From 56bc8dca66f274b44c3c1af87f578c75ddf1bd97 Mon Sep 17 00:00:00 2001
From: Mrnikbobjeff <schillinik@yahoo.de>
Date: Tue, 17 Jul 2018 22:08:22 +0200
Subject: [PATCH] GH-337 Android Low Pass Filter (#354)

* Implemented low pass filter as requested by issue 337

* Added value to sample page.

* Updated docs for ApplyLowPassFilter

* Enabled ApplyLowPassFilter Switch on Android, disabled on all other platforms

* Moved LowPassFilter to shared, removed unnecessary usings.

* Updated Docs
---
 Samples/Samples/View/CompassPage.xaml         |  6 +++
 Samples/Samples/ViewModel/CompassViewModel.cs | 11 ++++++
 Xamarin.Essentials/Compass/Compass.android.cs |  6 +++
 Xamarin.Essentials/Compass/Compass.shared.cs  |  3 +-
 .../Compass/LowPassFilter.shared.cs           | 39 +++++++++++++++++++
 .../xamarin-essentials-android.xml            |  1 +
 .../xamarin-essentials-ios.xml                |  1 +
 .../xamarin-essentials-uwp.xml                |  1 +
 .../en/FrameworksIndex/xamarin-essentials.xml |  1 +
 docs/en/Xamarin.Essentials/Compass.xml        | 20 ++++++++++
 10 files changed, 88 insertions(+), 1 deletion(-)
 create mode 100644 Xamarin.Essentials/Compass/LowPassFilter.shared.cs

diff --git a/Samples/Samples/View/CompassPage.xaml b/Samples/Samples/View/CompassPage.xaml
index 45e8f55..32194a5 100644
--- a/Samples/Samples/View/CompassPage.xaml
+++ b/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>
diff --git a/Samples/Samples/ViewModel/CompassViewModel.cs b/Samples/Samples/ViewModel/CompassViewModel.cs
index 68feb87..dd5cb5f 100644
--- a/Samples/Samples/ViewModel/CompassViewModel.cs
+++ b/Samples/Samples/ViewModel/CompassViewModel.cs
@@ -10,6 +10,7 @@ namespace Samples.ViewModel
     {
         bool compass1IsActive;
         bool compass2IsActive;
+        bool compassApplyLowPassFilter;
         double compass1;
         double compass2;
         int speed1 = 2;
@@ -43,6 +44,16 @@ namespace Samples.ViewModel
             set => SetProperty(ref compass2IsActive, value);
         }
 
+        public bool CompassApplyLowPassFilter
+        {
+            get => compassApplyLowPassFilter;
+            set
+            {
+                SetProperty(ref compassApplyLowPassFilter, value);
+                Compass.ApplyLowPassFilter = value;
+            }
+        }
+
         public double Compass1
         {
             get => compass1;
diff --git a/Xamarin.Essentials/Compass/Compass.android.cs b/Xamarin.Essentials/Compass/Compass.android.cs
index 0992f1f..c27f2ae 100644
--- a/Xamarin.Essentials/Compass/Compass.android.cs
+++ b/Xamarin.Essentials/Compass/Compass.android.cs
@@ -54,6 +54,7 @@ namespace Xamarin.Essentials
 
     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 +93,11 @@ namespace Xamarin.Essentials
                 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);
diff --git a/Xamarin.Essentials/Compass/Compass.shared.cs b/Xamarin.Essentials/Compass/Compass.shared.cs
index ae114d1..e45570d 100644
--- a/Xamarin.Essentials/Compass/Compass.shared.cs
+++ b/Xamarin.Essentials/Compass/Compass.shared.cs
@@ -1,5 +1,4 @@
 using System;
-using System.Threading;
 
 namespace Xamarin.Essentials
 {
@@ -11,6 +10,8 @@ namespace Xamarin.Essentials
 
         public static bool IsMonitoring { get; private set; }
 
+        public static bool ApplyLowPassFilter { get; set; }
+
         public static void Start(SensorSpeed sensorSpeed)
         {
             if (!IsSupported)
diff --git a/Xamarin.Essentials/Compass/LowPassFilter.shared.cs b/Xamarin.Essentials/Compass/LowPassFilter.shared.cs
new file mode 100644
index 0000000..5e47856
--- /dev/null
+++ b/Xamarin.Essentials/Compass/LowPassFilter.shared.cs
@@ -0,0 +1,39 @@
+using System;
+using System.Collections.Generic;
+
+namespace Xamarin.Essentials
+{
+    class LowPassFilter
+    {
+        const int length = 10;
+
+        float sin;
+        float cos;
+        Queue<float> history = new Queue<float>(length);
+
+        internal 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);
+            }
+        }
+
+        internal float Average()
+        {
+            var size = history.Count;
+
+            return (float)Math.Atan2(sin / size, cos / size);
+        }
+    }
+}
diff --git a/docs/en/FrameworksIndex/xamarin-essentials-android.xml b/docs/en/FrameworksIndex/xamarin-essentials-android.xml
index 57b06d2..82f7fdc 100644
--- a/docs/en/FrameworksIndex/xamarin-essentials-android.xml
+++ b/docs/en/FrameworksIndex/xamarin-essentials-android.xml
@@ -67,6 +67,7 @@
       <Member Id="E:Xamarin.Essentials.Compass.ReadingChanged" />
       <Member Id="M:Xamarin.Essentials.Compass.Start(Xamarin.Essentials.SensorSpeed)" />
       <Member Id="M:Xamarin.Essentials.Compass.Stop" />
+      <Member Id="P:Xamarin.Essentials.Compass.ApplyLowPassFilter" />
       <Member Id="P:Xamarin.Essentials.Compass.IsMonitoring" />
     </Type>
     <Type Name="Xamarin.Essentials.CompassChangedEventArgs" Id="T:Xamarin.Essentials.CompassChangedEventArgs">
diff --git a/docs/en/FrameworksIndex/xamarin-essentials-ios.xml b/docs/en/FrameworksIndex/xamarin-essentials-ios.xml
index ae6e09a..c174da9 100644
--- a/docs/en/FrameworksIndex/xamarin-essentials-ios.xml
+++ b/docs/en/FrameworksIndex/xamarin-essentials-ios.xml
@@ -67,6 +67,7 @@
       <Member Id="E:Xamarin.Essentials.Compass.ReadingChanged" />
       <Member Id="M:Xamarin.Essentials.Compass.Start(Xamarin.Essentials.SensorSpeed)" />
       <Member Id="M:Xamarin.Essentials.Compass.Stop" />
+      <Member Id="P:Xamarin.Essentials.Compass.ApplyLowPassFilter" />
       <Member Id="P:Xamarin.Essentials.Compass.IsMonitoring" />
       <Member Id="P:Xamarin.Essentials.Compass.ShouldDisplayHeadingCalibration" />
     </Type>
diff --git a/docs/en/FrameworksIndex/xamarin-essentials-uwp.xml b/docs/en/FrameworksIndex/xamarin-essentials-uwp.xml
index 2407913..eb2a7e6 100644
--- a/docs/en/FrameworksIndex/xamarin-essentials-uwp.xml
+++ b/docs/en/FrameworksIndex/xamarin-essentials-uwp.xml
@@ -67,6 +67,7 @@
       <Member Id="E:Xamarin.Essentials.Compass.ReadingChanged" />
       <Member Id="M:Xamarin.Essentials.Compass.Start(Xamarin.Essentials.SensorSpeed)" />
       <Member Id="M:Xamarin.Essentials.Compass.Stop" />
+      <Member Id="P:Xamarin.Essentials.Compass.ApplyLowPassFilter" />
       <Member Id="P:Xamarin.Essentials.Compass.IsMonitoring" />
     </Type>
     <Type Name="Xamarin.Essentials.CompassChangedEventArgs" Id="T:Xamarin.Essentials.CompassChangedEventArgs">
diff --git a/docs/en/FrameworksIndex/xamarin-essentials.xml b/docs/en/FrameworksIndex/xamarin-essentials.xml
index 15af2a5..72e1f47 100644
--- a/docs/en/FrameworksIndex/xamarin-essentials.xml
+++ b/docs/en/FrameworksIndex/xamarin-essentials.xml
@@ -67,6 +67,7 @@
       <Member Id="E:Xamarin.Essentials.Compass.ReadingChanged" />
       <Member Id="M:Xamarin.Essentials.Compass.Start(Xamarin.Essentials.SensorSpeed)" />
       <Member Id="M:Xamarin.Essentials.Compass.Stop" />
+      <Member Id="P:Xamarin.Essentials.Compass.ApplyLowPassFilter" />
       <Member Id="P:Xamarin.Essentials.Compass.IsMonitoring" />
     </Type>
     <Type Name="Xamarin.Essentials.CompassChangedEventArgs" Id="T:Xamarin.Essentials.CompassChangedEventArgs">
diff --git a/docs/en/Xamarin.Essentials/Compass.xml b/docs/en/Xamarin.Essentials/Compass.xml
index e9c0399..fd10fc2 100644
--- a/docs/en/Xamarin.Essentials/Compass.xml
+++ b/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" />