Revert to Rx's Maybe for singleValueEvent as Rx's Single does not has onComplete.

Update build tools to 2.3.0.
Update of gradle plugin to 3.3.
Update firebase version to 11.2.2.
Added braces for one line if statements.

Author: gswierczynski

app/build.gradle

@@ -2,7 +2,7 @@ buildscript {
     ext {
         rx_version = "2.1.0"
         rx_android_version = "2.0.1"
-        firebase_version = "11.2.0"
+        firebase_version = "11.2.2"
         support_version = "26.0.1"
     }
 }

app/src/main/java/durdinapps/rxfirebase2/RxFirebaseDatabase.java

@@ -25,10 +25,12 @@
 import io.reactivex.FlowableEmitter;
 import io.reactivex.FlowableOnSubscribe;
 import io.reactivex.Maybe;
+import io.reactivex.MaybeEmitter;
+import io.reactivex.MaybeOnSubscribe;
+import io.reactivex.MaybeSource;
 import io.reactivex.Single;
 import io.reactivex.SingleEmitter;
 import io.reactivex.SingleOnSubscribe;
-import io.reactivex.SingleSource;
 import io.reactivex.functions.Cancellable;
 import io.reactivex.functions.Function;
 import io.reactivex.functions.Predicate;
@@ -58,8 +60,9 @@ public void subscribe(final FlowableEmitter<DataSnapshot> emitter) throws Except
             final ValueEventListener valueEventListener = new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
-                  if (dataSnapshot.exists())
+                  if (dataSnapshot.exists()) {
                      emitter.onNext(dataSnapshot);
+                  }
                }
 
                @Override
@@ -86,15 +89,16 @@ public void cancel() throws Exception {
     * the given {@link DataSnapshot} exists.
     */
    @NonNull
-   public static Single<DataSnapshot> observeSingleValueEvent(@NonNull final Query query) {
-      return Single.create(new SingleOnSubscribe<DataSnapshot>() {
+   public static Maybe<DataSnapshot> observeSingleValueEvent(@NonNull final Query query) {
+      return Maybe.create(new MaybeOnSubscribe<DataSnapshot>() {
          @Override
-         public void subscribe(final SingleEmitter<DataSnapshot> emitter) throws Exception {
+         public void subscribe(final MaybeEmitter<DataSnapshot> emitter) throws Exception {
             query.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
-                  if (dataSnapshot.exists())
+                  if (dataSnapshot.exists()) {
                      emitter.onSuccess(dataSnapshot);
+                  }
                   emitter.onComplete();
                }
 
@@ -267,10 +271,10 @@ public void cancel() throws Exception {
     */
    @NonNull
    public static Flowable<DataSnapshot> observeMultipleSingleValueEvent(@NonNull DatabaseReference... whereRefs) {
-      return Single.merge(Flowable.fromArray(whereRefs)
-      .map(new Function<DatabaseReference, SingleSource<? extends DataSnapshot>>() {
+      return Maybe.merge(Flowable.fromArray(whereRefs)
+      .map(new Function<DatabaseReference, MaybeSource<? extends DataSnapshot>>() {
             @Override
-            public SingleSource<? extends DataSnapshot> apply(@io.reactivex.annotations.NonNull DatabaseReference databaseReference) throws
+            public MaybeSource<? extends DataSnapshot> apply(@io.reactivex.annotations.NonNull DatabaseReference databaseReference) throws
                   Exception {
                return observeSingleValueEvent(databaseReference);
             }

build.gradle

@@ -5,7 +5,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.2.2'
+        classpath 'com.android.tools.build:gradle:2.3.0'
         classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
-#Mon Dec 28 10:00:20 PST 2015
+#Tue Sep 19 19:36:52 CEST 2017
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-2.14.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-3.3-all.zip