MainActivity.kt

package com.example.nativeopencvandroidtemplate

import android.Manifest
import android.app.Activity
import android.content.pm.PackageManager
import android.os.Bundle
import androidx.core.app.ActivityCompat
import android.util.Log
import android.view.SurfaceView
import android.view.WindowManager
import android.widget.Toast
import org.opencv.android.BaseLoaderCallback
import org.opencv.android.CameraBridgeViewBase
import org.opencv.android.LoaderCallbackInterface
import org.opencv.android.OpenCVLoader
import org.opencv.core.Mat

class MainActivity : Activity(), CameraBridgeViewBase.CvCameraViewListener2 {

    private var mOpenCvCameraView: CameraBridgeViewBase? = null

    private val mLoaderCallback = object : BaseLoaderCallback(this) {
        override fun onManagerConnected(status: Int) {
            when (status) {
                LoaderCallbackInterface.SUCCESS -> {
                    Log.i(TAG, "OpenCV loaded successfully")

                    // Load native library after(!) OpenCV initialization
                    System.loadLibrary("native-lib")

                    mOpenCvCameraView!!.enableView()
                }
                else -> {
                    super.onManagerConnected(status)
                }
            }
        }
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        Log.i(TAG, "called onCreate")
        super.onCreate(savedInstanceState)
        window.addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON)

        // Permissions for Android 6+
        ActivityCompat.requestPermissions(
            this@MainActivity,
            arrayOf(Manifest.permission.CAMERA),
            CAMERA_PERMISSION_REQUEST
        )

        setContentView(R.layout.activity_main)

        mOpenCvCameraView = findViewById<CameraBridgeViewBase>(R.id.main_surface)

        mOpenCvCameraView!!.visibility = SurfaceView.VISIBLE

        mOpenCvCameraView!!.setCvCameraViewListener(this)
    }

    override fun onRequestPermissionsResult(
        requestCode: Int,
        permissions: Array<String>,
        grantResults: IntArray
    ) {
        when (requestCode) {
            CAMERA_PERMISSION_REQUEST -> {
                if (grantResults.isNotEmpty() && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    mOpenCvCameraView!!.setCameraPermissionGranted()
                } else {
                    val message = "Camera permission was not granted"
                    Log.e(TAG, message)
                    Toast.makeText(this, message, Toast.LENGTH_LONG).show()
                }
            }
            else -> {
                Log.e(TAG, "Unexpected permission request")
            }
        }
    }

    override fun onPause() {
        super.onPause()
        if (mOpenCvCameraView != null)
            mOpenCvCameraView!!.disableView()
    }

    override fun onResume() {
        super.onResume()
        if (!OpenCVLoader.initDebug()) {
            Log.d(TAG, "Internal OpenCV library not found. Using OpenCV Manager for initialization")
            OpenCVLoader.initAsync(OpenCVLoader.OPENCV_VERSION, this, mLoaderCallback)
        } else {
            Log.d(TAG, "OpenCV library found inside package. Using it!")
            mLoaderCallback.onManagerConnected(LoaderCallbackInterface.SUCCESS)
        }
    }

    override fun onDestroy() {
        super.onDestroy()
        if (mOpenCvCameraView != null)
            mOpenCvCameraView!!.disableView()
    }

    override fun onCameraViewStarted(width: Int, height: Int) {}

    override fun onCameraViewStopped() {}

    override fun onCameraFrame(frame: CameraBridgeViewBase.CvCameraViewFrame): Mat {
        // get current camera frame as OpenCV Mat object
        val mat = frame.gray()

        // native call to process current camera frame
        adaptiveThresholdFromJNI(mat.nativeObjAddr)

        // return processed frame for live preview
        return mat
    }

    private external fun adaptiveThresholdFromJNI(matAddr: Long)

    companion object {

        private const val TAG = "MainActivity"
        private const val CAMERA_PERMISSION_REQUEST = 1
    }
}