JSDeferred

JSDeferred Structure

Simple and clean asynchronous processing.

Sample

Download

jsdeferred.js
No comments: jsdeferred.nodoc.js
With jQuery supports: jsdeferred.jquery.js

Repository:

git clone git://github.com/cho45/jsdeferred.git

For userscript

Copy and paste following at end of your userscript:

jsdeferred.userscript.js

Example:

with (D()) {

next(fun...);

// normal xhr
http.get("...").
next(fun...);

// cross site
xhttp.get("...").
next(fun...);

}

// pasted code
function D () {
...JSDeferred...
}

See binding/userscript.js to get more information of utility functions (http.get/xhttp.get)

Documentation

See source.

Introduction

doc/index.html

Tests

test.html

License

MIT. See header of jsdeferred.js

Concept

Compact
Greasemonkey friendly (standalone and compact...)
Method chain
Short and meaning function names
Useful shorthand ways

Internal

This sections use some words as following meanings.

chain:: a sequence of processes. child:: the Deferred which returns from a callback. Deferred#foobar:: Deferred.prototype.foobar

Deferred structure and chain structure

A Deferred object has only one callback as its process. Deferred object packages a process (function) as callback and has reference to next Deferred (this is thought like continuation).

Example for understanding Deferred structure.

var d1 = new Deferred();
d1.callback.ok = function () {
    alert("1");
};

var d2 = new Deferred();
d2.callback.ok = function () {
    alert("2");
};

// Set d2 as continuation of d1.
d1._next = d2;

// Invoke the chain.
d1.call();

And example for usual use.

next(function () { // this `next` is global function
    alert("1");
}).
next(function () { // this `next` is Deferred#next
    alert("2");
}).
next(function () {
    alert("3");
});

Deferred#next creates new Deferred, sets the passed functions to process of it, sets it as continuation of this and returns it.

This structure makes easy to chain child Deferreds.

next(function () {
    alert("1");
}).
next(function () {
    alert("2");
    // child Deferred
    return next(function () {
        alert("3");
    });
}).
next(function () {
    alert("4");
});

When the callback returns Deferred, the Deferred calling the callback only sets its continuation (_next) to returned Deferred's continuation.

next(function () {
    alert("1");
}).
next(function () {
    alert("2");
    var d = next(function () {
        alert("3");
    });
    d._next = this._next;
    this.cancel();
}).
next(function () {
    alert("4");
});

After the process, above code is same as following:

next(function () {
    alert("1");
}).
next(function () {
    alert("2");
    next(function () {
        alert("3");
    }).
    next(function () {
        alert("4");
    });
});

Chain child deferred

Error processing and recovering

A Deferred has also error-back for error processing. Let's just see an example (this is from test):

next(function () {
    throw "Error";
}).
error(function (e) {
    expect("Errorback called", "Error", e);
    return e; // recovering error
}).
next(function (e) {
    expect("Callback called", "Error", e);
    throw "Error2";
}).
next(function (e) {
    // This process is not called because
    // the error is not recovered.
    ng("Must not be called!!");
}).
error(function (e) {
    expect("Errorback called", "Error2", e);
});

The error thrown in callback is propagated by error-back chain. If the error-back returns normal value, the error is considered as recovery, and the callback chain continues.

Difference between MochiKit and JSDeferred

JSDeferred is inspired by MochiKit Deferred object, so both is similar but JSDeferred drops some functions and simplified it.

MochiKit Deferred has chain by Array. JSDeferred has chain by chain of Deferred.
MochiKit Deferred separates parent chain and child chain, JSDeferred not.

Comparison of other deferred-like objects

vs jQuery's Deferred

Bad: Not supports child deferred
Bad: No utility functions
Good: Simple
Good: Cache result (it can be called after completed the task)
Good: jQuery embed (no more dependencies and compact)

vs Dojo's Deferred

Bad: Not supports child deferred
Bad: Very heavy dependencies for dojo
Good: Support progress

vs CommonJS's Promise

Bad: Not implemeneted?
Bad: Not supports child deferred
Bad: Too complex
Good: Highlier functional

Different-origin Deferred instances

JSDeferred can be used in inter-environment which is independent respectively like browser extension because JSDeferred determines a self-class identity by instance id.

Author

Portfolio: www.lowreal.net
Diary (Japanese): subtech

cho45.stfuawsc.com