[ios] WebStateList owns all WebState it manages.

ios/chrome/browser/tabs/tab_model_closing_web_state_observer.mm

+// Copyright 2017 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.
+
+#import "ios/chrome/browser/tabs/tab_model_closing_web_state_observer.h"
+
+#include "base/logging.h"
+#include "base/strings/string_piece.h"
+#include "components/sessions/core/tab_restore_service.h"
+#include "components/sessions/ios/ios_live_tab.h"
+#include "ios/chrome/browser/chrome_url_constants.h"
+#import "ios/chrome/browser/tabs/tab_model.h"
+#import "ios/shared/chrome/browser/tabs/web_state_list.h"
+#import "ios/web/public/navigation_item.h"
+#import "ios/web/public/navigation_manager.h"
+#import "ios/web/public/web_state/web_state.h"
+#include "url/gurl.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+@implementation TabModelClosingWebStateObserver {
+  __weak TabModel* _tabModel;
+  sessions::TabRestoreService* _restoreService;
+
+  // Records whether the last detached WebState was the active WebState.
+  // If YES, then the session is saved if the WebState is closed.

[rohitrao (ping after 24h), 2017/03/26 23:21:22]

We only save sessions when we close the active tab?  What happens when you close
a background tab?

[sdefresne, 2017/03/28 15:15:05]

We also save the session if the current Tab changes. See
TabModelSelectedTabObserver
-webStateList:didChangeActiveWebState:oldWebState:atIndex:userAction:. Since the
current Tab changes when the current Tab is closed this mean that we always save
the session when a Tab is closed. If we don't mind calling TabModel
-saveSessionImmediately: multiple time, then I can remove the
-webStateList:willDetachWebState:atIndex: as it is only there to allow the check
"atIndex == webStateList->active_index()".

+  BOOL _lastDetachedWebStateWasActive;
+}
+
+- (instancetype)initWithTabModel:(TabModel*)tabModel
+                  restoreService:(sessions::TabRestoreService*)restoreService {
+  DCHECK(tabModel);
+  self = [super init];
+  if (self) {
+    _tabModel = tabModel;
+    _restoreService = restoreService;
+    _lastDetachedWebStateWasActive = NO;
+  }
+  return self;
+}
+
+#pragma mark - WebStateListObserving
+
+- (void)webStateList:(WebStateList*)webStateList
+    willDetachWebState:(web::WebState*)webState
+               atIndex:(int)atIndex {
+  _lastDetachedWebStateWasActive = webStateList->active_index() == atIndex;
+  [self recordHistoryForWebState:webState atIndex:atIndex];
+}
+
+- (void)webStateList:(WebStateList*)webStateList
+    willCloseWebState:(web::WebState*)webState
+              atIndex:(int)atIndex {
+  if (_lastDetachedWebStateWasActive) {
+    _lastDetachedWebStateWasActive = NO;
+    [_tabModel saveSessionImmediately:NO];
+  }
+}
+
+#pragma mark - Private
+
+- (void)recordHistoryForWebState:(web::WebState*)webState atIndex:(int)atIndex {
+  // The RestoreService will be null if navigation is off the record.
+  if (!_restoreService)
+    return;
+
+  // No need to record history if the tab has no navigation or has only
+  // presented the NTP or the bookmark UI.
+  web::NavigationManager* navigationManager = webState->GetNavigationManager();
+  if (navigationManager->GetItemCount() <= 1) {
+    web::NavigationItem* item = navigationManager->GetLastCommittedItem();
+    if (!item)
+      return;
+
+    const base::StringPiece host = item->GetVirtualURL().host_piece();
+    if (host == kChromeUINewTabHost || host == kChromeUIBookmarksHost)
+      return;
+  }
+
+  _restoreService->CreateHistoricalTab(
+      sessions::IOSLiveTab::GetForWebState(webState), atIndex);
+}
+
+@end