fix: Intercept the back press only when the screen is visible

common/src/commonMain/kotlin/com/artemchep/keyguard/feature/navigation/backpress/BackPressInterceptorUtil.kt

@@ -1,20 +1,17 @@
 package com.artemchep.keyguard.feature.navigation.backpress
 
 import kotlinx.coroutines.CoroutineScope
-import kotlinx.coroutines.Job
-import kotlinx.coroutines.cancel
+import kotlinx.coroutines.awaitCancellation
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.launchIn
 import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.isActive
-import kotlinx.coroutines.plus
+import kotlinx.coroutines.launch
 
 fun BackPressInterceptorHost.interceptBackPress(
     scope: CoroutineScope,
     interceptorFlow: Flow<(() -> Unit)?>,
 ): () -> Unit {
-    val subScope = scope + Job()
-
     var callback: (() -> Unit)? = null
     // A registration to remove the back press interceptor
     // from the navigation entry. When it's not null it means
@@ -44,14 +41,25 @@ fun BackPressInterceptorHost.interceptBackPress(
         }
     }
 
-    interceptorFlow
-        .map { c ->
-            val newCallback = c.takeIf { subScope.isActive }
-            setCallback(newCallback)
+    val job = scope.launch {
+        interceptorFlow
+            .map { c ->
+                val newCallback = c.takeIf { this.isActive }
+                setCallback(newCallback)
+            }
+            .launchIn(this)
+
+        try {
+            awaitCancellation()
+        } finally {
+            // Unregister the existing interceptor,
+            // if there's any.
+            unregister?.invoke()
+            unregister = null
         }
-        .launchIn(subScope)
+    }
     return {
-        subScope.cancel()
+        job.cancel()
         // Unregister the existing interceptor,
         // if there's any.
         unregister?.invoke()

common/src/commonMain/kotlin/com/artemchep/keyguard/feature/navigation/state/FlowHolderViewModel.kt

@@ -46,7 +46,7 @@ class FlowHolderViewModel(
         screenName: String,
         context: LeContext,
         colorSchemeState: State<ColorScheme>,
-        init: RememberStateFlowScope.() -> T,
+        init: RememberStateFlowScopeZygote.() -> T,
     ): T = synchronized(this) {
         store.getOrPut(key) {
             val vmCoroutineScopeJob = SupervisorJob()

common/src/commonMain/kotlin/com/artemchep/keyguard/feature/navigation/state/RememberScreenStateFlow.kt

@@ -20,15 +20,15 @@ import kotlinx.coroutines.delay
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.SharingStarted
 import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.filter
 import kotlinx.coroutines.flow.firstOrNull
 import kotlinx.coroutines.flow.flattenConcat
 import kotlinx.coroutines.flow.flow
 import kotlinx.coroutines.flow.flowOn
 import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.merge
 import kotlinx.coroutines.flow.shareIn
 import kotlinx.coroutines.flow.withIndex
-import kotlinx.coroutines.launch
-import kotlinx.coroutines.suspendCancellableCoroutine
 import kotlinx.coroutines.withContext
 import kotlinx.datetime.Clock
 import kotlinx.serialization.json.Json
@@ -76,17 +76,7 @@ fun <T> rememberScreenStateFlow(
         *rargs,
     ) {
         val now = Clock.System.now()
-        val flow: RememberStateFlowScope.() -> Flow<T> = {
-            launch {
-//                Log.i(finalTag, "Initialized the state of '$finalKey'.")
-                try {
-                    // Suspend forever.
-                    suspendCancellableCoroutine<Unit> { }
-                } finally {
-//                    Log.i(finalTag, "Finished the state of '$finalKey'.")
-                }
-            }
-
+        val flow: RememberStateFlowScopeZygote.() -> Flow<T> = {
             val structureFlow = flow {
                 val shouldDelay = getDebugScreenDelay().firstOrNull() == true
                 if (shouldDelay) {
@@ -101,12 +91,17 @@ fun <T> rememberScreenStateFlow(
                 // We don't want to recreate a state producer
                 // each time we re-subscribe to it.
                 .shareIn(this, SharingStarted.Lazily, 1)
-            structureFlow
+            val stateFlow = structureFlow
                 .flattenConcat()
                 .withIndex()
                 .map { it.value }
                 .flowOn(Dispatchers.Default)
-                .persistingStateIn(this, SharingStarted.WhileSubscribed(), initial)
+            merge(
+                stateFlow,
+                keepAliveFlow
+                    .filter { false } as Flow<T>,
+            )
+                .persistingStateIn(screenScope, SharingStarted.WhileSubscribed(), initial)
         }
         val flow2 = viewModel.getOrPut(
             finalKey,

common/src/commonMain/kotlin/com/artemchep/keyguard/feature/navigation/state/RememberStateFlowScope.kt

@@ -86,6 +86,10 @@ interface RememberStateFlowScope : RememberStateFlowScopeSub, CoroutineScope, Tr
         interceptorFlow: Flow<(() -> Unit)?>,
     ): () -> Unit
 
+    fun launchUi(
+        block: CoroutineScope.() -> Unit,
+    ): () -> Unit
+
     //
     // Helpers
     //
@@ -101,6 +105,10 @@ interface RememberStateFlowScope : RememberStateFlowScopeSub, CoroutineScope, Tr
         )
 }
 
+interface RememberStateFlowScopeZygote : RememberStateFlowScope {
+    val keepAliveFlow: Flow<Unit>
+}
+
 fun RememberStateFlowScope.navigatePopSelf() {
     val intent = NavigationIntent.PopById(screenId, exclusive = false)
     navigate(intent)

common/src/commonMain/kotlin/com/artemchep/keyguard/feature/navigation/state/RememberStateFlowScopeImpl.kt

@@ -29,10 +29,14 @@ import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.Job
 import kotlinx.coroutines.cancel
+import kotlinx.coroutines.coroutineScope
 import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.MutableSharedFlow
 import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.distinctUntilChanged
 import kotlinx.coroutines.flow.launchIn
 import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.mapLatest
 import kotlinx.coroutines.flow.onEach
 import kotlinx.coroutines.plus
 import kotlinx.serialization.json.Json
@@ -52,7 +56,7 @@ class RememberStateFlowScopeImpl(
     private val colorSchemeState: State<ColorScheme>,
     override val screenName: String,
     override val context: LeContext,
-) : RememberStateFlowScope, CoroutineScope by scope {
+) : RememberStateFlowScopeZygote, CoroutineScope by scope {
     private val registry = mutableMapOf<String, Entry<Any?, Any?>>()
 
     override val colorScheme get() = colorSchemeState.value
@@ -98,6 +102,20 @@ class RememberStateFlowScopeImpl(
     override val screenId: String
         get() = screen
 
+    /**
+     * A flow that is getting observed while the user interface is
+     * visible on a screen. Used to provide lifecycle events for the
+     * screen.
+     */
+    private val keepAliveSharedFlow = MutableSharedFlow<Unit>()
+
+    private val isStartedFlow = keepAliveSharedFlow
+        .subscriptionCount
+        .map { it > 0 }
+        .distinctUntilChanged()
+
+    override val keepAliveFlow get() = keepAliveSharedFlow
+
     private fun getBundleKey(key: String) = "${this.key}:$key"
 
     override fun navigate(
@@ -163,10 +181,35 @@ class RememberStateFlowScopeImpl(
 
     override fun interceptBackPress(
         interceptorFlow: Flow<(() -> Unit)?>,
-    ) = backPressInterceptorHost.interceptBackPress(
-        scope = scope,
-        interceptorFlow = interceptorFlow,
-    )
+    ): () -> Unit {
+        // We want to launch back interceptor only when the
+        // screen is currently added to the composable. Otherwise
+        // it would intercept the back press event if visually invisible
+        // to a user.
+        return launchUi {
+            backPressInterceptorHost.interceptBackPress(
+                scope = this,
+                interceptorFlow = interceptorFlow,
+            )
+        }
+    }
+
+    override fun launchUi(block: CoroutineScope.() -> Unit): () -> Unit {
+        val job = isStartedFlow
+            .mapLatest { active ->
+                if (!active) {
+                    return@mapLatest
+                }
+
+                coroutineScope {
+                    block()
+                }
+            }
+            .launchIn(scope)
+        return {
+            job.cancel()
+        }
+    }
 
     override suspend fun loadDiskHandle(
         key: String,