如何用MediaCapture解决二维码扫描问题

二维码扫描的实现，简单的来说可以分三步走：“成像”、“截图”与“识别”。

UWP开发中，最常用的媒体工具非MediaCapture莫属了，下面就来简单介绍一下如何利用MediaCapture来实现扫描和截图并且利用Zxing识别二维码，以及会遇到的问题和需要注意的地方。

1. 初始化与成像

 1 private async void InitMediaCaptureAsync()
 2         {
 3             //寻找后置摄像头
 4             var allVideoDevices = await DeviceInformation.FindAllAsync(DeviceClass.VideoCapture);
 5             var cameraDevice = allVideoDevices.FirstOrDefault(x => x.EnclosureLocation != null && x.EnclosureLocation.Panel == Windows.Devices.Enumeration.Panel.Back);
 6 
 7             if (cameraDevice == null)
 8             {
 9                 Debug.WriteLine("No camera device found!");
10 
11                 return;
12             }
13 
14             var settings = new MediaCaptureInitializationSettings
15             {
16                 StreamingCaptureMode = StreamingCaptureMode.Video,
17                 //必须，否则截图的时候会很卡很慢
18                 PhotoCaptureSource = PhotoCaptureSource.VideoPreview,
19                 VideoDeviceId = cameraDevice.Id
20             };
21 
22             _mediaCapture = new MediaCapture();
23 
24             try
25             {
26                 await _mediaCapture.InitializeAsync(settings);
27                 _initialized = true;//初始化成功
28             }
29             catch (UnauthorizedAccessException)
30             {
31                 Debug.WriteLine("The app was denied access to the camera");
32             }
33             catch (Exception ex)
34             {
35                 Debug.WriteLine("Exception when initializing MediaCapture with {0}: {1}", cameraDevice.Id, ex.ToString());
36             }
37 
38             if (_initialized)
39             {
40                 var focusControl = _mediaCapture.VideoDeviceController.FocusControl;
41 
42                 if (focusControl.Supported)
43                 {
44                     var focusSettings = new FocusSettings()
45                     {
46                         Mode = focusControl.SupportedFocusModes.FirstOrDefault(f => f == FocusMode.Continuous),
47                         DisableDriverFallback = true,
48                         AutoFocusRange = focusControl.SupportedFocusRanges.FirstOrDefault(f => f == AutoFocusRange.FullRange),
49                         Distance = focusControl.SupportedFocusDistances.FirstOrDefault(f => f == ManualFocusDistance.Nearest)
50                     };
51 
52                     //设置聚焦，最好使用FocusMode.Continuous，否则影响截图会很模糊，不利于识别
53                     focusControl.Configure(focusSettings);
54                 }
55 
56                 captureElement.Source = _mediaCapture;
57                 captureElement.FlowDirection = FlowDirection.LeftToRight;
58 
59                 try
60                 {
61                     await _mediaCapture.StartPreviewAsync();
62                     _previewing = true;
63                 }
64                 catch (Exception ex)
65                 {
66                     Debug.WriteLine("Exception when starting the preview: {0}", ex.ToString());
67                 }
68 
69                 if (_previewing)
70                 {
71                     try
72                     {
73                         if (_mediaCapture.VideoDeviceController.FlashControl.Supported)
74                         {
75                             //关闭闪光灯
76                             _mediaCapture.VideoDeviceController.FlashControl.Enabled = false;
77                         }
78                     }
79                     catch
80                     {
81                     }
82 
83                     if (focusControl.Supported)
84                     {
85                         //开始聚焦
86                         await focusControl.FocusAsync();
87                     }
88                 }
89             }
90         }

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