derived children

[flipside]

I've been looking at how cached derived properties work and figured out a relatively easy way to have them behave like children.

derived: {
    settings: {
        deps: ['id'],
        fn: function () {
            // get settings model from global settingsCollection
            var settings = settingsCollection.get(this.id);
            if (settings) {
                var previousSettings = this._cache['settings'];
                // only update listener if model has changed
                if (previousSettings !== settings) {
                    // if there's a previous model, stop listening to it
                    if (previousSettings)
                        this.stopListening(previousSettings);
                    // listen to new model and bubble its events
                    this.listenTo(settings, 'all', this._getEventBubblingHandler('settings'));
                }
            }
            return settings;
        }
    }
}

This seems pretty useful and might be worth adding this to state with a {listen: true} option added to derived properties to handle all this.

Let me know if I should make a pull request!

[aaronmccall]

This may be a good candidate for an extended version of ampersand-state or a mixin to it—something like ampersand-state-derivedChildren or some such.

[flipside]

don't think this can be mixed in, it touches too many core parts like createDerivedProperty which is used in extend.

got an initial stab at an implementation here

[flipside]

figured out an elegant solution, by allowing type:"state" on derived properties, we can use dataTypes.state.compare to handle events while leaving the door open for custom data types to have similar functionality. Also makes detecting changes in derived properties behave the same as regular properties.

derived-type implementation

[flipside]

submitted a pull request so shutting this down.