## What are Computed Properties?

In a nutshell, computed properties let you declare functions as properties. You create one by defining a computed property as a function, which Ember will automatically call when you ask for the property. You can then use it the same way you would any normal, static property.

It's super handy for taking one or more normal properties and transforming or manipulating their data to create a new value.

### Computed properties in action

We'll start with a simple example:

```javascript
Person = Ember.Object.extend({
  // these will be supplied by `create`
  firstName: null,
  lastName: null,

  fullName: Ember.computed('firstName', 'lastName', function() {
    return `${this.get('firstName')} ${this.get('lastName')}`;
  })
});

let ironMan = Person.create({
  firstName: 'Tony',
  lastName:  'Stark'
});

ironMan.get('fullName'); // "Tony Stark"
```

This declares `fullName` to be a computed property, with `firstName` and `lastName` as the properties it depends on. The
first time you access the `fullName` property, the function backing the computed property (i.e. the last argument) will
be run and the results will be cached. Subsequent access of `fullName` will read from the cache without calling the
function.  Changing any of the dependent properties causes the cache to invalidate, so that the computed function runs
again on the next access.

When you want to depend on a property which belongs to an object, you can setup multiple dependent keys by using brace expansion:

```javascript
let obj = Ember.Object.extend({
  baz: {foo: 'BLAMMO', bar: 'BLAZORZ'},

  something: Ember.computed('baz.{foo,bar}', function() {
    return this.get('baz.foo') + ' ' + this.get('baz.bar');
  })
});
```

This allows you to observe both `foo` and `bar` on `baz` with much less duplication/redundancy
when your dependent keys are mostly similar.

### Chaining computed properties

You can use computed properties as values to create new computed properties. Let's add a `description` computed property to the previous example, and use the existing `fullName` property and add in some other properties:

```javascript
Person = Ember.Object.extend({
  firstName: null,
  lastName: null,
  age: null,
  country: null,

  fullName: Ember.computed('firstName', 'lastName', function() {
    return `${this.get('firstName')} ${this.get('lastName')}`;
  }),

  description: Ember.computed('fullName', 'age', 'country', function() {
    return `${this.get('fullName')}; Age: ${this.get('age')}; Country: ${this.get('country')}`;
  })
});

let captainAmerica = Person.create({
  firstName: 'Steve',
  lastName: 'Rogers',
  age: 80,
  country: 'USA'
});

captainAmerica.get('description'); // "Steve Rogers; Age: 80; Country: USA"
```

### Dynamic updating

Computed properties, by default, observe any changes made to the properties they depend on and are dynamically updated when they're called. Let's use computed properties to dynamically update.

```javascript
captainAmerica.set('firstName', 'William');

captainAmerica.get('description'); // "William Rogers; Age: 80; Country: USA"
```

So this change to `firstName` was observed by `fullName` computed property, which was itself observed by the `description` property.

Setting any dependent property will propagate changes through any computed properties that depend on them, all the way down the chain of computed properties you've created.

### Setting Computed Properties

You can also define what Ember should do when setting a computed property.
If you try to set a computed property, it will be invoked with the key (property name), and the value you want to set it to.
You must return the new intended value of the computed property from the setter function.

```javascript
Person = Ember.Object.extend({
  firstName: null,
  lastName: null,

  fullName: Ember.computed('firstName', 'lastName', {
    get(key) {
      return `${this.get('firstName')} ${this.get('lastName')}`;
    },
    set(key, value) {
      let [firstName, lastName] = value.split(/\s+/);
      this.set('firstName', firstName);
      this.set('lastName',  lastName);
      return value;
    }
  })
});


let captainAmerica = Person.create();
captainAmerica.set('fullName', 'William Burnside');
captainAmerica.get('firstName'); // William
captainAmerica.get('lastName'); // Burnside
```

### Computed property macros

Some types of computed properties are very common. Ember provides a number of
computed property macros, which are shorter ways of expressing certain types
of computed property.

In this example, the two computed properties are equivalent:

```javascript
Person = Ember.Object.extend({
  fullName: 'Tony Stark',

  isIronManLongWay: Ember.computed('fullName', function() {
    return this.get('fullName') === 'Tony Stark';
  }),

  isIronManShortWay: Ember.computed.equal('fullName', 'Tony Stark')
});
```

To see the full list of computed property macros, have a look at
[the API documentation](http://emberjs.com/api/classes/Ember.computed.html)
