How to achieve Pinch Zoom & Swipe on SKCanvasView? - xamarin.forms

Question

I am developing image viewer using xamarin forms. My source image is of infinite size which is way bigger then my viewport. So I am using SKCanvasView control to draw part by part so that I should swipe through the Image.
I am able to get Touch point through Touch event which is available on SKCanvasView. Here, I am able to detect double tap. Swipe and Panning.
I am not able to decide on when I swipe up/down how much image need to shift up/down?
Initially, I have tried placing SKCanvasView on top of ScrollView control and using scroll position value I was painting SKCanvas. Here scrolling and printing works perfect. But here Pinch zoom was not working as Pinch gesture event was not firing instead it was scrolling.

Answer 1

I did something similar to this sometime back for one of my friends for SKCanvas even though I have personally never used Skia.
In any case, it has Pinch, Pan and Tap zoom events feel free to make any changes as per your needs:
public class CustomSKCanvas : SKCanvasView
{
private const double MIN_SCALE = 1;
private const double MAX_SCALE = 4;
private const double OVERSHOOT = 0.15;
private double StartScale, LastScale;
private double StartX, StartY;
public CustomSKCanvas()
{
var pinch = new PinchGestureRecognizer();
pinch.PinchUpdated += OnPinchUpdated;
GestureRecognizers.Add(pinch);
var pan = new PanGestureRecognizer();
pan.PanUpdated += OnPanUpdated;
GestureRecognizers.Add(pan);
var tap = new TapGestureRecognizer { NumberOfTapsRequired = 2 };
tap.Tapped += OnTapped;
GestureRecognizers.Add(tap);
Scale = MIN_SCALE;
TranslationX = TranslationY = 0;
AnchorX = AnchorY = 0;
}
protected override SizeRequest OnMeasure(double widthConstraint, double heightConstraint)
{
Scale = MIN_SCALE;
TranslationX = TranslationY = 0;
AnchorX = AnchorY = 0;
return base.OnMeasure(widthConstraint, heightConstraint);
}
private void OnTapped(object sender, EventArgs e)
{
if (Scale > MIN_SCALE)
{
this.ScaleTo(MIN_SCALE, 250, Easing.CubicInOut);
this.TranslateTo(0, 0, 250, Easing.CubicInOut);
}
else
{
AnchorX = AnchorY = 0.5; //TODO tapped position
this.ScaleTo(MAX_SCALE, 250, Easing.CubicInOut);
}
}
private void OnPanUpdated(object sender, PanUpdatedEventArgs e)
{
switch (e.StatusType)
{
case GestureStatus.Started:
StartX = (1 - AnchorX) * Width;
StartY = (1 - AnchorY) * Height;
break;
case GestureStatus.Running:
AnchorX = Clamp(1 - (StartX + e.TotalX) / Width, 0, 1);
AnchorY = Clamp(1 - (StartY + e.TotalY) / Height, 0, 1);
break;
}
}
private void OnPinchUpdated(object sender, PinchGestureUpdatedEventArgs e)
{
switch (e.Status)
{
case GestureStatus.Started:
LastScale = e.Scale;
StartScale = Scale;
AnchorX = e.ScaleOrigin.X;
AnchorY = e.ScaleOrigin.Y;
break;
case GestureStatus.Running:
if (e.Scale < 0 || Math.Abs(LastScale - e.Scale) > (LastScale * 1.3) - LastScale)
{ return; }
LastScale = e.Scale;
var current = Scale + (e.Scale - 1) * StartScale;
Scale = Clamp(current, MIN_SCALE * (1 - OVERSHOOT), MAX_SCALE * (1 + OVERSHOOT));
break;
case GestureStatus.Completed:
if (Scale > MAX_SCALE)
this.ScaleTo(MAX_SCALE, 250, Easing.SpringOut);
else if (Scale < MIN_SCALE)
this.ScaleTo(MIN_SCALE, 250, Easing.SpringOut);
break;
}
}
private T Clamp<T>(T value, T minimum, T maximum) where T : IComparable
{
if (value.CompareTo(minimum) < 0)
return minimum;
else if (value.CompareTo(maximum) > 0)
return maximum;
else
return value;
}
}
Good luck
Revert in case of queries.