Doc tweak for Flow.Listener#go, unit test for that contract.

flow-sample/src/main/java/com/example/flow/MainActivity.java

@@ -108,8 +108,8 @@ protected void onCreate(Bundle savedInstanceState) {
     }
   }
 
-  @Override public void go(Backstack backstack, Flow.Direction direction) {
-    Object screen = backstack.current().getScreen();
+  @Override public void go(Backstack nextBackstack, Flow.Direction direction) {
+    Object screen = nextBackstack.current().getScreen();
     containerView.displayView(getView(screen), direction);
 
     setTitle(screen.getClass().getSimpleName());
@@ -155,7 +155,7 @@ class ActivityModule {
     }
 
     @Provides @Singleton Parcer<Object> provideParcer(Gson gson) {
-      return new GsonParcer<Object>(gson);
+      return new GsonParcer<>(gson);
     }
   }
 }

flow/src/main/java/flow/Flow.java

@@ -25,7 +25,12 @@ public enum Direction {
   }
 
   public interface Listener {
-    void go(Backstack backstack, Direction direction);
+    /**
+     * Notifies the listener that the backstack is about to change. Note that the change
+     * will not take effect until this method returns. That is, {@code nextBackstack} is where
+     * the Flow is going next, and {@link Flow#getBackstack()} is where it's coming from.
+     */
+    void go(Backstack nextBackstack, Direction direction);
   }
 
   private final Listener listener;

flow/src/test/java/flow/FlowTest.java

@@ -40,8 +40,8 @@ static class Tres implements HasParent<Dos> {
   Flow.Direction lastDirection;
 
   class FlowListener implements Flow.Listener {
-    @Override public void go(Backstack backstack, Flow.Direction direction) {
-      lastStack = backstack;
+    @Override public void go(Backstack nextBackstack, Flow.Direction direction) {
+      lastStack = nextBackstack;
       lastDirection = direction;
     }
   }
@@ -69,6 +69,21 @@ class FlowListener implements Flow.Listener {
     assertThat(flow.goBack()).isFalse();
   }
 
+  @Test public void backstackChangesAfterListenerCall() {
+    final Backstack firstBackstack = Backstack.single(new Uno());
+
+    class Ourrobouros implements Flow.Listener {
+      Flow flow = new Flow(firstBackstack ,this);
+
+      @Override public void go(Backstack nextBackstack, Flow.Direction direction) {
+        assertThat(firstBackstack).isSameAs(flow.getBackstack());
+      }
+    }
+
+    Ourrobouros listener = new Ourrobouros();
+    listener.flow.goTo(new Dos());
+  }
+
   @Test public void noUpNoUps() {
     Backstack backstack = Backstack.single(new Uno());
     Flow flow = new Flow(backstack, new FlowListener());
@@ -350,12 +365,6 @@ public int hashCode() {
     }
   }
 
-  private static abstract class ChildScreen extends Screen implements HasParent<Screen> {
-    ChildScreen(String name) {
-      super(name);
-    }
-  }
-
   static class Able extends Screen {
     Able() {
       super("Able");