Fix splashscreen theme not updating after theme change (#633)

[shaominngqing]

Summary

This PR fixes the issue where the splash screen doesn't reflect the user's dark theme preference when the app is cold-started after changing the theme setting.

Problem

When a user changes the theme from the app settings (e.g., from light to dark), the preference is saved but AppCompatDelegate.setDefaultNightMode() is not called. As a result, on the next cold start, the splash screen still uses the old theme (light) and then switches to the app's new theme (dark), causing an unpleasant flash.

Solution

This PR applies AppCompatDelegate.setDefaultNightMode() in two places:

1. SettingsViewModel.updateDarkThemeConfig(): Immediately apply the night mode when the user changes the setting
2. MainActivity.onCreate(): Apply the saved theme preference on app startup to ensure consistency

This ensures the system-level night mode is synchronized with the user's preference, making the splash screen display correctly from the start.

Testing

Manual testing:

1. Set device to light mode
2. Open the app and change theme to dark in settings
3. Force close the app
4. Cold start the app
5. ✅ Splash screen now shows in dark theme (previously showed light)

Changes

• SettingsViewModel.kt: Added AppCompatDelegate.setDefaultNightMode() call
• MainActivity.kt: Added startup logic to apply saved theme config

Reference

• Android Developer Guide: Implement dark theme

Fixes #633

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[gemini-code-assist]

Summary of Changes

Hello @shaominngqing, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a user experience inconsistency where the app's splash screen would briefly display with an incorrect theme (e.g., light mode) before switching to the user's preferred dark theme during a cold start. The changes ensure that the system-level theme is synchronized with the user's settings both immediately upon change and during initial application launch, providing a seamless visual experience from the very beginning.

Highlights

• Problem Addressed: Fixed an issue where the splash screen did not correctly reflect the user's dark theme preference on cold app starts, leading to a visual flicker before the main app theme loaded.
• Immediate Theme Application: Modified SettingsViewModel.updateDarkThemeConfig() to immediately apply the selected night mode using AppCompatDelegate.setDefaultNightMode() when the user changes the theme setting.
• Startup Theme Synchronization: Implemented logic in MainActivity.onCreate() to read the saved theme preference and apply it via AppCompatDelegate.setDefaultNightMode() during app startup, ensuring the splash screen uses the correct theme from the very beginning.

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Changelog

• app/src/main/kotlin/com/google/samples/apps/nowinandroid/MainActivity.kt
  • Imported AppCompatDelegate and DarkThemeConfig to support theme management.
  • Added a lifecycleScope.launch block within onCreate to observe the viewModel.uiState and dynamically apply the appropriate AppCompatDelegate.setDefaultNightMode() based on the user's darkThemeConfig during app startup.
• feature/settings/impl/src/main/kotlin/com/google/samples/apps/nowinandroid/feature/settings/impl/SettingsViewModel.kt
  • Imported AppCompatDelegate to enable programmatic theme control.
  • Added a call to AppCompatDelegate.setDefaultNightMode() within the updateDarkThemeConfig function to instantly apply the chosen theme setting across the application.

Activity

• The author, shaominngqing, has provided a comprehensive description detailing the problem, proposed solution, testing steps, and specific file changes.
• No further review comments or activity have been recorded in the provided context.

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[gemini-code-assist]

Code Review

This pull request effectively resolves the splash screen theme inconsistency by ensuring that the AppCompatDelegate.setDefaultNightMode() is applied both when the user changes the theme setting in SettingsViewModel and during the app's cold startup in MainActivity. This dual approach guarantees that the splash screen correctly reflects the user's preferred theme, eliminating the unpleasant flash previously observed. The changes align well with the problem description and proposed solution.