2.x: add tryOnError to create/XEmitter API

src/main/java/io/reactivex/CompletableEmitter.java

@@ -57,4 +57,19 @@ public interface CompletableEmitter {
      * @return true if the downstream disposed the sequence
      */
     boolean isDisposed();
+
+    /**
+     * Attempts to emit the specified {@code Throwable} error if the downstream
+     * hasn't cancelled the sequence or is otherwise terminated, returning false
+     * if the emission is not allowed to happen due to lifecycle restrictions.
+     * <p>
+     * Unlike {@link #onError(Throwable)}, the {@code RxJavaPlugins.onError} is not called
+     * if the error could not be delivered.
+     * @param t the throwable error to signal if possible
+     * @return true if successful, false if the downstream is not able to accept further
+     * events
+     * @since 2.1.1 - experimental
+     */
+    @Experimental
+    boolean tryOnError(@NonNull Throwable t);
 }

src/main/java/io/reactivex/FlowableEmitter.java

@@ -65,4 +65,19 @@ public interface FlowableEmitter<T> extends Emitter<T> {
      */
     @NonNull
     FlowableEmitter<T> serialize();
+
+    /**
+     * Attempts to emit the specified {@code Throwable} error if the downstream
+     * hasn't cancelled the sequence or is otherwise terminated, returning false
+     * if the emission is not allowed to happen due to lifecycle restrictions.
+     * <p>
+     * Unlike {@link #onError(Throwable)}, the {@code RxJavaPlugins.onError} is not called
+     * if the error could not be delivered.
+     * @param t the throwable error to signal if possible
+     * @return true if successful, false if the downstream is not able to accept further
+     * events
+     * @since 2.1.1 - experimental
+     */
+    @Experimental
+    boolean tryOnError(@NonNull Throwable t);
 }

src/main/java/io/reactivex/MaybeEmitter.java

@@ -65,4 +65,19 @@ public interface MaybeEmitter<T> {
      * @return true if the downstream cancelled the sequence
      */
     boolean isDisposed();
+
+    /**
+     * Attempts to emit the specified {@code Throwable} error if the downstream
+     * hasn't cancelled the sequence or is otherwise terminated, returning false
+     * if the emission is not allowed to happen due to lifecycle restrictions.
+     * <p>
+     * Unlike {@link #onError(Throwable)}, the {@code RxJavaPlugins.onError} is not called
+     * if the error could not be delivered.
+     * @param t the throwable error to signal if possible
+     * @return true if successful, false if the downstream is not able to accept further
+     * events
+     * @since 2.1.1 - experimental
+     */
+    @Experimental
+    boolean tryOnError(@NonNull Throwable t);
 }

src/main/java/io/reactivex/ObservableEmitter.java

@@ -56,4 +56,19 @@ public interface ObservableEmitter<T> extends Emitter<T> {
      */
     @NonNull
     ObservableEmitter<T> serialize();
+
+    /**
+     * Attempts to emit the specified {@code Throwable} error if the downstream
+     * hasn't cancelled the sequence or is otherwise terminated, returning false
+     * if the emission is not allowed to happen due to lifecycle restrictions.
+     * <p>
+     * Unlike {@link #onError(Throwable)}, the {@code RxJavaPlugins.onError} is not called
+     * if the error could not be delivered.
+     * @param t the throwable error to signal if possible
+     * @return true if successful, false if the downstream is not able to accept further
+     * events
+     * @since 2.1.1 - experimental
+     */
+    @Experimental
+    boolean tryOnError(@NonNull Throwable t);
 }

src/main/java/io/reactivex/SingleEmitter.java

@@ -60,4 +60,19 @@ public interface SingleEmitter<T> {
      * @return true if the downstream cancelled the sequence
      */
     boolean isDisposed();
+
+    /**
+     * Attempts to emit the specified {@code Throwable} error if the downstream
+     * hasn't cancelled the sequence or is otherwise terminated, returning false
+     * if the emission is not allowed to happen due to lifecycle restrictions.
+     * <p>
+     * Unlike {@link #onError(Throwable)}, the {@code RxJavaPlugins.onError} is not called
+     * if the error could not be delivered.
+     * @param t the throwable error to signal if possible
+     * @return true if successful, false if the downstream is not able to accept further
+     * events
+     * @since 2.1.1 - experimental
+     */
+    @Experimental
+    boolean tryOnError(@NonNull Throwable t);
 }

src/main/java/io/reactivex/internal/operators/completable/CompletableCreate.java

@@ -73,6 +73,13 @@ public void onComplete() {
 
         @Override
         public void onError(Throwable t) {
+            if (!tryOnError(t)) {
+                RxJavaPlugins.onError(t);
+            }
+        }
+
+        @Override
+        public boolean tryOnError(Throwable t) {
             if (t == null) {
                 t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
             }
@@ -86,10 +93,10 @@ public void onError(Throwable t) {
                             d.dispose();
                         }
                     }
-                    return;
+                    return true;
                 }
             }
-            RxJavaPlugins.onError(t);
+            return false;
         }
 
         @Override

src/main/java/io/reactivex/internal/operators/flowable/FlowableCreate.java

@@ -129,21 +129,27 @@ public void onNext(T t) {
 
         @Override
         public void onError(Throwable t) {
-            if (emitter.isCancelled() || done) {
-                RxJavaPlugins.onError(t);
-                return;
-            }
-            if (t == null) {
-                t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
-            }
-            if (error.addThrowable(t)) {
-                done = true;
-                drain();
-            } else {
+            if (!tryOnError(t)) {
                 RxJavaPlugins.onError(t);
             }
         }
 
+        @Override
+        public boolean tryOnError(Throwable t) {
+           if (emitter.isCancelled() || done) {
+               return false;
+           }
+           if (t == null) {
+               t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
+           }
+           if (error.addThrowable(t)) {
+               done = true;
+               drain();
+               return true;
+           }
+           return false;
+        }
+
         @Override
         public void onComplete() {
             if (emitter.isCancelled() || done) {
@@ -245,6 +251,10 @@ abstract static class BaseEmitter<T>
 
         @Override
         public void onComplete() {
+            complete();
+        }
+
+        protected void complete() {
             if (isCancelled()) {
                 return;
             }
@@ -256,19 +266,30 @@ public void onComplete() {
         }
 
         @Override
-        public void onError(Throwable e) {
+        public final void onError(Throwable e) {
+            if (!tryOnError(e)) {
+                RxJavaPlugins.onError(e);
+            }
+        }
+
+        @Override
+        public boolean tryOnError(Throwable e) {
+            return error(e);
+        }
+
+        protected boolean error(Throwable e) {
             if (e == null) {
                 e = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
             }
             if (isCancelled()) {
-                RxJavaPlugins.onError(e);
-                return;
+                return false;
             }
             try {
                 actual.onError(e);
             } finally {
                 serial.dispose();
             }
+            return true;
         }
 
         @Override
@@ -446,10 +467,9 @@ public void onNext(T t) {
         }
 
         @Override
-        public void onError(Throwable e) {
+        public boolean tryOnError(Throwable e) {
             if (done || isCancelled()) {
-                RxJavaPlugins.onError(e);
-                return;
+                return false;
             }
 
             if (e == null) {
@@ -459,6 +479,7 @@ public void onError(Throwable e) {
             error = e;
             done = true;
             drain();
+            return true;
         }
 
         @Override
@@ -507,9 +528,9 @@ void drain() {
                     if (d && empty) {
                         Throwable ex = error;
                         if (ex != null) {
-                            super.onError(ex);
+                            error(ex);
                         } else {
-                            super.onComplete();
+                            complete();
                         }
                         return;
                     }
@@ -536,9 +557,9 @@ void drain() {
                     if (d && empty) {
                         Throwable ex = error;
                         if (ex != null) {
-                            super.onError(ex);
+                            error(ex);
                         } else {
-                            super.onComplete();
+                            complete();
                         }
                         return;
                     }
@@ -589,17 +610,17 @@ public void onNext(T t) {
         }
 
         @Override
-        public void onError(Throwable e) {
+        public boolean tryOnError(Throwable e) {
             if (done || isCancelled()) {
-                RxJavaPlugins.onError(e);
-                return;
+                return false;
             }
             if (e == null) {
                 onError(new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources."));
             }
             error = e;
             done = true;
             drain();
+            return true;
         }
 
         @Override
@@ -648,9 +669,9 @@ void drain() {
                     if (d && empty) {
                         Throwable ex = error;
                         if (ex != null) {
-                            super.onError(ex);
+                            error(ex);
                         } else {
-                            super.onComplete();
+                            complete();
                         }
                         return;
                     }
@@ -677,9 +698,9 @@ void drain() {
                     if (d && empty) {
                         Throwable ex = error;
                         if (ex != null) {
-                            super.onError(ex);
+                            error(ex);
                         } else {
-                            super.onComplete();
+                            complete();
                         }
                         return;
                     }

src/main/java/io/reactivex/internal/operators/maybe/MaybeCreate.java

@@ -84,6 +84,13 @@ public void onSuccess(T value) {
 
         @Override
         public void onError(Throwable t) {
+            if (!tryOnError(t)) {
+                RxJavaPlugins.onError(t);
+            }
+        }
+
+        @Override
+        public boolean tryOnError(Throwable t) {
             if (t == null) {
                 t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
             }
@@ -97,10 +104,10 @@ public void onError(Throwable t) {
                             d.dispose();
                         }
                     }
-                    return;
+                    return true;
                 }
             }
-            RxJavaPlugins.onError(t);
+            return false;
         }
 
         @Override

src/main/java/io/reactivex/internal/operators/observable/ObservableCreate.java

@@ -70,6 +70,13 @@ public void onNext(T t) {
 
         @Override
         public void onError(Throwable t) {
+            if (!tryOnError(t)) {
+                RxJavaPlugins.onError(t);
+            }
+        }
+
+        @Override
+        public boolean tryOnError(Throwable t) {
             if (t == null) {
                 t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
             }
@@ -79,9 +86,9 @@ public void onError(Throwable t) {
                 } finally {
                     dispose();
                 }
-            } else {
-                RxJavaPlugins.onError(t);
+                return true;
             }
+            return false;
         }
 
         @Override
@@ -174,19 +181,25 @@ public void onNext(T t) {
 
         @Override
         public void onError(Throwable t) {
-            if (emitter.isDisposed() || done) {
+            if (!tryOnError(t)) {
                 RxJavaPlugins.onError(t);
-                return;
+            }
+        }
+
+        @Override
+        public boolean tryOnError(Throwable t) {
+            if (emitter.isDisposed() || done) {
+                return false;
             }
             if (t == null) {
                 t = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
             }
             if (error.addThrowable(t)) {
                 done = true;
                 drain();
-            } else {
-                RxJavaPlugins.onError(t);
+                return true;
             }
+            return false;
         }
 
         @Override