首先,这是一个非常具体的情况,故意以错误的方式将异步调用改造到非常同步的代码库中,该代码库有数千行之长,目前时间无法提供进行更改以“正确执行”的能力。它伤害了我身体的每一根纤维,但现实和理想往往不相符。我知道这很糟糕。
好了,我要怎么做才能:
function doSomething() {
var data;
function callBack(d) {
data = d;
}
myAsynchronousCall(param1, callBack);
// block here and return data when the callback is finished
return data;
}
示例(或缺少示例)都使用库和/或编译器,这两者都不适用于此解决方案。我需要一个具体的例子,如何使它块(例如不离开doSomething函数,直到回调被调用)而不冻结UI。如果这样的事情在JS中是可能的。
在http://taskjs.org/上有一个不错的解决方案
它使用了javascript的新生成器。所以目前大多数浏览器都没有实现它。我在firefox中测试了它,对我来说,这是包装异步函数的好方法。
下面是来自GitHub项目的示例代码
var { Deferred } = task;
spawn(function() {
out.innerHTML = "reading...\n";
try {
var d = yield read("read.html");
alert(d.responseText.length);
} catch (e) {
e.stack.split(/\n/).forEach(function(line) { console.log(line) });
console.log("");
out.innerHTML = "error: " + e;
}
});
function read(url, method) {
method = method || "GET";
var xhr = new XMLHttpRequest();
var deferred = new Deferred();
xhr.onreadystatechange = function() {
if (xhr.readyState === 4) {
if (xhr.status >= 400) {
var e = new Error(xhr.statusText);
e.status = xhr.status;
deferred.reject(e);
} else {
deferred.resolve({
responseText: xhr.responseText
});
}
}
};
xhr.open(method, url, true);
xhr.send();
return deferred.promise;
}
人们可能不会考虑的一件事是:如果你控制了异步函数(其他代码段依赖于它),并且它所采用的代码路径不一定是异步的,你可以通过创建一个可选参数使它成为同步的(而不破坏其他代码段)。
目前:
async function myFunc(args_etcetc) {
// you wrote this
return 'stuff';
}
(async function main() {
var result = await myFunc('argsetcetc');
console.log('async result:' result);
})()
考虑:
function myFunc(args_etcetc, opts={}) {
/*
param opts :: {sync:Boolean} -- whether to return a Promise or not
*/
var {sync=false} = opts;
if (sync===true)
return 'stuff';
else
return new Promise((RETURN,REJECT)=> {
RETURN('stuff');
});
}
// async code still works just like before:
(async function main() {
var result = await myFunc('argsetcetc');
console.log('async result:', result);
})();
// prints: 'stuff'
// new sync code works, if you specify sync mode:
(function main() {
var result = myFunc('argsetcetc', {sync:true});
console.log('sync result:', result);
})();
// prints: 'stuff'
当然,如果异步函数依赖于固有的异步操作(网络请求等),这是行不通的,在这种情况下,努力是徒劳的(没有有效地等待无理由的空闲旋转)。
另外,根据传入的选项返回值或Promise也是相当难看的。
("Why would I have written an async function if it didn't use async constructs?" one might ask? Perhaps some modalities/parameters of the function require asynchronicity and others don't, and due to code duplication you wanted a monolithic block rather than separate modular chunks of code in different functions... For example perhaps the argument is either localDatabase (which doesn't require await) or remoteDatabase (which does). Then you could runtime error if you try to do {sync:true} on the remote database. Perhaps this scenario is indicative of another problem, but there you go.)
在Node.js中,可以编写调用异步操作的同步代码。
节点光纤可以做到这一点。它是作为npm模块提供的第三方本地扩展。
它实现了光纤/协程,所以当一个特定的光纤被阻塞等待异步操作时,整个程序事件循环不会阻塞——另一个光纤(如果存在)继续它的工作。
使用纤维,你的代码看起来像这样:
var Fiber = require('fibers');
function doSomething() {
var fiber = Fiber.current;
function callBack(data) {
fiber.run(data);
}
myAsynchronousCall(param1, callBack);
// execution blocks here
var data = Fiber.yield();
return data;
}
// The whole program must be wrapped with Fiber
Fiber(function main() {
var data = doSomething();
console.log(data);
}).run();
注意,应该避免使用async/await。请看下面来自项目自述文件https://github.com/laverdet/node-fibers:的说明
NOTE OF OBSOLESCENCE -- The author of this project recommends you avoid its use if possible. The original version of this module targeted nodejs v0.1.x in early 2011 when JavaScript on the server looked a lot different. Since then async/await, Promises, and Generators were standardized and the ecosystem as a whole has moved in that direction.
I'll continue to support newer versions of nodejs as long as possible but v8 and nodejs are extraordinarily complex and dynamic platforms. It is inevitable that one day this library will abruptly stop working and no one will be able to do anything about it.
I'd like to say thank you to all the users of fibers, your support over the years has meant a lot to me.
在http://taskjs.org/上有一个不错的解决方案
它使用了javascript的新生成器。所以目前大多数浏览器都没有实现它。我在firefox中测试了它,对我来说,这是包装异步函数的好方法。
下面是来自GitHub项目的示例代码
var { Deferred } = task;
spawn(function() {
out.innerHTML = "reading...\n";
try {
var d = yield read("read.html");
alert(d.responseText.length);
} catch (e) {
e.stack.split(/\n/).forEach(function(line) { console.log(line) });
console.log("");
out.innerHTML = "error: " + e;
}
});
function read(url, method) {
method = method || "GET";
var xhr = new XMLHttpRequest();
var deferred = new Deferred();
xhr.onreadystatechange = function() {
if (xhr.readyState === 4) {
if (xhr.status >= 400) {
var e = new Error(xhr.statusText);
e.status = xhr.status;
deferred.reject(e);
} else {
deferred.resolve({
responseText: xhr.responseText
});
}
}
};
xhr.open(method, url, true);
xhr.send();
return deferred.promise;
}