How to create and implement a Android CustomOverlay for ZXing Scanner?

Question

I'm using ZXing barcode scanner plugin for my Xamarin.Forms project. According to some posts, I need to use custom overlay for ZXing scanner in order to have cancel button. But I'm not familiar with Android development in a Xamarin.Forms project.

So far, I know a scanner (MobileBarcodeScanner of ZXing plugin) accept an Android.Views.View as an overlay to replace the default scanning page view.

public async Task<string> ScanAsync()
{
    MobileBarcodeScanner scanner = new MobileBarcodeScanner();
    scanner.UseCustomOverlay = true;
    scanner.CustomOverlay = ???????;

    var scanResult = await scanner.Scan();
    if (scanResult != null)
    { return scanResult.Text; }
    return null;
}

But I don't know how to create a view page (either in XML design or programmatically) and set it to scanner.CustomOverlay.

I'm not sure if a common Android view can be used for scanner, any detailed reference would be really appreicated.

Thanks.

Answer 1

If you want to customize the overlay, you must create your own View for each platform. You can customize your overlay like this( create a custom overlay called ZxingOverlayView set it to the scanner.CustomOverlay):

var scanner = new ZXing.MobileMobileBarcodeScanner();
scanner.UseCustomOverlay = true;
myCustomOverlayInstance = new ZxingOverlayView(this, scanner);
scanner.CustomOverlay = myCustomOverlayInstance;

ZxingOverlayView should inherit the view then add your control to the custom view. You can see details about this link.

http://slackshotindustries.blogspot.com/2013/04/creating-custom-overlays-in-xzing.html

Edit

There are completely steps.

First of all, you could create a interface for dependence service.

 public interface IDeviceService
  {
    Task<string> ScanAsync();
  }

You can use this interface in the PCL code.

  private async void DependencyButton_Clicked(object sender, EventArgs e)
    {
        var result = await DependencyService.Get<IDeviceService>().ScanAsync();
        if (!string.IsNullOrEmpty(result))
        {
            await DisplayAlert(result, null, "OK");
        }
    }

In the android platform. You can design your layout. First of all, create a layout

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<ImageView
    android:id="@+id/imgClose"
    android:layout_width="30dp"
    android:layout_height="30dp"
    android:src="@drawable/close"
    android:layout_marginTop="20dp"
    android:layout_marginLeft="20dp"/>
  <RelativeLayout
    android:id="@+id/llScan"
    android:layout_width="240dp"
    android:layout_height="240dp"
    android:layout_centerInParent="true"
    android:background="@drawable/scan">
  <ImageView
        android:id="@+id/imgLine"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:adjustViewBounds="true"
        android:src="@drawable/scan_line"
        android:layout_centerHorizontal="true" />
     </RelativeLayout>
  <View
    android:id="@+id/viewTop"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_above="@id/llScan"
    android:background="@color/title_black"/>
 <View
    android:id="@+id/viewBottom"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_below="@id/llScan"
    android:background="@color/title_black"
    android:layout_alignParentBottom="true"/>
 <View
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_below="@id/viewTop"
    android:layout_toLeftOf="@id/llScan"
    android:layout_above="@id/viewBottom"
    android:background="@color/title_black"/>
 <View
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_below="@id/viewTop"
    android:layout_above="@id/viewBottom"
    android:layout_toRightOf="@id/llScan"
    android:background="@color/title_black"/>
 </RelativeLayout>

Then achieve the dependence service interface and use your new layout.

 [assembly: Xamarin.Forms.Dependency(typeof(DeviceService))]
 namespace Sample.Droid
 {
  public class DeviceService : IDeviceService
 {
    public async Task<string> ScanAsync()
    {
        var scanner = new ZXing.Mobile.MobileBarcodeScanner
        {
            UseCustomOverlay = true

        };
        //scanner.CustomOverlay = new CustomScanView(Application.Context);
        var options = new ZXing.Mobile.MobileBarcodeScanningOptions()
        {
            TryHarder = true,
            AutoRotate = false,
            UseFrontCameraIfAvailable = false,
            CameraResolutionSelector = new CameraResolutionSelectorDelegate(SelectLowestResolutionMatchingDisplayAspectRatio),
            PossibleFormats = new List<ZXing.BarcodeFormat>()
            {
                ZXing.BarcodeFormat.QR_CODE
            }
        };

        View scanView = LayoutInflater.From(Application.Context).Inflate(Resource.Layout.ScanView, null);
        ImageView imgLine = scanView.FindViewById<ImageView>(Resource.Id.imgLine);
        ImageView imgClose = scanView.FindViewById<ImageView>(Resource.Id.imgClose);
        imgClose.Click += delegate
        {
            scanner.Cancel();
        };
        scanner.CustomOverlay = scanView;

        ObjectAnimator objectAnimator = ObjectAnimator.OfFloat(imgLine, "Y", 0, DpToPixels(240));
        objectAnimator.SetDuration(2500);
        objectAnimator.RepeatCount = -1;
        objectAnimator.SetInterpolator(new LinearInterpolator());
        objectAnimator.RepeatMode = ValueAnimatorRepeatMode.Restart;
        objectAnimator.Start();

        ZXing.Result scanResults = await scanner.Scan(CrossCurrentActivity.Current.Activity, options);
        if (scanResults != null)
        {
            return scanResults.Text;
        }
        return string.Empty;
    }

    private int DpToPixels(double dp)
    {
        return (int)(dp * Application.Context.Resources.DisplayMetrics.Density);
    }

    private CameraResolution SelectLowestResolutionMatchingDisplayAspectRatio(List<CameraResolution> availableResolutions)
    {
        CameraResolution result = null;
        //a tolerance of 0.1 should not be visible to the user
        double aspectTolerance = 0.1;
        var displayOrientationHeight = DeviceDisplay.MainDisplayInfo.Orientation == DisplayOrientation.Portrait ? DeviceDisplay.MainDisplayInfo.Height : DeviceDisplay.MainDisplayInfo.Width;
        var displayOrientationWidth = DeviceDisplay.MainDisplayInfo.Orientation == DisplayOrientation.Portrait ? DeviceDisplay.MainDisplayInfo.Width : DeviceDisplay.MainDisplayInfo.Height;
        //calculatiing our targetRatio
        var targetRatio = displayOrientationHeight / displayOrientationWidth;
        var targetHeight = displayOrientationHeight;
        var minDiff = double.MaxValue;
        //camera API lists all available resolutions from highest to lowest, perfect for us
        //making use of this sorting, following code runs some comparisons to select the lowest resolution that matches the screen aspect ratio and lies within tolerance
        //selecting the lowest makes Qr detection actual faster most of the time
        foreach (var r in availableResolutions.Where(r => Math.Abs(((double)r.Width / r.Height) - targetRatio) < aspectTolerance))
        {
            //slowly going down the list to the lowest matching solution with the correct aspect ratio
            if (Math.Abs(r.Height - targetHeight) < minDiff)
                minDiff = Math.Abs(r.Height - targetHeight);
            result = r;
        }
        return result;
    }
   }

}

IOS code.

[assembly: Xamarin.Forms.Dependency(typeof(DeviceService))]
namespace Sample.iOS
{
public class DeviceService : IDeviceService
{
    public async Task<string> ScanAsync()
    {
        var scanner = new ZXing.Mobile.MobileBarcodeScanner()
        {
            UseCustomOverlay = true
        };

        var options = new ZXing.Mobile.MobileBarcodeScanningOptions()
        {
            TryHarder = true,
            AutoRotate = false,
            UseFrontCameraIfAvailable = false,
            PossibleFormats = new List<ZXing.BarcodeFormat>()
            {
                ZXing.BarcodeFormat.QR_CODE
            }
        };

        ScannerOverlayView customOverlay = new ScannerOverlayView();
        customOverlay.OnCancel += () =>
        {
            scanner?.Cancel();
        };
        customOverlay.OnResume += () =>
        {
            scanner?.ResumeAnalysis();
        };
        customOverlay.OnPause += () =>
        {
            scanner?.PauseAnalysis();
        };
        scanner.CustomOverlay = customOverlay;


        ZXing.Result scanResults = null;
        scanResults = await scanner.Scan(options);
        //customOverlay.Dispose();
        if (scanResults != null)
        {
            return scanResults.Text;
        }
        return string.Empty;
    }
}

Here is running GIF.

Here is a good simple code. you could refer to it. https://github.com/Wenfengcheng/ZXing.Sample_Xamarin