JavaScript 비동기 프로그래밍에 대한 초보자 가이드

function a(fn) {
//a函数任务...
setTimeout(function() {
fn();
}, 0);
console.log('a');
}
function b(fn) {
//b函数任务
console.log('b');
}
a(b);
以上代码打印值顺序如何呢？结果是先a后b。至于为什么，不得不说说setTimeout。再细微的东西，只要存在，我们都能从中弄出些精华，就比如setTimeout。
请看代码：
···
for (var i = 0; i < 5; i ++) {
setTimeout(function(){
console.log(i);
}, 0);
}
console.log(i);
//5 ; 5 ; 5 ; 5; 5
···

··
function f1(callback) {
setTimeout(function() {
f2(functions() {
setTimeout({
callback(...) ;
}, 0);
});
}, 0);
}
function(callback) {//...}
···

$('#btn').on('myEvent', function(e) {
console.log('There is my Event');
});
$('#btn').trigger('myEvent');
PubSub
var PubSub = function(){
this.handlers = {};
};
PubSub.prototype.subscribe = function(eventType, handler) {
if (!(eventType in this.handlers)) {
this.handlers[eventType] = [];
}
this.handlers[eventType].push(handler); //添加事件监听器
return this;//返回上下文环境以实现链式调用
};
PubSub.prototype.publish = function(eventType) {
var _args = Array.prototype.slice.call(arguments, 1);
for (var i = 0, _handlers = this.handlers[eventType]; i < _handlers.length; i++) {
_handlers[i].apply(this, _args);//遍历事件监听器
}
return this;
};
var event = new PubSub;//构造PubSub实例
event.subscribe(&#39;list&#39;, function(msg) {
console.log(msg);
});
event.publish(&#39;list&#39;, {data: [&#39;one,&#39;, &#39;two&#39;]});
//Object {data: Array[2]}

* jQuery之Promise
var prom = new $.Deferred();
prom.done(function(value) {
console.log(value + ' Done');
});
prom.fail(function(value) {
console.log(value + ' Fail');
});
prom.always(function(v) {
console.log(' always here');
});
prom.resolve('OK');
Deferred是Promise的超集，相对于Promise只提供then方法添加调用，触发这些调用需要其他支持，使用Deferred实例可直接触发调用。
Promise简单实现
function defer() {
var tasks = [],
progresses = [],
state = 'pending',
value,
reason;
return {
resolve: function(_value) {
if (tasks) {
value = ref(_value);
tasks.forEach(function(task) {
nextTick(function() {
value.then.apply(value, task);
});
});
tasks = null;
state = 'resolved';
}else {
if (state === 'resolved') {
throw new Error('A promise should been resolved once.');
}
}
},
reject: function(reason) {
if (tasks) {
value = ref(reason);
tasks.forEach(function(task) {
nextTick(function() {
value.then.apply(value, [task[1], task[0]]);
});
});
tasks = undefined;
state = 'rejected';
}else {
if (state === 'rejected') {
throw new Error('A promise should been rejected once.');
}
}
},
notify: function(progress) {
if (state === 'resolved' || state === 'rejected') {
return;
}
progresses.push(progress);
},
promise: {
then: function(_callback, _errback, _notifyback) {
var deferred = defer();
_callback = _callback || function(value) {
return value;
};
_errback = _errback || function(reason) {
return reject(reason);
};
_notifyback = _notifyback || function(progress) {
return progress;
};
var callback = function(value) {
var result;
try {
result = _callback(value);
}catch(e) {
deferred.reject(e);
}finally {
deferred.resolve(result);
}
};
var errback = function(reason) {
var result;
try {
result = _errback(reason);
}catch(e) {
deferred.reject(e);
}finally {
deferred.resolve(result);
}
}
nextTick(function() {
while (progresses.length) {
try {
_notifyback(progresses.shift());
}catch(e) {
deferred.reject(e);
break;
}
}
});
if (tasks) {
tasks.push([callback, errback]);
}else {
nextTick(function() {
if (state === 'rejected') {
value.then(errback);
}else if (state === 'resolved') {
value.then(callback);
}
});
}
return deferred.promise;
}
}
};
}
var ref = function(value) {
if (value && typeof value.then === 'function') {
return value;
}
return {
then: function(callback) {
return ref(callback(value));
}
}
};
var reject = function(reason) {
return {
then: function(callback, errback) {
return ref(errback(reason));
}
};
};
var nextTick = function(callback) {
setTimeout(callback, 0);
};
//Promise实例
var deferred = defer(),
promise = deferred.promise;
promise.then(function(value) {
console.log(value);
throw 'zhangsan';
}).then(function (v) {
console.log(v);
}, function (err) {
console.error(err);
done();
});
setTimeout(function () {
deferred.resolve('zhangsan1');
}, 1000);