Refactor Screen class as interface.

library/src/main/kotlin/com/github/terrakok/cicerone/Screen.kt

@@ -1,8 +1,8 @@
 package com.github.terrakok.cicerone
 
 /**
- * Screen is class for description application screen.
+ * Screen is interface for description application screen.
  */
-abstract class Screen {
-    open val screenKey: String = this::class.qualifiedName!!
+interface Screen {
+    val screenKey: String get() = this::class.java.name
 }

library/src/main/kotlin/com/github/terrakok/cicerone/androidx/AppScreen.kt

@@ -7,16 +7,19 @@ import androidx.fragment.app.Fragment
 import androidx.fragment.app.FragmentFactory
 import com.github.terrakok.cicerone.Screen
 
-sealed class AppScreen : Screen()
+sealed class AppScreen : Screen
 
 open class FragmentScreen(
-    override val screenKey: String,
+    private val key: String? = null,
     val createFragment: (FragmentFactory) -> Fragment
-) : AppScreen()
+) : AppScreen() {
+    override val screenKey: String get() = key ?: super.screenKey
+}
 
 open class ActivityScreen(
-    override val screenKey: String,
+    private val key: String? = null,
     val createIntent: (context: Context) -> Intent
 ) : AppScreen() {
+    override val screenKey: String get() = key ?: super.screenKey
     open val startActivityOptions: Bundle? = null
 }

sample/src/main/java/com/github/terrakok/cicerone/sample/Screens.kt

@@ -24,39 +24,39 @@ object Screens {
         SampleFragment.getNewInstance(number)
     }
 
-    fun Start() = ActivityScreen("Start") {
+    fun Start() = ActivityScreen {
         Intent(it, StartActivity::class.java)
     }
 
-    fun Main() = ActivityScreen("Main") {
+    fun Main() = ActivityScreen {
         Intent(it, MainActivity::class.java)
     }
 
-    fun BottomNavigation() = ActivityScreen("BottomNavigation") {
+    fun BottomNavigation() = ActivityScreen {
         Intent(it, BottomNavigationActivity::class.java)
     }
 
-    fun Tab(tabName: String) = FragmentScreen("Tab") {
+    fun Tab(tabName: String) = FragmentScreen {
         TabContainerFragment.getNewInstance(tabName)
     }
 
-    fun Forward(containerName: String, number: Int) = FragmentScreen("Forward") {
+    fun Forward(containerName: String, number: Int) = FragmentScreen {
         ForwardFragment.getNewInstance(containerName, number)
     }
 
-    fun Github() = ActivityScreen("Github") {
+    fun Github() = ActivityScreen {
         Intent(Intent.ACTION_VIEW, Uri.parse("https://github.com/terrakok/Cicerone"))
     }
 
-    fun Profile() = ActivityScreen("Profile") {
+    fun Profile() = ActivityScreen {
         Intent(it, ProfileActivity::class.java)
     }
 
-    fun ProfileInfo() = FragmentScreen("ProfileInfo") {
+    fun ProfileInfo() = FragmentScreen {
         ProfileFragment()
     }
 
-    fun SelectPhoto(resultKey: String) = FragmentScreen("SelectPhoto") {
+    fun SelectPhoto(resultKey: String) = FragmentScreen {
         SelectPhotoFragment.getNewInstance(resultKey)
     }
 }