issue-2764: add new operator onBackpressureDrop(Action1 onDrop)

Author: Steven Wu

src/main/java/rx/Observable.java

@@ -5258,6 +5258,27 @@ public final Observable<T> onBackpressureBuffer(long capacity, Action0 onOverflo
         return lift(new OperatorOnBackpressureBuffer<T>(capacity, onOverflow));
     }
 
+    /**
+     * Instructs an Observable that is emitting items faster than its observer can consume them to discard,
+     * rather than emit, those items that its observer is not prepared to observe.
+     * <p>
+     * <img width="640" height="245" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/bp.obp.drop.png" alt="">
+     * <p>
+     * If the downstream request count hits 0 then the Observable will refrain from calling {@code onNext} until
+     * the observer invokes {@code request(n)} again to increase the request count.
+     * <dl>
+     *  <dt><b>Scheduler:</b></dt>
+     *  <dd>{@code onBackpressureDrop} does not operate by default on a particular {@link Scheduler}.</dd>
+     * </dl>
+     *
+     * @param onDrop the action to invoke for each item dropped. onDrop action should be fast and should never block.
+     * @return the source Observable modified to drop {@code onNext} notifications on overflow
+     * @see <a href="http://reactivex.io/documentation/operators/backpressure.html">ReactiveX operators documentation: backpressure operators</a>
+     */
+    public final Observable<T> onBackpressureDrop(Action1 onDrop) {
+        return lift(new OperatorOnBackpressureDrop<T>(onDrop));
+    }
+
     /**
      * Instructs an Observable that is emitting items faster than its observer can consume them to discard,
      * rather than emit, those items that its observer is not prepared to observe.

src/main/java/rx/internal/operators/OperatorOnBackpressureDrop.java

@@ -20,21 +20,34 @@
 import rx.Observable.Operator;
 import rx.Producer;
 import rx.Subscriber;
+import rx.functions.Action1;
 
 public class OperatorOnBackpressureDrop<T> implements Operator<T, T> {
+
     /** Lazy initialization via inner-class holder. */
     private static final class Holder {
         /** A singleton instance. */
         static final OperatorOnBackpressureDrop<Object> INSTANCE = new OperatorOnBackpressureDrop<Object>();
     }
+
     /**
      * @return a singleton instance of this stateless operator.
      */
     @SuppressWarnings({ "unchecked" })
     public static <T> OperatorOnBackpressureDrop<T> instance() {
         return (OperatorOnBackpressureDrop<T>)Holder.INSTANCE;
     }
-    private OperatorOnBackpressureDrop() { }
+
+    private final Action1<T> onDrop;
+
+    private OperatorOnBackpressureDrop() {
+        this(null);
+    }
+
+    public OperatorOnBackpressureDrop(Action1<T> onDrop) {
+        this.onDrop = onDrop;
+    }
+
     @Override
     public Subscriber<? super T> call(final Subscriber<? super T> child) {
         final AtomicLong requested = new AtomicLong();
@@ -68,6 +81,11 @@ public void onNext(T t) {
                 if (requested.get() > 0) {
                     child.onNext(t);
                     requested.decrementAndGet();
+                } else {
+                    // item dropped
+                    if(onDrop != null) {
+                        onDrop.call(t);
+                    }
                 }
             }
 

src/test/java/rx/BackpressureTests.java

@@ -23,6 +23,7 @@
 
 import org.junit.*;
 
+import org.junit.rules.TestName;
 import rx.Observable.OnSubscribe;
 import rx.exceptions.MissingBackpressureException;
 import rx.functions.*;
@@ -33,6 +34,9 @@
 
 public class BackpressureTests {
 
+    @Rule
+    public TestName testName = new TestName();
+
     @After
     public void doAfterTest() {
         TestObstructionDetection.checkObstruction();
@@ -424,25 +428,69 @@ public void testOnBackpressureDrop() {
             .map(SLOW_PASS_THRU).take(NUM).subscribe(ts);
             ts.awaitTerminalEvent();
             ts.assertNoErrors();
-            
-            
+
             List<Integer> onNextEvents = ts.getOnNextEvents();
             assertEquals(NUM, onNextEvents.size());
 
             Integer lastEvent = onNextEvents.get(NUM - 1);
-            
+
             System.out.println("testOnBackpressureDrop => Received: " + onNextEvents.size() + "  Emitted: " + c.get() + " Last value: " + lastEvent);
             // it drop, so we should get some number far higher than what would have sequentially incremented
             assertTrue(NUM - 1 <= lastEvent.intValue());
         }
     }
+
+    @Test(timeout = 10000)
+    public void testOnBackpressureDropWithCallback() {
+        for (int i = 0; i < 100; i++) {
+            final AtomicInteger emitCount = new AtomicInteger();
+            final AtomicInteger dropCount = new AtomicInteger();
+            final AtomicInteger passCount = new AtomicInteger();
+            final int NUM = (int) (RxRingBuffer.SIZE * 1.5); // > 1 so that take doesn't prevent buffer overflow
+            TestSubscriber<Integer> ts = new TestSubscriber<Integer>();
+            firehose(emitCount).onBackpressureDrop(new Action1<Integer>() {
+                @Override
+                public void call(Integer i) {
+                    dropCount.incrementAndGet();
+                }
+            })
+            .doOnNext(new Action1<Integer>() {
+                @Override
+                public void call(Integer integer) {
+                    passCount.incrementAndGet();
+                }
+            })
+            .observeOn(Schedulers.computation())
+            .map(SLOW_PASS_THRU).take(NUM).subscribe(ts);
+            ts.awaitTerminalEvent();
+            ts.assertNoErrors();
+            
+            List<Integer> onNextEvents = ts.getOnNextEvents();
+            Integer lastEvent = onNextEvents.get(NUM - 1);
+            System.out.println(testName.getMethodName() + " => Received: " + onNextEvents.size() + " Passed: " + passCount.get() + " Dropped: " + dropCount.get() + "  Emitted: " + emitCount.get() + " Last value: " + lastEvent);
+            assertEquals(NUM, onNextEvents.size());
+            // in reality, NUM < passCount
+            assertTrue(NUM <= passCount.get());
+            // it drop, so we should get some number far higher than what would have sequentially incremented
+            assertTrue(NUM - 1 <= lastEvent.intValue());
+            assertTrue(0 < dropCount.get());
+            assertEquals(emitCount.get(), passCount.get() + dropCount.get());
+        }
+    }
+
     @Test(timeout = 10000)
     public void testOnBackpressureDropSynchronous() {
         for (int i = 0; i < 100; i++) {
+            final AtomicInteger dropCount = new AtomicInteger();
             int NUM = (int) (RxRingBuffer.SIZE * 1.1); // > 1 so that take doesn't prevent buffer overflow
             AtomicInteger c = new AtomicInteger();
             TestSubscriber<Integer> ts = new TestSubscriber<Integer>();
-            firehose(c).onBackpressureDrop()
+            firehose(c).onBackpressureDrop(new Action1<Integer>() {
+                @Override
+                public void call(Integer i) {
+                    dropCount.incrementAndGet();
+                }
+            })
             .map(SLOW_PASS_THRU).take(NUM).subscribe(ts);
             ts.awaitTerminalEvent();
             ts.assertNoErrors();
@@ -452,9 +500,12 @@ public void testOnBackpressureDropSynchronous() {
 
             Integer lastEvent = onNextEvents.get(NUM - 1);
 
-            System.out.println("testOnBackpressureDrop => Received: " + onNextEvents.size() + "  Emitted: " + c.get() + " Last value: " + lastEvent);
+            System.out.println("testOnBackpressureDrop => Received: " + onNextEvents.size() + " Dropped: " + dropCount.get() + "  Emitted: " + c.get() + " Last value: " + lastEvent);
             // it drop, so we should get some number far higher than what would have sequentially incremented
             assertTrue(NUM - 1 <= lastEvent.intValue());
+            // no drop in synchronous mode
+            assertEquals(0, dropCount.get());
+            assertEquals(c.get(), onNextEvents.size());
         }
     }