Adds async interface method to PersistentPrefStore.

chrome/common/persistent_pref_store.h

Index: chrome/common/persistent_pref_store.h
diff --git a/chrome/common/persistent_pref_store.h b/chrome/common/persistent_pref_store.h
index 620fee8fabdce6839b0132a36408a6ec207a4a75..0f007c5a6b1c3530e743eb4383093ea6dabcd5cd 100644
--- a/chrome/common/persistent_pref_store.h
+++ b/chrome/common/persistent_pref_store.h
@@ -34,6 +34,13 @@ class PersistentPrefStore : public PrefStore {
     PREF_READ_ERROR_FILE_NOT_SPECIFIED
   };
 
+  class ReadErrorDelegate {
+   public:
+    virtual ~ReadErrorDelegate() {}
+
+    virtual void OnError(PrefReadError error) = 0;
+  };
+
   // Equivalent to PrefStore::GetValue but returns a mutable value.
   virtual ReadResult GetMutableValue(const std::string& key,
                                      Value** result) = 0;
@@ -62,9 +69,16 @@ class PersistentPrefStore : public PrefStore {
   // read errors during startup.
   virtual bool ReadOnly() const = 0;
 
-  // Reads the preferences from disk.
+  // Reads the preferences from disk. Notifies observers via
+  // "PrefStore::OnInitializationCompleted" when done.
   virtual PrefReadError ReadPrefs() = 0;
 
+  // Reads the preferences from disk asynchronously. Notifies observers via
+  // "PrefStore::OnInitializationCompleted" when done. Also it fires

[Mattias Nissler (ping if slow), 2011/05/04 23:33:08]

So we still have two callback mechanisms? Seems like this is a hassle to
implementors. Can't we just make OnInitializationCompleted take a PrefReadError
parameter? And move the error handling code from PrefService to
PrefNotifierImpl?

[altimofeev, 2011/05/05 12:59:28]

Yes, there are two of them. But the only reason why ReadErrorDelegate was
created was that JsonPrefService can't handle error (show notification etc). So
owner of JsonPrefService provides this delegate to extend JsonPrefService if
this extensions is needed. At the same time, OnInitializationCompleted() is
general interface for all PrefStores, not only the ones which read initial data
from the file. It brings us to my first objection - OnInitializationCompleted()
is part of the PrefStore, not PersistentPrefStore. My second objection is that
moving error handling to PrefNotifierImpl implies passing error through the
chain - JsonPrefStore => PrefStoreKeeper => PrefValueStore =>
PrefNotificationImpl.

Note, in the future, if this interface will be broadly used, one can make
ReadErrorHandler public to be used by other PersistentPrefs clients.

[Mattias Nissler (ping if slow), 2011/05/07 00:24:14]

You are very right, in this case it makes sense to have two callback mechanisms,
since we don't want to pollute the PrefStore interface with PersistentPrefStore
stuff. Sorry for not thinking it through when writing the first comment.

+  // |error_delegate| if it is not NULL and reading error has occurred.
+  // Owns |error_delegate|.
+  virtual void ReadPrefsAsync(ReadErrorDelegate* error_delegate) = 0;
+
   // Writes the preferences to disk immediately.
   virtual bool WritePrefs() = 0;