MainActivity.cs

using Android;
using Android.App;
using Android.OS;
using Android.Runtime;
using Android.Util;
using Android.Widget;
using AndroidX.AppCompat.App;
using AndroidX.Camera.Core;
using AndroidX.Camera.Lifecycle;
using AndroidX.Camera.View;
using AndroidX.Core.App;
using AndroidX.Core.Content;
using Java.IO;
using Java.Lang;
using Java.Util;
using Java.Util.Concurrent;
using System.Linq;

namespace CameraX
{
    /// <summary>
    /// https://codelabs.developers.google.com/codelabs/camerax-getting-started#0
    /// </summary>

    [Activity(Label = "@string/app_name", Theme = "@style/AppTheme", MainLauncher = true)]
    public class MainActivity : AppCompatActivity
    {
        private const string TAG = "CameraXBasic";
        private const int REQUEST_CODE_PERMISSIONS = 10;
        private const string FILENAME_FORMAT = "yyyy-MM-dd-HH-mm-ss-SSS";

        ImageCapture imageCapture;
        File outputDirectory;
        IExecutorService cameraExecutor;

        PreviewView viewFinder;

        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);

            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.activity_main);

            this.viewFinder = this.FindViewById<PreviewView>(Resource.Id.viewFinder);
            var camera_capture_button = this.FindViewById<Button>(Resource.Id.camera_capture_button);

            // Request camera permissions   
            string[] permissions = new string[] { Manifest.Permission.Camera, Manifest.Permission.WriteExternalStorage };
            if (permissions.FirstOrDefault(x => ContextCompat.CheckSelfPermission(this, x) != Android.Content.PM.Permission.Granted) != null) //   ContextCompat.CheckSelfPermission(this, Manifest.Permission.Camera) == Android.Content.PM.Permission.Granted)
                ActivityCompat.RequestPermissions(this, permissions, REQUEST_CODE_PERMISSIONS);
            else
                StartCamera();

            // Set up the listener for take photo button
            camera_capture_button.SetOnClickListener(new OnClickListener(() => TakePhoto()));

            outputDirectory = GetOutputDirectory();

            cameraExecutor = Executors.NewSingleThreadExecutor();
        }

        protected override void OnDestroy()
        {
            base.OnDestroy();
            cameraExecutor.Shutdown();
        }

        public override void OnRequestPermissionsResult(int requestCode, string[] permissions, [GeneratedEnum] Android.Content.PM.Permission[] grantResults)
        {
            if (requestCode == REQUEST_CODE_PERMISSIONS)
            {
                //if (ContextCompat.CheckSelfPermission(this, Manifest.Permission.Camera) == Android.Content.PM.Permission.Granted)
                if (permissions.FirstOrDefault(x => ContextCompat.CheckSelfPermission(this, x) != Android.Content.PM.Permission.Granted) == null)
                {
                    StartCamera();
                }
                else
                {
                    Toast.MakeText(this, "Permissions not granted by the user.", ToastLength.Short).Show();
                    this.Finish();
                    return;
                }
            }

            //base.OnRequestPermissionsResult(requestCode, permissions, grantResults);              
        }

        /// <summary>
        /// https://codelabs.developers.google.com/codelabs/camerax-getting-started#3
        /// </summary>
        private void StartCamera()
        {
            var cameraProviderFuture = ProcessCameraProvider.GetInstance(this);

            cameraProviderFuture.AddListener(new Runnable(() =>
            {
                // Used to bind the lifecycle of cameras to the lifecycle owner
                var cameraProvider = (ProcessCameraProvider)cameraProviderFuture.Get();

                // Preview
                var preview = new Preview.Builder().Build();
                preview.SetSurfaceProvider(viewFinder.CreateSurfaceProvider());

                // Take Photo
                this.imageCapture = new ImageCapture.Builder().Build();

                // Frame by frame analyze
                var imageAnalyzer = new ImageAnalysis.Builder().Build();
                imageAnalyzer.SetAnalyzer(cameraExecutor, new LuminosityAnalyzer(luma =>
                    Log.Debug(TAG, $"Average luminosity: {luma}")
                    ));

                // Select back camera as a default, or front camera otherwise
                CameraSelector cameraSelector = null;
                if (cameraProvider.HasCamera(CameraSelector.DefaultBackCamera) == true)
                    cameraSelector = CameraSelector.DefaultBackCamera;
                else if (cameraProvider.HasCamera(CameraSelector.DefaultFrontCamera) == true)
                    cameraSelector = CameraSelector.DefaultFrontCamera;
                else
                    throw new System.Exception("Camera not found");

                try
                {
                    // Unbind use cases before rebinding
                    cameraProvider.UnbindAll();

                    // Bind use cases to camera
                    cameraProvider.BindToLifecycle(this, cameraSelector, preview, imageCapture, imageAnalyzer);
                }
                catch (Exception exc)
                {
                    Log.Debug(TAG, "Use case binding failed", exc);
                    Toast.MakeText(this, $"Use case binding failed: {exc.Message}", ToastLength.Short).Show();
                }

            }), ContextCompat.GetMainExecutor(this)); //GetMainExecutor: returns an Executor that runs on the main thread.
        }


        private void TakePhoto()
        {
            // Get a stable reference of the modifiable image capture use case   
            var imageCapture = this.imageCapture;
            if (imageCapture == null)
                return;

            // Create time-stamped output file to hold the image
            var photoFile = new File(outputDirectory, new Java.Text.SimpleDateFormat(FILENAME_FORMAT, Locale.Us).Format(JavaSystem.CurrentTimeMillis()) + ".jpg");

            // Create output options object which contains file + metadata
            var outputOptions = new ImageCapture.OutputFileOptions.Builder(photoFile).Build();

            // Set up image capture listener, which is triggered after photo has been taken
            imageCapture.TakePicture(outputOptions, ContextCompat.GetMainExecutor(this), new ImageSaveCallback(

                onErrorCallback: (exc) =>
                {
                    var msg = $"Photo capture failed: {exc.Message}";
                    Log.Error(TAG, msg, exc);
                    Toast.MakeText(this.BaseContext, msg, ToastLength.Short).Show();
                },

                onImageSaveCallback: (output) =>
                {
                    var savedUri = output.SavedUri;
                    var msg = $"Photo capture succeeded: {savedUri}";
                    Log.Debug(TAG, msg);
                    Toast.MakeText(this.BaseContext, msg, ToastLength.Short).Show();
                }
            ));
        }

        // Save photos to => /Pictures/CameraX/
        private File GetOutputDirectory()
        {
            //var mediaDir = GetExternalMediaDirs().FirstOrDefault();  
            var mediaDir = Environment.GetExternalStoragePublicDirectory(System.IO.Path.Combine(Environment.DirectoryPictures, Resources.GetString(Resource.String.app_name)));

            if (mediaDir != null && mediaDir.Exists())
                return mediaDir;

            var file = new File(mediaDir, string.Empty); 
            file.Mkdirs();
            return file;
        }
    }
}