jquery 之 Deferred 使用与实现

使用 Deferred

在 jQuery 中，实现观察者模式的就是 Deferred 了，我们先看它的使用。你也可以直接看 jQuery 的 Deferred 文档。

这个对象提供了主题和订阅的管理，使用它可以很容易实现一次性的观察者模式。

// 定义主题
var subject = (function(){
var dfd = $.Deferred();

return dfd;
})();

// 两个观察者
var fn1 = function(content){
console.log("fn1: " + content );
}

var fn2 = function(content){
console.log("fn2: " + content );
}

// 注册观察者
$.when( subject )
.done( fn1 )
.done( fn2 );

// 发布内容
subject.resolve("Alice");

通常我们在主题内部来决定什么时候，以及发布什么内容，而不允许在主题之外发布。通过 Deferred 对象的 promise 方法，我们可以只允许在主题之外注册观察者，有点像 .NET 中 event 的处理了。这样，我们的代码就成为下面的形式。

// 定义主题
var subject = (function(){
var dfd = $.Deferred();

var task = function()
{
// 发布内容
dfd.resolve("Alice");
}

setTimeout( task, 3000);

return dfd.promise();
})();

// 两个观察者
var fn1 = function(content){
console.log("fn1: " + content );
}

var fn2 = function(content){
console.log("fn2: " + content );
}

// 注册观察者
$.when( subject )
.done( fn1 )
.done( fn2 );

在 jQuery 中，甚至可以提供两个主题同时被观察, 需要注意的是，要等两个主题都触发之后，才会真正触发，每个观察者一次得到这两个主题，所以参数变成了 2 个。

// 定义主题
var subjectAlice = (function(){
var dfd = $.Deferred();

var task = function()
{
// 发布内容
dfd.resolve("Alice");
}

setTimeout( task, 3000);

return dfd.promise();
})();

var subjectTom = (function(){
var dfd = $.Deferred();

var task = function()
{
// 发布内容
dfd.resolve("Tom");
}

setTimeout( task, 1000);

return dfd.promise();
})();

// 两个观察者
var fn1 = function(content1, content2){
console.log("fn1: " + content1 );
console.log("fn1: " + content2 );
}

var fn2 = function(content1, content2){
console.log("fn2: " + content1 );
console.log("fn2: " + content2 );
}

// 注册观察者
$.when( subjectAlice, subjectTom )
.done( fn1 )
.done( fn2 );

实际上，在 jQuery 中，不仅可以发布成功完成的事件，主题还可以发布其它两种事件：失败和处理中。

失败事件，通过调用主题的 reject 方法可以发布失败的消息，对于观察者来说，需要通过 fail 来注册这个事件了。

处理中事件，通过调用主题的 notify 来发布处理中的消息，对于观察者来说，需要通过 progress 来注册这个事件。

要是观察者想一次性注册多个事件，那么，可以通过 then 来注册，这种方式可以处理主题的成功、失败和处理中三种事件。

$.get( "test.php" ).then(
function() {
alert( "$.get succeeded" );
}, function() {
alert( "$.get failed!" );
}
);

只考虑成功和失败的话，就通过 always 来处理。

$.get( "test.php" )
.always(function() {
alert( "$.get completed with success or error callback arguments" );
});

 

jQuery 中 Deferred 的使用

常用的是 ajax, get, post 等等 Ajax 函数了。它们内部都已经实现为了 Deferred ，返回的结果就是 Deferred 对象了。也就是说你只管写观察者就可以了，主题内部已经处理好了，比如当 ajax 成功之后调用 resolve 来触发 done 事件并传递参数的问题。你可以继续使用传统的回调方式，显然推荐你使用 Deferred 方式了。这样你的代码结构更加清晰。

$.post( "test.php", { name: "John", time: "2pm" })
.done(function( data ) {
alert( "Data Loaded: " + data );
});

 如果需要访问两个 Ajax ，则可以这样

$.when( $.post( "test.php", { name: "John", time: "2pm" }),
$.post( "other.php" ) )
.done(function( data1, data2 ) {
alert( "Data Loaded: " + data1 );
alert( "Data Loaded: " + data2 );
});

 

实现 Deferred

通过前面的使用，其实你一定可以想到，在 Deferred 这个对象的内部，必须有三个回调队列了，这里的成功和失败只能一次完成，所以这两个 Callbacks 都使用了 once 来定义。

var tuples = [
// action, add listener, listener list, final state
[ "resolve", "done", jQuery.Callbacks("once memory"), "resolved" ],
[ "reject", "fail", jQuery.Callbacks("once memory"), "rejected" ],
[ "notify", "progress", jQuery.Callbacks("memory") ]
],

当前处理的状态。

state = "pending",
promise = {
state: function() {
return state;
},

always 就是直接注册了两个事件。then 允许我们一次处理三种注册。

always: function() {
deferred.done( arguments ).fail( arguments );
return this;
},
then: function( /* fnDone, fnFail, fnProgress */ ) {
var fns = arguments;
return jQuery.Deferred(function( newDefer ) {
jQuery.each( tuples, function( i, tuple ) {
var action = tuple[ 0 ],
fn = jQuery.isFunction( fns[ i ] ) && fns[ i ];
// deferred[ done | fail | progress ] for forwarding actions to newDefer
deferred[ tuple[1] ](function() {
var returned = fn && fn.apply( this, arguments );
if ( returned && jQuery.isFunction( returned.promise ) ) {
returned.promise()
.done( newDefer.resolve )
.fail( newDefer.reject )
.progress( newDefer.notify );
} else {
newDefer[ action + "With" ]( this === promise ? newDefer.promise() : this, fn ? [ returned ] : arguments );

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}
});
});
fns = null;
}).promise();
},

deferred 就是一个对象。pipe 是已经过时的用法，是 then 的别名。

deferred = {};

// Keep pipe for back-compat
promise.pipe = promise.then;

// Add list-specific methods
jQuery.each( tuples, function( i, tuple ) {
var list = tuple[ 2 ],
stateString = tuple[ 3 ];

// promise[ done | fail | progress ] = list.add
promise[ tuple[1] ] = list.add;

// Handle state
if ( stateString ) {
list.add(function() {
// state = [ resolved | rejected ]
state = stateString;

// [ reject_list | resolve_list ].disable; progress_list.lock
}, tuples[ i ^ 1 ][ 2 ].disable, tuples[ 2 ][ 2 ].lock );
}

// deferred[ resolve | reject | notify ]
deferred[ tuple[0] ] = function() {
deferred[ tuple[0] + "With" ]( this === deferred ? promise : this, arguments );
return this;
};
deferred[ tuple[0] + "With" ] = list.fireWith;
});

// Make the deferred a promise
promise.promise( deferred );

// Call given func if any
if ( func ) {
func.call( deferred, deferred );
}

// All done!
return deferred;
},

when 是一个助手方法，支持多个主题。

// Deferred helper
when: function( subordinate /* , ..., subordinateN */ ) {
var i = 0,
resolveValues = core_slice.call( arguments ),
length = resolveValues.length,

// the count of uncompleted subordinates
remaining = length !== 1 || ( subordinate && jQuery.isFunction( subordinate.promise ) ) ? length : 0,

// the master Deferred. If resolveValues consist of only a single Deferred, just use that.
deferred = remaining === 1 ? subordinate : jQuery.Deferred(),

// Update function for both resolve and progress values
updateFunc = function( i, contexts, values ) {
return function( value ) {
contexts[ i ] = this;
values[ i ] = arguments.length > 1 ? core_slice.call( arguments ) : value;
if( values === progressValues ) {
deferred.notifyWith( contexts, values );
} else if ( !( --remaining ) ) {
deferred.resolveWith( contexts, values );
}
};
},

progressValues, progressContexts, resolveContexts;

// add listeners to Deferred subordinates; treat others as resolved
if ( length > 1 ) {
progressValues = new Array( length );
progressContexts = new Array( length );
resolveContexts = new Array( length );
for ( ; i < length; i++ ) {
if ( resolveValues[ i ] && jQuery.isFunction( resolveValues[ i ].promise ) ) {
resolveValues[ i ].promise()
.done( updateFunc( i, resolveContexts, resolveValues ) )
.fail( deferred.reject )
.progress( updateFunc( i, progressContexts, progressValues ) );
} else {
--remaining;
}
}
}

// if we're not waiting on anything, resolve the master
if ( !remaining ) {
deferred.resolveWith( resolveContexts, resolveValues );
}

return deferred.promise();
}

完整的代码如下所示：

jQuery.extend({

Deferred: function( func ) {
var tuples = [
// action, add listener, listener list, final state
[ "resolve", "done", jQuery.Callbacks("once memory"), "resolved" ],
[ "reject", "fail", jQuery.Callbacks("once memory"), "rejected" ],
[ "notify", "progress", jQuery.Callbacks("memory") ]
],
state = "pending",
promise = {
state: function() {
return state;
},
always: function() {
deferred.done( arguments ).fail( arguments );
return this;
},
then: function( /* fnDone, fnFail, fnProgress */ ) {
var fns = arguments;
return jQuery.Deferred(function( newDefer ) {
jQuery.each( tuples, function( i, tuple ) {
var action = tuple[ 0 ],
fn = jQuery.isFunction( fns[ i ] ) && fns[ i ];
// deferred[ done | fail | progress ] for forwarding actions to newDefer
deferred[ tuple[1] ](function() {
var returned = fn && fn.apply( this, arguments );
if ( returned && jQuery.isFunction( returned.promise ) ) {
returned.promise()
.done( newDefer.resolve )
.fail( newDefer.reject )
.progress( newDefer.notify );
} else {
newDefer[ action + "With" ]( this === promise ? newDefer.promise() : this, fn ? [ returned ] : arguments );
}
});
});
fns = null;
}).promise();
},
// Get a promise for this deferred
// If obj is provided, the promise aspect is added to the object
promise: function( obj ) {
return obj != null ? jQuery.extend( obj, promise ) : promise;
}
},
deferred = {};

// Keep pipe for back-compat
promise.pipe = promise.then;

// Add list-specific methods
jQuery.each( tuples, function( i, tuple ) {
var list = tuple[ 2 ],
stateString = tuple[ 3 ];

// promise[ done | fail | progress ] = list.add
promise[ tuple[1] ] = list.add;

// Handle state
if ( stateString ) {
list.add(function() {
// state = [ resolved | rejected ]
state = stateString;

// [ reject_list | resolve_list ].disable; progress_list.lock
}, tuples[ i ^ 1 ][ 2 ].disable, tuples[ 2 ][ 2 ].lock );
}

// deferred[ resolve | reject | notify ]
deferred[ tuple[0] ] = function() {
deferred[ tuple[0] + "With" ]( this === deferred ? promise : this, arguments );
return this;
};
deferred[ tuple[0] + "With" ] = list.fireWith;
});

// Make the deferred a promise
promise.promise( deferred );

// Call given func if any
if ( func ) {
func.call( deferred, deferred );
}

// All done!
return deferred;
},

// Deferred helper
when: function( subordinate /* , ..., subordinateN */ ) {
var i = 0,
resolveValues = core_slice.call( arguments ),
length = resolveValues.length,

// the count of uncompleted subordinates
remaining = length !== 1 || ( subordinate && jQuery.isFunction( subordinate.promise ) ) ? length : 0,

// the master Deferred. If resolveValues consist of only a single Deferred, just use that.
deferred = remaining === 1 ? subordinate : jQuery.Deferred(),

// Update function for both resolve and progress values
updateFunc = function( i, contexts, values ) {
return function( value ) {
contexts[ i ] = this;
values[ i ] = arguments.length > 1 ? core_slice.call( arguments ) : value;
if( values === progressValues ) {
deferred.notifyWith( contexts, values );
} else if ( !( --remaining ) ) {
deferred.resolveWith( contexts, values );
}
};
},

progressValues, progressContexts, resolveContexts;

// add listeners to Deferred subordinates; treat others as resolved
if ( length > 1 ) {
progressValues = new Array( length );
progressContexts = new Array( length );
resolveContexts = new Array( length );
for ( ; i < length; i++ ) {
if ( resolveValues[ i ] && jQuery.isFunction( resolveValues[ i ].promise ) ) {
resolveValues[ i ].promise()
.done( updateFunc( i, resolveContexts, resolveValues ) )
.fail( deferred.reject )
.progress( updateFunc( i, progressContexts, progressValues ) );
} else {
--remaining;
}
}
}

// if we're not waiting on anything, resolve the master
if ( !remaining ) {
deferred.resolveWith( resolveContexts, resolveValues );
}

return deferred.promise();
}
});