Aurelia creating a Binding Behaviour that wraps SignalBindingBehaviour

I've currently got a simple Value Converter which uses momentjs to convert Dates to strings:

export class MomentValueConverter {
  public toView(value: Date, format: string): string {
    return moment(value).format(format);
  }
}

However, wherever I use it I end up having to combine it with the aurelia-translation-signal so that its updated if the user changes the current language.

${fileSaved | moment:'ll LTS' & signal:'aurelia-translation-signal'}

How do I instead create a Binding Behavior that automatically takes care of the signalling from aurelia-translation-signal?

Then I could use it like:

${fileSaved & moment:'ll LTS'}

Solution

There is a good example in aurelia-i18n library, https://github.com/aurelia/i18n/blob/master/src/t.js#L89-L122

import {ValueConverter} from 'aurelia-binding';
import {SignalBindingBehavior} from 'aurelia-templating-resources';

export class TBindingBehavior {
  static inject = [SignalBindingBehavior];

  constructor(signalBindingBehavior) {
    this.signalBindingBehavior = signalBindingBehavior;
  }

  bind(binding, source) {
    // bind the signal behavior
    this.signalBindingBehavior.bind(binding, source, 'aurelia-translation-signal');

    // rewrite the expression to use the TValueConverter.
    // pass through any args to the binding behavior to the TValueConverter
    let sourceExpression = binding.sourceExpression;

    // do create the sourceExpression only once
    if (sourceExpression.rewritten) {
      return;
    }
    sourceExpression.rewritten = true;

    let expression = sourceExpression.expression;
    sourceExpression.expression = new ValueConverter(
      expression,
      't',
      sourceExpression.args,
      [expression, ...sourceExpression.args]);
  }

  unbind(binding, source) {
    // unbind the signal behavior
    this.signalBindingBehavior.unbind(binding, source);
  }
}

UPDATE: Signals are internally supported by value converters already http://aurelia.io/docs/binding/value-converters#signalable-value-converters

import {signalBindings} from 'aurelia-framework';

signalBindings('locale-changed');


export class FlightTimeValueConverter {
  signals = ['locale-changed'];

  toView(date) {
    return date.toLocaleString(window.currentLocale);
  }
}