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[英]How do I convert this chained promises code with callback to async/await
[英]How do I convert an existing callback API to promises?
我想使用 promises,但我有一個回調 API,格式如下:
window.onload; // set to callback
...
window.onload = function() {
};
function request(onChangeHandler) {
...
}
request(function() {
// change happened
...
});
function getStuff(dat, callback) {
...
}
getStuff("dataParam", function(err, data) {
...
})
API;
API.one(function(err, data) {
API.two(function(err, data2) {
API.three(function(err, data3) {
...
});
});
});
Promise 具有狀態,它們以待處理狀態開始,並且可以解決:
Promise 返回函數不應該 throw ,而是應該返回拒絕。 從承諾返回函數中拋出將迫使您同時使用} catch {
和.catch
。 使用 Promisified API 的人並不期望 Promise 會拋出。 如果您不確定異步 API 在 JS 中的工作方式 - 請先查看此答案。
因此,創建 Promise 通常意味着指定它們何時結算 - 這意味着它們何時移動到已完成或已拒絕階段以指示數據可用(並且可以使用.then
訪問)。
使用支持Promise
構造函數的現代 Promise 實現,如原生 ES6 Promise:
function load() {
return new Promise(function(resolve, reject) {
window.onload = resolve;
});
}
然后,您將像這樣使用生成的承諾:
load().then(function() {
// Do things after onload
});
使用支持延遲的庫(我們在此示例中使用 $q,但稍后我們還將使用 jQuery):
function load() {
var d = $q.defer();
window.onload = function() { d.resolve(); };
return d.promise;
}
或者使用類似 jQuery 的 API,掛鈎一次發生的事件:
function done() {
var d = $.Deferred();
$("#myObject").once("click",function() {
d.resolve();
});
return d.promise();
}
這些 API 相當普遍,因為……回調在 JS 中很常見。 讓我們看一下onSuccess
和onFail
的常見情況:
function getUserData(userId, onLoad, onFail) { …
使用支持Promise
構造函數的現代 Promise 實現,如原生 ES6 Promise:
function getUserDataAsync(userId) {
return new Promise(function(resolve, reject) {
getUserData(userId, resolve, reject);
});
}
使用支持延遲的庫(我們在此示例中使用 jQuery,但我們在上面也使用了 $q):
function getUserDataAsync(userId) {
var d = $.Deferred();
getUserData(userId, function(res){ d.resolve(res); }, function(err){ d.reject(err); });
return d.promise();
}
jQuery 還提供了一個$.Deferred(fn)
形式,它的優點是允許我們編寫一個非常接近於new Promise(fn)
形式的表達式,如下所示:
function getUserDataAsync(userId) {
return $.Deferred(function(dfrd) {
getUserData(userId, dfrd.resolve, dfrd.reject);
}).promise();
}
注意:這里我們利用了jQuery deferred 的resolve
和reject
方法是“可分離的”這一事實; IE。 它們綁定到 jQuery.Deferred() 的實例。 並非所有庫都提供此功能。
節點樣式回調 (nodebacks) 具有特定格式,其中回調始終是最后一個參數,其第一個參數是錯誤。 讓我們首先手動承諾一個:
getStuff("dataParam", function(err, data) { …
至:
function getStuffAsync(param) {
return new Promise(function(resolve, reject) {
getStuff(param, function(err, data) {
if (err !== null) reject(err);
else resolve(data);
});
});
}
使用 deferreds 您可以執行以下操作(讓我們在此示例中使用 Q,盡管 Q 現在支持您應該喜歡的新語法):
function getStuffAsync(param) {
var d = Q.defer();
getStuff(param, function(err, data) {
if (err !== null) d.reject(err);
else d.resolve(data);
});
return d.promise;
}
一般來說,你不應該過多地手動承諾事情,大多數在設計時考慮到 Node 的承諾庫以及 Node 8+ 中的本機承諾都有一個用於承諾 nodebacks 的內置方法。 例如
var getStuffAsync = Promise.promisify(getStuff); // Bluebird
var getStuffAsync = Q.denodeify(getStuff); // Q
var getStuffAsync = util.promisify(getStuff); // Native promises, node only
這里沒有金科玉律,你一一答應他們。 但是,一些 Promise 實現允許您批量執行此操作,例如在 Bluebird 中,將 nodeback API 轉換為 Promise API 非常簡單:
Promise.promisifyAll(API);
或者使用Node中的原生承諾:
const { promisify } = require('util');
const promiseAPI = Object.entries(API).map(([key, v]) => ({key, fn: promisify(v)}))
.reduce((o, p) => Object.assign(o, {[p.key]: p.fn}), {});
筆記:
.then
處理程序中時,您不需要承諾任何事情。 從.then
處理程序返回一個 Promise 將使用該 Promise 的值解析或拒絕。 從.then
處理程序中拋出也是一種很好的做法,並且會拒絕承諾——這就是著名的承諾拋出安全性。onload
情況下,您應該使用addEventListener
而不是onX
。今天,我可以在Node.js
中使用Promise
作為一個普通的 Javascript 方法。
一個簡單而基本的Promise
示例(使用KISS方式):
純Javascript 異步 API 代碼:
function divisionAPI (number, divider, successCallback, errorCallback) {
if (divider == 0) {
return errorCallback( new Error("Division by zero") )
}
successCallback( number / divider )
}
Promise
Javascript 異步 API 代碼:
function divisionAPI (number, divider) {
return new Promise(function (fulfilled, rejected) {
if (divider == 0) {
return rejected( new Error("Division by zero") )
}
fulfilled( number / divider )
})
}
(我建議訪問這個美麗的來源)
Promise
也可以與ES7
中的async\await
一起使用,以使程序流等待fullfiled
的結果,如下所示:
function getName () {
return new Promise(function (fulfilled, rejected) {
var name = "John Doe";
// wait 3000 milliseconds before calling fulfilled() method
setTimeout (
function() {
fulfilled( name )
},
3000
)
})
}
async function foo () {
var name = await getName(); // awaits for a fulfilled result!
console.log(name); // the console writes "John Doe" after 3000 milliseconds
}
foo() // calling the foo() method to run the code
使用.then()
方法使用相同代碼的另一種用法
function getName () {
return new Promise(function (fulfilled, rejected) {
var name = "John Doe";
// wait 3000 milliseconds before calling fulfilled() method
setTimeout (
function() {
fulfilled( name )
},
3000
)
})
}
// the console writes "John Doe" after 3000 milliseconds
getName().then(function(name){ console.log(name) })
Promise
也可以在任何基於 Node.js 的平台上使用,例如react-native
。
獎勵:混合方法
(回調方法假設有error和result兩個參數)
function divisionAPI (number, divider, callback) {
return new Promise(function (fulfilled, rejected) {
if (divider == 0) {
let error = new Error("Division by zero")
callback && callback( error )
return rejected( error )
}
let result = number / divider
callback && callback( null, result )
fulfilled( result )
})
}
上述方法可以響應老式回調和 Promise 用法的結果。
希望這可以幫助。
在將函數轉換為 Node.JS 中的 Promise 之前
var request = require('request'); //http wrapped module
function requestWrapper(url, callback) {
request.get(url, function (err, response) {
if (err) {
callback(err);
}else{
callback(null, response);
}
})
}
requestWrapper(url, function (err, response) {
console.log(err, response)
})
轉換后
var request = require('request');
function requestWrapper(url) {
return new Promise(function (resolve, reject) { //returning promise
request.get(url, function (err, response) {
if (err) {
reject(err); //promise reject
}else{
resolve(response); //promise resolve
}
})
})
}
requestWrapper('http://localhost:8080/promise_request/1').then(function(response){
console.log(response) //resolve callback(success)
}).catch(function(error){
console.log(error) //reject callback(failure)
})
如果您需要處理多個請求
var allRequests = [];
allRequests.push(requestWrapper('http://localhost:8080/promise_request/1'))
allRequests.push(requestWrapper('http://localhost:8080/promise_request/2'))
allRequests.push(requestWrapper('http://localhost:8080/promise_request/5'))
Promise.all(allRequests).then(function (results) {
console.log(results);//result will be array which contains each promise response
}).catch(function (err) {
console.log(err)
});
我認為@Benjamin 的window.onload
建議不會一直有效,因為它不會檢測到它是否在加載后被調用。 我已經被那個咬過很多次了。 這是一個應該始終有效的版本:
function promiseDOMready() {
return new Promise(function(resolve) {
if (document.readyState === "complete") return resolve();
document.addEventListener("DOMContentLoaded", resolve);
});
}
promiseDOMready().then(initOnLoad);
Node.js 8.0.0 包含一個新的util.promisify()
API,它允許將標准 Node.js 回調樣式 API 包裝在返回 Promise 的函數中。 util.promisify()
的使用示例如下所示。
const fs = require('fs');
const util = require('util');
const readFile = util.promisify(fs.readFile);
readFile('/some/file')
.then((data) => { /* ... */ })
.catch((err) => { /* ... */ });
const promisify = (fn, ...args) => {
return new Promise((resolve, reject) => {
fn(...args, (err, data) => {
if (err) {
return reject(err);
}
resolve(data);
});
});
};
promisify
接受一個帶有回調的函數: const cb = (result) => `The result is ${result}`;
const sum = (a, b, cb) => {
const result = a + b;
cb(result); // passing args to the callback function
}
// using the util
promise = promisify(sum, 3, 1, cb);
promise.then(x => console.log(x)) // 4
您可能不是在尋找這個答案,但這將有助於了解可用工具的內部工作原理
在 Node.js 8.0.0 的候選版本中,有一個新的實用程序util.promisify
(我寫過關於util.promisify的文章),它封裝了承諾任何功能的能力。
它與其他答案中建議的方法沒有太大區別,但具有作為核心方法的優勢,並且不需要額外的依賴項。
const fs = require('fs');
const util = require('util');
const readFile = util.promisify(fs.readFile);
然后你有一個返回原生Promise
的readFile
方法。
readFile('./notes.txt')
.then(txt => console.log(txt))
.catch(...);
您可以將 JavaScript 原生 Promise 與 Node JS 一起使用。
My Cloud 9 代碼鏈接: https ://ide.c9.io/adx2803/native-promises-in-node
/**
* Created by dixit-lab on 20/6/16.
*/
var express = require('express');
var request = require('request'); //Simplified HTTP request client.
var app = express();
function promisify(url) {
return new Promise(function (resolve, reject) {
request.get(url, function (error, response, body) {
if (!error && response.statusCode == 200) {
resolve(body);
}
else {
reject(error);
}
})
});
}
//get all the albums of a user who have posted post 100
app.get('/listAlbums', function (req, res) {
//get the post with post id 100
promisify('http://jsonplaceholder.typicode.com/posts/100').then(function (result) {
var obj = JSON.parse(result);
return promisify('http://jsonplaceholder.typicode.com/users/' + obj.userId + '/albums')
})
.catch(function (e) {
console.log(e);
})
.then(function (result) {
res.end(result);
})
})
var server = app.listen(8081, function () {
var host = server.address().address
var port = server.address().port
console.log("Example app listening at http://%s:%s", host, port)
})
//run webservice on browser : http://localhost:8081/listAlbums
使用普通的老式 javaScript,這是一個承諾 api 回調的解決方案。
function get(url, callback) {
var xhr = new XMLHttpRequest();
xhr.open('get', url);
xhr.addEventListener('readystatechange', function () {
if (xhr.readyState === 4) {
if (xhr.status === 200) {
console.log('successful ... should call callback ... ');
callback(null, JSON.parse(xhr.responseText));
} else {
console.log('error ... callback with error data ... ');
callback(xhr, null);
}
}
});
xhr.send();
}
/**
* @function promisify: convert api based callbacks to promises
* @description takes in a factory function and promisifies it
* @params {function} input function to promisify
* @params {array} an array of inputs to the function to be promisified
* @return {function} promisified function
* */
function promisify(fn) {
return function () {
var args = Array.prototype.slice.call(arguments);
return new Promise(function(resolve, reject) {
fn.apply(null, args.concat(function (err, result) {
if (err) reject(err);
else resolve(result);
}));
});
}
}
var get_promisified = promisify(get);
var promise = get_promisified('some_url');
promise.then(function (data) {
// corresponds to the resolve function
console.log('successful operation: ', data);
}, function (error) {
console.log(error);
});
kriskowal 的 Q 庫包括回調到承諾的函數。 像這樣的方法:
obj.prototype.dosomething(params, cb) {
...blah blah...
cb(error, results);
}
可以用 Q.ninvoke 轉換
Q.ninvoke(obj,"dosomething",params).
then(function(results) {
});
當您有幾個函數需要回調並且您希望它們返回一個 Promise 時,您可以使用此函數進行轉換。
function callbackToPromise(func){
return function(){
// change this to use what ever promise lib you are using
// In this case i'm using angular $q that I exposed on a util module
var defered = util.$q.defer();
var cb = (val) => {
defered.resolve(val);
}
var args = Array.prototype.slice.call(arguments);
args.push(cb);
func.apply(this, args);
return defered.promise;
}
}
在內置 promises 和 async 的 node v7.6+ 下:
// promisify.js
let promisify = fn => (...args) =>
new Promise((resolve, reject) =>
fn(...args, (err, result) => {
if (err) return reject(err);
return resolve(result);
})
);
module.exports = promisify;
如何使用:
let readdir = require('fs').readdir;
let promisify = require('./promisify');
let readdirP = promisify(readdir);
async function myAsyncFn(path) {
let entries = await readdirP(path);
return entries;
}
在 Node.js 8 中,您可以使用此 npm 模塊即時承諾對象方法:
https://www.npmjs.com/package/doasync
它使用util.promisify和Proxies使您的對象保持不變。 記憶化也是使用 Wea kMaps完成的)。 這里有些例子:
與對象:
const fs = require('fs');
const doAsync = require('doasync');
doAsync(fs).readFile('package.json', 'utf8')
.then(result => {
console.dir(JSON.parse(result), {colors: true});
});
帶功能:
doAsync(request)('http://www.google.com')
.then(({body}) => {
console.log(body);
// ...
});
您甚至可以使用本機call
並apply
來綁定一些上下文:
doAsync(myFunc).apply(context, params)
.then(result => { /*...*/ });
你可以在 ES6 中使用原生 Promise ,例如處理 setTimeout:
enqueue(data) {
const queue = this;
// returns the Promise
return new Promise(function (resolve, reject) {
setTimeout(()=> {
queue.source.push(data);
resolve(queue); //call native resolve when finish
}
, 10); // resolve() will be called in 10 ms
});
}
在這個例子中,Promise 沒有失敗的理由,所以從不調用reject()
。
回調風格的函數總是這樣(node.js 中幾乎所有的函數都是這種風格):
//fs.readdir(path[, options], callback)
fs.readdir('mypath',(err,files)=>console.log(files))
這種風格具有相同的特點:
回調函數由最后一個參數傳遞。
回調函數總是接受錯誤對象作為它的第一個參數。
因此,您可以編寫一個函數來轉換具有這種風格的函數,如下所示:
const R =require('ramda')
/**
* A convenient function for handle error in callback function.
* Accept two function res(resolve) and rej(reject) ,
* return a wrap function that accept a list arguments,
* the first argument as error, if error is null,
* the res function will call,else the rej function.
* @param {function} res the function which will call when no error throw
* @param {function} rej the function which will call when error occur
* @return {function} return a function that accept a list arguments,
* the first argument as error, if error is null, the res function
* will call,else the rej function
**/
const checkErr = (res, rej) => (err, ...data) => R.ifElse(
R.propEq('err', null),
R.compose(
res,
R.prop('data')
),
R.compose(
rej,
R.prop('err')
)
)({err, data})
/**
* wrap the callback style function to Promise style function,
* the callback style function must restrict by convention:
* 1. the function must put the callback function where the last of arguments,
* such as (arg1,arg2,arg3,arg...,callback)
* 2. the callback function must call as callback(err,arg1,arg2,arg...)
* @param {function} fun the callback style function to transform
* @return {function} return the new function that will return a Promise,
* while the origin function throw a error, the Promise will be Promise.reject(error),
* while the origin function work fine, the Promise will be Promise.resolve(args: array),
* the args is which callback function accept
* */
const toPromise = (fun) => (...args) => new Promise(
(res, rej) => R.apply(
fun,
R.append(
checkErr(res, rej),
args
)
)
)
為了更簡潔,上面的例子使用了 ramda.js。 Ramda.js 是一個優秀的函數式編程庫。 在上面的代碼中,我們使用了它的apply (如 javascript function.prototype.apply
)和 append(如 javascript function.prototype.push
)。 因此,我們現在可以將回調樣式函數轉換為 Promise 樣式函數:
const {readdir} = require('fs')
const readdirP = toPromise(readdir)
readdir(Path)
.then(
(files) => console.log(files),
(err) => console.log(err)
)
toPromise和checkErr函數歸berserk庫所有,它是ramda.js的函數式編程庫 fork(由我創建)。
希望這個答案對你有用。
es6-promisify
將基於回調的函數轉換為基於 Promise 的函數。
const promisify = require('es6-promisify');
const promisedFn = promisify(callbackedFn, args);
參考: https ://www.npmjs.com/package/es6-promisify
你可以做這樣的事情
// @flow
const toPromise = (f: (any) => void) => {
return new Promise<any>((resolve, reject) => {
try {
f((result) => {
resolve(result)
})
} catch (e) {
reject(e)
}
})
}
export default toPromise
然后使用它
async loadData() {
const friends = await toPromise(FriendsManager.loadFriends)
console.log(friends)
}
我的callback
函數的 Promisify 版本是P
函數:
var P = function() { var self = this; var method = arguments[0]; var params = Array.prototype.slice.call(arguments, 1); return new Promise((resolve, reject) => { if (method && typeof(method) == 'function') { params.push(function(err, state) { if (!err) return resolve(state) else return reject(err); }); method.apply(self, params); } else return reject(new Error('not a function')); }); } var callback = function(par, callback) { var rnd = Math.floor(Math.random() * 2) + 1; return rnd > 1 ? callback(null, par) : callback(new Error("trap")); } callback("callback", (err, state) => err ? console.error(err) : console.log(state)) callback("callback", (err, state) => err ? console.error(err) : console.log(state)) callback("callback", (err, state) => err ? console.error(err) : console.log(state)) callback("callback", (err, state) => err ? console.error(err) : console.log(state)) P(callback, "promise").then(v => console.log(v)).catch(e => console.error(e)) P(callback, "promise").then(v => console.log(v)).catch(e => console.error(e)) P(callback, "promise").then(v => console.log(v)).catch(e => console.error(e)) P(callback, "promise").then(v => console.log(v)).catch(e => console.error(e))
P
函數要求回調簽名必須是callback(error,result)
。
下面是如何將函數(回調 API)轉換為 Promise 的實現。
function promisify(functionToExec) {
return function() {
var array = Object.values(arguments);
return new Promise((resolve, reject) => {
array.push(resolve)
try {
functionToExec.apply(null, array);
} catch (error) {
reject(error)
}
})
}
}
// USE SCENARIO
function apiFunction (path, callback) { // Not a promise
// Logic
}
var promisedFunction = promisify(apiFunction);
promisedFunction('path').then(()=>{
// Receive the result here (callback)
})
// Or use it with await like this
let result = await promisedFunction('path');
// given you've defined this `Future` fn somewhere:
const Future = fn => {return new Promise((r,t) => fn(r,t))}
// define an eventFn that takes a promise `resolver`
const eventFn = resolve => {
// do event related closure actions here. When finally done, call `resolve()`
something.oneventfired = e => {resolve(e)}
}
// invoke eventFn in an `async` workflowFn using `Future`
// to obtain a `promise` wrapper
const workflowFn = async () => {await Future(eventFn)}
特別是對於諸如
indexedDb
事件包裝器之類的東西以簡化使用。
或者您可能會發現Future
的這種變體更通用
class PromiseEx extends Promise {
resolve(v,...a) {
this.settled = true; this.settledValue = v;
return(this.resolve_(v,...a))
}
reject(v,...a) {
this.settled = false; this.settledValue = v;
return(this.reject_(v,...a))
}
static Future(fn,...args) {
let r,t,ft = new PromiseEx((r_,t_) => {r=r_;t=t_})
ft.resolve_ = r; ft.reject_ = t; fn(ft,...args);
return(ft)
}
}
死靈一點點,這個鏈接可能有用....
cb(error,result)
或new Promise (...)
格式promiseToCB
轉換並導出先前已編碼為返回承諾的現有函數cbToPromise
轉換和導出一個現有函數,該函數之前已編碼為使用 (error,result) 調用最后一個參數
cb(undefined,path,stat)
---> resolve([path,stat])
/ cb(undefined,[path,stat])
asPromise
允許您編寫一個新函數來返回一個 Promise,但它可以以任何一種方式調用asCallback
允許您編寫一個新函數來調用cb(err,result)
,但它可以以任何一種方式調用每個樣本有 2 個參數,並根據隨機數解決/拒絕/錯誤。
arg2 也可用於強制通過或失敗。 (尋找“-pass”或“-fail”)。
promiseToCB(function myFunc(){},newThis);
)
promiseToCB(function sampleFunc1(arg1,arg2) {
console.log("deciding:",arg1,arg2);
return new Promise(function(resolve,reject){
const timer = setTimeout(function(){reject([arg1,arg2,"ouch"].join("-"));},5000);
setTimeout(function(){
if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
console.log("complete:",arg1,arg2);
clearTimeout(timer);
resolve([arg1,arg2,"all good"].join("-"));
}
},2000);
});
});
cbToPromise('sampleFunc2',function someOtherName(arg1,arg2,cb) {
console.log("deciding:",arg1,arg2);
const timer = setTimeout(function(){cb([arg1,arg2,"ouch"].join("-"));},5000);
setTimeout(function(){
if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
console.log("complete:",arg1,arg2);
clearTimeout(timer);
cb(undefined,[arg1,arg2,"all good"].join("-"));
}
},2000);
},local);
function sampleFunc3(arg1,arg2) {return asPromise(arguments,function(resolve,reject){
console.log("deciding:",arg1,arg2);
const timer = setTimeout(function(){reject([arg1,arg2,"ouch"].join("-"));},5000);
setTimeout(function(){
if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
console.log("complete:",arg1,arg2);
clearTimeout(timer);
resolve([arg1,arg2,"all good"].join("-"));
}
},2000);
});}
function sampleFunc4(arg1,arg2) {return asCallback(arguments,function(cb){
console.log("deciding:",arg1,arg2);
const timer = setTimeout(function(){cb([arg1,arg2,"ouch"].join("-"));},5000);
setTimeout(function(){
if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
console.log("complete:",arg1,arg2);
clearTimeout(timer);
cb(undefined,[arg1,arg2,"all good"].join("-"));
}
},2000);
});}
const local = {};
promiseToCB(function sampleFunc1(arg1,arg2) {
console.log("deciding:",arg1,arg2);
return new Promise(function(resolve,reject){
const timer = setTimeout(function(){reject([arg1,arg2,"ouch"].join("-"));},5000);
setTimeout(function(){
if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
console.log("complete:",arg1,arg2);
clearTimeout(timer);
resolve([arg1,arg2,"all good"].join("-"));
}
},2000);
});
});
cbToPromise('sampleFunc2',function someOtherName(arg1,arg2,cb) {
console.log("deciding:",arg1,arg2);
const timer = setTimeout(function(){cb([arg1,arg2,"ouch"].join("-"));},5000);
setTimeout(function(){
if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
console.log("complete:",arg1,arg2);
clearTimeout(timer);
cb(undefined,[arg1,arg2,"all good"].join("-"));
}
},2000);
},local);
function sampleFunc3(arg1,arg2) {return asPromise(arguments,function(resolve,reject){
console.log("deciding:",arg1,arg2);
const timer = setTimeout(function(){reject([arg1,arg2,"ouch"].join("-"));},5000);
setTimeout(function(){
if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
console.log("complete:",arg1,arg2);
clearTimeout(timer);
resolve([arg1,arg2,"all good"].join("-"));
}
},2000);
});}
function sampleFunc4(arg1,arg2) {return asCallback(arguments,function(cb){
console.log("deciding:",arg1,arg2);
const timer = setTimeout(function(){cb([arg1,arg2,"ouch"].join("-"));},5000);
setTimeout(function(){
if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
console.log("complete:",arg1,arg2);
clearTimeout(timer);
cb(undefined,[arg1,arg2,"all good"].join("-"));
}
},2000);
});}
const log=console.log.bind(console),info=console.info.bind(console),error=console.error.bind(console);
sampleFunc1("sample1","promise").then (log).catch(error);
local.sampleFunc2("sample2","promise").then (log).catch(error);
sampleFunc3("sample3","promise").then (log).catch(error);
sampleFunc4("sample4","promise").then (log).catch(error);
sampleFunc1("sample1","callback",info);
local.sampleFunc2("sample2","callback",info);
sampleFunc3("sample3","callback",info);
sampleFunc4("sample4","callback",info);
sampleFunc1("sample1","promise-pass").then (log).catch(error);
local.sampleFunc2("sample2","promise-pass").then (log).catch(error);
sampleFunc3("sample3","promise-pass").then (log).catch(error);
sampleFunc4("sample4","promise-pass").then (log).catch(error);
sampleFunc1("sample1","callback-pass",info);
local.sampleFunc2("sample2","callback-pass",info);
sampleFunc3("sample3","callback-pass",info);
sampleFunc4("sample4","callback-pass",info);
sampleFunc1("sample1","promise-fail").then (log).catch(error);
local.sampleFunc2("sample2","promise-fail").then (log).catch(error);
sampleFunc3("sample3","promise-fail").then (log).catch(error);
sampleFunc4("sample4","promise-fail").then (log).catch(error);
sampleFunc1("sample1","callback-fail",info);
local.sampleFunc2("sample2","callback-fail",info);
sampleFunc3("sample3","callback-fail",info);
sampleFunc4("sample4","callback-fail",info);
var cpArgs = Array.prototype.slice.call.bind(Array.prototype.slice); function promiseToCB (nm,fn,THIS) { if (typeof nm==='function') { THIS=fn;fn=nm;nm=fn.name; } THIS=THIS||this; const func = function () { let args = cpArgs(arguments); if (typeof args[args.length-1]==='function') { const cb = args.pop(); return fn.apply(THIS,args).then(function(r){ cb (undefined,r); }).catch(cb); } else { return fn.apply(THIS,args); } }; Object.defineProperty(func,'name',{value:nm,enumerable:false,configurable: true}); if (THIS[nm]) delete THIS[nm]; Object.defineProperty(THIS,nm,{value:func,enumerable:false,configurable: true}); return func; } function cbToPromise (nm,fn,THIS) { if (typeof nm==='function') { THIS=fn;fn=nm;nm=fn.name; } THIS=THIS||this; const func = function () { let args = cpArgs(arguments); if (typeof args[args.length-1]==='function') { return fn.apply(THIS,args); } else { return new Promise(function(resolve,reject){ args.push(function(err,result){ if (err) return reject(err); if (arguments.length==2) { return resolve(result); } return resolve(cpArgs(arguments,1)); }); fn.apply(THIS,args); }); } }; Object.defineProperty(func,'name',{value:nm,enumerable:false,configurable: true}); if (THIS[nm]) delete THIS[nm]; Object.defineProperty(THIS,nm,{value:func,enumerable:false,configurable: true}); return func; } function asPromise (args,resolver,no_err) { const cb = args[args.length-1], promise = new Promise(resolver); return (typeof cb==='function') ? promise.then(function(result){return cb(no_err,result)}).catch(cb) : promise; } function asCallback (args,wrap,no_err) { const cb = args[args.length-1], promise=new Promise(function resolver(resolve,reject) { return wrap (function (err,result) { if (err) return reject(err); resolve(result); }); }); return (typeof cb==='function') ? promise.then(function(result){return cb(no_err,result)}).catch(cb) : promise; } function cbPromiseTest(){ /*global sampleFunc1,sampleFunc2*/ const local = {}; promiseToCB(function sampleFunc1(arg1,arg2) { console.log("deciding:",arg1,arg2); return new Promise(function(resolve,reject){ const timer = setTimeout(function(){reject([arg1,arg2,"ouch"].join("-"));},5000); setTimeout(function(){ if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) { console.log("complete:",arg1,arg2); clearTimeout(timer); resolve([arg1,arg2,"all good"].join("-")); } },2000); }); }); cbToPromise('sampleFunc2',function someOtherName(arg1,arg2,cb) { console.log("deciding:",arg1,arg2); const timer = setTimeout(function(){cb([arg1,arg2,"ouch"].join("-"));},5000); setTimeout(function(){ if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) { console.log("complete:",arg1,arg2); clearTimeout(timer); cb(undefined,[arg1,arg2,"all good"].join("-")); } },2000); },local); function sampleFunc3(arg1,arg2) {return asPromise(arguments,function(resolve,reject){ console.log("deciding:",arg1,arg2); const timer = setTimeout(function(){reject([arg1,arg2,"ouch"].join("-"));},5000); setTimeout(function(){ if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) { console.log("complete:",arg1,arg2); clearTimeout(timer); resolve([arg1,arg2,"all good"].join("-")); } },2000); });} function sampleFunc4(arg1,arg2) {return asCallback(arguments,function(cb){ console.log("deciding:",arg1,arg2); const timer = setTimeout(function(){cb([arg1,arg2,"ouch"].join("-"));},5000); setTimeout(function(){ if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) { console.log("complete:",arg1,arg2); clearTimeout(timer); cb(undefined,[arg1,arg2,"all good"].join("-")); } },2000); });} const log=console.log.bind(console),info=console.info.bind(console),error=console.error.bind(console); sampleFunc1("sample1","promise").then (log).catch(error); local.sampleFunc2("sample2","promise").then (log).catch(error); sampleFunc3("sample3","promise").then (log).catch(error); sampleFunc4("sample4","promise").then (log).catch(error); sampleFunc1("sample1","callback",info); local.sampleFunc2("sample2","callback",info); sampleFunc3("sample3","callback",info); sampleFunc4("sample4","callback",info); sampleFunc1("sample1","promise-pass").then (log).catch(error); local.sampleFunc2("sample2","promise-pass").then (log).catch(error); sampleFunc3("sample3","promise-pass").then (log).catch(error); sampleFunc4("sample4","promise-pass").then (log).catch(error); sampleFunc1("sample1","callback-pass",info); local.sampleFunc2("sample2","callback-pass",info); sampleFunc3("sample3","callback-pass",info); sampleFunc4("sample4","callback-pass",info); sampleFunc1("sample1","promise-fail").then (log).catch(error); local.sampleFunc2("sample2","promise-fail").then (log).catch(error); sampleFunc3("sample3","promise-fail").then (log).catch(error); sampleFunc4("sample4","promise-fail").then (log).catch(error); sampleFunc1("sample1","callback-fail",info); local.sampleFunc2("sample2","callback-fail",info); sampleFunc3("sample3","callback-fail",info); sampleFunc4("sample4","callback-fail",info); } cbPromiseTest();
承諾總是有resolve
和reject
。 當您編寫異步包裝器時,只需調用 resolve 即可。
您可以為幾乎任何采用回調的函數編寫包裝函數,如下所示:
const myAsyncWrapper = (...params) =>
new Promise((resolve, reject) =>
someFunctionWithCallback(...params, (error, response) =>
error ? reject(error) : resolve(response)
)
);
您可以進一步編寫回調到 Promise 的轉換函數:
const promisify =
(functionWithCallback) =>
(...params) =>
new Promise((resolve, reject) =>
functionWithCallback(...params, (error, response) =>
error ? reject(error) : resolve(response)
)
);
在使用較舊的庫或 SDK 時,包裝函數的概念特別有用。 例如,考慮 Facebook Graph API 的 JavaScript SDK,它使用類似的回調結構來發出 API 請求。
FB.api(apiURL, options, function (request) {
if (request.error || !request) return;
// handle request
});
在現代應用程序中,使用基於 Promise 的 API 更為有用。 如果您只使用一次或兩次函數,最好單獨承諾響應:
// in an async function
const response = await new Promise((resolve, reject) =>
FB.api(apiURL, (res) => (res?.error ? reject(res?.error) : resolve(res)))
);
如果你經常使用這個函數,你可以使用相同的包裝器概念來編寫一個函數,如下所示:
const apiWrapper = (...params) =>
new Promise((resolve, reject) =>
FB.api(...params, (res) => (res?.error ? reject(res?.error) : resolve(res)))
);
盡管承諾者有時很棒,但它們不適用於像這樣的特定情況。 在這種情況下,在 Github 上尋找現代包裝器,或者像這樣編寫自己的包裝器。
由於我們事先知道基於回調的函數的特性,我們可以創建一個函數,將基於回調的函數轉換為返回 Promise 的等效函數。
回調是函數的最后一個參數
如果有錯誤,它總是第一個參數傳遞給回調
錯誤后的任何返回值都傳遞給回調
function promisify(yourCallbackApi) { return function promisified(...args) { return new Promise((resolve, reject) => { // newArgs=[..args,callback] const newArgs = [ ...args, function (err, result) { if (err) { return reject(err); } resolve(result); }, ]; // invoke yourCallbackApi with the new list of arguments yourCallbackApi(...newArgs); }); }; }
我創建了可以重用的通用函數。
const wrapIntoPromise: (fnToExecute) => {
return new Promise((resolve, reject) => {
return fnToExecute(resolve, reject);
});
}
//example
async function run() {
await wrapIntoPromise((resolve, reject) => {
/*execute any function here with callbacks etc
when done just call return resolve(valueToReturn)
*/
return backup({
callback: function() { return resolve()}
});
});
}
好像晚了 5 年,但我想在這里發布我的 promesify 版本,它從回調 API 中獲取函數並將它們轉換為 Promise
const promesify = fn => {
return (...params) => ({
then: cbThen => ({
catch: cbCatch => {
fn(...params, cbThen, cbCatch);
}
})
});
};
在這里看看這個非常簡單的版本: https ://gist.github.com/jdtorregrosas/aeee96dd07558a5d18db1ff02f31e21a
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