[Offline Pages] Set up the initial prefetching service.

chrome/browser/android/offline_pages/suggestions_observer.h

+// 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.
+
+#ifndef CHROME_BROWSER_ANDROID_OFFLINE_PAGES_SUGGESTIONS_OBSERVER_H_
+#define CHROME_BROWSER_ANDROID_OFFLINE_PAGES_SUGGESTIONS_OBSERVER_H_
+
+#include <memory>
+
+#include "base/callback.h"
+#include "base/memory/weak_ptr.h"
+#include "components/ntp_snippets/content_suggestions_service.h"
+#include "components/offline_pages/core/prefetch/prefetch_service.h"
+
+namespace content {
+class BrowserContext;
+}  // namespace content
+
+namespace ntp_snippets {
+class Category;
+}
+
+namespace offline_pages {
+
+// Observes the ContentSuggestionsService, listening for new suggestions of a
+// single category (initially ARTICLES).  When those suggestions arrive, it
+// then forwards them to the Prefetch Service, which does not know about Content
+// Suggestions specifically.
+class SuggestionsObserver
+    : public ntp_snippets::ContentSuggestionsService::Observer,
+      base::SupportsUserData::Data {
+ public:
+  // Delegate exists to allow for dependency injection in unit tests.
+  // SuggestionsObserver implements its own delegate, |DefaultDelegate| in the
+  // .cc file that forwards to the ContentSuggestionsService and the
+  // PrefetchServiceFactory.  Code inside |DefaultDelegate| should be as simple
+  // as possible, since it will only be covered by instrumentation/browser

[carlosk, 2017/04/18 00:36:09]

nit: if by covered you mean overridden or re-implemented I think these latter
two words are clearer.

[dewittj, 2017/04/18 18:20:25]

no, I mean test coverage in particular.

[carlosk, 2017/04/18 20:17:22]

Acknowledged.

+  // tests.
+  class Delegate {

[jianli, 2017/04/17 21:52:58]

It just seems to me that using this technique though make the test code a bit
easy to write, but it makes the production code more complicated and a bit hard
to follow. Can we use SetTestingFactoryAndUse for GetPrefetchService? For
GetSuggestions, can we pass a ContentSuggestionService instance which can be
mocked in the test code?

[dewittj, 2017/04/17 22:22:18]

Mocking ContentSuggestionService turned out to be a lot of code.  I actually
implemented that before, and it required extensive harness setup.  So once we
have a delegate or even a raw base::Callback<...> for content suggestions it
doesn't make sense to have multiple strategies for injection.

+   public:
+    virtual const std::vector<ntp_snippets::ContentSuggestion>& GetSuggestions(
+        const ntp_snippets::Category& category) = 0;
+    virtual PrefetchService* GetPrefetchService(
+        content::BrowserContext* context) = 0;
+    virtual ~Delegate() = default;
+  };
+
+  // This API creates a new SuggestionsObserver and adds it as an
+  // observer to the ContentSuggestionsService provided.  Its lifetime is
+  // managed by the ContentSuggestionsService.
+  static void ObserveContentSuggestionsService(
+      content::BrowserContext* browser_context,
+      ntp_snippets::ContentSuggestionsService* service);
+
+  SuggestionsObserver(content::BrowserContext* browser_context,
+                      const ntp_snippets::Category& category,
+                      std::unique_ptr<Delegate> delegate);
+  ~SuggestionsObserver() override;
+
+  // ContentSuggestionsService::Observer overrides.
+  void OnNewSuggestions(ntp_snippets::Category category) override;
+  void OnCategoryStatusChanged(
+      ntp_snippets::Category category,
+      ntp_snippets::CategoryStatus new_status) override;
+  void OnSuggestionInvalidated(
+      const ntp_snippets::ContentSuggestion::ID& suggestion_id) override;
+  void OnFullRefreshRequired() override;
+  void ContentSuggestionsServiceShutdown() override;
+
+ private:
+  content::BrowserContext* browser_context_;
+  ntp_snippets::CategoryStatus category_status_ =
+      ntp_snippets::CategoryStatus::INITIALIZING;
+  ntp_snippets::Category category_;
+  std::unique_ptr<Delegate> delegate_;
+
+  DISALLOW_COPY_AND_ASSIGN(SuggestionsObserver);
+};
+
+}  // namespace offline_pages
+
+#endif  // CHROME_BROWSER_ANDROID_OFFLINE_PAGES_SUGGESTIONS_OBSERVER_H_