[iOS Clean] Created OverlayService.

ios/clean/chrome/browser/ui/overlays/overlay_scheduler.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/clean/chrome/browser/ui/overlays/overlay_scheduler.h"
+
+#include <list>
+
+#include "base/logging.h"
+#import "ios/chrome/browser/web_state_list/web_state_list.h"
+#import "ios/clean/chrome/browser/ui/commands/tab_grid_commands.h"
+#import "ios/clean/chrome/browser/ui/overlays/overlay_queue.h"
+#import "ios/web/public/web_state/web_state.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+@interface OverlayScheduler () {
+  // Backing object for property of the same name.
+  WebStateList* _webStateList;
+  // The WebStates that have queued overlays.
+  std::list<web::WebState*> _webStates;

[marq (ping after 24h), 2017/06/14 10:02:32]

Several places below make assertions about what's at the head of this list; the
semantics of that should be spelled out here in comments.

[kkhorimoto, 2017/06/15 08:26:28]

Done.

+}
+
+// The WebStateList passed on initialization.
+@property(nonatomic, readonly, assign) WebStateList& webStateList;
+
+// The dispatcher passed on initialization.
+@property(nonatomic, readonly, weak)
+    id<TabGridCommands, OverlayPresentationCommands>
+        dispatcher;
+
+// Attempts to show the next queued overlay.
+- (void)tryToStartNextOverlay;
+
+@end
+
+@implementation OverlayScheduler
+
+@synthesize dispatcher = _dispatcher;
+
+- (instancetype)initWithWebStateList:(WebStateList*)webStateList
+                          dispatcher:
+                              (id<TabGridCommands, OverlayPresentationCommands>)
+                                  dispatcher {
+  DCHECK(webStateList);
+  DCHECK(dispatcher);
+  if ((self = [super init])) {
+    _webStateList = webStateList;
+    _dispatcher = dispatcher;
+  }
+  return self;
+}
+
+#pragma mark - Accessors
+
+- (WebStateList&)webStateList {
+  return *_webStateList;
+}
+
+#pragma mark - OverlaySchedulerCommands
+
+- (void)scheduleOverlayForWebState:(web::WebState*)webState {
+  DCHECK(webState);
+  _webStates.push_back(webState);
+  [self tryToStartNextOverlay];
+}
+
+- (void)visibleOverlayWasReplacedForWebState:(web::WebState*)webState {
+  DCHECK(webState);
+  DCHECK_EQ(_webStates.front(), webState);

[marq (ping after 24h), 2017/06/14 10:02:32]

This makes me slightly nervous, because it makes the correctness of a command
sent by a part of the code that has no information or guarantees about how the
comment is implemented coupled to an implementation detail of this class. 

In general, the point of the dispatcher is to strongly decouple command
implementations from the callers of those commands. Commands should generally be
interpreted as requests from other parts of the application, and the caller
should assume little about how, by who,, or if the command is actually
fulfilled.

(this is more of a reason to make this information communicated by observation,
but it still shouldn't depend on the semantics of _webStates).

[kkhorimoto, 2017/06/15 08:26:29]

It's been converted to an observer pattern now.  The new semantics make more
sense as to why these assumptions have been made.

+  DCHECK(OverlayQueue::FromWebState(webState)->IsShowingOverlay());
+  _webStates.push_front(webState);
+}
+
+- (void)overlayWasStoppedForWebState:(web::WebState*)webState {
+  DCHECK(!_webStates.empty());
+  DCHECK_EQ(_webStates.front(), webState);
+  _webStates.pop_front();
+  [self tryToStartNextOverlay];
+}
+
+- (void)cancelOverlaysForWebState:(web::WebState*)webState {
+  DCHECK(webState);
+  // Remove all scheduled instances of |webState| from the queue.
+  auto i = _webStates.begin();
+  while (i != _webStates.end()) {
+    if (*i == webState)
+      _webStates.erase(i);
+  }
+  // Remove all overlays from |webState|'s queue.
+  OverlayQueue::FromWebState(webState)->CancelOverlays();
+  [self tryToStartNextOverlay];
+}
+
+#pragma mark -
+
+- (void)tryToStartNextOverlay {
+  // Early return if there are no overlays to show.
+  if (_webStates.empty())
+    return;
+  // Also early return if the front queue is already presenting an overlay.
+  web::WebState* webState = _webStates.front();
+  OverlayQueue* queue = OverlayQueue::FromWebState(webState);
+  if (queue->IsShowingOverlay())
+    return;
+  // Show the next WebState's content view and start its overlay.
+  int newActiveIndex = self.webStateList.GetIndexOfWebState(webState);
+  DCHECK_NE(newActiveIndex, WebStateList::kInvalidIndex);
+  self.webStateList.ActivateWebStateAt(newActiveIndex);
+  [self.dispatcher showTabGridTabForActiveWebState];
+  [self.dispatcher startNextOverlayForWebState:webState];
+}
+
+@end