[javascript] Promise简单学习使用详解编程语言

<script> 
//Promise代码部分（我选择狗带） 
Promise = function() { 
this.queue = []; 
this.value = null; 
this.status = 'pending';// pending fulfilled rejected 
}; 
Promise.prototype.getQueue = function() { 
return this.queue; 
}; 
Promise.prototype.getStatus = function() { 
return this.status; 
}; 
Promise.prototype.setStatus = function(s, value) { 
if (s === 'fulfilled' || s === 'rejected') { 
this.status = s; 
this.value = value || null; 
this.queue = []; 
var freezeObject = Object.freeze || function(){}; 
freezeObject(this);// promise的状态是不可逆的 
} else { 
throw new Error({ 
message: "doesn't support status: " + s 
}); 
} 
}; 
Promise.prototype.isFulfilled = function() { 
return this.status === 'fulfilled'; 
}; 
Promise.prototype.isRejected = function() { 
return this.status === 'rejected'; 
} 
Promise.prototype.isPending = function() { 
return this.status === 'pending'; 
} 
Promise.prototype.then = function(onFulfilled, onRejected) { 
var handler = { 
'fulfilled': onFulfilled, 
'rejected': onRejected 
}; 
handler.deferred = new Deferred(); 
if (!this.isPending()) {//这里允许先改变promise状态后添加回调 
utils.procedure(this.status, handler, this.value); 
} else { 
this.queue.push(handler);//then may be called multiple times on the same promise;规范2.2.6 
    } 
return handler.deferred.promise;//then must return a promise;规范2.2.7 
}; 
var utils = (function(){ 
var makeSignaler = function(deferred, type) { 
return function(result) { 
transition(deferred, type, result); 
} 
}; 
var procedure = function(type, handler, result) { 
var func = handler[type]; 
var def = handler.deferred; 
if (func) { 
try { 
var newResult = func(result); 
if (newResult && typeof newResult.then === 'function') {//thenable 
// 此种写法存在闭包容易造成内存泄露，我们通过高阶函数解决 
// newResult.then(function(data) { 
//     def.resolve(data); 
// }, function(err) { 
//     def.reject(err); 
// }); 
//PromiseA+规范，x代表newResult，promise代表def.promise 
//If x is a promise, adopt its state [3.4]: 
//If x is pending, promise must remain pending until x is fulfilled or rejected. 
//If/when x is fulfilled, fulfill promise with the same value. 
//If/when x is rejected, reject promise with the same reason. 
newResult.then(makeSignaler(def, 'fulfilled'), makeSignaler(def, 'rejected'));//此处的本质是利用了异步闭包 
} else { 
transition(def, type, newResult); 
} 
} catch(err) { 
transition(def, 'rejected', err); 
} 
} else { 
transition(def, type, result); 
} 
}; 
var transition = function(deferred, type, result) { 
if (type === 'fulfilled') { 
deferred.resolve(result); 
} else if (type === 'rejected') { 
deferred.reject(result); 
} else if (type !== 'pending') { 
throw new Error({ 
'message': "doesn't support type: " + type 
}); 
} 
}; 
return { 
'procedure': procedure 
} 
})(); 
Deferred = function() { 
this.promise = new Promise(); 
}; 
Deferred.prototype.resolve = function(result) { 
if (!this.promise.isPending()) { 
return; 
} 
var queue = this.promise.getQueue(); 
for (var i = 0, len = queue.length; i < len; i++) { 
utils.procedure('fulfilled', queue[i], result); 
} 
this.promise.setStatus('fulfilled', result); 
}; 
Deferred.prototype.reject = function(err) { 
if (!this.promise.isPending()) { 
return; 
} 
var queue = this.promise.getQueue(); 
for (var i = 0, len = queue.length; i < len; i++) { 
utils.procedure('rejected', queue[i], err); 
} 
this.promise.setStatus('rejected', err); 
} 
/*****************************上面看不懂，分割线************************************/ 
//测试部分 
ajax = function(url) { 
var def = new Deferred(); 
var xhr = new XMLHttpRequest(); 
xhr.onreadystatechange = function() { 
if (xhr.readyState === 4) { 
if ((xhr.status >=200 && xhr.status < 300) || xhr.status === 304) { 
def.resolve(xhr.responseText) 
} else {//简化ajax，没有提供错误回调 
def.reject(new Error({ 
message: xhr.status 
})); 
} 
} 
}; 
xhr.open('get', url, true); 
xhr.send(null); 
return def.promise; 
} 
ajax('test.php?act=1').then(function(data1) { 
console.log(data1);//处理data1 
return ajax('test.php?act=2'); 
}).then(function(data2) { 
console.log(data2);//处理data2 
return ajax('test.php?act=3'); 
}, function(err) { 
console.error(err); 
}).then(function(data3) { 
console.log(data3); 
alert('success'); 
}, function(err) { 
console.error(err); 
}); 
</script>

<?php 
if($_GET['act']==1){ 
echo json_encode(array("code"=>200)); 
}else if($_GET['act']==2){ 
echo json_encode(array("code"=>300)); 
}else if($_GET['act']==3){ 
echo json_encode(array("code"=>400)); 
}