Access computed properties without injecting to the cache

[Patrik-Lundqvist]

It would be nice if the computed properties worked with items which have not yet been inserted in to the cache. Example:

var User = DS.defineResource({
  name: 'user',
  computed: {
    fullName: ['first', 'last', function (first, last) {
      return first + ' ' + last;
    }],
  }
});

var newUser = User.createInstance({first:'John', last:'Doe'});

newUser.fullName // Should return 'John Doe'

[jmdobry]

Unfortunately, the way dirty checking works cannot support this. With angular's $scope, dirty-checking does not start until a value is added to the scope. Likewise, dirty-checking does not start until items are injected. In order to setup dirty-checking, observers have to be activated. In order to complete the solution the observers have to be cleaned up when items are ejected. That's the part that is impossible for non-injected items, and this is why I can't implement dirty-checking for non-injected items. Computed properties rely on dirty-checking, so computed properties cannot automatically work for non-injected items. You can however, compute the properties yourself:

function compute (resource, instance) {
  DS.utils.forOwn(resource.computed, function (fn, field) {
    var args = [];
    angular.forEach(fn.deps, function (dep) {
      args.push(instance[dep]);
    });
    // compute property
    instance[field] = fn[fn.length - 1].apply(instance, args);
  });
}

var User = defineResource({
  name: 'name',
  computed: {
    fullName: ['first', 'last', function (first, last) {
      return first + ' ' + last;
    }
  }
});

var user = User.createInstance({ first: 'John', last: 'Doe' });

compute(User, user);

user.fullName; // "John Doe"

It's possible I could add that compute method to the prototype of resource instances, so you could manually re-compute properties, but it would be unavailable if devs use useClass: false.

[Patrik-Lundqvist]

Oh I see, thanks for the explanation! A compute method to invoke seems like a good solution to me.

[jmdobry]

Closing in favor of #136