Turns async function into sync via JavaScript wrapper of Node event loop
npm install deasync2DeAsync.js
=======
DeAsync turns async function into sync, implemented with a blocking mechanism by calling Node.js event loop at JavaScript layer. The core of deasync is writen in C++.
this is a fork of fork add support promise
* bluelovers/deasync
* malyutinegor/deasync
* abbr/deasync
nodemon
npm install deasync2
`
Motivation
Suppose you maintain a library that exposes a function getData. Your users call it to get actual data:
var myData = getData();
Under the hood data is saved in a file so you implemented getData using Node.js built-in fs.readFileSync. It's obvious both getData and fs.readFileSync are sync functions. One day you were told to switch the underlying data source to a repo such as MongoDB which can only be accessed asynchronously. You were also told to avoid pissing off your users, getData API cannot be changed to return merely a promise or demand a callback parameter. How do you meet both requirements?
You may tempted to use node-fibers or a module derived from it, but node fibers can only wrap async function call into a sync function inside a fiber. In the case above you cannot assume all callers are inside fibers. On the other hand, if you start a fiber in getData then getData itself will still return immediately without waiting for the async call result. For similar reason ES6 generators introduced in Node v0.11 won't work either.
What really needed is a way to block subsequent JavaScript from running without blocking entire thread by yielding to allow other events in the event loop to be handled. Ideally the blockage is removed as soon as the result of async function is available. A less ideal but often acceptable alternative is a sleep function which you can use to implement the blockage like `while(!done) sleep(100);`. It is less ideal because sleep duration has to be guessed. It is important the sleep function not only shouldn't block entire thread, but also shouldn't incur busy wait that pegs the CPU to 100%.
DeAsync supports both alternatives.
Usages
* await.demo.js
* Generic wrapper of async function with standard API signature function(p1,...pn,function cb(error,result){}). Returns result and throws error as exception if not null:
`javascript
var deasync = require('deasync2');
var cp = require('child_process');
var exec = deasync(cp.exec);
// output result of ls -la
try{
console.log(exec('ls -la'));
}
catch(err){
console.log(err);
}
// done is printed last, as supposed, with cp.exec wrapped in deasync; first without.
console.log('done');
`
* For async function with non-standard API, for instance function asyncFunction(p1,function cb(res){}), use loopWhile(predicateFunc) where predicateFunc is a function that returns boolean loop condition
`javascript
var done = false;
var data;
asyncFunction(p1,function cb(res){
data = res;
done = true;
});
require('deasync2').loopWhile(function(){return !done;});
// data is now populated
`
* Sleep (a wrapper of setTimeout)
`javascript
function SyncFunction(){
var ret;
setTimeout(function(){
ret = "hello";
},3000);
while(ret === undefined) {
require('deasync2').sleep(100);
}
// returns hello with sleep; undefined without
return ret;
}
`
Installation
Except on a few platforms + Node version combinations where binary distribution is included, DeAsync uses node-gyp to compile C++ source code so you may need the compilers listed in node-gyp. You may also need to update npm's bundled node-gyp.
To install, run
`npm install deasync2`
Recommendation
Unlike other (a)sync js packages that mostly have only syntactic impact, DeAsync also changes code execution sequence. As such, it is intended to solve niche cases like the above one. If all you are facing is syntatic problem such as callback hell, using a less drastic package implemented in pure js is recommended.
Support
Pull requests and issue reporting are welcome. For issues to be considered by maintainer
1. they must be reproducible
2. there must be evidence the issue is related to DeAsync
To that end, the issue should contain platform information, error message relevant to DeAsync, and preferably code snippet. If code snippet is supplied, it must be self-contained, i.e. independent from your runtime environment or other modules not explictly specified via require` in the code snippet.