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Differential Revision: https://phabricator.services.mozilla.com/D19329 --HG-- extra : moz-landing-system : lando
727 lines
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ReStructuredText
727 lines
25 KiB
ReStructuredText
.. role:: html(code)
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:language: html
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.. role:: js(code)
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:language: javascript
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=============================
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Fluent for Firefox Developers
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=============================
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This tutorial is intended for Firefox engineers already familiar with the previous
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localization systems offered by Gecko - `DTD`_ and `StringBundle`_ - and assumes
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prior experience with those systems.
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Using Fluent in Gecko
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=====================
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`Fluent`_ is a modern localization system currently being progressively introduced into
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the Gecko platform with a focus on quality, performance, maintenance and completeness.
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In order to ensure that Fluent is ready for engineers to work with, the initial
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migrations are performed manually with a lot of oversight from the involved
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stakeholders.
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In this initial phase, `Firefox Preferences`_ is being migrated as the first target
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and, as a result, the first bindings to be stabilized are for chrome-privileged
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XUL context.
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From there we plan to focus on two areas:
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- `Unprivileged Contexts`_
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- `System Add-ons`_
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The end goal is replacing all uses of DTD and StringBundle within Firefox's codebase.
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If you want to use Fluent, and your code involves one of the areas currently unsupported,
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we'd like to work with you on getting Fluent ready for your code.
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Getting a Review
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----------------
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If you end up working on any patch which touches FTL files, we have a temporary
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hook in place that will reject your patch unless you get an r+ from one of the following
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L10n Drivers:
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- Francesco Lodolo (:flod)
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- Zibi Braniecki (:gandalf)
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- Axel Hecht (:pike)
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- Staś Małolepszy (:stas)
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Major Benefits
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==============
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Not only was the previous system designed over 20 years ago using file formats
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never intended for localization, but also the Web stack which Fluent ties into has
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completely changed over the same period, and the domain of internationalization
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got a powerful foundation in the form of `Unicode`_, `CLDR`_ and `ICU`_ which Fluent tightly
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`interoperates with`__.
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__ https://github.com/projectfluent/fluent/wiki/Fluent-and-Standards
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While it is beyond the scope of this document to cover all the benefits of Fluent in detail,
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below is an attempt to select some most observable changes for each group of consumers.
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Developers
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----------
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- Support for XUL, XHTML, HTML, Web Components, React, JS, Python and Rust
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- Strings are available in a single, unified localization context available for both DOM and runtime code
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- Full internationalization (i18n) support: date and time formatting, number formatting, plurals, genders etc.
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- Strong focus on `declarative API via DOM attributes`__
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- Extensible with custom formatters, Mozilla-specific APIs etc.
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- `Separation of concerns`__: localization details, and the added complexity of some languages, don't leak onto the source code and are no concern for developers
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- Compound messages link a single translation unit to a single UI element
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- `DOM Overlays`__ allow for localization of DOM fragments
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- Simplified build system model
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- No need for pre-processing instructions
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- Support for pseudolocalization
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__ https://github.com/projectfluent/fluent/wiki/Get-Started
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__ https://github.com/projectfluent/fluent/wiki/Design-Principles
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__ https://github.com/projectfluent/fluent.js/wiki/DOM-Overlays
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Product Quality
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------------------
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- A robust, multilevel, `error fallback system`__ prevents XML errors and runtime errors
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- Simplified l10n API reduces the amount of l10n specific code and resulting bugs
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- Runtime localization allows for dynamic language changes and updates over-the-air
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- DOM Overlays increase localization security
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Many other smaller improvements will be noticed by the users of the system over time
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and, with the new foundation, the Fluent team is `currently working`__ on multiple highly
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requested features which will further improve the experience of developing
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localizable UIs.
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__ https://github.com/projectfluent/fluent/wiki/Error-Handling
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__ https://github.com/projectfluent/fluent/wiki/Roadmap
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Fluent Translation List - FTL
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=============================
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Fluent introduces a new localization format designed specifically for easy readability
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and localization features offered by the system.
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At first glance the format resembles `.properties` file. It may look like this:
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.. code-block:: properties
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home-page-header = Home Page
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# The label of a button opening a new tab
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new-tab-open = Open New Tab
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But the FTL file format is significantly more powerful and the additional features
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quickly add up. In order to familiarize yourself with the basic features,
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consider reading through the `Fluent Syntax Guide`_ to understand
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a more complex example like:
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.. code-block:: properties
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### These messages correspond to security and privacy user interface.
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###
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### Please, choose simple and non-threatening language when localizing
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### to help user feel in control when interacting with the UI.
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## General Section
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-brand-short-name = Firefox
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.gender = masculine
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pref-pane =
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.title =
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{ PLATFORM() ->
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[windows] Options
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*[other] Preferences
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}
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.accesskey = C
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# Variables:
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# $tabCount (Number) - number of container tabs to be closed
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containers-disable-alert-ok-button =
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{ $tabCount ->
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[one] Close { $tabCount } Container Tab
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*[other] Close { $tabCount } Container Tabs
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}
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update-application-info =
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You are using { -brand-short-name } Version: { $version }.
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<span>Please, read the <a>privacy policy</a>.</span>
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The above, of course, is a particular selection of complex strings intended to exemplify
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the new features and concepts introduced by Fluent.
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.. important::
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While in Fluent it’s possible to use both lowercase and uppercase characters in message
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identifiers, the naming convention in Gecko is to use lowercase and hyphens, avoiding
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CamelCase and underscores. For example, `allow-button` should be preferred to
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`allow_button` or `allowButton`, unless there are technically constraints – like
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identifiers generated at run-time from external sources – that make this impractical.
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In order to ensure the quality of the output, a lot of new checks and tooling
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has been added to the build system.
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`Pontoon`_, the main localization tool used to translate Firefox, has been rebuilding
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its user experience to support localizers in their work.
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.. _fluent-tutorial-social-contract:
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Social Contract
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===============
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Fluent uses the concept of a `social contract` between developer and localizers.
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This contract is established by the selection of a unique identifier, called :js:`l10n-id`,
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which carries a promise of being used in a particular place to carry a particular meaning.
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The use of unique identifiers is not new for Firefox engineers, but it is important
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to recognize that Fluent formalizes this relationship.
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.. important::
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An important part of the contract is that the developer commits to treat the
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localization output as `opaque`. That means that no concatenations, replacements
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or splitting should happen after the translation is completed to generate the
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desired output.
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In return, localizers enter the social contract by promising to provide an accurate
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and clean translation of the messages that match the request.
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In previous localization systems, developers were responsible for differentiating
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string variants based on a platform via pre-processing instructions, or
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selecting which strings should be formatted using `PluralForms.jsm`.
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In Fluent, the developer is not to be bothered with inner logic and complexity that the
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localization will use to construct the response. Whether `declensions`__ or other
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variant selection techniques are used is up to a localizer and their particular translation.
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From the developer perspective, Fluent returns a final string to be presented to
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the user, with no l10n logic required in the running code.
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__ https://en.wikipedia.org/wiki/Declension
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Markup Localization
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===================
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Fluent fully replaces the use of `DTD`_ in localization.
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To localize an element in Fluent, the developer adds a new message to
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an FTL file and then has to associate an :js:`l10n-id` with the element
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by defining a :js:`data-l10n-id` attribute:
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.. code-block:: html
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<h1 data-l10n-id="home-page-header" />
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<button data-l10n-id="pref-pane" />
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Fluent will take care of the rest, populating the element with the message value
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in its content and all localizable attributes if defined.
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The difference compared to the use of DTD is that the developer provides only a single
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message to localize the whole element, rather than a separate entity for
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the value and each of the attributes.
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The other change is that the developer can localize a whole fragment of DOM:
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.. code-block:: html
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<p data-l10n-id="update-application-info" data-l10n-args="{'version': '60.0'}">
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<span class="bold">
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<a href="http://www.mozilla.org/privacy" />
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</span>
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</p>
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.. code-block:: properties
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-brand-short-name = Firefox
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update-application-info =
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You are using { -brand-short-name } Version: { $version }.
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<span>Please, read the <a>privacy policy</a>.</span>
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Fluent will overlay the translation onto the source fragment preserving attributes like
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:code:`class` and :code:`href` from the source and adding translations for the elements
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inside. The resulting localized content will look like this:
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.. code-block:: html
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<p data-l10n-id="update-application-info" data-l10n-args="{'version': '60.0'}">
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You are using Firefox Version: 60.0.
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<span class="bold">
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Please, read the <a href="http://www.mozilla.org/privacy">privacy policy</a>.
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</span>
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</p>
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This operation is sanitized, and Fluent takes care of selecting which elements and
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attributes can be safely provided by the localization.
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The list of allowed elements and attributes is `maintained by the W3C`__, and if
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the developer needs to allow for localization of additional attributes, they can
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whitelist them using :code:`data-l10n-attrs` list:
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.. code-block:: html
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<label data-l10n-id="search-input" data-l10n-attrs="style" />
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The above example adds an attribute :code:`style` to be allowed on this
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particular :code:`label` element.
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External Arguments
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------------------
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Notice in the previous example the attribute :code:`data-l10n-args`, which is
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a JSON object storing variables exposed by the developer to the localizer.
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This is the main channel for the developer to provide additional variables
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to be used in the localization.
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Arguments are rarely needed for situations where it’s currently possible to use
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DTD, since such variables would need to be computed from the code at runtime.
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It's worth noting that, when the :code:`l10n-args` are set in
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the runtime code, they are in fact encoded as JSON and stored together with
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:code:`l10n-id` as an attribute of the element.
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__ https://www.w3.org/TR/2011/WD-html5-20110525/text-level-semantics.html
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Runtime Localization
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====================
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Fluent fully replaces the use of `StringBundle`_ in localization.
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In almost every case the JS runtime code will operate on a particular document, either
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XUL, XHTML or HTML.
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If the document has its markup already localized, then Fluent exposes a new
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attribute on the :js:`document` element - :js:`document.l10n`.
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This property is an object of type :js:`DOMLocalization` which maintains the main
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localization context for this document and exposes it to runtime code as well.
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With a focus on `declarative localization`__, the primary method of localization is
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to alter the localization attributes in the DOM. Fluent provides a method to facilitate this:
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.. code-block:: javascript
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document.l10n.setAttributes(element, "new-panel-header");
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This will set the :code:`data-l10n-id` on the element and translate it before the next
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animation frame.
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The reason to use this API over manually setting the attribute is that it also
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facilitates encoding l10n arguments as JSON:
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.. code-block:: javascript
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document.l10n.setAttributes(element "containers-disable-alert-ok-button", {
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tabCount: 5
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}
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__ https://github.com/projectfluent/fluent/wiki/Good-Practices-for-Developers
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Non-Markup Localization
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-----------------------
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In rare cases, when the runtime code needs to retrieve the translation and not
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apply it onto the DOM, Fluent provides an API to retrieve it:
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.. code-block:: javascript
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let [ msg ] = await document.l10n.formatValues([
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{id: "remove-containers-description"}
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]);
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alert(msg);
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This model is heavily discouraged and should be used only in cases where the
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DOM annotation is not possible.
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.. note::
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This API is currently only available as asynchronous. In case of Firefox,
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the only non-DOM localizable calls are used where the output goes to
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a third-party like Bluetooth, Notifications etc.
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All those cases should already be asynchronous.
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Internationalization
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====================
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The majority of internationalization issues are implicitly handled by Fluent without
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any additional requirement. Full Unicode support, `bidirectionality`__, and
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correct number formatting work without any action required from either
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developer or localizer.
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__ https://github.com/projectfluent/fluent/wiki/BiDi-in-Fluent
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.. code-block:: javascript
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document.l10n.setAttributes(element, "welcome-message", {
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userName: "اليسع",
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count: 5
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});
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A message like this localized to American English will correctly wrap the user
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name in directionality marks, allowing the layout engine to determine how to
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display the bidirectional text.
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On the other hand, the same message localized to Arabic will use the Eastern Arabic
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numeral for number "5".
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Plural Rules
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------------
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The most common localization feature is the ability to provide different variants
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of the same string depending on plural categories.
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Fluent replaces the use of the proprietary :code:`PluralForms.jsm` with a Unicode CLDR
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standard called `Plural Rules`_.
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In order to allow localizers to use it, all the developer has to do is to pass
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an external argument number:
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.. code-block:: javascript
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document.l10n.setAttributes(element, "unread-warning", { unreadCount: 5 });
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Localizers can use the argument to build a multi variant message if their
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language requires that:
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.. code-block:: properties
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unread-warning =
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{ $unreadCount ->
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[one] You have { $unreadCount } unread message
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*[other] You have { $unreadCount } unread messages
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}
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Fluent guesses that, since the variant selection is performed based on a number,
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its `plural category`__ should be retrieved.
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If the given translation doesn't need pluralization for the string (for example
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Japanese often will not), the localizer can replace it with:
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.. code-block:: properties
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unread-warning = You have { $unreadCount } unread messages
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and the message will preserve the social contract.
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One additional feature is that the localizer can further improve the message by
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specifying variants for particular values:
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.. code-block:: properties
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unread-warning =
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{ $unreadCount ->
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[0] You have no unread messages
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[1] You have one unread message
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*[other] You have { $unreadCount } unread messages
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}
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The advantage here is that per-locale choices don't leak onto the source code
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and the developer is not affected.
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.. note::
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There is an important distinction between a variant keyed on plural category
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`one` and digit `1`. Although in English the two are synonymous, in other
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languages category `one` may be used for other numbers.
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For example in `Bosnian`__, category `one` is used for numbers like `1`, `21`, `31`
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and so on, and also for fractional numbers like `0.1`.
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__ https://unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html
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__ https://unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html#bs
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Partially-formatted variables
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-----------------------------
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When it comes to formatting data, Fluent allows the developer to provide
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a set of parameters for the formatter, and the localizer can fine tune some of them.
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This technique is called `partially-formatted variables`__.
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For example, when formatting a date, the developer can just pass a JS :js:`Date` object,
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but its default formatting will be pretty expressive. In most cases, the developer
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may want to use some of the :js:`Intl.DateTimeFormat` options to select the default
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representation of the date in string:
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.. code-block:: javascript
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document.l10n.setAttributes(element, "welcome-message", {
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startDate: FluentDateTime(new Date(), {
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year: "numeric",
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month: "long",
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day: "numeric"
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})
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});
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.. code-block:: properties
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welcome-message = Your session will start date: { $startDate }
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In most cases, that will be enough and the date would get formatted in the current
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Firefox as `February 28, 2018`.
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But if in some other locale the string would get too long, the localizer can fine
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tune the options as well:
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.. code-block:: properties
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welcome-message = Początek Twojej sesji: { DATETIME($startDate, month: "short") }
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This will adjust the length of the month token in the message to short and get formatted
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in Polish as `28 lut 2018`.
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At the moment Fluent supports two formatters that match JS Intl API counterparts:
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* **NUMBER**: `Intl.NumberFormat`__
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* **DATETIME**: `Intl.DateTimeFormat`__
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With time more formatters will be added.
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__ https://projectfluent.org/fluent/guide/functions.html#partially-formatted-variables
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__ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NumberFormat
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__ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DateTimeFormat
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Registering New L10n Files
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||
==========================
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In the previous system, a new localization file had to be registered in order to
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add it in the `jar.mn` file for packaging.
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||
Fluent uses a wildcard statement, packaging all localization resources into
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their component's `/localization/` directory.
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||
That means that, if a new file is added to a component of Firefox already
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covered by Fluent like `browser`, it's enough to add the new file to the
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repository in a path like `browser/locales/en-US/browser/component/file.ftl`, and
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the toolchain will package it into `browser/localization/browser/component/file.ftl`.
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At runtime Firefox uses a special registry for all localization data. It will
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register the browser's `/localization/` directory and make all files inside it
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available to be referenced.
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To make the document localized using Fluent, all the developer has to do is add
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localizable resources for Fluent API to use:
|
||
|
||
.. code-block:: html
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<link rel="localization" href="branding/brand.ftl"/>
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<link rel="localization" href="browser/preferences/preferences.ftl"/>
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|
||
The URI provided to the :html:`<link/>` element are relative paths within the localization
|
||
system.
|
||
|
||
|
||
Custom Contexts
|
||
===============
|
||
|
||
The above method creates a single localization context per document.
|
||
In almost all scenarios that's sufficient.
|
||
|
||
In rare edge cases where the developer needs to fetch additional resources, or
|
||
the same resources in another language, it is possible to create additional
|
||
contexts manually using the `Localization` class:
|
||
|
||
.. code-block:: javascript
|
||
|
||
const { Localization } =
|
||
ChromeUtils.import("resource://gre/modules/Localization.jsm", {});
|
||
|
||
|
||
const myL10n = new Localization([
|
||
"branding/brand.ftl",
|
||
"browser/preferences/preferences.ftl"
|
||
]);
|
||
|
||
|
||
let [isDefaultMsg, isNotDefaultMsg] =
|
||
myL10n.formatValues({id: "is-default"}, {id: "is-not-default"});
|
||
|
||
|
||
.. admonition:: Example
|
||
|
||
An example of a use case is the Preferences UI in Firefox, which uses the
|
||
main context to localize the UI but also to build a search index.
|
||
|
||
It is common to build such search index both in a current language and additionally
|
||
in English, since a lot of documentation and online help exist only in English.
|
||
|
||
A developer may create manually a new context with the same resources as the main one,
|
||
but hardcode it to `en-US` and then build the search index using both contexts.
|
||
|
||
Designing Localizable APIs
|
||
==========================
|
||
|
||
When designing localizable APIs, the most important rule is to resolve localization as
|
||
late as possible. That means that instead of resolving strings somewhere deep in the
|
||
codebase and then passing them on, or even caching, it is highly recommended to pass
|
||
around :code:`l10n-id` or :code:`[l10n-id, l10n-args]` pairs until the top-most code
|
||
resolves them or applies them onto the DOM element.
|
||
|
||
|
||
Testing
|
||
=======
|
||
|
||
When writing tests that involve both I18n and L10n, the general rule is that
|
||
result strings are opaque. That means that the developer should not assume any particular
|
||
value and should never test against it.
|
||
|
||
In case of raw i18n the :js:`resolvedOptions` method on all :js:`Intl.*` formatters
|
||
makes it relatively easy. In case of localization, the recommended way is to test that
|
||
the code sets the right :code:`l10n-id`/:code:`l10n-args` attributes like this:
|
||
|
||
.. code-block:: javascript
|
||
|
||
testedFunction();
|
||
|
||
const l10nAttrs = document.l10n.getAttributes(element);
|
||
|
||
deepEquals(l10nAttrs, {
|
||
id: "my-expected-id",
|
||
args: {
|
||
unreadCount: 5
|
||
}
|
||
});
|
||
|
||
If the code really has to test for particular values in the localized UI, it is
|
||
always better to scan for a variable:
|
||
|
||
.. code-block:: javascript
|
||
|
||
testedFunction();
|
||
|
||
equals(element.textContent.contains("John"));
|
||
|
||
.. important::
|
||
|
||
Testing against whole values is brittle and will break when we insert Unicode
|
||
bidirectionality marks into the result string or adapt the output in other ways.
|
||
|
||
|
||
Pseudolocalization
|
||
==================
|
||
|
||
When working with a Fluent-backed UI, the developer gets a new tool to test their UI
|
||
against several classes of problems.
|
||
|
||
Pseudolocalization is a mechanism which transforms messages on the fly, using
|
||
specific logic to help emulate how the UI will look once it gets localized.
|
||
|
||
The three classes of potential problems that this can help with are:
|
||
|
||
- Hardcoded strings.
|
||
|
||
Turning on pseudolocalization should expose any strings that were left
|
||
hardcoded in the source, since they won't get transformed.
|
||
|
||
|
||
- UI space not adapting to longer text.
|
||
|
||
Many languages use longer strings than English. For example, German strings
|
||
may be 30% longer (or more). Turning on pseudolocalization is a quick way to
|
||
test how the layout handles such locales.
|
||
|
||
|
||
- Bidi adaptation.
|
||
|
||
For many developers, testing the UI in right-to-left mode is hard. Mozilla
|
||
offers a pref :js:`intl.uidirection` which switches the direction of the layout,
|
||
but that doesn't expose problems related to right-to-left text.
|
||
Pseudolocalization shows how a right-to-left locale will look like.
|
||
|
||
To turn on pseudolocalization, add a new string pref :js:`intl.l10n.pseudo` and
|
||
select the strategy to be used:
|
||
|
||
- :js:`accented` - Ȧȧƈƈḗḗƞŧḗḗḓ Ḗḗƞɠŀīīşħ
|
||
|
||
This strategy replaces all Latin characters with their accented equivalents,
|
||
and duplicates some vowels to create roughly 30% longer strings.
|
||
|
||
|
||
- :js:`bidi` - ɥsıʅƃuƎ ıpıԐ
|
||
|
||
This strategy replaces all Latin characters with their 180 degree rotated versions
|
||
and enforces right to left text flow using Unicode UAX#9 `Explicit Directional Embeddings`__.
|
||
In this mode, the UI directionality will also be set to right-to-left.
|
||
|
||
__ https://www.unicode.org/reports/tr9/#Explicit_Directional_Embeddings
|
||
|
||
Inner Structure of Fluent
|
||
=========================
|
||
|
||
The inner structure of Fluent in Gecko is out of scope of this tutorial, but
|
||
since the class and file names may show up during debugging or profiling,
|
||
below is a list of major components, each with a corresponding file in `/intl/l10n`
|
||
modules in Gecko.
|
||
|
||
FluentBundle
|
||
--------------
|
||
|
||
FluentBundle is the lowest level API. It's fully synchronous, contains a parser for the
|
||
FTL file format and a resolver for the logic. It is not meant to be used by
|
||
consumers directly.
|
||
|
||
In the future we intend to offer this layer for standardization and it may become
|
||
part of the :js:`mozIntl.*` or even :js:`Intl.*` API sets.
|
||
|
||
That part of the codebase is also the first that we'll be looking to port to Rust.
|
||
|
||
|
||
Localization
|
||
------------
|
||
|
||
Localization is a higher level API which uses :js:`FluentBundle` internally but
|
||
provides a full layer of compound message formatting and robust error fall-backing.
|
||
|
||
It is intended for use in runtime code and contains all fundamental localization
|
||
methods.
|
||
|
||
|
||
DOMLocalization
|
||
---------------
|
||
|
||
DOMLocalization extends :js:`Localization` with functionality to operate on HTML, XUL
|
||
and the DOM directly including DOM Overlays and Mutation Observers.
|
||
|
||
DocumentL10n
|
||
------------
|
||
|
||
DocumentL10n implements the DocumentL10n WebIDL API and allows Document to
|
||
communicate with mozDOMLocalization.
|
||
|
||
L10nRegistry
|
||
------------
|
||
|
||
L10nRegistry is our resource management service. It replaces :js:`ChromeRegistry` and
|
||
maintains the state of resources packaged into the build and language packs,
|
||
providing an asynchronous iterator of :js:`FluentBundle` objects for a given locale set
|
||
and resources that the :js:`Localization` class uses.
|
||
|
||
|
||
.. _Fluent: https://projectfluent.org/
|
||
.. _DTD: https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Tutorial/Localization
|
||
.. _StringBundle: https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Tutorial/Property_Files
|
||
.. _Firefox Preferences: https://bugzilla.mozilla.org/show_bug.cgi?id=1415730
|
||
.. _Unprivileged Contexts: https://bugzilla.mozilla.org/show_bug.cgi?id=1407418
|
||
.. _System Add-ons: https://bugzilla.mozilla.org/show_bug.cgi?id=1425104
|
||
.. _CLDR: http://cldr.unicode.org/
|
||
.. _ICU: http://site.icu-project.org/
|
||
.. _Unicode: https://www.unicode.org/
|
||
.. _Fluent Syntax Guide: https://projectfluent.org/fluent/guide/
|
||
.. _Pontoon: https://pontoon.mozilla.org/
|
||
.. _Plural Rules: http://cldr.unicode.org/index/cldr-spec/plural-rules
|