2.x: add TestSubscriber.withTag

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

@@ -48,6 +48,8 @@ public abstract class BaseTestConsumer<T, U extends BaseTestConsumer<T, U>> impl
 
     protected int establishedFusionMode;
 
+    protected CharSequence tag;
+
     public BaseTestConsumer() {
         this.values = new ArrayList<T>();
         this.errors = new ArrayList<Throwable>();
@@ -129,6 +131,15 @@ protected final AssertionError fail(String message) {
         .append("values = ").append(values.size()).append(", ")
         .append("errors = ").append(errors.size()).append(", ")
         .append("completions = ").append(completions)
+        ;
+
+        CharSequence tag = this.tag;
+        if (tag != null) {
+            b.append(", tag = ")
+            .append(tag);
+        }
+
+        b
         .append(')')
         ;
 
@@ -747,4 +758,18 @@ public final U assertEmpty() {
                 .assertNoErrors()
                 .assertNotComplete();
     }
+
+    /**
+     * Set the tag displayed along with an assertion failure's
+     * other state information.
+     * @param tag the string to display (null won't print any tag)
+     * @return this
+     * @since 2.0.7 - experimental
+     */
+    @SuppressWarnings("unchecked")
+    @Experimental
+    public final U withTag(CharSequence tag) {
+        this.tag = tag;
+        return (U)this;
+    }
 }

src/test/java/io/reactivex/observers/TestObserverTest.java

@@ -1381,4 +1381,20 @@ public void assertValueAtInvalidIndex() {
             }
         });
     }
+
+    @Test
+    public void withTag() {
+        try {
+            for (int i = 1; i < 3; i++) {
+                Observable.just(i)
+                .test()
+                .withTag("testing with item=" + i)
+                .assertResult(1)
+                ;
+            }
+            fail("Should have thrown!");
+        } catch (AssertionError ex) {
+            assertTrue(ex.toString(), ex.toString().contains("testing with item=2"));
+        }
+    }
 }

src/test/java/io/reactivex/subscribers/TestSubscriberTest.java

@@ -1777,4 +1777,20 @@ public void requestMore() {
         .requestMore(3)
         .assertResult(1, 2, 3, 4, 5);
     }
+
+    @Test
+    public void withTag() {
+        try {
+            for (int i = 1; i < 3; i++) {
+                Flowable.just(i)
+                .test()
+                .withTag("testing with item=" + i)
+                .assertResult(1)
+                ;
+            }
+            fail("Should have thrown!");
+        } catch (AssertionError ex) {
+            assertTrue(ex.toString(), ex.toString().contains("testing with item=2"));
+        }
+    }
 }