Change NTPSnippetsService to implement ContentSuggestionsProvider

components/ntp_snippets/content_suggestions_category_status.h

Index: components/ntp_snippets/content_suggestions_category_status.h
diff --git a/components/ntp_snippets/content_suggestions_category_status.h b/components/ntp_snippets/content_suggestions_category_status.h
index d3b5ba474baa18b8f345a80c890ea9ef466869d0..cfc095a06c158d47f988cec97c062ab0bc0506ee 100644
--- a/components/ntp_snippets/content_suggestions_category_status.h
+++ b/components/ntp_snippets/content_suggestions_category_status.h
@@ -11,6 +11,10 @@ namespace ntp_snippets {
 // On Android builds, a Java counterpart will be generated for this enum.
 // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.chrome.browser.ntp.snippets
 enum class ContentSuggestionsCategoryStatus {
+  // The provider is still loading and it is not yet determined whether content
+  // suggestions will be available or not.
+  LOADING,

[Marc Treib, 2016/07/08 13:26:02]

Do we need both LOADING and AVAILABLE_LOADING?

[Philipp Keck, 2016/07/08 14:02:54]

My idea was that a distinction between "MAY be available some time" and "WILL be
available soon" could be useful. Without focusing too much on the current
Android UI, any UI might display an empty section with a loading icon for
AVAILABLE_LOADING, but not for LOADING.

The article provider should do the following (though currently the order is the
wrong way round):
- Set status LOADING
- Check dependencies, switch to some unavailable state if not satisfied.
- Set status AVAILABLE_LOADING
- Load data
- Set status AVAILABLE

The distinction is only useful if the dependencies checking is asynchronous or
takes a long time. Similarly, if something with the dependencies changes and the
new state needs to be clarified asynchronously, it could be required to remove
the currently displayed suggestion cards, which would be achieved by switching
to the LOADING state, which is not considered an available state and thus clears
the cards from the UI.

The Android UI would always treat LOADING and AVAILABLE_LOADING sections the
same, so I guess we could just get rid of LOADING.

[Marc Treib, 2016/07/08 15:17:23]

Hm, okay, I guess that makes sense. I'd like a different name though, to make
the difference clearer. INITIALIZING?
Or alternatively, if nobody is interested in that state (yet), we might as well
not introduce it for now.

[Philipp Keck, 2016/07/08 16:17:01]

I was about to just remove it, but it might prove useful if the providers use
this status enum also for their internal state handling (then calling it
INITIALIZING would indeed make sense).

The reason I initially wanted to avoid "init" is because I thought that a
provider can also fall into this state during runtime, e.g., after discovering
that sync was disabled and before it has figured out what its new state will be.
I thought that happens asynchronously because of the way the
NTPSnippetsStatusService communicates, but it is in fact synchronous (and most
likely will always be synchronous). So the service would never need to go back
to that "LOADING" state where it is unsure, but could directly determine the
right ones of the disabled-states, if applicable, or be confident to go back to
available.

Depending on what we decide for state_ and category_status_ I will rename this
to INITIALIZING only if it is used and otherwise remove it.

+
   // Content suggestions are available (though the list of available suggestions
   // may be empty simply because there are no reasonable suggestions to be made
   // at the moment).
@@ -36,9 +40,10 @@ enum class ContentSuggestionsCategoryStatus {
   PASSPHRASE_ENCRYPTION_ENABLED,
   // Content suggestions are not available because history sync is disabled.
   HISTORY_SYNC_DISABLED,
-  // Content suggestions are not available because the history sync service is
-  // not yet completely initialized and its status is unknown.
-  HISTORY_SYNC_STATE_UNKNOWN
+
+  // Content suggestions are not available because an error occured when loading
+  // or updating them.
+  ERROR
 };
 
 // Determines whether the given status is one of the AVAILABLE statuses.