CODEWITHSUNDEEP
ember.js
Asfand Qazi
Asfand Qazi

Reputation: 6875

Why is the computed property being updated even though I didn't specify its dependencies?

I have a schema structured something like this:

App = {};

App.Outer = Ember.Object.extend({
    inner: null,
    quantity: 0,

    count: function () {
        var self = this, inner = self.get('inner');
        return self.get('quantity') * inner.get('count');
    }.property('nothing')
});

App.Inner = Ember.Object.extend({
    count: 0
});

Yes, the 'count' computed property really is set to depend on a totally nonexistent property 'nothing'. However it seems to get updated anyway:

var o1 = App.Outer.create({
    quantity: 2,
    inner: App.Inner.create({count: 4})
});

console.log(o1.get('count')); // => 8
o1.get('inner').set('count', 5);
console.log(o1.get('count')); // => 10
o1.set('inner', App.Inner.create({count: 10}));
console.log(o1.get('count')); // => 20

Am I missing something? It knows what to update without me telling it what to depend on... can't be right, can it? What am I misunderstanding about Ember computed properties?

Thanks

Upvotes: 2

Views: 1648

Answers (2)

Luke Melia
Luke Melia

Reputation: 8389

As of Ember 0.9.5, property values are not cached unless cacheable() is called on them. e.g.

...

count: function () {
    var self = this, inner = self.get('inner');
    return self.get('quantity') * inner.get('count');
}.property('nothing').cacheable()

...

For more background, see the discussion on this GitHub issue: https://github.com/emberjs/ember.js/issues/38

Upvotes: 3

Tom Whatmore
Tom Whatmore

Reputation: 1327

By using this.get('quantity'), inner.get('count') you are telling it what it depends on. Every time you call .get('count') the function will go off and get the current values for those properties and therefore return the up to date result.

The .property() part comes into play when you bind the computed property count to something else e.g. a view. When you do that then making a change to quantity will automatically recalculate the count, and this new value will be propagated to whatever you have bound the count too.

You can see the difference in action here: http://jsfiddle.net/tomwhatmore/6gz8x/

Upvotes: 7

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