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

private async void InitMediaCaptureAsync()
        {
            //寻找后置摄像头
            var allVideoDevices = await DeviceInformation.FindAllAsync(DeviceClass.VideoCapture);
            var cameraDevice = allVideoDevices.FirstOrDefault(x => x.EnclosureLocation != null && x.EnclosureLocation.Panel == Windows.Devices.Enumeration.Panel.Back);

            if (cameraDevice == null)
            {
                Debug.WriteLine("No camera device found!");

                return;
            }

            var settings = new MediaCaptureInitializationSettings
            {
                StreamingCaptureMode = StreamingCaptureMode.Video,
                //必须，否则截图的时候会很卡很慢
                PhotoCaptureSource = PhotoCaptureSource.VideoPreview,
                VideoDeviceId = cameraDevice.Id
            };

            _mediaCapture = new MediaCapture();

            try
            {
                await _mediaCapture.InitializeAsync(settings);
                _initialized = true;//初始化成功
            }
            catch (UnauthorizedAccessException)
            {
                Debug.WriteLine("The app was denied access to the camera");
            }
            catch (Exception ex)
            {
                Debug.WriteLine("Exception when initializing MediaCapture with {0}: {1}", cameraDevice.Id, ex.ToString());
            }

            if (_initialized)
            {
                var focusControl = _mediaCapture.VideoDeviceController.FocusControl;

                if (focusControl.Supported)
                {
                    var focusSettings = new FocusSettings()
                    {
                        Mode = focusControl.SupportedFocusModes.FirstOrDefault(f => f == FocusMode.Continuous),
                        DisableDriverFallback = true,
                        AutoFocusRange = focusControl.SupportedFocusRanges.FirstOrDefault(f => f == AutoFocusRange.FullRange),
                        Distance = focusControl.SupportedFocusDistances.FirstOrDefault(f => f == ManualFocusDistance.Nearest)
                    };

                    //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
                    focusControl.Configure(focusSettings);
                }

                captureElement.Source = _mediaCapture;
                captureElement.FlowDirection = FlowDirection.LeftToRight;

                try
                {
                    await _mediaCapture.StartPreviewAsync();
                    _previewing = true;
                }
                catch (Exception ex)
                {
                    Debug.WriteLine("Exception when starting the preview: {0}", ex.ToString());
                }

                if (_previewing)
                {
                    try
                    {
                        if (_mediaCapture.VideoDeviceController.FlashControl.Supported)
                        {
                            //关闭闪光灯
                            _mediaCapture.VideoDeviceController.FlashControl.Enabled = false;
                        }
                    }
                    catch
                    {
                    }

                    if (focusControl.Supported)
                    {
                        //开始聚焦
                        await focusControl.FocusAsync();
                    }
                }
            }
        }

2. 截图与识别

private void InitTimer()
        {
            _timer = new DispatcherTimer();
            //每50毫秒截一次图
            _timer.Interval = TimeSpan.FromMilliseconds(50);
            _timer.Tick += _timer_Tick;
            _timer.Start();
        }

private async void _timer_Tick(object sender, object e)
        {
            using (var stream = new InMemoryRandomAccessStream())
            {
                var previewProperties = _mediaCapture.VideoDeviceController.GetMediaStreamProperties(MediaStreamType.VideoPreview) as VideoEncodingProperties;
                //将截图写入内存流中
                await _mediaCapture.CapturePhotoToStreamAsync(ImageEncodingProperties.CreateJpeg(), stream);

                //利用Zxing识别，成功：停止timer；失败：继续
                var reader = new BarcodeReader();
                var bitmapWriteable = new WriteableBitmap((int)previewProperties.Width, (int)previewProperties.Height);
                bitmapWriteable.SetSource(stream);
                var result = reader.Decode(bitmapWriteable);

                if (!string.IsNullOrEmpty(result.Text))
                {
                    _timer.Stop();
                }
            }
        }

这里顺便说一下如何安装Zxing，打开nuget管理器 命令窗口输入 Install-Package ZXing.Net ，回车; 关于Zxing如何使用，到网上搜索一下有很多教程，这里不再赘述

3. 问题与优化

A） 截图有响声

使用CapturePhotoToStreamAsync来截取图片有的时候会有“咔擦咔擦”声，很影响用户体验，最理想的做法是找到一种能从视频流中直接截取图片的方法，在这里不得不说一句MediaCapture真的真的很强大，MediaCapture给我们提供了直接从视频流中取出其中一帧的方法GetPreviewFrameAsync，于是我把代码进行了如下修改，即流畅又没有烦人的“咔擦咔擦”声

private async void _timer_Tick(object sender, object e)
        {
            var previewProperties = _mediaCapture.VideoDeviceController.GetMediaStreamProperties(MediaStreamType.VideoPreview) as VideoEncodingProperties;

            using (var videoFrame = new VideoFrame(BitmapPixelFormat.Rgba8, (int)previewProperties.Width, (int)previewProperties.Height))
            {
                using (var currentFrame = await _mediaCapture.GetPreviewFrameAsync(videoFrame))
                {
                    using (var previewFrame = currentFrame.SoftwareBitmap)
                    {
                        var buffer = new Windows.Storage.Streams.Buffer((uint)(4 * previewFrame.PixelWidth * previewFrame.PixelHeight));
                        previewFrame.CopyToBuffer(buffer);

                        using (var stream = buffer.AsStream().AsRandomAccessStream())
                        {
                            //利用Zxing识别，成功：停止timer；失败：继续
                            var reader = new BarcodeReader();
                            var bitmapWriteable = new WriteableBitmap((int)previewProperties.Width, (int)previewProperties.Height);
                            bitmapWriteable.SetSource(stream);
                            var result = reader.Decode(bitmapWriteable);

                            if (!string.IsNullOrEmpty(result.Text))
                            {
                                _timer.Stop();
                            }
                        }
                    }
                }
            }            
        }

顺便提一下记得要使用如下两个命名空间

using System.IO;
using System.Runtime.InteropServices.WindowsRuntime;

否则无法实现buffer.AsStream().AsRandomAccessStream()

B） 连续聚焦

并不是所有机型都支持连续聚焦的（FocusMode.Continuous），这个时候只能自己实现间断性持续聚焦了

C） 截图之后图片处理

有的时候为了实现一些功能（比如说扫描框）或者提高识别率，我们需要对截取出来的图片进行一些二次处理，或剪裁或缩放或旋转，我们可以使用BitmapDecoder和BitmapEncoder来实现

using (var stream = buffer.AsStream().AsRandomAccessStream())
{
      var decoder = await BitmapDecoder.CreateAsync(BitmapDecoder.JpegDecoderId, stream);
      var destStream = new InMemoryRandomAccessStream();

      var encoder = await BitmapEncoder.CreateForTranscodingAsync(destStream, decoder);
                            
      //剪裁
      encoder.BitmapTransform.Bounds = new BitmapBounds() { X = 0, Y = 0, Width = 100, Height = 100 };
      //缩放
      encoder.BitmapTransform.ScaledWidth = 100;
      encoder.BitmapTransform.ScaledHeight = 100;
      //旋转
      encoder.BitmapTransform.Rotation = BitmapRotation.Clockwise90Degrees;

      await encoder.FlushAsync();
      await destStream.FlushAsync();
}

D） 挂起和唤醒

另外值得注意的是，程序在Suspending和Resuming还有Activated时出现的一系列状态转换，这时候很容易引起bug，需要处理好避免crash。

4. 最后

识别出来的字符串处理一般也就超链接和普通文本两种，当然也可以增加条码扫描功能，识别出的是编码，不管怎样，大家可以根据项目具体需求做相应的处理。