[expo-notifications] Add NotificationChannelsManager

NotificationChannelsManager singleton module should be capable of providing modules with a fallback notification channel.

packages/expo-notifications/android/src/main/java/expo/modules/notifications/NotificationsPackage.java

@@ -7,10 +7,10 @@
 import org.unimodules.core.interfaces.SingletonModule;
 
 import java.util.Arrays;
-import java.util.Collections;
 import java.util.List;
 
 import expo.modules.notifications.installationid.InstallationIdProvider;
+import expo.modules.notifications.notifications.channels.ExpoNotificationChannelsManager;
 import expo.modules.notifications.tokens.PushTokenManager;
 import expo.modules.notifications.tokens.PushTokenModule;
 
@@ -25,6 +25,9 @@ public List<ExportedModule> createExportedModules(Context context) {
 
   @Override
   public List<SingletonModule> createSingletonModules(Context context) {
-    return Collections.singletonList((SingletonModule) new PushTokenManager());
+    return Arrays.asList(
+        new PushTokenManager(),
+        new ExpoNotificationChannelsManager(context)
+    );
   }
 }

packages/expo-notifications/android/src/main/java/expo/modules/notifications/notifications/channels/ExpoNotificationChannelsManager.java

@@ -0,0 +1,63 @@
+package expo.modules.notifications.notifications.channels;
+
+import android.app.NotificationChannel;
+import android.app.NotificationManager;
+import android.content.Context;
+import android.os.Build;
+
+import org.unimodules.core.interfaces.SingletonModule;
+
+import androidx.annotation.RequiresApi;
+import expo.modules.notifications.R;
+import expo.modules.notifications.notifications.interfaces.NotificationChannelsManager;
+
+public class ExpoNotificationChannelsManager implements SingletonModule, NotificationChannelsManager {
+  private final static String SINGLETON_NAME = "NotificationChannelsManager";
+
+  private final static String FALLBACK_CHANNEL_ID = "expo_notifications_fallback_notification_channel";
+
+  @RequiresApi(api = Build.VERSION_CODES.N)
+  private final static int FALLBACK_CHANNEL_IMPORTANCE = NotificationManager.IMPORTANCE_HIGH;
+
+  private Context mContext;
+
+  public ExpoNotificationChannelsManager(Context context) {
+    mContext = context;
+  }
+
+  @Override
+  public String getName() {
+    return SINGLETON_NAME;
+  }
+
+  @Override
+  public NotificationChannel getFallbackNotificationChannel() {
+    if (Build.VERSION.SDK_INT < Build.VERSION_CODES.O) {
+      return null;
+    }
+
+    NotificationChannel channel = getNotificationManager().getNotificationChannel(FALLBACK_CHANNEL_ID);
+    if (channel != null) {
+      return channel;
+    }
+
+    return createFallbackChannel();
+  }
+
+  @RequiresApi(api = Build.VERSION_CODES.O)
+  protected NotificationChannel createFallbackChannel() {
+    NotificationChannel channel = new NotificationChannel(FALLBACK_CHANNEL_ID, getFallbackChannelName(), FALLBACK_CHANNEL_IMPORTANCE);
+    channel.setShowBadge(true);
+    channel.enableVibration(true);
+    getNotificationManager().createNotificationChannel(channel);
+    return channel;
+  }
+
+  protected String getFallbackChannelName() {
+    return mContext.getString(R.string.expo_notifications_fallback_channel_name);
+  }
+
+  private NotificationManager getNotificationManager() {
+    return (NotificationManager) mContext.getSystemService(Context.NOTIFICATION_SERVICE);
+  }
+}

packages/expo-notifications/android/src/main/java/expo/modules/notifications/notifications/interfaces/NotificationChannelsManager.java

@@ -0,0 +1,7 @@
+package expo.modules.notifications.notifications.interfaces;
+
+import android.app.NotificationChannel;
+
+public interface NotificationChannelsManager {
+  NotificationChannel getFallbackNotificationChannel();
+}

packages/expo-notifications/android/src/main/res/values/strings.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+  <string name="expo_notifications_fallback_channel_name">Miscellaneous</string>
+</resources>