[javascript] Promise简单学习使用
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Promise的诞生与javascript中异步编程息息相关,js中异步编程主要指的是setTimout/setInterval、DOM事件机制、ajax,通过传入回调函数实现控制反转。异步编程为js带来强大灵活性的同时,也带来了嵌套回调的问题。详细来说主要有两点,第一嵌套太深代码可读性太差,第二并行逻辑必须串行执行。
1 request = function(url, cb, eb) { 2 var xhr = new XMLHttpRequest(); 3 xhr.onreadystatechange = function() { 4 if (xhr.readyState === 4) { 5 if ((xhr.status >=200 && xhr.status < 300) || xhr.status === 304) { 6 cb(xhr.responseText); 7 } else { 8 eb(new Error({ 9 message: xhr.status 10 })); 11 } 12 } 13 }; 14 xhr.open(\'get\', url, true); 15 xhr.send(null); 16 }
这个例子中程序要依次处理data1、data2、data3,嵌套太多可读性太差
1 //回调函数嵌套过深 2 request(\'data1.json\', function(data1){ 3 console.log(data1);//处理data1 4 request(\'data2.json\', function(data2) { 5 console.log(data2);//处理data2 6 request(\'data3.json\', function(data3) { 7 console.log(data3);//处理data3 8 9 alert(\'success\'); 10 }, function(err) { 11 console.error(err); 12 }); 13 }, function(err) { 14 console.error(err); 15 }); 16 }, function(err) { 17 console.error(err); 18 });
这个例子中程序需要请求data1、data2、data3数据,得到三个数据后才进行下一步处理。数据并不需要串行请求,但我们的代码却需要串行执行,增加了等待时间。
1 //并行逻辑串行执行 2 request(\'data1\', function(data1) { 3 request(\'data2\', function(data2) { 4 request(\'data3\', function(data3) { 5 console.log(data1, data2, data3);//处理全部数据 6 7 alert(\'success\'); 8 }, function(err) { 9 console.error(err); 10 }); 11 }, function(err) { 12 console.error(err); 13 }); 14 }, function(err) { 15 console.error(err); 16 });
Promise机制
Promise机制便是上述问题的一种解决方案。与他相关的规范有PromiseA和PromiseA+,PromiseA中对Promise进行了整体描述,PromiseA+对A进行了补充,在then函数的行为方面进行了更加详尽的阐述。
then
method, which registers callbacks to receive either a promise’s eventual value or the reason why the promise cannot be fulfilled.跟promise交互的主要方式是通过他的then方法来注册回调函数去接收promise的最终结果值或者是promise不能完成的原因。我们可以简单总结一下规范。每个promise都有三个状态:pending(默认)、fulfilled(完成)、rejected(失败);默认状态可以转变为完成态或失败态,完成态与失败态之间无法相互转换,转变的过程是不可逆的,转变一旦完成promise对象就不能被修改。通过promise提供的then函数注册onFulfill(成功回调)、onReject(失败回调)、onProgres(进度回调)来与promise交互。Then函数返回一个promise对象(称为promise2,前者成为promise1),promise2受promise1状态的影响,具体请查看A+规范。
上两个规范中并没有说明promise的状态如何改变,大部分前端框架中使用Deferred来改变promise的状态(resolve()、reject())。二者关系请看下图。
这里根据规范,我们实现一下promise
1 Promise = function() { 2 this.queue = []; 3 this.value = null; 4 this.status = \'pending\';// pending fulfilled rejected 5 }; 6 7 Promise.prototype.getQueue = function() { 8 return this.queue; 9 }; 10 Promise.prototype.getStatus = function() { 11 return this.status; 12 }; 13 Promise.prototype.setStatus = function(s, value) { 14 if (s === \'fulfilled\' || s === \'rejected\') { 15 this.status = s; 16 this.value = value || null; 17 this.queue = []; 18 var freezeObject = Object.freeze || function(){}; 19 freezeObject(this);// promise的状态是不可逆的 20 } else { 21 throw new Error({ 22 message: "doesn\'t support status: " + s 23 }); 24 } 25 }; 26 Promise.prototype.isFulfilled = function() { 27 return this.status === \'fulfilled\'; 28 }; 29 Promise.prototype.isRejected = function() { 30 return this.status === \'rejected\'; 31 } 32 Promise.prototype.isPending = function() { 33 return this.status === \'pending\'; 34 } 35 Promise.prototype.then = function(onFulfilled, onRejected) { 36 var handler = { 37 \'fulfilled\': onFulfilled, 38 \'rejected\': onRejected 39 }; 40 handler.deferred = new Deferred(); 41 42 if (!this.isPending()) {//这里允许先改变promise状态后添加回调 43 utils.procedure(this.status, handler, this.value); 44 } else { 45 this.queue.push(handler);//then may be called multiple times on the same promise;规范2.2.6 46 } 47 return handler.deferred.promise;//then must return a promise;规范2.2.7 48 }; 49 50 var utils = (function(){ 51 var makeSignaler = function(deferred, type) { 52 return function(result) { 53 transition(deferred, type, result); 54 } 55 }; 56 57 var procedure = function(type, handler, result) { 58 var func = handler[type]; 59 var def = handler.deferred; 60 61 if (func) { 62 try { 63 var newResult = func(result); 64 if (newResult && typeof newResult.then === \'function\') {//thenable 65 // 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 66 // newResult.then(function(data) { 67 // def.resolve(data); 68 // }, function(err) { 69 // def.reject(err); 70 // }); 71 //PromiseA+规范,x代表newResult,promise代表def.promise 72 //If x is a promise, adopt its state [3.4]: 73 //If x is pending, promise must remain pending until x is fulfilled or rejected. 74 //If/when x is fulfilled, fulfill promise with the same value. 75 //If/when x is rejected, reject promise with the same reason. 76 newResult.then(makeSignaler(def, \'fulfilled\'), makeSignaler(def, \'rejected\'));//此处的本质是利用了异步闭包 77 } else { 78 transition(def, type, newResult); 79 } 80 } catch(err) { 81 transition(def, \'rejected\', err); 82 } 83 } else { 84 transition(def, type, result); 85 } 86 }; 87 88 var transition = function(deferred, type, result) { 89 if (type === \'fulfilled\') { 90 deferred.resolve(result); 91 } else if (type === \'rejected\') { 92 deferred.reject(result); 93 } else if (type !== \'pending\') { 94 throw new Error({ 95 \'message\': "doesn\'t support type: " + type 96 }); 97 } 98 }; 99 100 return { 101 \'procedure\': procedure 102 } 103 })(); 104 105 Deferred = function() { 106 this.promise = new Promise(); 107 }; 108 109 Deferred.prototype.resolve = function(result) { 110 if (!this.promise.isPending()) { 111 return; 112 } 113 114 var queue = this.promise.getQueue(); 115 for (var i = 0, len = queue.length; i < len; i++) { 116 utils.procedure(\'fulfilled\', queue[i], result); 117 } 118 this.promise.setStatus(\'fulfilled\', result); 119 }; 120 121 Deferred.prototype.reject = function(err) { 122 if (!this.promise.isPending()) { 123 return; 124 } 125 126 var queue = this.promise.getQueue(); 127 for (var i = 0, len = queue.length; i < len; i++) { 128 utils.procedure(\'rejected\', queue[i], err); 129 } 130 this.promise.setStatus(\'rejected\', err); 131 }
通过Promise机制我们的编程方式可以变成这样:
1 request = function(url) { 2 var def = new Deferred(); 3 4 var xhr = new XMLHttpRequest(); 5 xhr.onreadystatechange = function() { 6 if (xhr.readyState === 4) { 7 if ((xhr.status >=200 && xhr.status < 300) || xhr.status === 304) { 8 def.resolve(xhr.responseText) 9 } else {//简化ajax,没有提供错误回调 10 def.reject(new Error({ 11 message: xhr.status 12 })); 13 } 14 } 15 }; 16 xhr.open(\'get\', url, true); 17 xhr.send(null); 18 19 return def.promise; 20 } 21 22 request(\'data1.json\').then(function(data1) { 23 console.log(data1);//处理data1 24 return request(\'data2.json\'); 25 }).then(function(data2) { 26 console.log(data2);//处理data2 27 return request(\'data3.json\'); 28 }, function(err) { 29 console.error(err); 30 }).then(function(data3) { 31 console.log(data3); 32 alert(\'success\'); 33 }, function(err) { 34 console.error(err); 35 });
对于并行逻辑串行执行问题我们可以这样解决
1 //所有异步操作都完成时,进入完成态, 2 //其中一项异步操作失败则进入失败态 3 all = function(requestArray) { 4 // var some = Array.prototype.some; 5 var def = new Deferred(); 6 var results = []; 7 var total = 0; 8 requestArray.some(function(r, idx) { 9 //为数组中每一项注册回调函数 10 r.then(function(data) { 11 if (def.promise.isPending()) { 12 total++; 13 results[idx] = data; 14 15 if (total === requestArray.length) { 16 def.resolve(results); 17 } 18 } 19 }, function(err) { 20 def.reject(err); 21 }); 22 //如果不是等待状态则停止,比如requestArray[0]失败的话,剩下数组则不用继续注册 23 return !def.promise.isPending(); 24 }); 25 26 return def.promise; 27 } 28 29 all( 30 [request(\'data1.json\'), 31 request(\'data2.json\'), 32 request(\'data3.json\')] 33 ).then( 34 function(results){ 35 console.log(results);// 处理data1,data2,data3 36 alert(\'success\'); 37 }, function(err) { 38 console.error(err); 39 });
以下是几个测试案例
1 //链式调用 2 var p1 = new Deferred(); 3 p1.promise.then(function(result) { 4 console.log(\'resolve: \', result); 5 return result; 6 }, function(err) { 7 console.log(\'reject: \', err); 8 return err; 9 }).then(function(result) { 10 console.log(\'resolve2: \', result); 11 return result; 12 }, function(err) { 13 console.log(\'reject2: \', err); 14 return err; 15 }).then(function(result) { 16 console.log(\'resolve3: \', result); 17 return result; 18 }, function(err) { 19 console.log(\'reject3: \', err); 20 return err; 21 }); 22 p1.resolve(\'success\'); 23 //p1.reject(\'failed\'); 24 p1.promise.then(function(result) { 25 console.log(\'after resolve: \', result); 26 return result; 27 }, function(err) { 28 console.log(\'after reject: \', err); 29 return err; 30 }).then(function(result) { 31 console.log(\'after resolve2: \', result); 32 return result; 33 }, function(err) { 34 console.log(\'after reject2: \', err); 35 return err; 36 }).then(function(result) { 37 console.log(\'after resolve2: \', result); 38 return result; 39 }, function(err) { 40 console.log(\'after reject2: \', err); 41 return err; 42 }); 43 44 //串行异步 45 var p2 = new Deferred(); 46 p2.promise.then(function(result) { 47 var def = new Deferred(); 48 setTimeout(function(){ 49 console.log(\'resolve: \', result); 50 def.resolve(result); 51 }) 52 return def.promise; 53 }, function(err) { 54 console.log(\'reject: \', err); 55 return err; 56 }).then(function(result) { 57 var def = new Deferred(); 58 setTimeout(function(){ 59 console.log(\'resolve2: \', result); 60 def.reject(result); 61 }) 62 return def.promise; 63 }, function(err) { 64 console.log(\'reject2: \', err); 65 return err; 66 }).then(function(result) { 67 console.log(\'resolve3: \', result); 68 return result; 69 }, function(err) { 70 console.log(\'reject3: \', err); 71 return err; 72 }); 73 p2.resolve(\'success\'); 74 75 //并行异步 76 var p1 = function(){ 77 var def = new Deferred(); 78 setTimeout(function() { 79 console.log(\'p1 success\'); 80 def.resolve(\'p1 success\'); 81 }, 20); 82 83 return def.promise; 84 } 85 var p2 = function(){ 86 var def = new Deferred(); 87 setTimeout(function() { 88 console.log(\'p2 failed\'); 89 def.reject(\'p2 failed\'); 90 }, 10); 91 92 return def.promise; 93 } 94 95 var p3 = function(){ 96 var def = new Deferred(); 97 setTimeout(function() { 98 console.log(\'p3 success\'); 99 def.resolve(\'p3 success\'); 100 }, 15); 101 102 return def.promise; 103 } 104 105 all([p1(), p2(), p3()]).then(function(results) { 106 console.log(results); 107 }, function(err) { 108 console.error(err); 109 });
Promise优点
对比使用Promise前后我们可以发现,传统异步编程通过嵌套回调函数的方式,等待异步操作结束后再执行下一步操作。过多的嵌套导致意大利面条式的代码,可读性差、耦合度高、扩展性低。通过Promise机制,扁平化的代码机构,大大提高了代码可读性;用同步编程的方式来编写异步代码,保存线性的代码逻辑,极大的降低了代码耦合性而提高了程序的可扩展性。
Note:下图是我整理的dojo/Deferred模块的脉络图,使用dojo的道友可以看一下
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