MainActivity.kt

class MainActivity : AppCompatActivity() {
 
   var reviewManager: ReviewManager? = null
   var reviewInfo: ReviewInfo? = null
 
   override fun onCreate(savedInstanceState: Bundle?) {
       super.onCreate(savedInstanceState)
       setContentView(R.layout.activity_main)

       getReviewInfo();
       
       someButton.setOnClickListener {
           startReviewFlow()
       }
   }
 
   /**
    * Call this method at app start etc to pre-cache the reviewInfo object to use to show 
    * in-app review dialog later.
    */
   private fun getReviewInfo() {
       reviewManager = ReviewManagerFactory.create(applicationContext)
       val manager = reviewManager?.requestReviewFlow()
       manager?.addOnCompleteListener { task: Task<ReviewInfo?> ->
           if (task.isSuccessful) {
               reviewInfo = task.result
           } else {
               Toast.makeText(
                   applicationContext,
                   "In App ReviewFlow failed to start",
                   Toast.LENGTH_LONG
               ).show()
           }
       }
   }
   
   /**
    * Call this method when you want to show the in-app rating dialog
    */
   fun startReviwFlow() {
       if (reviewInfo != null) {
           val flow = reviewManager!!.launchReviewFlow(this, reviewInfo!!)
           flow.addOnCompleteListener {
               Toast.makeText(
                   applicationContext,
                   "Rating complete",
                   Toast.LENGTH_LONG
               ).show()
           }
       } else {
           Toast.makeText(applicationContext, "Rating failed", Toast.LENGTH_LONG).show()
       }
   }
}