GH-337 Android Low Pass Filter

Samples/Samples/View/CompassPage.xaml

@@ -11,6 +11,12 @@
     <StackLayout>
         <Label Text="Monitor compass for changes." FontAttributes="Bold" Margin="12" />
 
+        <Switch IsToggled="{Binding CompassApplyLowPassFilter}" >
+            <Switch.IsVisible>
+                <OnPlatform x:TypeArguments="x:Boolean" Android="True" Default="False"/>
+            </Switch.IsVisible>
+        </Switch>
+
         <ScrollView>
             <Grid Padding="12,0,12,12">
                 <Grid.RowDefinitions>

Samples/Samples/ViewModel/CompassViewModel.cs

@@ -10,6 +10,7 @@ class CompassViewModel : BaseViewModel
     {
         bool compass1IsActive;
         bool compass2IsActive;
+        bool compassApplyLowPassFilter;
         double compass1;
         double compass2;
         int speed1 = 2;
@@ -43,6 +44,16 @@ public bool Compass2IsActive
             set => SetProperty(ref compass2IsActive, value);
         }
 
+        public bool CompassApplyLowPassFilter
+        {
+            get => compassApplyLowPassFilter;
+            set
+            {
+                SetProperty(ref compassApplyLowPassFilter, value);
+                Compass.ApplyLowPassFilter = value;
+            }
+        }
+
         public double Compass1
         {
             get => compass1;

Xamarin.Essentials/Compass/Compass.android.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using Android.Hardware;
 using Android.Runtime;
 
@@ -52,8 +53,43 @@ internal static void PlatformStop()
         }
     }
 
+    class LowPassFilter
+    {
+        const int length = 10;
+
+        float sin;
+        float cos;
+        Queue<float> history = new Queue<float>(length);
+
+        public void Add(float radians)
+        {
+            sin += (float)Math.Sin(radians);
+
+            cos += (float)Math.Cos(radians);
+
+            history.Enqueue(radians);
+
+            if (history.Count > length)
+            {
+                var old = history.Dequeue();
+
+                sin -= (float)Math.Sin(old);
+
+                cos -= (float)Math.Cos(old);
+            }
+        }
+
+        public float Average()
+        {
+            var size = history.Count;
+
+            return (float)Math.Atan2(sin / size, cos / size);
+        }
+    }
+
     class SensorListener : Java.Lang.Object, ISensorEventListener, IDisposable
     {
+        LowPassFilter filter = new LowPassFilter();
         float[] lastAccelerometer = new float[3];
         float[] lastMagnetometer = new float[3];
         bool lastAccelerometerSet;
@@ -92,6 +128,11 @@ void ISensorEventListener.OnSensorChanged(SensorEvent e)
                 SensorManager.GetRotationMatrix(r, null, lastAccelerometer, lastMagnetometer);
                 SensorManager.GetOrientation(r, orientation);
                 var azimuthInRadians = orientation[0];
+                if (Compass.ApplyLowPassFilter)
+                {
+                    filter.Add(azimuthInRadians);
+                    azimuthInRadians = filter.Average();
+                }
                 var azimuthInDegress = (Java.Lang.Math.ToDegrees(azimuthInRadians) + 360.0) % 360.0;
 
                 var data = new CompassData(azimuthInDegress);

Xamarin.Essentials/Compass/Compass.shared.cs

@@ -11,6 +11,8 @@ public static partial class Compass
 
         public static bool IsMonitoring { get; private set; }
 
+        public static bool ApplyLowPassFilter { get; set; }
+
         public static void Start(SensorSpeed sensorSpeed)
         {
             if (!IsSupported)

docs/en/Xamarin.Essentials/Compass.xml

@@ -17,6 +17,26 @@
     </remarks>
   </Docs>
   <Members>
+    <Member MemberName="ApplyLowPassFilter">
+      <MemberSignature Language="C#" Value="public static bool ApplyLowPassFilter { get; set; }" />
+      <MemberSignature Language="ILAsm" Value=".property bool ApplyLowPassFilter" />
+      <MemberSignature Language="DocId" Value="P:Xamarin.Essentials.Compass.ApplyLowPassFilter" />
+      <MemberType>Property</MemberType>
+      <AssemblyInfo>
+        <AssemblyName>Xamarin.Essentials</AssemblyName>
+        <AssemblyVersion>1.0.0.0</AssemblyVersion>
+      </AssemblyInfo>
+      <ReturnValue>
+        <ReturnType>System.Boolean</ReturnType>
+      </ReturnValue>
+      <Docs>
+        <summary>Applies a moving average filter on the Android implementation. Unused on other platforms</summary>
+        <value>If a low pass filter is applied</value>
+        <remarks>
+          <para></para>
+        </remarks>
+      </Docs>
+    </Member>
     <Member MemberName="IsMonitoring">
       <MemberSignature Language="C#" Value="public static bool IsMonitoring { get; }" />
       <MemberSignature Language="ILAsm" Value=".property bool IsMonitoring" />