PinchGestureTestPage.cs

using System;
using System.Diagnostics;
using Xamarin.Forms.Internals;

namespace Xamarin.Forms.Controls
{
	public class PinchToZoomContainer : ContentView
	{
		public PinchToZoomContainer ()
		{
			
		}

		public void AddPinch ()
		{

			var pinch = new PinchGestureRecognizer ();

			double xOffset = 0;
			double yOffset = 0;
			double startScale = 1;

			pinch.PinchUpdated += (sender, e) => {
				
				if (e.Status == GestureStatus.Started) {
					startScale = Content.Scale;
					Content.AnchorX = Content.AnchorY = 0;
				}
				if (e.Status == GestureStatus.Running) {

					_currentScale += (e.Scale - 1) * startScale;
					_currentScale = Math.Max (1, _currentScale);

					var renderedX = Content.X + xOffset;
					var deltaX = renderedX / Width;
					var deltaWidth = Width / (Content.Width * startScale);
					var originX = (e.ScaleOrigin.X - deltaX) * deltaWidth;

					var renderedY = Content.Y + yOffset;
					var deltaY = renderedY / Height;
					var deltaHeight = Height / (Content.Height * startScale);
					var originY = (e.ScaleOrigin.Y - deltaY) * deltaHeight;

					double targetX = xOffset - (originX * Content.Width) * (_currentScale - startScale);
					double targetY = yOffset - (originY * Content.Height) * (_currentScale - startScale);

					Content.TranslationX = targetX.Clamp (-Content.Width * (_currentScale - 1), 0);
					Content.TranslationY = targetY.Clamp (-Content.Height * (_currentScale - 1), 0);

					Content.Scale = _currentScale;
				}
				if (e.Status == GestureStatus.Completed) {
					xOffset = Content.TranslationX;
					yOffset = Content.TranslationY;
				}
			};

			GestureRecognizers.Add (pinch);
		}

		public bool AlwaysZoomCenter { get; set; }

		double _currentScale = 1;
	}

	public class PinchGestureTestPage : ContentPage
	{
		public PinchGestureTestPage ()
		{
			var stack = new StackLayout { VerticalOptions = LayoutOptions.Start, HorizontalOptions = LayoutOptions.Center };
			var textBoxScale = new Label { VerticalOptions = LayoutOptions.Start, HorizontalOptions = LayoutOptions.Center };
			var textBox = new Label { VerticalOptions = LayoutOptions.Start, HorizontalOptions = LayoutOptions.Center };
			var textBoxPoint = new Label { VerticalOptions = LayoutOptions.Start, HorizontalOptions = LayoutOptions.Center };
			stack.Children.Add (textBox);
			stack.Children.Add (textBoxScale);
			stack.Children.Add (textBoxPoint);

			var box = new Image { Source = "crimson.jpg", BackgroundColor = Color.Red, WidthRequest = 200, HeightRequest = 200, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center };

			var zoomContainer = new PinchToZoomContainer ();
			zoomContainer.Content = box;

			var btn = new Button { Text = "add pinch gesture", Command = new Command (() => zoomContainer.AddPinch ()) };
			var btnRemove = new Button { Text = "remove pinch gesture", Command = new Command (() => zoomContainer.GestureRecognizers.Clear ()) };

			Content = new StackLayout { Children = { btn, btnRemove, new Grid { Children = { zoomContainer }, Padding = new Thickness (20) } } };
		}

		double _currentScale = 1;
	}
}