Class Observable

public

Overview

This mixin provides properties and property observing functionality, core features of the Ember object model.

Properties and observers allow one object to observe changes to a property on another object. This is one of the fundamental ways that models, controllers and views communicate with each other in an Ember application.

Any object that has this mixin applied can be used in observer operations. That includes EmberObject and most objects you will interact with as you write your Ember application.

Note that you will not generally apply this mixin to classes yourself, but you will use the features provided by this module frequently, so it is important to understand how to use it.

Using get() and set()

Because of Ember's support for bindings and observers, you will always access properties using the get method, and set properties using the set method. This allows the observing objects to be notified and computed properties to be handled properly.

More documentation about get and set are below.

Observing Property Changes

You typically observe property changes simply by using the observer function in classes that you write.

For example:

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import { observer } from '@ember/object';
import EmberObject from '@ember/object';

EmberObject.extend({
  valueObserver: observer('value', function(sender, key, value, rev) {
    // Executes whenever the "value" property changes
    // See the addObserver method for more information about the callback arguments
  })
});

Although this is the most common way to add an observer, this capability is actually built into the EmberObject class on top of two methods defined in this mixin: addObserver and removeObserver. You can use these two methods to add and remove observers yourself if you need to do so at runtime.

To add an observer for a property, call:

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object.addObserver('propertyKey', targetObject, targetAction)

This will call the targetAction method on the targetObject whenever the value of the propertyKey changes.

Note that if propertyKey is a computed property, the observer will be called when any of the property dependencies are changed, even if the resulting value of the computed property is unchanged. This is necessary because computed properties are not computed until get is called.

Show:

key
String
The key to observe
target
Object
The target object to invoke
method
String|Function
The method to invoke
returns
Observable

Adds an observer on a property.

This is the core method used to register an observer for a property.

Once you call this method, any time the key's value is set, your observer will be notified. Note that the observers are triggered any time the value is set, regardless of whether it has actually changed. Your observer should be prepared to handle that.

Observer Methods

Observer methods have the following signature:

component.js
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import Component from '@ember/component';

export default Component.extend({
  init() {
    this._super(...arguments);
    this.addObserver('foo', this, 'fooDidChange');
  },

  fooDidChange(sender, key, value, rev) {
    // your code
  }
});

The sender is the object that changed. The key is the property that changes. The value property is currently reserved and unused. The rev is the last property revision of the object when it changed, which you can use to detect if the key value has really changed or not.

Usually you will not need the value or revision parameters at the end. In this case, it is common to write observer methods that take only a sender and key value as parameters or, if you aren't interested in any of these values, to write an observer that has no parameters at all.

keyName
String
returns
Object
The cached value of the computed property, if any

Returns the cached value of a computed property, if it exists. This allows you to inspect the value of a computed property without accidentally invoking it if it is intended to be generated lazily.

keyName
String
The name of the property to decrement
decrement
Number
The amount to decrement by. Defaults to 1
returns
Number
The new property value

Set the value of a property to the current value minus some amount.

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player.decrementProperty('lives');
orc.decrementProperty('health', 5);
keyName
String
The property to retrieve
returns
Object
The property value or undefined.

Retrieves the value of a property from the object.

This method is usually similar to using object[keyName] or object.keyName, however it supports both computed properties and the unknownProperty handler.

Because get unifies the syntax for accessing all these kinds of properties, it can make many refactorings easier, such as replacing a simple property with a computed property, or vice versa.

Computed Properties

Computed properties are methods defined with the property modifier declared at the end, such as:

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import { computed } from '@ember/object';

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

When you call get on a computed property, the function will be called and the return value will be returned instead of the function itself.

Unknown Properties

Likewise, if you try to call get on a property whose value is undefined, the unknownProperty() method will be called on the object. If this method returns any value other than undefined, it will be returned instead. This allows you to implement "virtual" properties that are not defined upfront.

list
String...|Array
of keys to get
returns
Object

To get the values of multiple properties at once, call getProperties with a list of strings or an array:

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record.getProperties('firstName', 'lastName', 'zipCode');
// { firstName: 'John', lastName: 'Doe', zipCode: '10011' }

is equivalent to:

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record.getProperties(['firstName', 'lastName', 'zipCode']);
// { firstName: 'John', lastName: 'Doe', zipCode: '10011' }
keyName
String
The name of the property to retrieve
defaultValue
Object
The value to return if the property value is undefined
returns
Object
The property value or the defaultValue.

Retrieves the value of a property, or a default value in the case that the property returns undefined.

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person.getWithDefault('lastName', 'Doe');
keyName
String
The name of the property to increment
increment
Number
The amount to increment by. Defaults to 1
returns
Number
The new property value

Set the value of a property to the current value plus some amount.

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person.incrementProperty('age');
team.incrementProperty('score', 2);
keyName
String
The property key to be notified about.
returns
Observable

Notify the observer system that a property has just changed.

Sometimes you need to change a value directly or indirectly without actually calling get() or set() on it. In this case, you can use this method instead. Calling this method will notify all observers that the property has potentially changed value.

key
String
The key to observe
target
Object
The target object to invoke
method
String|Function
The method to invoke
returns
Observable

Remove an observer you have previously registered on this object. Pass the same key, target, and method you passed to addObserver() and your target will no longer receive notifications.

keyName
String
The property to set
value
Object
The value to set or `null`.
returns
Object
The passed value

Sets the provided key or path to the value.

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record.set("key", value);

This method is generally very similar to calling object["key"] = value or object.key = value, except that it provides support for computed properties, the setUnknownProperty() method and property observers.

Computed Properties

If you try to set a value on a key that has a computed property handler defined (see the get() method for an example), then set() will call that method, passing both the value and key instead of simply changing the value itself. This is useful for those times when you need to implement a property that is composed of one or more member properties.

Unknown Properties

If you try to set a value on a key that is undefined in the target object, then the setUnknownProperty() handler will be called instead. This gives you an opportunity to implement complex "virtual" properties that are not predefined on the object. If setUnknownProperty() returns undefined, then set() will simply set the value on the object.

Property Observers

In addition to changing the property, set() will also register a property change with the object. Unless you have placed this call inside of a beginPropertyChanges() and endPropertyChanges(), any "local" observers (i.e. observer methods declared on the same object), will be called immediately. Any "remote" observers (i.e. observer methods declared on another object) will be placed in a queue and called at a later time in a coalesced manner.

hash
Object
the hash of keys and values to set
returns
Object
The passed in hash

Sets a list of properties at once. These properties are set inside a single beginPropertyChanges and endPropertyChanges batch, so observers will be buffered.

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record.setProperties({ firstName: 'Charles', lastName: 'Jolley' });
keyName
String
The name of the property to toggle
returns
Boolean
The new property value

Set the value of a boolean property to the opposite of its current value.

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starship.toggleProperty('warpDriveEngaged');