re-compute only on value change

controllers.application.js

import Ember from 'ember';

let get = Ember.get;
let defaultFilter = function(value) { return value; };

function changeGate(dependentKey, filter) {
  filter = filter || defaultFilter;

  let computed = Ember.computed(function handler(key) {
    let lastValueKey = `__changeGate${key}LastValue`;

    let isFirstRun = !this.hasOwnProperty(lastValueKey);
    if (isFirstRun) { //setup an observer which is responsible for notifying property changes
      this[lastValueKey] = filter.call(this, get(this, dependentKey));

      this.addObserver(dependentKey, function() {
        let newValue = filter.call(this, get(this, dependentKey));
        let lastValue = this[lastValueKey];

        if (newValue !== lastValue) {
          this[lastValueKey] = newValue;
          //should be private API?
          //this.notifyPropertyChange(key);
          this.set(key, newValue);
        }
      });
    }

    return this[lastValueKey];
  });
  
  return computed;
};

export default Ember.Controller.extend({
  
  text: "some text",
  
  normalCount: Ember.computed('text', function() {
    return this.get('text').split(' ').length;
  }),
 
  gatedCount: changeGate('text', function() {
    return this.get('text').split(' ').length;
  }),
  
  normalObserverCount: 0,
  normalObserver: Ember.observer('normalCount', function() {
    this.incrementProperty('normalObserverCount');
  }),
  
  gatedObserverCount: 0,
  gatedObserver: Ember.observer('gatedCount', function() {
    this.incrementProperty('gatedObserverCount');
  }),
 
});
  

templates.application.hbs

<h1>re-compute only on value change</h1>

<br>
<br>
{{input value=text style="width: 300px;"}}
<br>
normal count: {{normalCount}}, {{normalObserverCount}}
<br>
gated count: {{gatedCount}}, {{gatedObserverCount}}
<br>
{{outlet}}
<br>
<br>

twiddle.json

{
  "version": "0.10.6",
  "EmberENV": {
    "FEATURES": {},
    "EXTEND_PROTOTYPES": {
      "Date": false
    }
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.4.5",
    "ember-template-compiler": "2.4.5",
    "ember-testing": "2.4.5",
    "ember-data": "2.9.0"
  },
  "addons": {}
}