Android Studio, "Unresolved Reference: activity_main"

I am following this tutorial and cannot figure out how to resolve the errors "Unresolved Reference: activity_main" and "Unresolved Reference: app_name". The tutorial was created fairly recently, so I don't think it's deprecated, but I have only recently reinstalled Android Studio, so everything should be default.

When trying to create a resource with that name, both resources fail to create because they already exist.

Here is the MainActivity.kt:

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.Manifest
import android.content.pm.PackageManager
import android.net.Uri
import android.util.Log
import android.widget.Toast
import androidx.core.app.ActivityCompat
import androidx.core.content.ContextCompat
import java.util.concurrent.Executors
import androidx.camera.core.*
import androidx.camera.lifecycle.ProcessCameraProvider
import kotlinx.android.synthetic.main.activity_main.*
import java.io.File
import java.nio.ByteBuffer
import java.text.SimpleDateFormat
import java.util.*
import java.util.concurrent.ExecutorService
typealias LumaListener = (luma: Double) -> Unit

class MainActivity : AppCompatActivity() {
   private var imageCapture: ImageCapture? = null

   private lateinit var outputDirectory: File
   private lateinit var cameraExecutor: ExecutorService

   override fun onCreate(savedInstanceState: Bundle?) {
       super.onCreate(savedInstanceState)
       setContentView(R.layout.activity_main)

       // Request camera permissions
       if (allPermissionsGranted()) {
           startCamera()
       } else {
           ActivityCompat.requestPermissions(
               this, REQUIRED_PERMISSIONS, REQUEST_CODE_PERMISSIONS)
       }

       // Setup the listener for take photo button
       camera_capture_button.setOnClickListener { takePhoto() }

       outputDirectory = getOutputDirectory()

       cameraExecutor = Executors.newSingleThreadExecutor()
   }

   private fun takePhoto() {}

   private fun startCamera() {}

   private fun allPermissionsGranted() = REQUIRED_PERMISSIONS.all {
       ContextCompat.checkSelfPermission(
           baseContext, it) == PackageManager.PERMISSION_GRANTED
   }

   private fun getOutputDirectory(): File {
       val mediaDir = externalMediaDirs.firstOrNull()?.let {
           File(it, resources.getString(R.string.app_name)).apply { mkdirs() } }
       return if (mediaDir != null && mediaDir.exists())
           mediaDir else filesDir
   }

   override fun onDestroy() {
       super.onDestroy()
       cameraExecutor.shutdown()
   }

   companion object {
       private const val TAG = "CameraXBasic"
       private const val FILENAME_FORMAT = "yyyy-MM-dd-HH-mm-ss-SSS"
       private const val REQUEST_CODE_PERMISSIONS = 10
       private val REQUIRED_PERMISSIONS = arrayOf(Manifest.permission.CAMERA)
   }
}

R.layout.activity_main and R.string.app_name are the lines in question.

Here is the Module: app build.gradle:

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'

android {
    compileSdkVersion 30
    buildToolsVersion "30.0.1"

    defaultConfig {
        applicationId "com.example.cameraxapp"
        minSdkVersion 21
        targetSdkVersion 30
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }

    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
}

dependencies {
    implementation fileTree(dir: "libs", include: ["*.jar"])
    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    implementation 'androidx.core:core-ktx:1.1.0'
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    def camerax_version = "1.0.0-beta07"
// CameraX core library using camera2 implementation
    implementation "androidx.camera:camera-camera2:$camerax_version"
// CameraX Lifecycle Library
    implementation "androidx.camera:camera-lifecycle:$camerax_version"
// CameraX View class
    implementation "androidx.camera:camera-view:1.0.0-alpha14"
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'

}

I've seen people suggest removing import android.R, but I cannot find anything in my project with this import statement. Any suggestions as to which file this statement might be in would be appreciated.

You need to add in line 0 of MainActivity.kt package [NAME_PROJECT] with project name, for example package com.example.myapplication

The example that you pasted in didn't include the package name on the first line. To restore the package name, add this line to the top of MainActivity.kt:

package com.example.myapplication

Except replace com.example.myapplication with your package name.

I had this issue and it was because I accidentally edited the line that loads this info. Make sure you have the line below in your MainActivity.kt

import android.R.*