使对象可见

Question

I've been looking into JavaScript frameworks such as Angular and Meteor lately, and I was wondering how they know when an object property has changed so that they could update the DOM.

I was a bit surprised that Angular used plain old JS objects rather than requiring you to call some kind of getter/setter so that it could hook in and do the necessary updates. My understanding is that they just poll the objects regularly for changes.

But with the advent of getters and setters in JS 1.8.5 , we can do better than that, can't we?

As a little proof-of-concept, I put together this script:

( Edit: updated code to add dependent-property/method support)

function dependentProperty(callback, deps) {
    callback.__dependencies__ = deps;
    return callback;
}

var person = {
    firstName: 'Ryan',
    lastName: 'Gosling',
    fullName: dependentProperty(function() {
        return person.firstName + ' ' + person.lastName;
    }, ['firstName','lastName'])
};

function observable(obj) {
    if (!obj.__properties__) Object.defineProperty(obj, '__properties__', {
        __proto__: null,
        configurable: false,
        enumerable: false,
        value: {},
        writable: false
    });
    for (var prop in obj) {
        if (obj.hasOwnProperty(prop)) {
            if(!obj.__properties__[prop]) obj.__properties__[prop] = {
                value: null,
                dependents: {},
                listeners: []
            };
            if(obj[prop].__dependencies__) {
                for(var i=0; i<obj[prop].__dependencies__.length; ++i) {
                    obj.__properties__[obj[prop].__dependencies__[i]].dependents[prop] = true;
                }
                delete obj[prop].__dependencies__;
            }
            obj.__properties__[prop].value = obj[prop];
            delete obj[prop];
            (function (prop) {
                Object.defineProperty(obj, prop, {
                    get: function () {
                        return obj.__properties__[prop].value;
                    },
                    set: function (newValue) {
                        var oldValue = obj.__properties__[prop].value;
                        if(oldValue !== newValue) {
                            var oldDepValues = {};
                            for(var dep in obj.__properties__[prop].dependents) {
                                if(obj.__properties__[prop].dependents.hasOwnProperty(dep)) {
                                    oldDepValues[dep] = obj.__properties__[dep].value();
                                }
                            }
                            obj.__properties__[prop].value = newValue;
                            for(var i=0; i<obj.__properties__[prop].listeners.length; ++i) {
                                obj.__properties__[prop].listeners[i](oldValue, newValue);
                            }
                            for(dep in obj.__properties__[prop].dependents) {
                                if(obj.__properties__[prop].dependents.hasOwnProperty(dep)) {
                                    var newDepValue = obj.__properties__[dep].value();
                                    for(i=0; i<obj.__properties__[dep].listeners.length; ++i) {
                                        obj.__properties__[dep].listeners[i](oldDepValues[dep], newDepValue);
                                    }
                                }
                            }
                        }
                    }
                });
            })(prop);
        }
    }
    return obj;
}

function listen(obj, prop, callback) {
    if(!obj.__properties__) throw 'object is not observable';
    obj.__properties__[prop].listeners.push(callback);
}

observable(person);

listen(person, 'fullName', function(oldValue, newValue) {
    console.log('Name changed from "'+oldValue+'" to "'+newValue+'"');
});

person.lastName = 'Reynolds';

Which logs:

Name changed from "Ryan Gosling" to "Ryan Reynolds"

The only problem I see is with defining methods such as fullName() on the person object which would depend on the other two properties. This requires a little extra markup on the object to allow developers to specify the dependency.

Other than that, are there any downsides to this approach?

JsFiddle

Answer 1

advent of getters and setters in JS 1.8.5 - are there any downsides to this approach?

• You don't capture any property changes apart from the observed ones. Sure, this is enough for modeled entity objects, and for anything else we could use Proxies.
• It's limited to browsers that support getters/setters, and maybe even proxies. But hey, who does care about outdated browsers? :-) And in restricted environments (Node.js) this doesn't hold at all.
• Accessor properties (with getter and setter) are much slower than real get/set methods. Of course I don't expect them to be used in critical sections, and they can make code looking much fancier. Yet you need to keep that in the back of your mind. Also, the fancy-looking code can lead to misconceptions - normally you would expect property assignment/accessing to be a short ( O(1) ) operation, while with getters/setters there might be a lot of more happening. You will need to care not forgetting that, and the use of actual methods could help.

So if we know what we are doing, yes, we can do better.

Still, there is one huge point we need to remember: the synchronity/asynchronity (also have a look at this excellent answer ). Angular's dirty checking allows you to change a bunch of properties at once, before the event fires in the next event loop turn. This helps to avoid (the propagation of) semantically invalid states.

Yet I see the synchronous getters/setters as a chance as well. They do allow us to declare the dependencies between properties and define the valid states by this. It will automatically ensure the correctness of the model, while we only have to change one property at a time (instead of changing firstName and fullName all the time, firstName is enough). Nevertheless, during dependency resolving that might not hold true so we need to care about it.

So, the listeners that are not related to the dependencies management should be fired asynchronous. Just setImmediate their loop.