Dark theme / DayNight: Add option to follow system

fix maybe #1748. Maybe not, because actually I don't understand why
issue #1748 even happened because we don't use the system background for
the bubble but an own one (see conversation_item_sent.xml:50). Probably
the system is somehow trying to forcefully create a dark mode.

So, let's see whether this PR fixes the issue, if not, I'll try again.

res/values-v29/arrays.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources xmlns:tools="http://schemas.android.com/tools">
+
+	<string-array name="pref_theme_entries" tools:ignore="InconsistentArrays">
+		<item>@string/pref_system_default</item>
+		<item>@string/pref_light_theme</item>
+		<item>@string/pref_dark_theme</item>
+	</string-array>
+
+	<string-array name="pref_theme_values" translatable="false" tools:ignore="InconsistentArrays">
+		<item>system</item>
+		<item>light</item>
+		<item>dark</item>
+	</string-array>
+
+</resources>

res/values/arrays.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<resources>
+<resources xmlns:tools="http://schemas.android.com/tools">
 
   <!-- take care, language_entries and languages_values are in sync ...
        ... some one has to clean up this mess some day ... -->
@@ -84,12 +84,12 @@
       <item>te</item>
   </string-array>
 
-    <string-array name="pref_theme_entries">
+    <string-array name="pref_theme_entries" tools:ignore="InconsistentArrays">
       <item>@string/pref_light_theme</item>
       <item>@string/pref_dark_theme</item>
   </string-array>
 
-  <string-array name="pref_theme_values" translatable="false">
+  <string-array name="pref_theme_values" translatable="false" tools:ignore="InconsistentArrays">
       <item>light</item>
       <item>dark</item>
   </string-array>

res/values/strings.xml

@@ -457,6 +457,7 @@
     <string name="pref_silent">Silent</string>
     <string name="pref_privacy">Privacy</string>
     <string name="pref_chats_and_media">Chats and media</string>
+    <string name="pref_system_default">System default</string>
     <!-- Translators: "light" in the meaning "opposite of dark" -->
     <string name="pref_light_theme">Light</string>
     <string name="pref_dark_theme">Dark</string>

res/xml-v29/preferences_appearance.xml

@@ -0,0 +1,36 @@
+<?xml version="1.0" encoding="utf-8"?>
+
+<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">
+
+    <ListPreference
+            android:key="pref_theme"
+            android:title="@string/pref_theme"
+            android:entries="@array/pref_theme_entries"
+            android:entryValues="@array/pref_theme_values"
+            android:defaultValue="system"/>
+
+    <Preference
+        android:key="pref_chat_background"
+        android:title="@string/pref_background"/>
+
+    <ListPreference
+        android:key="pref_message_body_text_size"
+        android:title="@string/pref_message_text_size"
+        android:entries="@array/pref_message_font_size_entries"
+        android:entryValues="@array/pref_message_font_size_values"
+        android:defaultValue="16"/>
+
+    <org.thoughtcrime.securesms.components.SwitchPreferenceCompat
+        android:defaultValue="false"
+        android:key="pref_system_emoji"
+        android:title="@string/pref_use_system_emoji"
+        android:summary="@string/pref_use_system_emoji_explain" />
+
+    <ListPreference
+            android:key="pref_language"
+            android:title="@string/pref_language"
+            android:entries="@array/language_entries"
+            android:entryValues="@array/language_values"
+            android:defaultValue="zz"/>
+
+</PreferenceScreen>

src/org/thoughtcrime/securesms/util/DynamicNoActionBarTheme.java

@@ -1,16 +1,15 @@
 package org.thoughtcrime.securesms.util;
 
-import android.app.Activity;
+import androidx.annotation.StyleRes;
 
 import org.thoughtcrime.securesms.R;
 
 public class DynamicNoActionBarTheme extends DynamicTheme {
-  @Override
+  protected @StyleRes int getLightThemeStyle() {
-  protected int getSelectedTheme(Activity activity) {
-    String theme = Prefs.getTheme(activity);
-
-    if (theme.equals("dark")) return R.style.TextSecure_DarkNoActionBar;
-
     return R.style.TextSecure_LightNoActionBar;
   }
+
+  protected @StyleRes int getDarkThemeStyle() {
+    return R.style.TextSecure_DarkNoActionBar;
+  }
 }

src/org/thoughtcrime/securesms/util/DynamicTheme.java

@@ -3,19 +3,37 @@ package org.thoughtcrime.securesms.util;
 import android.app.Activity;
 import android.content.Context;
 import android.content.Intent;
+import android.content.res.Configuration;
+import android.os.Build;
+
+import androidx.annotation.NonNull;
+import androidx.annotation.StyleRes;
 
 import org.thoughtcrime.securesms.R;
 
 public class DynamicTheme {
 
-  public static final String DARK  = "dark";
+  public static final String DARK   = "dark";
-  public static final String LIGHT = "light";
+  public static final String LIGHT  = "light";
+  public static final String SYSTEM = "system";
+
+  //private static boolean isDarkTheme;
 
   private int currentTheme;
 
   public void onCreate(Activity activity) {
+    //boolean wasDarkTheme = isDarkTheme;
+
     currentTheme = getSelectedTheme(activity);
+    //isDarkTheme  = isDarkTheme(activity);
+
     activity.setTheme(currentTheme);
+
+    // In case you introduce a CachedInflater and there are problems with the dark mode, uncomment
+    // this line and the line in onResume():
+    //if (isDarkTheme != wasDarkTheme) {
+      //CachedInflater.from(activity).clear();
+    //}
   }
 
   public void onResume(Activity activity) {
@@ -25,19 +43,45 @@ public class DynamicTheme {
       OverridePendingTransition.invoke(activity);
       activity.startActivity(intent);
       OverridePendingTransition.invoke(activity);
+      //CachedInflater.from(activity).clear();
     }
   }
 
-  protected int getSelectedTheme(Activity activity) {
+  private @StyleRes int getSelectedTheme(Activity activity) {
-    String theme = Prefs.getTheme(activity);
+    if (isDarkTheme(activity)) {
-
+      return getDarkThemeStyle();
-    if (theme.equals(DARK)) return R.style.TextSecure_DarkTheme;
+    } else {
+      return getLightThemeStyle();
+    }
+  }
 
+  protected @StyleRes int getLightThemeStyle() {
     return R.style.TextSecure_LightTheme;
   }
 
-  public static boolean isDarkTheme(Context context) {
+  protected @StyleRes int getDarkThemeStyle() {
-    return Prefs.getTheme(context).equals(DARK);
+    return R.style.TextSecure_DarkTheme;
+  }
+
+  static boolean systemThemeAvailable() {
+    return Build.VERSION.SDK_INT >= 29;
+  }
+
+  /**
+   * Takes the system theme into account.
+   */
+  public static boolean isDarkTheme(@NonNull Context context) {
+    String theme = Prefs.getTheme(context);
+
+    if (theme.equals(SYSTEM) && systemThemeAvailable()) {
+      return isSystemInDarkTheme(context);
+    } else {
+      return theme.equals(DARK);
+    }
+  }
+
+  private static boolean isSystemInDarkTheme(@NonNull Context context) {
+    return (context.getResources().getConfiguration().uiMode & Configuration.UI_MODE_NIGHT_MASK) == Configuration.UI_MODE_NIGHT_YES;
   }
 
   private static final class OverridePendingTransition {

src/org/thoughtcrime/securesms/util/Prefs.java

@@ -138,7 +138,7 @@ public class Prefs {
   }
 
   public static String getTheme(Context context) {
-    return getStringPreference(context, THEME_PREF, "light");
+    return getStringPreference(context, THEME_PREF, DynamicTheme.systemThemeAvailable() ? DynamicTheme.SYSTEM : DynamicTheme.LIGHT);
   }
 
   public static boolean isScreenLockTimeoutEnabled(Context context) {