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Syncing Languages and Input Methods

Background

In M41, Chromium OS started syncing preferences for languages, keyboard layouts, and extension IMEs. Because devices may have different physical keyboard layouts, as well as different available locales and input methods, changes on one device should not automatically propagate to another device. However, we still wanted to provide a convenient out-of-box experience for users who go beyond their device's default input method when they Powerwash their system or sign in to a new device.

Overview

Instead of syncing input methods both ways, we send changes to input methods from the device to the sync server. This lets us "remember" the most recent settings without overwriting existing preferences on other machines. When a user signs in to a device for the first time, we combine the input method used to sign in with the information from the sync server, adding the user's saved input method preferences to the local preferences without overwriting any input methods already set up.

For instance, a user should only have to manually add the Dvorak layout to a device once. Then, we automatically add this layout to new devices for this user, without affecting devices that have already been set up. If the layout isn't available on a device, it's simply not added. In the worst case, we may add an input method that is incompatible with the hardware keyboard, but the user would still retain any input methods that they enabled before the first sync.

Design

Preferences

We kept the original unsyncable preferences, which describe the local settings:
  • settings.language.preferred_languages: enabled language IDs (e.g., "en-US,fr,ko")
  • settings.language.preload_engines: preloaded (active) component input method IDs (e.g., "pinyin,mozc")
  • settings.language.enabled_extension_imes: enabled extension IME IDs
We added three syncable preferences: 
  • settings.language.preferred_languages_syncable
  • settings.language.preload_engines_syncable
  • settings.language.enabled_extension_imes_syncable
During the first sync, the values of these preferences are merged with the local preferences above. After that, these preferences are only used to keep track of the user's latest settings. They never affect the settings on a device that has already synced. In other words, they can be thought of as one-way, syncing from the device to the sync server -- except during the first sync.

A fourth preference was added to indicate whether the initial merging should happen: settings.language.merge_input_methods. This is set to true during OOBE. After the first sync, this is always false. This ensures that devices set up before M41 won't have their input settings changed.

First Sync

chromeos::input_method::InputMethodSyncer was created to handle sync logic for input methods. After the first ever sync, MergeSyncedPrefs adds the IME settings from the syncable prefs to the local prefs using the following algorithm for each pref:
  1. Append unique tokens from the syncable pref to the local pref.
  2. Set the syncable pref equal to the merged list.*
  3. Remove tokens corresponding to values not available on this system.
  4. Set the local pref equal to this validated list.
MergeSyncedPrefs also sets merge_input_methods to false, ensuring that it won't be called again.

Notice that this solution doesn't assume the first sync happens right after OOBE. For cases where users have set encryption passphrases, they could easily add languages and input methods before enabling sync, so we support arbitrary lists instead of single values during the merge.

Updates

When a local input method pref changes, InputMethodSyncer updates the syncable prefs to correspond to the local pref values.* We update all three prefs at the same time to ensure the settings are consistent -- since these will be the values used the next time the user completes OOBE, the languages should correspond to the input methods.

* For preload_engines_syncable (input method IDs), we transform the list to use legacy component IDs so the preference can sync between Chrome OS and Chromium OS. During first sync, we convert this back to locally valid component IDs.
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