.volatile deprecation in 3.9 needs an alternative for "classic" classes

[kategengler]

In 3.9 .volatile is deprecated. The deprecation guide suggests moving to native classes, but that isn't yet possible in all cases, and so we need an alternative to .volatile to allow the deprecation to be resolved where native classes cannot yet be used.

h/t @pzuraq

[rwjblue]

Over in the RFC I mentioned this as well (see emberjs/rfcs#370 (comment)):

---

Because the only path forward involves using native classes, does the deprecation of .volatile() need to be tied to the deprecation of Ember.Object.extend({}) style syntax?

No 😸, and native classes aren't the only way forward (though the alternatives are less ergonomic than ES classes).

For example, you can migrate from:

export default Component.extend({
  init() {
    this._super(...arguments);
    this._foo = 0;
  },

  foo: computed(function() {
    return this._foo++;
  }).volatile()
});

To:

const FooBar = Component.extend({
  init() {
    this._super(...arguments);
    this._foo = 0;
  }
});

Object.defineProperty(FooBar.prototype, 'foo', {
  get() {
    return ++this._foo;
  }
});

export default FooBar;

[kategengler]

Then it may be that just the deprecation guide needs to change.

[kanongil]

I have 2 use cases that aren't covered:

Class extend inheritance:

const A = Component.extend({
  foo: computed(function() {
    return { foo: true };
  }).volatile()
});

const B = Component.extend({
  foo: computed(function() {
    let obj = this._super();
    obj.bar = 123;
    return obj;
  }).volatile()
});

The second case is for an internal addon, which exposes a ComputedProperty instance that needs to call the _getter on all get() calls. This case fundamentally clashes with the deprecation.

[kanongil]

3rd case that is not covered: Relying on dependent properties.

Example:

  currentLimitMs: computed('program.{start,duration}', function () {
    cancel(this._invalidateNow);

    let start = this.get('program.start');
    let duration = this.get('program.duration');

    let now = Date.now();
    let limitMs = now - start;
    if (limitMs < duration) {
      this._invalidateNow = later(() => this.notifyPropertyChange('currentLimitMs'), 1000);
    } else {
      limitMs = duration;
    }

    return limitMs;
  }).volatile(),

Any chance that the deprecation can be reverted?