Implement LambdaConsumerIntrospection (#5590)

* Implement HasDefaultErrorConsumer

Followup from #5569, and allows you to introspect if the resulting observer has missing error consumption and subsequently supplies a default (throwing) one.

* Add `@since`

* Add tests

* Add support in relevant completable observers

* Add support in ConsumerSingleObserver

* Add support in MaybeCallbackObserverTest

* Add support in LambdaSubscriber

* Switch to CompositeObserver and onErrorImplemented()

* Update wording to use Introspection

* Update tests and flip implementation logic to match naming

Author: ZacSweers

src/main/java/io/reactivex/internal/observers/CallbackCompletableObserver.java

@@ -20,10 +20,12 @@
 import io.reactivex.exceptions.*;
 import io.reactivex.functions.*;
 import io.reactivex.internal.disposables.DisposableHelper;
+import io.reactivex.observers.LambdaConsumerIntrospection;
 import io.reactivex.plugins.RxJavaPlugins;
 
 public final class CallbackCompletableObserver
-extends AtomicReference<Disposable> implements CompletableObserver, Disposable, Consumer<Throwable> {
+extends AtomicReference<Disposable>
+        implements CompletableObserver, Disposable, Consumer<Throwable>, LambdaConsumerIntrospection {
 
 
     private static final long serialVersionUID = -4361286194466301354L;
@@ -82,4 +84,9 @@ public void dispose() {
     public boolean isDisposed() {
         return get() == DisposableHelper.DISPOSED;
     }
+
+    @Override
+    public boolean hasCustomOnError() {
+        return onError != this;
+    }
 }

src/main/java/io/reactivex/internal/observers/ConsumerSingleObserver.java

@@ -20,11 +20,13 @@
 import io.reactivex.exceptions.*;
 import io.reactivex.functions.Consumer;
 import io.reactivex.internal.disposables.*;
+import io.reactivex.internal.functions.Functions;
+import io.reactivex.observers.LambdaConsumerIntrospection;
 import io.reactivex.plugins.RxJavaPlugins;
 
 public final class ConsumerSingleObserver<T>
 extends AtomicReference<Disposable>
-implements SingleObserver<T>, Disposable {
+implements SingleObserver<T>, Disposable, LambdaConsumerIntrospection {
 
 
     private static final long serialVersionUID = -7012088219455310787L;
@@ -74,4 +76,9 @@ public void dispose() {
     public boolean isDisposed() {
         return get() == DisposableHelper.DISPOSED;
     }
+
+    @Override
+    public boolean hasCustomOnError() {
+        return onError != Functions.ON_ERROR_MISSING;
+    }
 }

src/main/java/io/reactivex/internal/observers/EmptyCompletableObserver.java

@@ -19,11 +19,12 @@
 import io.reactivex.disposables.Disposable;
 import io.reactivex.exceptions.OnErrorNotImplementedException;
 import io.reactivex.internal.disposables.DisposableHelper;
+import io.reactivex.observers.LambdaConsumerIntrospection;
 import io.reactivex.plugins.RxJavaPlugins;
 
 public final class EmptyCompletableObserver
 extends AtomicReference<Disposable>
-implements CompletableObserver, Disposable {
+implements CompletableObserver, Disposable, LambdaConsumerIntrospection {
 
 
     private static final long serialVersionUID = -7545121636549663526L;
@@ -55,4 +56,8 @@ public void onSubscribe(Disposable d) {
         DisposableHelper.setOnce(this, d);
     }
 
+    @Override
+    public boolean hasCustomOnError() {
+        return false;
+    }
 }

src/main/java/io/reactivex/internal/observers/LambdaObserver.java

@@ -20,9 +20,12 @@
 import io.reactivex.exceptions.*;
 import io.reactivex.functions.*;
 import io.reactivex.internal.disposables.DisposableHelper;
+import io.reactivex.internal.functions.Functions;
+import io.reactivex.observers.LambdaConsumerIntrospection;
 import io.reactivex.plugins.RxJavaPlugins;
 
-public final class LambdaObserver<T> extends AtomicReference<Disposable> implements Observer<T>, Disposable {
+public final class LambdaObserver<T> extends AtomicReference<Disposable>
+        implements Observer<T>, Disposable, LambdaConsumerIntrospection {
 
     private static final long serialVersionUID = -7251123623727029452L;
     final Consumer<? super T> onNext;
@@ -101,4 +104,9 @@ public void dispose() {
     public boolean isDisposed() {
         return get() == DisposableHelper.DISPOSED;
     }
+
+    @Override
+    public boolean hasCustomOnError() {
+        return onError != Functions.ON_ERROR_MISSING;
+    }
 }

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

@@ -20,6 +20,8 @@
 import io.reactivex.exceptions.*;
 import io.reactivex.functions.*;
 import io.reactivex.internal.disposables.DisposableHelper;
+import io.reactivex.internal.functions.Functions;
+import io.reactivex.observers.LambdaConsumerIntrospection;
 import io.reactivex.plugins.RxJavaPlugins;
 
 /**
@@ -29,7 +31,7 @@
  */
 public final class MaybeCallbackObserver<T>
 extends AtomicReference<Disposable>
-implements MaybeObserver<T>, Disposable {
+implements MaybeObserver<T>, Disposable, LambdaConsumerIntrospection {
 
 
     private static final long serialVersionUID = -6076952298809384986L;
@@ -96,5 +98,8 @@ public void onComplete() {
         }
     }
 
-
+    @Override
+    public boolean hasCustomOnError() {
+        return onError != Functions.ON_ERROR_MISSING;
+    }
 }

src/main/java/io/reactivex/internal/subscribers/LambdaSubscriber.java

@@ -15,6 +15,8 @@
 
 import java.util.concurrent.atomic.AtomicReference;
 
+import io.reactivex.internal.functions.Functions;
+import io.reactivex.observers.LambdaConsumerIntrospection;
 import org.reactivestreams.Subscription;
 
 import io.reactivex.FlowableSubscriber;
@@ -24,7 +26,8 @@
 import io.reactivex.internal.subscriptions.SubscriptionHelper;
 import io.reactivex.plugins.RxJavaPlugins;
 
-public final class LambdaSubscriber<T> extends AtomicReference<Subscription> implements FlowableSubscriber<T>, Subscription, Disposable {
+public final class LambdaSubscriber<T> extends AtomicReference<Subscription>
+        implements FlowableSubscriber<T>, Subscription, Disposable, LambdaConsumerIntrospection {
 
     private static final long serialVersionUID = -7251123623727029452L;
     final Consumer<? super T> onNext;
@@ -115,4 +118,9 @@ public void request(long n) {
     public void cancel() {
         SubscriptionHelper.cancel(this);
     }
+
+    @Override
+    public boolean hasCustomOnError() {
+        return onError != Functions.ON_ERROR_MISSING;
+    }
 }

src/main/java/io/reactivex/observers/LambdaConsumerIntrospection.java

@@ -0,0 +1,23 @@
+package io.reactivex.observers;
+
+import io.reactivex.annotations.Experimental;
+
+/**
+ * An interface that indicates that the implementing type is composed of individual components and exposes information
+ * about their behavior.
+ *
+ * <p><em>NOTE:</em> This is considered a read-only public API and is not intended to be implemented externally.
+ *
+ * @since 2.1.4 - experimental
+ */
+@Experimental
+public interface LambdaConsumerIntrospection {
+
+    /**
+     * @return {@code true} if a custom onError consumer implementation was supplied. Returns {@code false} if the
+     * implementation is missing an error consumer and thus using a throwing default implementation.
+     */
+    @Experimental
+    boolean hasCustomOnError();
+
+}

src/test/java/io/reactivex/internal/observers/CallbackCompletableObserverTest.java

@@ -0,0 +1,25 @@
+package io.reactivex.internal.observers;
+
+import io.reactivex.internal.functions.Functions;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public final class CallbackCompletableObserverTest {
+
+    @Test
+    public void emptyActionShouldReportNoCustomOnError() {
+        CallbackCompletableObserver o = new CallbackCompletableObserver(Functions.EMPTY_ACTION);
+
+        assertFalse(o.hasCustomOnError());
+    }
+
+    @Test
+    public void customOnErrorShouldReportCustomOnError() {
+        CallbackCompletableObserver o = new CallbackCompletableObserver(Functions.<Throwable>emptyConsumer(),
+                Functions.EMPTY_ACTION);
+
+        assertTrue(o.hasCustomOnError());
+    }
+
+}

src/test/java/io/reactivex/internal/observers/ConsumerSingleObserverTest.java

@@ -0,0 +1,26 @@
+package io.reactivex.internal.observers;
+
+import io.reactivex.internal.functions.Functions;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public final class ConsumerSingleObserverTest {
+
+    @Test
+    public void onErrorMissingShouldReportNoCustomOnError() {
+        ConsumerSingleObserver<Integer> o = new ConsumerSingleObserver<Integer>(Functions.<Integer>emptyConsumer(),
+                Functions.ON_ERROR_MISSING);
+
+        assertFalse(o.hasCustomOnError());
+    }
+
+    @Test
+    public void customOnErrorShouldReportCustomOnError() {
+        ConsumerSingleObserver<Integer> o = new ConsumerSingleObserver<Integer>(Functions.<Integer>emptyConsumer(),
+                Functions.<Throwable>emptyConsumer());
+
+        assertTrue(o.hasCustomOnError());
+    }
+
+}

src/test/java/io/reactivex/internal/observers/EmptyCompletableObserverTest.java

@@ -0,0 +1,15 @@
+package io.reactivex.internal.observers;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertFalse;
+
+public final class EmptyCompletableObserverTest {
+
+    @Test
+    public void defaultShouldReportNoCustomOnError() {
+        EmptyCompletableObserver o = new EmptyCompletableObserver();
+
+        assertFalse(o.hasCustomOnError());
+    }
+}