when

API

提供了一个基于一个或多个对象的状态来执行回调函数的功能,通常是基于具有异步事件的异步队列

jQuery.when( deferreds )

example

$.when(
   $.ajax(),
   $.ajax(),
   $.ajax(),
).done(whenDone).fail(whenFail)

//whenDone里得到的数组是ajax里全部success时的得到的参数

结构

这个的代码相对好理解.

function adoptValue( value, resolve, reject ) {
    var method;
    try {
        //  检查副本
        if ( value && jQuery.isFunction( ( method = value.promise ) ) ) {
            method.call( value ).done( resolve ).fail( reject );
        //  检查then
        } else if ( value && jQuery.isFunction( ( method = value.then ) ) ) {
            method.call( value, resolve, reject );
        //  其他情况
        } else {
            resolve.call( undefined, value );
        }
    } catch ( value ) {
        reject.call( undefined, value );
    }
}

jQuery.extend( {
    when: function( singleValue ) {
        var

            // count of uncompleted subordinates
            remaining = arguments.length,

            // count of unprocessed arguments
            i = remaining,

            // subordinate fulfillment data
            resolveContexts = Array( i ),
            resolveValues = slice.call( arguments ),

            // the master Deferred
            master = jQuery.Deferred(),

            // subordinate callback factory
            updateFunc = function( i ) {
                return function( value ) {
                    resolveContexts[ i ] = this;
                    resolveValues[ i ] = arguments.length > 1 ? slice.call( arguments ) : value;
                    if ( !( --remaining ) ) {
                        master.resolveWith( resolveContexts, resolveValues );
                    }
                };
            };

        // Single- and empty arguments are adopted like Promise.resolve
        if ( remaining <= 1 ) {
            adoptValue( singleValue, master.done( updateFunc( i ) ).resolve, master.reject );

            // Use .then() to unwrap secondary thenables (cf. gh-3000)
            if ( master.state() === "pending" ||
                jQuery.isFunction( resolveValues[ i ] && resolveValues[ i ].then ) ) {

                return master.then();
            }
        }

        // Multiple arguments are aggregated like Promise.all array elements
        while ( i-- ) {
            adoptValue( resolveValues[ i ], updateFunc( i ), master.reject );
        }

        return master.promise();

})