From c5bd06af9048bafc90342aa918d35b6a18772d03 Mon Sep 17 00:00:00 2001
From: Daniel Belcher <dabelc@microsoft.com>
Date: Mon, 1 Apr 2019 17:20:21 -0700
Subject: [PATCH] - Mouse wheel can be used to zoom in/out of the graph.

---
 src/GraphControl/Control/Grapher.cpp | 28 ++++++++++++++++++++++++++++
 src/GraphControl/Control/Grapher.h   |  5 +++--
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/src/GraphControl/Control/Grapher.cpp b/src/GraphControl/Control/Grapher.cpp
index 86809619..aefbd1af 100644
--- a/src/GraphControl/Control/Grapher.cpp
+++ b/src/GraphControl/Control/Grapher.cpp
@@ -417,4 +417,32 @@ namespace GraphControl
             e->Handled = true;
         }
     }
+
+    void Grapher::OnPointerWheelChanged(PointerRoutedEventArgs^ args)
+    {
+        PointerPoint^ currentPointer = args->GetCurrentPoint(/*relative to*/ this);
+
+        double delta = currentPointer->Properties->MouseWheelDelta;
+
+        // The maximum delta is 120 according to:
+        // https://docs.microsoft.com/en-us/uwp/api/windows.ui.input.pointerpointproperties.mousewheeldelta#Windows_UI_Input_PointerPointProperties_MouseWheelDelta
+        // Apply a dampening effect so that small mouse movements have a smoother zoom.
+        constexpr double scrollDamper = 0.15;
+        double scale = 1.0 + (abs(delta) / WHEEL_DELTA) * scrollDamper;
+
+        // positive delta if wheel scrolled away from the user
+        if (delta >= 0)
+        {
+            scale = 1.0 / scale;
+        }
+
+        // For scaling, the graphing engine interprets x,y position between the range [-1, 1].
+        // Translate the pointer position to the [-1, 1] bounds.
+        double centerX = (currentPointer->Position.X - ActualWidth / 2) / (ActualWidth / 2);
+        double centerY = (ActualHeight / 2 - currentPointer->Position.Y) / (ActualHeight / 2);
+
+        ScaleRange(centerX, centerY, scale);
+
+        args->Handled = true;
+    }
 }
diff --git a/src/GraphControl/Control/Grapher.h b/src/GraphControl/Control/Grapher.h
index d6b57d1c..f33ac4fc 100644
--- a/src/GraphControl/Control/Grapher.h
+++ b/src/GraphControl/Control/Grapher.h
@@ -110,6 +110,7 @@ namespace GraphControl
         void OnPointerEntered(Windows::UI::Xaml::Input::PointerRoutedEventArgs^ e) override;
         void OnPointerMoved(Windows::UI::Xaml::Input::PointerRoutedEventArgs^ e) override;
         void OnPointerExited(Windows::UI::Xaml::Input::PointerRoutedEventArgs^ e) override;
+        void OnPointerWheelChanged(Windows::UI::Xaml::Input::PointerRoutedEventArgs^ e) override;
         #pragma endregion
 
     private:
@@ -125,6 +126,7 @@ namespace GraphControl
         void OnDataSourceChanged(GraphControl::InspectingDataSource^ sender, GraphControl::DataSourceChangedEventArgs args);
 
         void OnEquationsChanged(Windows::UI::Xaml::DependencyPropertyChangedEventArgs^ args);
+        void OnEquationsVectorChanged(Windows::Foundation::Collections::IObservableVector<GraphControl::Equation ^> ^sender, Windows::Foundation::Collections::IVectorChangedEventArgs ^event);
         void OnEquationChanged();
 
         void UpdateGraph();
@@ -158,6 +160,5 @@ namespace GraphControl
 
         const std::unique_ptr<Graphing::IMathSolver> m_solver;
         const std::shared_ptr<Graphing::IGraph> m_graph;
-        void OnEquationsVectorChanged(Windows::Foundation::Collections::IObservableVector<GraphControl::Equation ^> ^sender, Windows::Foundation::Collections::IVectorChangedEventArgs ^event);
-};
+    };
 }