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diff --git a/background_scripts/sync.coffee b/background_scripts/sync.coffee
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+++ b/background_scripts/sync.coffee
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+#
+# * Sync.set() and Sync.clear() propagate local changes to chrome.storage.sync.
+# * Sync.handleStorageUpdate() listens for changes to chrome.storage.sync and propagates those
+#   changes to localStorage and into vimium's internal state.
+# * Sync.fetchAsync() polls chrome.storage.sync at startup, similarly propagating
+#   changes to localStorage and into vimium's internal state.
+#
+# Changes are propagated into vimium's state using the same mechanism
+# (Settings.performPostUpdateHook) that is used when options are changed on
+# the options page.
+#
+# The effect is best-effort synchronization of vimium options/settings between
+# chrome/vimium instances.
+#
+# NOTE:
+#   Values handled within this module are ALWAYS already JSON.stringifed, so
+#   they're always non-empty strings.
+#
+
+root = exports ? window
+root.Sync = Sync =
+
+  # April 19 2014: Leave logging statements in, but disable debugging. We may need to come back to this, so
+  # removing logging now would be premature. However, if users report problems, they are unlikely to notice
+  # and make sense of console logs on background pages. So disable it, by default. For genuine errors, we
+  # call console.log directly.
+  debug: false
+  storage: chrome.storage.sync
+  doNotSync: ["settingsVersion", "previousVersion"]
+
+  # This is called in main.coffee.
+  init: ->
+    chrome.storage.onChanged.addListener (changes, area) -> Sync.handleStorageUpdate changes, area
+    @fetchAsync()
+
+  # Asynchronous fetch from synced storage, called only at startup.
+  fetchAsync: ->
+    @storage.get null, (items) =>
+      # Chrome sets chrome.runtime.lastError if there is an error.
+      if chrome.runtime.lastError is undefined
+        for own key, value of items
+          @log "fetchAsync: #{key} <- #{value}"
+          @storeAndPropagate key, value
+      else
+        console.log "callback for Sync.fetchAsync() indicates error"
+        console.log chrome.runtime.lastError
+
+  # Asynchronous message from synced storage.
+  handleStorageUpdate: (changes, area) ->
+    for own key, change of changes
+      @log "handleStorageUpdate: #{key} <- #{change.newValue}"
+      @storeAndPropagate key, change?.newValue
+
+  # Only ever called from asynchronous synced-storage callbacks (fetchAsync and handleStorageUpdate).
+  storeAndPropagate: (key, value) ->
+    return if not key of Settings.defaults
+    return if not @shouldSyncKey key
+    return if value and key of localStorage and localStorage[key] is value
+    defaultValue = Settings.defaults[key]
+    defaultValueJSON = JSON.stringify(defaultValue)
+
+    if value and value != defaultValueJSON
+      # Key/value has been changed to non-default value at remote instance.
+      @log "storeAndPropagate update: #{key}=#{value}"
+      localStorage[key] = value
+      Settings.performPostUpdateHook key, JSON.parse(value)
+    else
+      # Key has been reset to default value at remote instance.
+      @log "storeAndPropagate clear: #{key}"
+      if key of localStorage
+        delete localStorage[key]
+      Settings.performPostUpdateHook key, defaultValue
+
+  # Only called synchronously from within vimium, never on a callback.
+  # No need to propagate updates to the rest of vimium, that's already been done.
+  set: (key, value) ->
+    if @shouldSyncKey key
+      @log "set scheduled: #{key}=#{value}"
+      key_value = {}
+      key_value[key] = value
+      @storage.set key_value, =>
+        # Chrome sets chrome.runtime.lastError if there is an error.
+        if chrome.runtime.lastError
+          console.log "callback for Sync.set() indicates error: #{key} <- #{value}"
+          console.log chrome.runtime.lastError
+
+  # Only called synchronously from within vimium, never on a callback.
+  clear: (key) ->
+    if @shouldSyncKey key
+      @log "clear scheduled: #{key}"
+      @storage.remove key, =>
+        # Chrome sets chrome.runtime.lastError if there is an error.
+        if chrome.runtime.lastError
+          console.log "for Sync.clear() indicates error: #{key}"
+          console.log chrome.runtime.lastError
+
+  # Should we synchronize this key?
+  shouldSyncKey: (key) ->
+    key not in @doNotSync
+
+  log: (msg) ->
+    console.log "Sync: #{msg}" if @debug