawingFigure>())
    {
        // Check if the finger is touching one of the ellipses
        if (fig.IsInEllipse(ConvertToPixel(args.Location)))
        {
            // Tentatively assume this is a dragging operation
            isDragOperation = true;

            // Loop through all the figures currently being dragged
            foreach (EllipseDrawingFigure draggedFigure in draggingFigures.Values)
            {
                // If there's a match, we'll need to dig deeper
                if (fig == draggedFigure)
                {
                    isDragOperation = false;
                    break;
                }
            }

            if (isDragOperation)
            {
                fig.LastFingerLocation = args.Location;
                draggingFigures.Add(args.Id, fig);
                break;
            }
        }
    }

    if (isDragOperation)
    {
        // Move the dragged ellipse to the end of completedFigures so it's drawn on top
        EllipseDrawingFigure fig = draggingFigures[args.Id];
        completedFigures.Remove(fig);
        completedFigures.Add(fig);
    }
    else // start making a new ellipse
    {
        // Random bytes for random color
        byte[] buffer = new byte[4];
        random.NextBytes(buffer);

        EllipseDrawingFigure figure = new EllipseDrawingFigure
        {
            Color = new SKColor(buffer[0], buffer[1], buffer[2], buffer[3]),
            StartPoint = ConvertToPixel(args.Location),
            EndPoint = ConvertToPixel(args.Location)
        };
        inProgressFigures.Add(args.Id, figure);
    }
    canvasView.InvalidateSurface();
    break;

The other SkiaSharp example is the Finger Paint page. You can select a stroke color and stroke width from two Picker views and then draw with one or more fingers:

This example also requires a separate class to represent each line painted on the screen:

class FingerPaintPolyline
{
    public FingerPaintPolyline()
    {
        Path = new SKPath();
    }

    public SKPath Path { set; get; }

    public Color StrokeColor { set; get; }

    public float StrokeWidth { set; get; }
}

An SKPath object is used to render each line. The FingerPaint.xaml.cs file maintains two collections of these objects, one for those polylines currently being drawn and another for the completed polylines:

Dictionary<long, FingerPaintPolyline> inProgressPolylines = new Dictionary<long, FingerPaintPolyline>();
List<FingerPaintPolyline> completedPolylines = new List<FingerPaintPolyline>();

The Pressed processing creates a new FingerPaintPolyline, calls MoveTo on the path object to store the initial point, and adds that object to the inProgressPolylines dictionary. The Moved processing calls LineTo on the path object with the new finger position, and the Released processing transfers the completed polyline from inProgressPolylines to completedPolylines. Once again, the actual SkiaSharp drawing code is relatively simple:

SKPaint paint = new SKPaint
{
    Style = SKPaintStyle.Stroke,
    StrokeCap = SKStrokeCap.Round,
    StrokeJoin = SKStrokeJoin.Round
};
...
void OnCanvasViewPaintSurface(object sender, SKPaintSurfaceEventArgs args)
{
    SKCanvas canvas = args.Surface.Canvas;
    canvas.Clear();

    foreach (FingerPaintPolyline polyline in completedPolylines)
    {
        paint.Color = polyline.StrokeColor.ToSKColor();
        paint.StrokeWidth = polyline.StrokeWidth;
        canvas.DrawPath(polyline.Path, paint);
    }

    foreach (FingerPaintPolyline polyline in inProgressPolylines.Values)
    {
        paint.Color = polyline.StrokeColor.ToSKColor();
        paint.StrokeWidth = polyline.StrokeWidth;
        canvas.DrawPath(polyline.Path, paint);
    }
}

Tracking View-to-View Touch

All the previous examples have set the Capture property of the TouchEffect to true, either when the TouchEffect was created or when the Pressed event occurred. This ensures that the same element receives all the events associated with the finger that first pressed the view. The final sample does not set Capture to true. This causes different behavior when a finger in contact with the screen moves from one element to another. The element that the finger moves from receives an event with a Type property set to TouchActionType.Exited and the second element receives an event with a Type setting of TouchActionType.Entered.

This type of touch processing is very useful for a music keyboard. A key should be able to detect when it's pressed, but also when a finger slides from one key to another.

The Silent Keyboard page defines small WhiteKey and BlackKey classes that derive from Key, which derives from BoxView.

The Key class is ready to be used in an actual music program. It defines public properties named IsPressed and KeyNumber, which is intended to be set to the key code established by the MIDI standard. The Key class also defines an event named StatusChanged, which is invoked when the IsPressed property changes.

Multiple fingers are allowed on each key. For this reason, the Key class maintains a List of the touch ID numbers of all the fingers currently touching that key:

List<long> ids = new List<long>();

The TouchAction event handler adds an ID to the ids list for both a Pressed event type and an Entered type, but only when the IsInContact property is true for the Entered event. The ID is removed from the List for a Released or Exited event:

void OnTouchEffectAction(object sender, TouchActionEventArgs args)
{
    switch (args.Type)
    {
      case TouchActionType.Pressed:
          AddToList(args.Id);
          break;

        case TouchActionType.Entered:
            if (args.IsInContact)
            {
                AddToList(args.Id);
            }
            break;

        case TouchActionType.Moved:
            break;

        case TouchActionType.Released:
        case TouchActionType.Exited:
            RemoveFromList(args.Id);
            break;
    }
}

The AddToList and RemoveFromList methods both check if the List has changed between empty and non-empty, and if so, invokes the StatusChanged event.

The various WhiteKey and BlackKey elements are arranged in the page's XAML file, which looks best when the phone is held in a landscape mode:

If you sweep your finger across the keys, you'll see by the slight changes in color that the touch events are transferred from one key to another.

Summary

This article has demonstrated how to invoke events in an effect, and how to write and use an effect that implements low-level multi-touch processing.

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