Optimized handling of dark/light mode

- Activity handles uiMode config change to prevent re-creating the whole activity
- Wrap uiState logic into its classes
- Optimize startup to prevent recomposing twice on startup
- Optimize startup to call enableEdgeToEdge only once or when change occurs

Change-Id: I6f7a48b3b6ce9b55db4ab2ec1770583028e9bc50

app/src/main/AndroidManifest.xml

@@ -39,6 +39,7 @@
 
         <activity
             android:name=".MainActivity"
+            android:configChanges="uiMode"
             android:exported="true">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />

app/src/main/kotlin/com/google/samples/apps/nowinandroid/MainActivity.kt

@@ -22,12 +22,9 @@ import androidx.activity.SystemBarStyle
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
 import androidx.activity.viewModels
-import androidx.compose.foundation.isSystemInDarkTheme
 import androidx.compose.material3.windowsizeclass.ExperimentalMaterial3WindowSizeClassApi
 import androidx.compose.material3.windowsizeclass.calculateWindowSizeClass
-import androidx.compose.runtime.Composable
 import androidx.compose.runtime.CompositionLocalProvider
-import androidx.compose.runtime.DisposableEffect
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.setValue
@@ -38,27 +35,22 @@ import androidx.lifecycle.lifecycleScope
 import androidx.lifecycle.repeatOnLifecycle
 import androidx.metrics.performance.JankStats
 import androidx.tracing.trace
-import com.google.samples.apps.nowinandroid.MainActivityUiState.Loading
-import com.google.samples.apps.nowinandroid.MainActivityUiState.Success
 import com.google.samples.apps.nowinandroid.core.analytics.AnalyticsHelper
 import com.google.samples.apps.nowinandroid.core.analytics.LocalAnalyticsHelper
 import com.google.samples.apps.nowinandroid.core.data.repository.UserNewsResourceRepository
 import com.google.samples.apps.nowinandroid.core.data.util.NetworkMonitor
 import com.google.samples.apps.nowinandroid.core.data.util.TimeZoneMonitor
 import com.google.samples.apps.nowinandroid.core.designsystem.theme.NiaTheme
-import com.google.samples.apps.nowinandroid.core.model.data.DarkThemeConfig
-import com.google.samples.apps.nowinandroid.core.model.data.ThemeBrand
 import com.google.samples.apps.nowinandroid.core.ui.LocalTimeZone
 import com.google.samples.apps.nowinandroid.ui.NiaApp
 import com.google.samples.apps.nowinandroid.ui.rememberNiaAppState
+import com.google.samples.apps.nowinandroid.util.isSystemInDarkTheme
 import dagger.hilt.android.AndroidEntryPoint
-import kotlinx.coroutines.flow.collect
-import kotlinx.coroutines.flow.onEach
+import kotlinx.coroutines.flow.combine
+import kotlinx.coroutines.flow.distinctUntilChanged
 import kotlinx.coroutines.launch
 import javax.inject.Inject
 
-private const val TAG = "MainActivity"
-
 @OptIn(ExperimentalMaterial3WindowSizeClassApi::class)
 @AndroidEntryPoint
 class MainActivity : ComponentActivity() {
@@ -82,60 +74,64 @@ class MainActivity : ComponentActivity() {
     lateinit var userNewsResourceRepository: UserNewsResourceRepository
 
     val viewModel: MainActivityViewModel by viewModels()
-
     override fun onCreate(savedInstanceState: Bundle?) {
         val splashScreen = installSplashScreen()
         super.onCreate(savedInstanceState)
 
-        var uiState: MainActivityUiState by mutableStateOf(Loading)
+        // We keep this as a mutable state, so that we can track changes inside the composition.
+        // This allows us to react to dark/light mode changes.
+        var themeSettings by mutableStateOf(
+            ThemeSettings(
+                darkTheme = resources.configuration.isSystemInDarkTheme,
+                androidTheme = false,
+                disableDynamicTheming = true,
+            ),
+        )
 
         // Update the uiState
         lifecycleScope.launch {
             lifecycle.repeatOnLifecycle(Lifecycle.State.STARTED) {
-                viewModel.uiState
-                    .onEach { uiState = it }
-                    .collect()
+                combine(
+                    isSystemInDarkTheme(),
+                    viewModel.uiState,
+                ) { systemDark, uiState ->
+                    ThemeSettings(
+                        darkTheme = uiState.shouldUseDarkTheme(systemDark),
+                        androidTheme = uiState.shouldUseAndroidTheme,
+                        disableDynamicTheming = uiState.shouldDisableDynamicTheming,
+                    )
+                }
+                    .distinctUntilChanged()
+                    .collect { newThemeSettings ->
+                        trace("niaEdgeToEdge") {
+                            // Turn off the decor fitting system windows, which allows us to handle insets,
+                            // including IME animations, and go edge-to-edge.
+                            // This is the same parameters as the default enableEdgeToEdge call, but we manually
+                            // resolve whether or not to show dark theme using uiState, since it can be different
+                            // than the configuration's dark theme value based on the user preference.
+                            enableEdgeToEdge(
+                                statusBarStyle = SystemBarStyle.auto(
+                                    lightScrim = android.graphics.Color.TRANSPARENT,
+                                    darkScrim = android.graphics.Color.TRANSPARENT,
+                                ) { newThemeSettings.darkTheme },
+                                navigationBarStyle = SystemBarStyle.auto(
+                                    lightScrim = lightScrim,
+                                    darkScrim = darkScrim,
+                                ) { newThemeSettings.darkTheme },
+                            )
+                        }
+
+                        themeSettings = newThemeSettings
+                    }
             }
         }
 
         // Keep the splash screen on-screen until the UI state is loaded. This condition is
         // evaluated each time the app needs to be redrawn so it should be fast to avoid blocking
         // the UI.
-        splashScreen.setKeepOnScreenCondition {
-            when (uiState) {
-                Loading -> true
-                is Success -> false
-            }
-        }
-
-        // Turn off the decor fitting system windows, which allows us to handle insets,
-        // including IME animations, and go edge-to-edge
-        // This also sets up the initial system bar style based on the platform theme
-        trace("niaEdgeToEdge") { enableEdgeToEdge() }
+        splashScreen.setKeepOnScreenCondition { viewModel.uiState.value.shouldKeepSplashScreen() }
 
         setContent {
-            val darkTheme = shouldUseDarkTheme(uiState)
-
-            // Update the edge to edge configuration to match the theme
-            // This is the same parameters as the default enableEdgeToEdge call, but we manually
-            // resolve whether or not to show dark theme using uiState, since it can be different
-            // than the configuration's dark theme value based on the user preference.
-            DisposableEffect(darkTheme) {
-                trace("niaEdgeToEdge") {
-                    enableEdgeToEdge(
-                        statusBarStyle = SystemBarStyle.auto(
-                            android.graphics.Color.TRANSPARENT,
-                            android.graphics.Color.TRANSPARENT,
-                        ) { darkTheme },
-                        navigationBarStyle = SystemBarStyle.auto(
-                            lightScrim,
-                            darkScrim,
-                        ) { darkTheme },
-                    )
-                    onDispose {}
-                }
-            }
-
             val appState = rememberNiaAppState(
                 windowSizeClass = calculateWindowSizeClass(this),
                 networkMonitor = networkMonitor,
@@ -150,9 +146,9 @@ class MainActivity : ComponentActivity() {
                 LocalTimeZone provides currentTimeZone,
             ) {
                 NiaTheme(
-                    darkTheme = darkTheme,
-                    androidTheme = shouldUseAndroidTheme(uiState),
-                    disableDynamicTheming = shouldDisableDynamicTheming(uiState),
+                    darkTheme = themeSettings.darkTheme,
+                    androidTheme = themeSettings.androidTheme,
+                    disableDynamicTheming = themeSettings.disableDynamicTheming,
                 ) {
                     NiaApp(appState)
                 }
@@ -171,47 +167,6 @@ class MainActivity : ComponentActivity() {
     }
 }
 
-/**
- * Returns `true` if the Android theme should be used, as a function of the [uiState].
- */
-@Composable
-private fun shouldUseAndroidTheme(
-    uiState: MainActivityUiState,
-): Boolean = when (uiState) {
-    Loading -> false
-    is Success -> when (uiState.userData.themeBrand) {
-        ThemeBrand.DEFAULT -> false
-        ThemeBrand.ANDROID -> true
-    }
-}
-
-/**
- * Returns `true` if the dynamic color is disabled, as a function of the [uiState].
- */
-@Composable
-private fun shouldDisableDynamicTheming(
-    uiState: MainActivityUiState,
-): Boolean = when (uiState) {
-    Loading -> false
-    is Success -> !uiState.userData.useDynamicColor
-}
-
-/**
- * Returns `true` if dark theme should be used, as a function of the [uiState] and the
- * current system context.
- */
-@Composable
-private fun shouldUseDarkTheme(
-    uiState: MainActivityUiState,
-): Boolean = when (uiState) {
-    Loading -> isSystemInDarkTheme()
-    is Success -> when (uiState.userData.darkThemeConfig) {
-        DarkThemeConfig.FOLLOW_SYSTEM -> isSystemInDarkTheme()
-        DarkThemeConfig.LIGHT -> false
-        DarkThemeConfig.DARK -> true
-    }
-}
-
 /**
  * The default light scrim, as defined by androidx and the platform:
  * https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:activity/activity/src/main/java/androidx/activity/EdgeToEdge.kt;l=35-38;drc=27e7d52e8604a080133e8b842db10c89b4482598
@@ -223,3 +178,13 @@ private val lightScrim = android.graphics.Color.argb(0xe6, 0xFF, 0xFF, 0xFF)
  * https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:activity/activity/src/main/java/androidx/activity/EdgeToEdge.kt;l=40-44;drc=27e7d52e8604a080133e8b842db10c89b4482598
  */
 private val darkScrim = android.graphics.Color.argb(0x80, 0x1b, 0x1b, 0x1b)
+
+/**
+ * Class for the system theme settings.
+ * This wrapping class allows us to combine all the changes and prevent unnecessary recompositions.
+ */
+data class ThemeSettings(
+    val darkTheme: Boolean,
+    val androidTheme: Boolean,
+    val disableDynamicTheming: Boolean,
+)

app/src/main/kotlin/com/google/samples/apps/nowinandroid/MainActivityViewModel.kt

@@ -21,6 +21,8 @@ import androidx.lifecycle.viewModelScope
 import com.google.samples.apps.nowinandroid.MainActivityUiState.Loading
 import com.google.samples.apps.nowinandroid.MainActivityUiState.Success
 import com.google.samples.apps.nowinandroid.core.data.repository.UserDataRepository
+import com.google.samples.apps.nowinandroid.core.model.data.DarkThemeConfig
+import com.google.samples.apps.nowinandroid.core.model.data.ThemeBrand
 import com.google.samples.apps.nowinandroid.core.model.data.UserData
 import dagger.hilt.android.lifecycle.HiltViewModel
 import kotlinx.coroutines.flow.SharingStarted
@@ -44,5 +46,40 @@ class MainActivityViewModel @Inject constructor(
 
 sealed interface MainActivityUiState {
     data object Loading : MainActivityUiState
-    data class Success(val userData: UserData) : MainActivityUiState
+
+    data class Success(val userData: UserData) : MainActivityUiState {
+        override val shouldDisableDynamicTheming = !userData.useDynamicColor
+
+        override val shouldUseAndroidTheme: Boolean = when (userData.themeBrand) {
+            ThemeBrand.DEFAULT -> false
+            ThemeBrand.ANDROID -> true
+        }
+
+        override fun shouldUseDarkTheme(isSystemDarkTheme: Boolean) =
+            when (userData.darkThemeConfig) {
+                DarkThemeConfig.FOLLOW_SYSTEM -> isSystemDarkTheme
+                DarkThemeConfig.LIGHT -> false
+                DarkThemeConfig.DARK -> true
+            }
+    }
+
+    /**
+     * Returns `true` if the state wasn't loaded yet and it should keep showing the splash screen.
+     */
+    fun shouldKeepSplashScreen() = this is Loading
+
+    /**
+     * Returns `true` if the dynamic color is disabled.
+     */
+    val shouldDisableDynamicTheming: Boolean get() = false
+
+    /**
+     * Returns `true` if the Android theme should be used.
+     */
+    val shouldUseAndroidTheme: Boolean get() = false
+
+    /**
+     * Returns `true` if dark theme should be used.
+     */
+    fun shouldUseDarkTheme(isSystemDarkTheme: Boolean) = isSystemDarkTheme
 }

app/src/main/kotlin/com/google/samples/apps/nowinandroid/util/UiExtensions.kt

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.samples.apps.nowinandroid.util
+
+import android.content.res.Configuration
+import androidx.activity.ComponentActivity
+import androidx.core.util.Consumer
+import kotlinx.coroutines.channels.awaitClose
+import kotlinx.coroutines.flow.callbackFlow
+import kotlinx.coroutines.flow.conflate
+import kotlinx.coroutines.flow.distinctUntilChanged
+
+/**
+ * Convenience wrapper for dark mode checking
+ */
+val Configuration.isSystemInDarkTheme
+    get() = (uiMode and Configuration.UI_MODE_NIGHT_MASK) == Configuration.UI_MODE_NIGHT_YES
+
+/**
+ * Registers listener for configuration changes to retrieve whether system is in dark theme or not.
+ * Immediately upon subscribing, it sends the current value and then registers listener for changes.
+ */
+fun ComponentActivity.isSystemInDarkTheme() = callbackFlow {
+    channel.trySend(resources.configuration.isSystemInDarkTheme)
+
+    val listener = Consumer<Configuration> {
+        channel.trySend(it.isSystemInDarkTheme)
+    }
+
+    addOnConfigurationChangedListener(listener)
+
+    awaitClose { removeOnConfigurationChangedListener(listener) }
+}
+    .distinctUntilChanged()
+    .conflate()