one way mutable value that resets

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  didReceiveAttrs() {
    this._super(...arguments);
    this.set('downstream', this.get('upstream'));
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  upstream: 100,

  actions: {
    add100ToUpstream() {
      this.set('upstream', this.get('upstream') + 100);
    }
  }

});

templates.application.hbs

<h2>From the application controller</h2>

Upstream: {{upstream}}<br/>

<br/>

<a href="#" {{action "add100ToUpstream"}}>Add 100 to upstream</a> <br/>

<br />

<hr />

{{my-component upstream=upstream}}

templates.components.my-component.hbs

<h2>From the component</h2>

Upstream: {{upstream}}<br/>
Downstream: {{downstream}}<br/>

<br/>

<a href="#" {{action (mut downstream) 300}}>Set downstream to 300</a> <br/>

twiddle.json

{
  "version": "0.11.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.10.2",
    "ember-data": "2.11.0",
    "ember-template-compiler": "2.10.2",
    "ember-testing": "2.10.2"
  },
  "addons": {}
}