push and pop mechanics

lib/android/app/src/main/java/com/reactnativenavigation/viewcontrollers/StackController.java

 	}
 
 	public void push(final ViewController child) {
+		ViewController previousTop = peek();
 		child.setParentStackController(this);
 		childControllers.push(child);
 		getView().addView(child.getView());
+		if (previousTop != null) {
+			getView().removeView(previousTop.getView());
+		}
 	}
 
 	public boolean canPop() {
 
 	public void pop() {
 		if (canPop()) {
-			childControllers.pop();
+			ViewController poppedController = childControllers.pop();
+			getView().removeView(poppedController.getView());
+			ViewController previousTop = peek();
+			getView().addView(previousTop.getView());
 		}
 	}
 

lib/android/app/src/test/java/com/reactnativenavigation/viewcontrollers/StackControllerTest.java

 		uut.push(child1);
 		assertThat(uut.getView().getChildCount()).isEqualTo(1);
 	}
+
+	@Test
+	public void pushRemovesPreviousFromTree() throws Exception {
+		assertThat(uut.getView().getChildCount()).isZero();
+		uut.push(child1);
+		assertThat(uut.getView().getChildCount()).isEqualTo(1);
+		uut.push(child2);
+		assertThat(uut.getView().getChildCount()).isEqualTo(1);
+	}
+
+	@Test
+	public void popReplacesViewWithPrevious() throws Exception {
+		uut.push(child1);
+		uut.push(child2);
+		assertThat(uut.getView().getChildCount()).isEqualTo(1);
+		assertThat(uut.getView().getChildAt(0)).isEqualTo(child2.getView());
+		uut.pop();
+		assertThat(uut.getView().getChildCount()).isEqualTo(1);
+		assertThat(uut.getView().getChildAt(0)).isEqualTo(child1.getView());
+	}
 }