Merge branch '88-test-listener-removes-itself' into 'dev'

QueryObserverTest: test observer can be removed during callback.

Closes #88

See merge request objectbox/objectbox-java!75

Author: greenrobot-team

tests/objectbox-java-test/src/test/java/io/objectbox/query/QueryObserverTest.java

@@ -29,10 +29,12 @@
 import io.objectbox.Box;
 import io.objectbox.TestEntity;
 import io.objectbox.reactive.DataObserver;
+import io.objectbox.reactive.DataSubscription;
 
 
 import static io.objectbox.TestEntity_.simpleInt;
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
 public class QueryObserverTest extends AbstractObjectBoxTest {
@@ -44,6 +46,44 @@ public void setUpBox() {
         box = getTestEntityBox();
     }
 
+    @Test
+    public void observer_removeDuringCallback_works() throws InterruptedException {
+        SelfRemovingObserver testObserver = new SelfRemovingObserver();
+        // Note: use onlyChanges to not trigger observer on subscribing.
+        testObserver.dataSubscription = box.query().build()
+                .subscribe()
+                .onlyChanges()
+                .observer(testObserver);
+
+        // Trigger event.
+        putTestEntitiesScalars();
+
+        // Should have gotten past dataSubscription.cancel() without crashing.
+        assertTrue(testObserver.latch.await(5, TimeUnit.SECONDS));
+
+        // Just to make sure: trigger another event, should not be received.
+        testObserver.latch = new CountDownLatch(1);
+        putTestEntitiesScalars();
+        assertFalse(testObserver.latch.await(5, TimeUnit.SECONDS));
+    }
+
+    private static class SelfRemovingObserver implements DataObserver<List<TestEntity>> {
+
+        CountDownLatch latch = new CountDownLatch(1);
+        DataSubscription dataSubscription;
+
+        @Override
+        public void onData(List<TestEntity> data) {
+            if (dataSubscription != null) {
+                System.out.println("Cancelling subscription");
+                dataSubscription.cancel();
+                dataSubscription = null;
+            }
+            // Once here, cancel did not crash.
+            latch.countDown();
+        }
+    }
+
     @Test
     public void testObserver() {
         int[] valuesInt = {2003, 2007, 2002};