如何用MediaCapture解決二維碼掃描問題

二維碼掃描的實現，簡單的來說可以分三步走：“成像”、“截圖”與“識別”。

UWP開發中，最常用的媒體工具非MediaCapture莫屬了，下面就來簡單介紹一下如何利用MediaCapture來實現掃描和截圖並且利用Zxing識別二維碼，以及會遇到的問題和需要注意的地方。

1. 初始化與成像

private async void InitMediaCaptureAsync()
        {
            //尋找后置攝像頭
            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,
                //必須，否則截圖的時候會很卡很慢
                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)
                    };

                    //設置聚焦，最好使用FocusMode.Continuous，否則影響截圖會很模糊，不利於識別
                    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)
                        {
                            //關閉閃光燈
                            _mediaCapture.VideoDeviceController.FlashControl.Enabled = false;
                        }
                    }
                    catch
                    {
                    }

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

2. 截圖與識別

private void InitTimer()
        {
            _timer = new DispatcherTimer();
            //每50毫秒截一次圖
            _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;
                //將截圖寫入內存流中
                await _mediaCapture.CapturePhotoToStreamAsync(ImageEncodingProperties.CreateJpeg(), stream);

                //利用Zxing識別，成功：停止timer；失敗：繼續
                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();
                }
            }
        }

這里順便說一下如何安裝Zxing，打開nuget管理器 命令窗口輸入 Install-Package ZXing.Net ，回車; 關於Zxing如何使用，到網上搜索一下有很多教程，這里不再贅述

3. 問題與優化

A） 截圖有響聲

使用CapturePhotoToStreamAsync來截取圖片有的時候會有“咔擦咔擦”聲，很影響用戶體驗，最理想的做法是找到一種能從視頻流中直接截取圖片的方法，在這里不得不說一句MediaCapture真的真的很強大，MediaCapture給我們提供了直接從視頻流中取出其中一幀的方法GetPreviewFrameAsync，於是我把代碼進行了如下修改，即流暢又沒有煩人的“咔擦咔擦”聲

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識別，成功：停止timer；失敗：繼續
                            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();
                            }
                        }
                    }
                }
            }            
        }

順便提一下記得要使用如下兩個命名空間

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

否則無法實現buffer.AsStream().AsRandomAccessStream()

B） 連續聚焦

並不是所有機型都支持連續聚焦的（FocusMode.Continuous），這個時候只能自己實現間斷性持續聚焦了

C） 截圖之后圖片處理

有的時候為了實現一些功能（比如說掃描框）或者提高識別率，我們需要對截取出來的圖片進行一些二次處理，或剪裁或縮放或旋轉，我們可以使用BitmapDecoder和BitmapEncoder來實現

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 };
      //縮放
      encoder.BitmapTransform.ScaledWidth = 100;
      encoder.BitmapTransform.ScaledHeight = 100;
      //旋轉
      encoder.BitmapTransform.Rotation = BitmapRotation.Clockwise90Degrees;

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

D） 掛起和喚醒

另外值得注意的是，程序在Suspending和Resuming還有Activated時出現的一系列狀態轉換，這時候很容易引起bug，需要處理好避免crash。

4. 最后

識別出來的字符串處理一般也就超鏈接和普通文本兩種，當然也可以增加條碼掃描功能，識別出的是編碼，不管怎樣，大家可以根據項目具體需求做相應的處理。