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

ios/clean/chrome/browser/browser_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 "base/logging.h"
 #import "ios/clean/chrome/browser/browser_coordinator+internal.h"
@@ skipping 25 matching lines @@
   if (self = [super init]) {
     _context = [[CoordinatorContext alloc] init];
     _childCoordinators = [NSMutableSet set];
   }
   return self;
 }
 
 #pragma mark - Public API
 
 - (void)start {
-  // Default implementation is a no-op.
+  [self.parentCoordinator childCoordinatorDidStart:self];
 }
 
 - (void)stop {
-  // Default implementation is a no-op.
+  [self.parentCoordinator childCoordinatorWillStop:self];
 }
 
 @end
 
 @implementation BrowserCoordinator (Internal)
 // Concrete implementations must implement a |viewController| property.
 @dynamic viewController;
 
 - (NSSet*)children {
   return [self.childCoordinators copy];
@@ skipping 54 matching lines @@
          self.childCoordinators.count == 0;
 }
 
 - (void)removeChildCoordinator:(BrowserCoordinator*)coordinator {
   if (![self.childCoordinators containsObject:coordinator])
     return;
   [self.childCoordinators removeObject:coordinator];
   coordinator.parentCoordinator = nil;
 }
 
+- (void)childCoordinatorDidStart:(BrowserCoordinator*)coordinator {
+  // Default implementation is a no-op.
+}
+
+- (void)childCoordinatorWillStop:(BrowserCoordinator*)coordinator {
+  // Default implementation is a no-op.
+}
+
 @end