We are currently working on a Polymer app, and my most recent task has been to add localization. As the language files tend to get quite large, and we expect only a small percentage of users to change languages after the app has started, we want to only load the appropriate data, and fetch more as required.

The Polymer Project has a section in their Toolbox on localization so that seems like a good place to start. They even provide a nice behavior called Polymer.AppLocalizeBehavior. This might be easy!

Ok, lets use the Polymer CLI to generate a quick app to experiment with:

mkdir example && cd example polymer init application

Answer the questions, or accept the defaults. When it is has finished generating the app, and installing the dependencies, serve it up:

polymer serve

Open it in your browser, here. You should see something like this:

Hello example-app

Next, we need to install the behavior as a dependency:

bower install --save PolymerElements/app-localize-behavior

Open example-app.html, and import the behavior, and update the code, following the example in the documentation for app-localize-behavior:

<link rel="import" href="../../bower_components/polymer/polymer.html"> <link rel="import" href="../../bower_components/app-localize-behavior/app-localize-behavior.html"> <dom-module id="example-app"> <template> <div>{{localize('hello', 'name', 'Batman')}}</div> </template> <script> Polymer({ is: 'example-app', // include the behavior behaviors: [ Polymer.AppLocalizeBehavior ], properties: { language: { value: 'en' }, }, // load localized messages attached: function() { this.loadResources(this.resolveUrl('locales.json')); } }); </script> </dom-module>

Now, we need to create the file locales.json in the same directory as example-app.html:

{ "en": { "hello": "My name is {name}." }, "fr": { "hello": "Je m'apelle {name}." } }

Reload the app, and you should see:

My name is Batman.

Change the value of language to ‘fr’, and reload the app. You should see:

Je m'apelle Batman.

Awesome! Ok, to solve our requirement of only loading the data we need, lets break locales.json into two files, en.json:

{ "en": { "hello": "My name is {name}." } }

and fr.json:

{ "fr": { "hello": "Je m'apelle {name}." } }

and lets change the attached method:

attached: function() { this.loadResources(this.resolveUrl(this.language + '.json')); }

Reload the app. It should still work. Change the language, and reload again. It still works. Thats awesome, we are now only loading the data we need. But, we can’t change the language.

Lets add a little code to allow the user to switch languages. Add the following markup inside the template:

<div id="languages"> <span id="en">en</span> <span id="fr">fr</span> </div>

And add the following ready function inside the script section:

ready: function() { document.getElementById('languages').addEventListener('click', function(event) { this.language = event.target.id; }.bind(this)); }

Update: The bug documented here has been fixed in version 0.10.0, but I leave this paragraph here so I can brag about my pull request:

Reload the page, and click ‘fr’. Hmm, that’s weird, we got an error!

app-localize-behavior.html:213 Uncaught TypeError: Cannot read property 'hello' of undefined

Turns out, there is what I consider to be a bug in the behavior. Great! A chance to submit a pull request, and give back! Until that is accepted, we can continue using my fork.

Now, reload the app, and click on ‘fr’. The text disappears. That’s because we haven’t loaded fr.json yet.

Add an observer to the language property, like this:

properties: { language: { value: 'en', observer: '_languageChanged' } }

Remove the attached function, and add this function:

_languageChanged: function(newValue, oldValue) { this.loadResources(this.resolveUrl(newValue + '.json')); }

Reload the app, click ‘fr’. It works! Click ‘en’. It works too! We accomplished both the goals we set out above, and it wasn’t that hard!

Here is the final version of example-app.html: