Natively Recognize Barcodes/QR Codes in iOS 7 with Xamarin.iOS

There have been great barcode-reading libraries available for Xamarin for some time, but iOS 7 has built-in barcode-recognition support.

There’s only one tricky bit: you have to tell the AVCaptureMetadataOutput what types of barcodes you’re interested in after you’ve added it to the AVCaptureSession. (I suppose what happens behind the scene is that the AVCaptureSession registers with the AVCaptureMetadataOutput the various types of barcodes it could recognize.)

UPDATE: This also works for realtime face detection! s/AVMetadataMachineReadableCodeObject/AVMetadataFaceObject/ and set AVMetadataObject.TypeFace!

Here’s a complete program in Xamarin.iOS that recognizes QR and standard Ean13 barcodes:

using System;
using System.Collections.Generic;
using System.Linq;
using MonoTouch.Foundation;
using MonoTouch.UIKit;
using System.Drawing;
using MonoTouch.CoreGraphics;
using MonoTouch.AVFoundation;
using MonoTouch.CoreFoundation;

namespace SingleFileSolution
{
	public class ContentView : UIView
	{
		AVCaptureVideoPreviewLayer layer; 

		public ContentView(UIColor fillColor, AVCaptureVideoPreviewLayer layer, MyMetadataOutputDelegate metadataSource)
		{
			BackgroundColor = fillColor;


			this.layer = layer;
			layer.MasksToBounds = true;
			layer.VideoGravity = AVCaptureVideoPreviewLayer.GravityResizeAspectFill;

			Frame = UIScreen.MainScreen.Bounds;
			layer.Frame = Frame;
			Layer.AddSublayer(layer);

			var label = new UILabel(new RectangleF(40, 80, UIScreen.MainScreen.Bounds.Width - 80, 80));
			AddSubview(label);

			metadataSource.MetadataFound += (s, e) => label.Text = e.StringValue;

		}

		public override void LayoutSubviews()
		{
			base.LayoutSubviews();
			layer.Frame = Bounds;
		}
	}

	public class MyMetadataOutputDelegate : AVCaptureMetadataOutputObjectsDelegate
	{
		public override void DidOutputMetadataObjects(AVCaptureMetadataOutput captureOutput, AVMetadataObject[] metadataObjects, AVCaptureConnection connection)
		{
			foreach(var m in metadataObjects)
			{
				if(m is AVMetadataMachineReadableCodeObject)
				{
					MetadataFound(this, m as AVMetadataMachineReadableCodeObject);
				}
			}
		}

		public event EventHandler<AVMetadataMachineReadableCodeObject> MetadataFound = delegate {};
	}

	public class SimpleViewController : UIViewController
	{
		AVCaptureSession session;
		AVCaptureMetadataOutput metadataOutput;

		public SimpleViewController() : base()
		{
		}

		public override void DidReceiveMemoryWarning()
		{
			// Releases the view if it doesn't have a superview.
			base.DidReceiveMemoryWarning();
		}

		public override void ViewDidLoad()
		{
			base.ViewDidLoad();
			
			session = new AVCaptureSession();
			var camera = AVCaptureDevice.DefaultDeviceWithMediaType(AVMediaType.Video);
			var input = AVCaptureDeviceInput.FromDevice(camera);
			session.AddInput(input);

			//Add the metadata output channel
			metadataOutput = new AVCaptureMetadataOutput();
			var metadataDelegate = new MyMetadataOutputDelegate();
			metadataOutput.SetDelegate(metadataDelegate, DispatchQueue.MainQueue);
			session.AddOutput(metadataOutput);
			//Confusing! *After* adding to session, tell output what to recognize...
			foreach(var t in metadataOutput.AvailableMetadataObjectTypes)
			{
				Console.WriteLine(t);
			}
			metadataOutput.MetadataObjectTypes = new NSString[] {
				AVMetadataObject.TypeQRCode,
				AVMetadataObject.TypeEAN13Code
			};

			var previewLayer = new AVCaptureVideoPreviewLayer(session);
			var view = new ContentView(UIColor.Blue, previewLayer, metadataDelegate);

			session.StartRunning();
		
			this.View = view;
		}
	}

	[Register("AppDelegate")]
	public  class AppDelegate : UIApplicationDelegate
	{
		UIWindow window;
		SimpleViewController viewController;

		public override bool FinishedLaunching(UIApplication app, NSDictionary options)
		{
			window = new UIWindow(UIScreen.MainScreen.Bounds);

			viewController = new SimpleViewController();
			window.RootViewController = viewController;

			window.MakeKeyAndVisible();
			
			return true;
		}
	}

	public class Application
	{
		static void Main(string[] args)
		{
			UIApplication.Main(args, null, "AppDelegate");
		}
	}
}
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