Use a more reliable fragment id generation

app/src/main/java/io/timelimit/android/Application.kt

@@ -1,5 +1,5 @@
 /*
- * TimeLimit Copyright <C> 2019 - 2021 Jonas Lochmann
+ * TimeLimit Copyright <C> 2019 - 2023 Jonas Lochmann
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -16,9 +16,19 @@
 package io.timelimit.android
 
 import android.app.Application
+import android.view.View
 import com.jakewharton.threetenabp.AndroidThreeTen
 
 class Application : Application() {
+    // two legacy screens use small id numbers as they want; by running generateViewId() often enough,
+    // all ids that are harcoded this way are not returned from generateViewId
+    init { (0..1024).forEach { _ -> View.generateViewId() } }
+
+    // allocate some view ids for Fragments that are not used for anything else
+    // by running this in the Application class, there is a high chance that these
+    // are always the same ids so that there is no trouble when restoring state
+    val viewIdPool = (0..4).map { View.generateViewId() }
+
     override fun onCreate() {
         super.onCreate()
 

app/src/main/java/io/timelimit/android/ui/MainActivity.kt

@@ -79,7 +79,6 @@ class MainActivity : AppCompatActivity(), ActivityViewModelHolder, U2fManager.De
         private const val EXTRA_AUTH_HANDOVER = "authHandover"
         private const val MAIN_MODEL_STATE = "mainModelState"
         private const val FRAGMENT_IDS_STATE = "fragmentIds"
-        private const val NEXT_FRAGMENT_ID = "nextFragmentId"
 
         private var authHandover: Triple<Long, Long, AuthenticatedUser>? = null
 
@@ -114,7 +113,6 @@ class MainActivity : AppCompatActivity(), ActivityViewModelHolder, U2fManager.De
     }
 
     private val mainModel by viewModels<MainModel>()
-    private var fragmentIds = mutableSetOf<Int>()
     private val syncModel: SyncStatusModel by lazy {
         ViewModelProviders.of(this).get(SyncStatusModel::class.java)
     }
@@ -134,8 +132,7 @@ class MainActivity : AppCompatActivity(), ActivityViewModelHolder, U2fManager.De
 
         if (savedInstanceState != null) {
             mainModel.state.value = savedInstanceState.getSerializable(MAIN_MODEL_STATE) as State
-            fragmentIds.addAll(savedInstanceState.getIntegerArrayList(FRAGMENT_IDS_STATE) ?: emptyList())
+            mainModel.fragmentIds.addAll(savedInstanceState.getIntegerArrayList(FRAGMENT_IDS_STATE) ?: emptyList())
-            mainModel.nextFragmentId = savedInstanceState.getInt(NEXT_FRAGMENT_ID)
         }
 
         lifecycleScope.launch {
@@ -279,7 +276,7 @@ class MainActivity : AppCompatActivity(), ActivityViewModelHolder, U2fManager.De
                                 screen = screen,
                                 executeCommand = ::execute,
                                 fragmentManager = supportFragmentManager,
-                                fragmentIds = fragmentIds,
+                                fragmentIds = mainModel.fragmentIds,
                                 modifier = Modifier
                                     .fillMaxSize()
                                     .padding(paddingValues)
@@ -300,8 +297,7 @@ class MainActivity : AppCompatActivity(), ActivityViewModelHolder, U2fManager.De
         super.onSaveInstanceState(outState)
 
         outState.putSerializable(MAIN_MODEL_STATE, mainModel.state.value)
-        outState.putIntegerArrayList(FRAGMENT_IDS_STATE, ArrayList(fragmentIds))
+        outState.putIntegerArrayList(FRAGMENT_IDS_STATE, ArrayList(mainModel.fragmentIds))
-        outState.putInt(NEXT_FRAGMENT_ID, mainModel.nextFragmentId)
     }
 
     override fun onStart() {
@@ -373,7 +369,7 @@ class MainActivity : AppCompatActivity(), ActivityViewModelHolder, U2fManager.De
     }
 
     private fun cleanupFragments() {
-        fragmentIds
+        mainModel.fragmentIds
             .filter { fragmentId ->
                 var v = mainModel.state.value as State?
 
@@ -385,8 +381,7 @@ class MainActivity : AppCompatActivity(), ActivityViewModelHolder, U2fManager.De
 
                 true
             }
-            .map { supportFragmentManager.findFragmentById(it) }
+            .mapNotNull { supportFragmentManager.findFragmentById(it) }
-            .filterNotNull()
             .filter { it.isDetached }
             .forEach {
                 if (BuildConfig.DEBUG) {
@@ -399,7 +394,7 @@ class MainActivity : AppCompatActivity(), ActivityViewModelHolder, U2fManager.De
                     .remove(it)
                     .commitAllowingStateLoss()
 
-                fragmentIds.remove(id)
+                mainModel.fragmentIds.remove(id)
             }
     }
 

app/src/main/java/io/timelimit/android/ui/model/MainModel.kt

@@ -81,7 +81,7 @@ class MainModel(application: Application): AndroidViewModel(application) {
 
     val activityCommand: ReceiveChannel<ActivityCommand> = activityCommandInternal
     val state = MutableStateFlow(State.LaunchState as State)
-    var nextFragmentId = 1
+    var fragmentIds = mutableSetOf<Int>()
 
     val screen: Flow<Screen> = flow {
         while (true) {
@@ -93,9 +93,17 @@ class MainModel(application: Application): AndroidViewModel(application) {
                 is State.DiagnoseScreen.DeviceOwner -> emitAll(DeviceOwnerHandling.processState(logic, scope, authenticationModelApi, state))
                 is FragmentState -> emitAll(state.transformWhile {
                     if (it is FragmentState && it !is State.Overview) {
-                        if (it.containerId == null) it.containerId = nextFragmentId++
+                        val containerId = it.containerId ?: run {
+                            ((application as io.timelimit.android.Application).viewIdPool - fragmentIds).firstOrNull()?.also { id ->
+                                it.containerId = id
+                            }
+                        }
 
-                        emit(Screen.FragmentScreen(it, it.toolbarIcons, it.toolbarOptions, it, it.containerId!!))
+                        if (containerId != null) {
+                            fragmentIds.add(containerId)
+
+                            emit(Screen.FragmentScreen(it, it.toolbarIcons, it.toolbarOptions, it, containerId))
+                        }
 
                         true
                     } else false