FetchMore functionality backend

components/ntp_snippets/remote/ntp_snippets_fetcher.h

 // 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.
 
 #ifndef COMPONENTS_NTP_SNIPPETS_REMOTE_NTP_SNIPPETS_FETCHER_H_
 #define COMPONENTS_NTP_SNIPPETS_REMOTE_NTP_SNIPPETS_FETCHER_H_
 
 #include <memory>
 #include <set>
 #include <string>
@@ skipping 47 matching lines @@
     ~FetchedCategory();                             // = default, in .cc
     FetchedCategory& operator=(FetchedCategory&&);  // = default, in .cc
   };
   using FetchedCategoriesVector = std::vector<FetchedCategory>;
   using OptionalFetchedCategories = base::Optional<FetchedCategoriesVector>;
 
   // |snippets| contains parsed snippets if a fetch succeeded. If problems
   // occur, |snippets| contains no value (no actual vector in base::Optional).
   // Error details can be retrieved using last_status().
   using SnippetsAvailableCallback =
-      base::Callback<void(OptionalFetchedCategories fetched_categories)>;
+      base::OnceCallback<void(OptionalFetchedCategories fetched_categories)>;

[Marc Treib, 2016/10/18 08:17:45]

If the callback is now for one fetch only, it should be passed directly to
FetchSnippets - no point in a separate SetCallback.

One thing to consider: What happends when FetchSnippets is called again before
the previous one has completed?

[fhorschig, 2016/10/20 13:10:38]

The callback is passed into FetchSnippets.

The descriptive comment of this function states: 
"If an ongoing fetch exists, it will be cancelled and a new one without
triggering an additional callback [...]"
I haven't verified this behavior but I have faith in whoever wrote this.

[Marc Treib, 2016/10/20 16:51:39]

Well, that's what the previous behavior amounted to, but you're changing that.
See other comment.

[tschumann, 2016/10/24 06:38:25]

never trust untested code ;-) A test would be really nice for that scenario.

[Marc Treib, 2016/10/28 14:49:48]

We decided to fix this after this CL (crbug.com/659931).

 
   // Enumeration listing all possible outcomes for fetch attempts. Used for UMA
   // histograms, so do not change existing values. Insert new values at the end,
   // and update the histogram definition.
   enum class FetchResult {
     SUCCESS,
     DEPRECATED_EMPTY_HOSTS,
     URL_REQUEST_STATUS_ERROR,
     HTTP_ERROR,
     JSON_PARSE_ERROR,
@@ skipping 18 matching lines @@
       PrefService* pref_service,
       CategoryFactory* category_factory,
       translate::LanguageModel* language_model,
       const ParseJSONCallback& parse_json_callback,
       const std::string& api_key,
       const UserClassifier* user_classifier);
   ~NTPSnippetsFetcher() override;
 
   // Set a callback that is called when a new set of snippets are downloaded,
   // overriding any previously set callback.
-  void SetCallback(const SnippetsAvailableCallback& callback);
+  void SetCallback(SnippetsAvailableCallback callback);
 
   // Fetches snippets from the server. |hosts| restricts the results to a set of
   // hosts, e.g. "www.google.com". If |hosts| is empty, no host restrictions are
   // applied.
   //
   // |excluded_ids| will be reported to the server; the server should not return
   // suggestions with those IDs.
   //
   // If an ongoing fetch exists, it will be cancelled and a new one started,
   // without triggering an additional callback (i.e. not noticeable by
@@ skipping 165 matching lines @@
   // When a token request gets canceled, we want to retry once.
   bool oauth_token_retried_;
 
   base::WeakPtrFactory<NTPSnippetsFetcher> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(NTPSnippetsFetcher);
 };
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_REMOTE_NTP_SNIPPETS_FETCHER_H_