$.when()方法翻译

地址：http://api.jquery.com/jQuery.when/

jQuery.when( deferreds )，returns Promise

正文

Description: Provides a way to execute callback functions based on zero or more Thenable objects, usually Deferred objects that represent asynchronous events.

描述：提供一个执行回调函数的方法，它是基于0或多个继发对象的，这些对象通常是表示异步事件的deferred对象。

deferreds

Type: Deferred or Promise or Thenable

Zero or more Thenable objects.

jQuery.when( deferreds )方法是 JQ1.5版本后添加的方法。传入的参数deferreds类型为deferred，promise，thenable，0或多个继发对象。

If no arguments are passed to jQuery.when(), it will return a resolved Promise.

如果不传递参数给when，该方法将返回一个已完成状态的promise。关于Promise（参看：http://www.jianshu.com/p/063f7e490e9a）。

If a single Deferred is passed to jQuery.when(), its Promise object (a subset of the Deferred methods) is returned by the method. Additional methods of the Promise object can be called to attach callbacks, such as deferred.then. When the Deferred is resolved or rejected, usually by the code that created the Deferred originally, the appropriate callbacks will be called. For example, the jqXHR object returned by jQuery.ajax() is a Promise-compatible object and can be used this way:

$.when( $.ajax( "test.aspx" ) ).then(function( data, textStatus, jqXHR ) {

alert( jqXHR.status ); // Alerts 200

});

如果仅将一个deferred传给jQuery.when()，一个Promise对象（deferred对象的子集）将被这个方法返回。promise对象的附加方法将可以附着到回调上被调用。跟deferred一样。通常一个deferred被创建它的原始脚本完成或者失败后，对应的回调将被调用。比如，被jQuery.ajax()返回的jqXHR对象是一个混合的promise对象，可以如下使用：例子见上，不赘述。

If a single argument is passed to jQuery.when() and it is not a Deferred or a Promise, it will be treated as a resolved Deferred and any doneCallbacks attached will be executed immediately. The doneCallbacks are passed the original argument. In this case any failCallbacks you might set are never called since the Deferred is never rejected. For example:

$.when( { testing: 123 } ).done(function( x ) {

alert( x.testing ); // Alerts "123"

});

 如果仅将一个参数传入jQuery.when()，并且它不是一个deferred或者promise对象，它将被当作一个已完成的deferred对象，任何done附着的done回调将立即被执行。done回调将被传入原始参数。这样的话，任何的fail回调都不能设置或者调用，因为deferred永远不会失败。举例如下：例子见引用。

If you don't pass it any arguments at all, jQuery.when() will return a resolved promise.

$.when().then(function( x ) {

alert( "I fired immediately" );

});

如果不传递任何参数，jQuery.when()将返回一个完成状态的promise。

In the case where multiple Deferred objects are passed to jQuery.when(), the method returns the Promise from a new "master" Deferred object that tracks the aggregate state of all the Deferreds it has been passed. The method will resolve its master Deferred as soon as all the Deferreds resolve, or reject the master Deferred as soon as one of the Deferreds is rejected. If the master Deferred is resolved, the doneCallbacks for the master Deferred are executed. The arguments passed to the doneCallbacks provide the resolved values for each of the Deferreds, and matches the order the Deferreds were passed to jQuery.when(). For example:

var d1 = $.Deferred();

var d2 = $.Deferred();

$.when( d1, d2 ).done(function ( v1, v2 ) {

console.log( v1 ); // "Fish"

console.log( v2 ); // "Pizza"

});

d1.resolve( "Fish" );

d2.resolve( "Pizza" );

既然这样，如果传递复合的deferred对象给jQuery.when()，方法将返回一个promise对象，它来自于一个新的主deferred对象，这个对象会跟踪记录下所有传入的deferreds的全部轨迹。这个方法在所有defferreds完成后将立即完成它的主deferred，或者会在所有deferreds失败的时候后立即置为失败。如果主deferred已经完成，它的done回调会被执行。传入到done回调的所有参数提供了所有deferreds的完成值，并且按照传递给jQuery.when()的所有deferred的顺序，例如：例子见上。

再举一个例子：

var dtd = $.Deferred();
var wait=function(dtd){setTimeout(task, 8000);function task(){console.log("i'm jiangtian!");dtd.resolve();}return dtd;
}$.when(wait(dtd)).done(function(){console.log("累死我了，终于执行完了。");
})

阮一峰这篇文章写的相当详细，推荐：《jQuery的deferred对象详解 - 阮一峰的网络日志》

In the event a Deferred was resolved with no value, the corresponding doneCallback argument will be undefined. If a Deferred resolved to a single value, the corresponding argument will hold that value. In the case where a Deferred resolved to multiple values, the corresponding argument will be an array of those values. For example:

var d1 = $.Deferred();

var d2 = $.Deferred();

var d3 = $.Deferred();

$.when( d1, d2, d3 ).done(function ( v1, v2, v3 ) {

console.log( v1 ); // v1 is undefined

console.log( v2 ); // v2 is "abc"

console.log( v3 ); // v3 is an array [ 1, 2, 3, 4, 5 ]

});

d1.resolve();

d2.resolve( "abc" );

d3.resolve( 1, 2, 3, 4, 5 );