Implement tap focus

Give the Canvas a name and add a Tap handler, lock focus parameters on
successful tap focus and release them when picture has been taken. Note
that taps and the region have to be converted into the coordinate space of
the preview image which is always 640x480 independent of the app
orientation. We use that as a clipping rectangle for the focus area to
avoid getting invalid argument errors due to the FocusRegion falling
outside the limits.

CameraExplorer/MainPage.xaml

@@ -33,7 +33,7 @@
         </StackPanel>
 
         <Grid x:Name="ContentPanel" Grid.Row="1">
-            <Canvas>
+            <Canvas x:Name="VideoCanvas">
                 <Canvas.Background>
                     <VideoBrush x:Name="videoBrush"/>
                 </Canvas.Background>

CameraExplorer/MainPage.xaml.cs

@@ -3,8 +3,12 @@ using Microsoft.Phone.Controls;
 using Microsoft.Phone.Shell;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.IO;
+using System.Threading;
 using System.Threading.Tasks;
+using System.Windows;
+using System.Windows.Input;
 using System.Windows.Media;
 using System.Windows.Navigation;
 using Windows.Phone.Media.Capture;
@@ -21,6 +25,9 @@ namespace CameraExplorer
         private CameraExplorer.DataContext _dataContext = CameraExplorer.DataContext.Singleton;
         private ProgressIndicator _progressIndicator = new ProgressIndicator();
         private bool _capturing = false;
+        private Semaphore _focusSemaphore = new Semaphore(1, 1);
+        private bool _manuallyFocused = false;
+        private Windows.Foundation.Size _focusRegionSize = new Windows.Foundation.Size(50, 50);
 
         public MainPage()
         {
@@ -31,6 +38,7 @@ namespace CameraExplorer
             menuItem.IsEnabled = false;
             ApplicationBar.MenuItems.Add(menuItem);
             menuItem.Click += new EventHandler(aboutMenuItem_Click);
+            VideoCanvas.Tap += new EventHandler<GestureEventArgs>(videoCanvas_Tap);
 
             DataContext = _dataContext;
 
@@ -188,7 +196,10 @@ namespace CameraExplorer
         /// </summary>
         private async void captureButton_Click(object sender, EventArgs e)
         {
-            await AutoFocus();
+            if (!_manuallyFocused)
+            {
+                await AutoFocus();
+            }
 
             await Capture();
         }
@@ -201,6 +212,42 @@ namespace CameraExplorer
             NavigationService.Navigate(new Uri("/AboutPage.xaml", UriKind.Relative));
         }
 
+        /// <summary>
+        /// Set autofocus area to tap location and refocus.
+        /// </summary>
+        /// <param name="sender"></param>
+        /// <param name="e"></param>
+        private async void videoCanvas_Tap(object sender, GestureEventArgs e)
+        {
+            System.Windows.Point tapPoint = e.GetPosition(VideoCanvas);
+            if (_focusSemaphore.WaitOne(0))
+            {
+                double xRatio = VideoCanvas.ActualWidth / _dataContext.Device.PreviewResolution.Width;
+                double yRatio = VideoCanvas.ActualHeight / _dataContext.Device.PreviewResolution.Height; 
+                Windows.Foundation.Point origin = new Windows.Foundation.Point(
+                    (tapPoint.X - _focusRegionSize.Width / 2) / xRatio,
+                    (tapPoint.Y - _focusRegionSize.Height / 2) / yRatio);
+                Windows.Foundation.Rect focusrect = new Windows.Foundation.Rect(origin, _focusRegionSize);
+                Windows.Foundation.Rect viewPortRect = new Windows.Foundation.Rect(0, 0, _dataContext.Device.PreviewResolution.Width, _dataContext.Device.PreviewResolution.Height);
+                focusrect.Intersect(viewPortRect);
+                _dataContext.Device.FocusRegion = focusrect;
+
+                CameraFocusStatus status = await _dataContext.Device.FocusAsync();
+                if (status == CameraFocusStatus.Locked)
+                {
+                    _manuallyFocused = true;
+                    _dataContext.Device.SetProperty(KnownCameraPhotoProperties.LockedAutoFocusParameters,
+                        AutoFocusParameters.Exposure & AutoFocusParameters.Focus & AutoFocusParameters.WhiteBalance);
+                }
+                else
+                {
+                    _manuallyFocused = false;
+                    _dataContext.Device.SetProperty(KnownCameraPhotoProperties.LockedAutoFocusParameters, AutoFocusParameters.None);
+                }
+                _focusSemaphore.Release();
+            }
+        }
+
         /// <summary>
         /// Starts displaying progress indicator.
         /// </summary>
@@ -289,14 +336,20 @@ namespace CameraExplorer
 
                 NavigationService.Navigate(new Uri("/PreviewPage.xaml", UriKind.Relative));
             }
+            _manuallyFocused = false;
+            _dataContext.Device.FocusRegion = null;
+            _dataContext.Device.SetProperty(KnownCameraPhotoProperties.LockedAutoFocusParameters, AutoFocusParameters.None);
         }
 
         /// <summary>
-        /// Half-pressing the shutter key initiates autofocus.
+        /// Half-pressing the shutter key initiates autofocus unless tapped to focus.
         /// </summary>
         private async void ShutterKeyHalfPressed(object sender, EventArgs e)
         {
-            await AutoFocus();
+            if (!_manuallyFocused)
+            {
+                await AutoFocus();
+            }
         }
 
         /// <summary>