can-observe

Use

Using can-observe allows you to create observable objects where any property added is immediately observable, including nested objects. This makes can-observe ideal for use-cases where the data may be dynamic, or where the more rigid approach of can-define is unneeded.

To use can-observe call the observe() method with an object. This will return a Proxy object where any changes will reflect back on the target object. Nested objects will be observed lazily when they are accessed or set dynamically after initialization on a can-observe proxy object.

To listen for changes on a property, use canReflect.onKeyValue. Pass the handler as the third argument which will be triggered when a new value is set on the proxy. In the example below, updating dog.name to "Wilbur" will trigger the callback that will console.log the new value "Wilbur".

var canReflect = require("can-reflect");
var dog = observe({});

canReflect.onKeyValue(dog, 'name', function(newVal){
    console.log(newVal); //-> "Wilbur"
});

dog.name = "Wilbur";

can-observe can be combined with any other CanJS observable type, like can-define or can-compute. In this example we create a compute that changes when a can-observe proxy changes. Note that with computes we use canReflect.onValue to set up the event listener and handler.

var compute = require("can-compute");
var observe = require("can-observe");
var canReflect = require("can-reflect");

var person = observe({});

var fullName = compute(function(){
    return person.first + " " + person.last;
});

fullName.on("change", function(ev, newVal){
    console.log(newVal); // -> Chasen Le Hara
});


person.first = "Chasen";
person.last = "Le Hara";

Nested Objects

Any Object property in a can-observe will be replaced with a can-observe observed Proxy on read or write. This allows deep path traversal in objects, with observable changes all along the way.

var observe = require("can-observe");

var name = { first: "Justin", last: "Meyer" };
var person = { 
    name: name
};

var observed = observe(person); 
observed;       // -> observed is a Proxy;
observed.name;  // -> also a Proxy
person.name;    // -> this is a plain object instead

observed.address = { city: "Chicago" };  // this gets proxified on set, so...
person.address // -> this is a Proxy

ES6 Classes

can-observe is specifically designed to work with ES6 classes. To make view models for your can-components from ES6 classes, only a few lines of constructor code are necessary:

import observe from ("can-observe");
import canComponent from ("can-component");
import stache from ("can-stache")

class WidgetViewModel {
    constructor(obj) {
        // view model instances receive properties as an object on instantiation
        Object.assign(this, obj);
        return observe(this);
    }
    get fixedMessage() {
        return "Hello"
    }
    // ... more static and prototype functions.
}

canComponent.extend({
    tag: "my-widget",
    view: stache("<p>{{fixedMessage}}, {{messageFromParent}}</p>"),
    ViewModel: WidgetViewModel
});

<my-widget messageFromParent:from="'world'" />

<!-- above tag will contain "<p>Hello, world!</p>" on render --> 

Browser support

can-observe uses the Proxy feature of JavaScript to observe arbitrary properties. Proxies are available in all modern browsers.

A polyfill is available that brings Proxies back to IE9, with the caveat that only existing properties on the target object can be observed. This means this code:

var person = observe({first: '', last: ''});

The first and last properties are observable in older browsers, but any other property added would not be. To ensure maximum compatibility make sure to give all properties a default value.