# Class: Observable

A class with a get/set interface that makes it easy to manipulate data
on an object allowing others to observe all or only specific changes.

### Extends

[EventEmitter](./events.md#EventEmitter)

## Method: set

Set a property that will trigger a change event if the value is
different.

### Syntax

	observable.set(property, value);
	observable.set(properties);

### Arguments

- Two arguments (property, value)
    - property - (_string_) A key corresponding to the same name of one
      of the model's `fields`.
    - value - (_mixed_) The value to set for the speific field.
- One argument (properties)
    - properties - (_object_) An object of key-value pairs that work
      like calling `set` with each pair individually as the two-argument
      path.

### Returns

- (object) This Observable instance.

## Method: get

Retrieve a value from the object. This allows computed properties to
work seemlessly.

### Syntax

	var value = observable.get(keyOrKeys);

### Arguments

- keyOrKeys - (_string or array_) A string or array of strings of
  properties to return the value of.

### Returns

- (_string or array_) The value of the property requested, or an array of the values
  if an array was passed as an argument.

## Method: observe

Specify a function that should be called when the observed property
changes. A shortcut for listening to change events for only a specific
property.

### Syntax

	observable.observe(property, callback);

### Arguments

- property - (_string_) The property name to observe.
- callback - (_function_) The function to execute when the property
  changes. It will be passed as arguments the new value, and the old
  value: `function (newVal, oldVal) {}`

## Mutator: onEvent

Any property matching the regular expression `/^on[A-Z]/` (such as
onClick) will become a default handler for every instance of the class.

It can also be used at instantiation for a single instance.

### Example

	var Foo = new Class({
		Extends: Observable,
		onFoo: function() {
			console.log('bar!');
		}
	});

	var f = new Foo({
		onBar: function() {
			console.log('baz');
		}
	});

	f.emit('foo'); // logged "bar!"
	f.emit('bar'); // logged "baz"

## Static Method: property

A helper to declare computed properties and the properties they depend
on.

### Syntax

	var computedProp = property(fn, keys...);

### Arguments

- fn - (_function_) The function to execute when being `set` or `get`.
- keys - (_strings_) A string of each property that this one depends on.

### Example

	var Person = new Class({
		Extends: Observable,
		firstName: 'John',
		lastName: 'Doe',
		fullName: Observable.property(function(name) {
			if (arguments.length === 0) {
				// getter
				return this.get('firstName') + ' ' + this.get('lastName');
			} else {
				// setter
				var split = name.split(' ');
				this.set({
					'firstName': split[0],
					'lastName': split[1]
				});
			}
		}, 'firstName', 'lastName')
	});