Child coordinators notify their parent upon -start and -stop.

ios/clean/chrome/browser/ui/tab/tab_coordinator.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // ======                        New Architecture                         =====
 // =         This code is only used in the new iOS Chrome architecture.       =
 // ============================================================================
 
 #import "ios/clean/chrome/browser/ui/tab/tab_coordinator.h"
 
@@ skipping 53 matching lines @@
 
   ToolbarCoordinator* toolbarCoordinator = [[ToolbarCoordinator alloc] init];
   toolbarCoordinator.webState = self.webState;
   [self addChildCoordinator:toolbarCoordinator];
 
   // Unset the base view controller, so |toolbarCoordinator| doesn't present
   // its view controller.
   toolbarCoordinator.context.baseViewController = nil;
   [toolbarCoordinator start];
 
-  self.viewController.toolbarViewController = toolbarCoordinator.viewController;
-  self.viewController.contentViewController = webCoordinator.viewController;
-
   // PLACEHOLDER: Replace this placeholder with an actual tab strip view
   // controller.
   UIButton* button = [UIButton buttonWithType:UIButtonTypeCustom];
   [button addTarget:nil
                 action:@selector(hideTabStrip:)
       forControlEvents:UIControlEventTouchUpInside];
   [button setTitle:@"Hide Strip" forState:UIControlStateNormal];
   button.frame = CGRectMake(10, 10, 100, 100);
 
   UIViewController* tabStripViewController = [[UIViewController alloc] init];
   tabStripViewController.view.backgroundColor = [UIColor blackColor];
   [tabStripViewController.view addSubview:button];
   self.viewController.tabStripViewController = tabStripViewController;
 
   [self.context.baseViewController presentViewController:self.viewController
                                                 animated:self.context.animated
                                               completion:nil];
+  [super start];
 }
 
 - (void)stop {
+  [super stop];
   [self.viewController.presentingViewController
       dismissViewControllerAnimated:self.context.animated
                          completion:nil];
   _webStateObserver.reset();
 }
 
+- (void)childCoordinatorDidStart:(BrowserCoordinator*)coordinator {
+  if ([coordinator isKindOfClass:[ToolbarCoordinator class]]) {
+    self.viewController.toolbarViewController = coordinator.viewController;
+  } else if ([coordinator isKindOfClass:[WebCoordinator class]]) {
+    self.viewController.contentViewController = coordinator.viewController;
+  }
+}
+
 - (BOOL)canAddOverlayCoordinator:(BrowserCoordinator*)overlayCoordinator {
   // This coordinator will always accept overlay coordinators.
   return YES;
 }
 
 #pragma mark - Experiment support
 
 // Create and return a new view controller for use as a tab container;
 // experimental configurations determine which subclass of
 // TabContainerViewController to return.
@@ skipping 35 matching lines @@
 - (id<UIViewControllerAnimatedTransitioning>)
 animationControllerForDismissedController:(UIViewController*)dismissed {
   ZoomTransitionAnimator* animator = [[ZoomTransitionAnimator alloc] init];
   animator.presenting = NO;
   animator.presentationKey = self.presentationKey;
   [animator selectDelegate:@[ dismissed.presentingViewController ]];
   return animator;
 }
 
 @end