ampersand-state
An observable, extensible state object with derived watchable properties.
Ampersand-state serves as a base object for ampersand-model but is useful any time you want to track complex state.
ampersand-model extends ampersand-state to include assumptions that you'd want if you're using models to model date from a REST API. But by itself ampersand-state is useful for anytime you want something to model state, that fires events for changes and lets you define and listen to derived properties.
Part of the Ampersand.js toolkit for building clientside applications.
browser support
install
npm install ampersand-state
In pursuit of the ultimate observable JS object.
So much of building an application is managing state. Your app needs a single unadulterated source of truth. But in order to fully de-couple it from everything that cares about it, it needs to be observable.
Typically that's done by allowing you to register handlers for when things change.
In our case it looks like this:
var State = require('ampersand-state');
var Person = State.extend({
props: {
name: 'string',
isDancing: 'boolean'
}
});
var person = new Person({name: 'henrik'});
person.on('change:isDancing', function () {
console.log('shake it!');
});
person.isDancing = true;
So what?! That's boring.
Agreed. Though, there is some more subtle awesomeness in being able to observe changes that are set with a simple assigment: person.isDancing = true
as opposed to person.set('isDancing', true)
(either works, btw), but that's nothing groundbreaking.
So, what else? Well, as it turns out, a huge amount of code that you write in a project is really in describing and tracking relationships between variables.
So, what if our observable layer did that for us too?
Say you wanted to describe a draggable element on a page so you wanted it to follow a set of a rules. You want it to only be considered to have been dragged if it's total delta is > 10 pixels.
var DraggedElementModel = State.extend({
props: {
x: 'number',
y: 'number'
},
derived: {
dragged: {
deps: ['x', 'y'],
fn: function () {
return Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2)) > 10;
}
}
}
});
var element = new DraggedElementModel({x: 0, y: 0});
element.on('change:dragged', function (model, val) {
if (val) {
console.log('element has moved more than 10px');
} else {
console.log('element has moved less than 10px');
}
});
You didn't invent derived properties, pal. </sarcasm>
True, derived properties aren't a new idea. But, being able to clearly declare and derive watchable properties from a model is super useful and in our case, they're just accessed without calling a method. For example, using the draggable example above, the derived property is just element.dragged
.
Handling relationships between objects/models with derived properties
Say you've got an observable that you're using to model data from a RESTful API. Say that you've got a /users
endpoint and when fetching a user, the user data includes a groupID that links them to another collection of groups that we've already fetched and created models for. From our user model we want to be able to easily access the group model. So, when passed to a template we can just access related group information.
Cached, derived properties are perfect for handling this relationship:
var UserModel = State.extend({
props: {
name: 'string',
groupId: 'string'
},
derirved: {
groupModel: {
deps: ['groupId'],
fn: function () {
return ourGroupCollection.get(this.groupId);
}
}
}
});
var user = new UserModel({name: 'henrik', groupId: '2341'});
user.groupModel;
user.on('change:groupModel', function (model, newGroupModel) {
console.log('group changed!', newGroupModel);
});
Cached, derived properties are da shiznit
So, say you have a more "expensive" computation for model. Say you're parsing a long string for URLs and turning them into HTML and then wanting to reference that later. Again, this is built in.
By default, derived properties are cached.
var linkify = require('urlify');
var MySmartDescriptionModel = State.extend({
description: 'string',
derived: {
linkified: {
deps: ['description'],
fn: function () {
return linkify(this.description);
}
}
}
});
var myDescription = new MySmartDescriptionModel({
description: "Some text with a link. http://twitter.com/henrikjoreteg"
});
myDescription.linkified;
With the model above, the descrition will only be run through that linkifier method once, unless of course the description changes.
Derived properties are intelligently triggered
Just because an underlying property has changed, doesn't mean the derived property has.
Cached derived properties will only trigger a change
if the resulting calculated value has changed.
This is super useful if you've bound a derived property to a DOM property. This ensures that you won't ever touch the DOM unless the resulting value is actually different. Avoiding unecessary DOM changes is a huge boon for performance.
This is also important for cases where you're dealing with fast changing attributes.
Say you're drawing a realtime graph of tweets from the Twitter firehose, instead of binding your graph to increment with each tweet, if you know your graph only ticks with every thousand tweets you can easily create a property to watch.
var MyGraphDataModel = State.extend({
props: {
numberOfTweets: 'number'
},
derived: {
thousandTweets: {
deps: ['numberOfTweets'],
fn: function () {
return Math.floor(this.numberOfTweets / 1000);
}
}
}
});
var data = new MyGraphDataModel({numberOfTweets: 555});
var increment = function () {
data.number += 1;
}
setInterval(increment, 50);
data.on('change:thousandTweets', function () {
});
Derived properties don't have to be cached.
Say you want to calculate a value whenever it's accessed. Sure, you can create a non-cached derived property.
If you say cache: false
then it will fire a change
event anytime any of the deps
changes and it will be re-calculated each time its accessed.
State can be extended as many times as you want
Each state object you define will have and extend
method on the constructor.
That means you can extend as much as you want and the definitions will get merged.
var Person = State.extend({
props: {
name: 'string'
},
sayHi: function () {
return 'hi, ' + this.name;
}
});
var AwesomePerson = Person.extend({
props: {
awesomeness: 'number'
}
});
var awesome = new AwesomePerson({
name: 'henrik',
awesomeness: 8
});
awesome.sayHi();
awesome instanceof AwesomePerson;
awesome instanceof Person;
Changelog
- 0.1.0 - lots of cleanup, grabbing tests from human-model, now maintains the prototype chain so
instanceof
checks pass no matter how many times it's been extended. - 0.0.2 - improved doc
- 0.0.1 - initial publish
Credits
@HenrikJoreteg
License
MIT