[multiball] Create the SceneController and make it an observer.

Adds a SceneState observer protocol, and makes the SceneController be
created by the SceneDelegate and listen to this protocol.

Bug: none
Change-Id: I10378f6034f13a4b47687fcbcec6a38d5c078666
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1789585
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697979}

ios/chrome/browser/ui/main/scene_controller.h

@@ -7,8 +7,18 @@
 
 #import <UIKit/UIKit.h>
 
+#import "ios/chrome/browser/ui/main/scene_state.h"
+
 // The controller object for a scene. Reacts to scene state changes.
-@interface SceneController : NSObject
+@interface SceneController : NSObject <SceneStateObserver>
+
+- (instancetype)init NS_UNAVAILABLE;
+- (instancetype)initWithSceneState:(SceneState*)sceneState
+    NS_DESIGNATED_INITIALIZER;
+
+// The state of the scene controlled by this object.
+@property(nonatomic, weak, readonly) SceneState* sceneState;
+
 @end
 
 #endif  // IOS_CHROME_BROWSER_UI_MAIN_SCENE_CONTROLLER_H_

ios/chrome/browser/ui/main/scene_controller.mm

@@ -10,4 +10,19 @@
 
 @implementation SceneController
 
+- (instancetype)initWithSceneState:(SceneState*)sceneState {
+  self = [super init];
+  if (self) {
+    _sceneState = sceneState;
+    [_sceneState addObserver:self];
+  }
+  return self;
+}
+
+#pragma mark - SceneStateObserver
+
+- (void)sceneState:(SceneState*)sceneState
+    transitionedToActivationLevel:(SceneActivationLevel)level {
+}
+
 @end

ios/chrome/browser/ui/main/scene_controller_unittest.mm

@@ -16,8 +16,9 @@ namespace {
 class SceneControllerTest : public PlatformTest {
  protected:
   SceneControllerTest() {
-    scene_controller_ = [[SceneController alloc] init];
     scene_state_ = [[SceneState alloc] init];
+    scene_controller_ =
+        [[SceneController alloc] initWithSceneState:scene_state_];
   }
 
   SceneController* scene_controller_;

ios/chrome/browser/ui/main/scene_delegate.h

@@ -7,6 +7,7 @@
 
 #import <UIKit/UIKit.h>
 
+#import "ios/chrome/browser/ui/main/scene_controller.h"
 #import "ios/chrome/browser/ui/main/scene_state.h"
 
 // An object acting as a scene delegate for UIKit. Updates the scene state.
@@ -17,6 +18,9 @@
 // The object that holds the state of the scene associated with this delegate.
 @property(nonatomic, strong) SceneState* sceneState;
 
+// The controller created and owned by this object.
+@property(nonatomic, strong) SceneController* sceneController;
+
 @end
 
 #endif  // IOS_CHROME_BROWSER_UI_MAIN_SCENE_DELEGATE_H_

ios/chrome/browser/ui/main/scene_delegate.mm

@@ -13,6 +13,7 @@
 - (SceneState*)sceneState {
   if (!_sceneState) {
     _sceneState = [[SceneState alloc] init];
+    _sceneController = [[SceneController alloc] initWithSceneState:_sceneState];
   }
   return _sceneState;
 }

ios/chrome/browser/ui/main/scene_state.h

@@ -7,6 +7,8 @@
 
 #import <UIKit/UIKit.h>
 
+@class SceneState;
+
 // Describes the possible scene states.
 // This is an iOS 12 compatible version of UISceneActivationState enum.
 typedef NS_ENUM(NSUInteger, SceneActivationLevel) {
@@ -23,6 +25,17 @@ typedef NS_ENUM(NSUInteger, SceneActivationLevel) {
   SceneActivationLevelForegroundActive
 };
 
+@protocol SceneStateObserver <NSObject>
+
+@optional
+
+// Called whenever the scene state transitions between different activity
+// states.
+- (void)sceneState:(SceneState*)sceneState
+    transitionedToActivationLevel:(SceneActivationLevel)level;
+
+@end
+
 // An object containing the state of a UIWindowScene. One state object
 // corresponds to one scene.
 @interface SceneState : NSObject
@@ -33,6 +46,13 @@ typedef NS_ENUM(NSUInteger, SceneActivationLevel) {
 // Window for the associated scene, if any.
 @property(nonatomic, weak) UIWindow* window;
 
+// Adds an observer to this scene state. The observers will be notified about
+// scene state changes per SceneStateObserver protocol.
+- (void)addObserver:(id<SceneStateObserver>)observer;
+// Removes the observer. It's safe to call this at any time, including from
+// SceneStateObserver callbacks.
+- (void)removeObserver:(id<SceneStateObserver>)observer;
+
 @end
 
 #endif  // IOS_CHROME_BROWSER_UI_MAIN_SCENE_STATE_H_

ios/chrome/browser/ui/main/scene_state.mm

@@ -4,9 +4,85 @@
 
 #import "ios/chrome/browser/ui/main/scene_state.h"
 
+#import "base/ios/crb_protocol_observers.h"
+
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
+@interface SceneStateObserverList : CRBProtocolObservers <SceneStateObserver>
+@end
+
+@implementation SceneStateObserverList
+@end
+
+#pragma mark - SceneState
+
+@interface SceneState ()
+
+// Container for this object's observers.
+@property(nonatomic, strong) SceneStateObserverList* observers;
+
+@end
+
 @implementation SceneState
+
+- (instancetype)init {
+  self = [super init];
+  if (self) {
+    _observers = [SceneStateObserverList
+        observersWithProtocol:@protocol(SceneStateObserver)];
+  }
+  return self;
+}
+
+#pragma mark - public
+
+- (void)addObserver:(id<SceneStateObserver>)observer {
+  [self.observers addObserver:observer];
+}
+
+- (void)removeObserver:(id<SceneStateObserver>)observer {
+  [self.observers removeObserver:observer];
+}
+
+#pragma mark - Setters & Getters.
+
+- (void)setActivationLevel:(SceneActivationLevel)newLevel {
+  if (_activationLevel == newLevel) {
+    return;
+  }
+  _activationLevel = newLevel;
+
+  [self.observers sceneState:self transitionedToActivationLevel:newLevel];
+}
+
+#pragma mark - debug
+
+- (NSString*)description {
+  NSString* activityString = nil;
+  switch (self.activationLevel) {
+    case SceneActivationLevelUnattached: {
+      activityString = @"Unattached";
+      break;
+    }
+
+    case SceneActivationLevelBackground: {
+      activityString = @"Background";
+      break;
+    }
+    case SceneActivationLevelForegroundInactive: {
+      activityString = @"Foreground, Inactive";
+      break;
+    }
+    case SceneActivationLevelForegroundActive: {
+      activityString = @"Active";
+      break;
+    }
+  }
+
+  return
+      [NSString stringWithFormat:@"SceneState %p (%@)", self, activityString];
+}
+
 @end