async-caf

Cancelable Async Functions (CAF)

CAF (/ˈkahf/) is a wrapper for function* generators that treats them like async functions, but with support for external cancelation.

Environment Support

This utility uses ES6 (aka ES2015) features. If you need to support environments prior to ES6, transpile it first (with Babel, etc).

At A Glance

CAF (Cancelable Async Functions) wraps a function* generator so it behaves like an async function that can be externally canceled:

var token = new CAF.cancelToken();

// wrap a generator to make it look like a normal async
// function that when called, returns a promise.
var main = CAF( function *main(signal,url){
    var resp = yield ajax( url );

    // want to be able to cancel so we never get here?!?
    console.log( resp );
    return resp;
} );

// run the fake async function, listen to its
// returned promise
main( token.signal, "http://some.tld/other" )
.then( onResponse, onCancelOrError );

// only wait 5 seconds for the request!
setTimeout( function(){
    token.abort( "Request took too long!" );
}, 5000 );

Create a cancelation token (via new CAF.cancelToken()) to pass into your wrapped function* generator, and then if you cancel the token, the function* generator will abort itself immediately, even if it's presently waiting on a promise to resolve.

Moreover, the generator itself is provided the cancelation token's signal, so you can call another function* generator via CAF and pass along that shared signal. In this way, a single cancelation signal can cascade across all the CAF-wrapped functions in the chain of execution:

var token = new CAF.cancelToken();

var one = CAF( function *one(signal,v){
    return yield two( signal, v );
} );

var two = CAF( function *two(signal,v){
    return yield three( signal, v );
} );

var three = CAF( function* three(signal,v){
    return yield ajax( `http://some.tld/?v=${v}` );
} );

one( token.signal, 42 );

// only wait 5 seconds for the request!
setTimeout( function(){
    token.abort( "Request took too long!" );
}, 5000 );

In this snippet, one(..) calls and waits on two(..), two(..) calls and waits on three(..), and three(..) calls and waits on ajax(..). Because the same cancelation token is used for the 3 generators, if token.abort() is executed while they're all still paused, they will all immediately abort.

Note: In this example, the cancelation token has no effect on the actual ajax(..) call itself, since that utility ostensibly doesn't provide cancelation capability; the Ajax request itself would still run to its completion (or error or whatever). We've only canceled the one(..), two(..), and three(..) functions that were waiting to process its response. See AbortController(..) and Manual Cancelation Signal Handling below for addressing this concern.

Background/Motivation

Generally speaking, an async function and a function* generator (driven with a generator-runner) look very similar. For that reason, most people just prefer the async function form since it's a little nicer syntax and doesn't require a library for the runner.

However, there are limitations to async functions inherent to having the syntax and engine make implicit assumptions that you otherwise have to explicitly handle with function* generators.

One clear example of these limitations is that an async function cannot be externally canceled once it starts running. If you want to be able to cancel it, you have to intrusively modify its definition to have it consult an external value source -- like a boolean or promise -- at each line that you care about being a cancelation point. This is ugly and error-prone.

function* generators by contrast can be aborted at any time, using the iterator's return(..) method. But the downside of using function* generators is either needing a runner utility or the repetitive boilerplate of handling the iterator manually.

The best solution would be a function* generator that can be called like a normal async function, but with a cancelation token to signal it to cancel. That's what CAF provides.

The CAF(..) function takes a function* generator, and returns a regular function that expects any arguments, much the same as if it was a normal async function. Other than minor syntactic aesthetics, the most observable difference is that a CAF-wrapped function should be provided a cancelation token's signal as its first argument, with any other arguments passed subsequent, as desired.

Overview

These two functions are essentially equivalent; one(..) is an actual async function, whereas two(..) will behave like an async function in that it also returns a promise:

async function one(v) {
    await delay( 100 );
    return v * 2;
}

var two = CAF( function *two(cancelToken,v){
    yield delay( 100 );
    return v * 2;
} );

Both one(..) and two(..) can be called directly with argument(s), and both return a promise for their completion:

one( 21 )
.then( console.log, console.error );   // 42

var token = new CAF.cancelToken();

two( token.signal, 21 )
.then( console.log, console.error );   // 42

If token.abort(..) is executed while two(..) is still running, the signal's promise will be rejected. If you pass a cancelation reason (any value, but typically a string) to token.abort(..), that will be the promise rejection reason:

two( token, 21 )
.then( console.log, console.error );    // Took too long!

token.abort( "Took too long!" );

finally { .. }

Canceling a CAF-wrapped function* generator that is paused causes it to abort right away, but if there's a pending finally {..} clause, that will still have a chance to run.

Moreover, a return of any non-undefined value in that pending finally {..} clause will override the promise rejection reason:

var token = new CAF.cancelToken();

var main = CAF( function *main(signal,url){
    try {
        return yield ajax( url );
    }
    finally {
        return 42;
    }
} );

main( token.signal, "http://some.tld/other" )
.catch( console.log );   // 42 <-- not "Aborting!"

token.abort( "Aborting!" );

Whatever value is passed to abort(..), if any, is normally set as the promise rejection reason. But in this case, return 42 overrides the "Aborting!" rejection reason.

AbortController(..)

CAF.cancelToken(..) instantiates AbortController, the DOM standard for canceling/aborting operations like fetch(..) calls. As such, a cancelation token's signal can be passed directly to a DOM method like fetch(..), which will respond to it accordingly:

var token = new CAF.cancelToken();

var main = CAF(function *main(signal,url) {
    var resp = yield fetch( url, { signal } );

    console.log( resp );
    return resp;
});

main( token.signal, "http://some.tld/other" )
.catch( console.log );   // "Aborting!"

token.abort( "Aborting!" );

Note: If the standard AbortController is not defined in the environment, it's polyfilled by CAF. In such a case, fetch(..) and other such DOM methods will likely not actually respond to the cancelation signal.

Manual Cancelation Signal Handling

Even if you aren't calling a cancelation signal-aware utility (like fetch(..)), you can still manually respond to the cancelation signal via its attached promise:

var token = new CAF.cancelToken();

var main = CAF( function *main(signal,url){
    // listen to the signal's promise rejection directly
    signal.pr.catch( reason => {
        // reason == "Aborting!"
    } );

    var resp = yield ajax( url );

    console.log( resp );
    return resp;
} );

main( token.signal, "http://some.tld/other" )
.catch( console.log );   // "Aborting!"

token.abort( "Aborting!" );

Note: The catch(..) handler inside of main(..) will still run, even though main(..) itself will be aborted at its waiting yield statement. If there was a way to manually cancel the ajax(..) call, that code could run in the catch(..) handler.

And even if you aren't running a CAF-wrapped function, you can still respond to the cancelation signal's promise manually to affect flow control:

var token = new CAF.cancelToken();

// normal async function
async function main(signal,url) {
    try {
        var resp = await Promise.race( [
            ajax( url ),
            signal.pr
        ] );

        // this won't run if `signal.pr` rejects
        console.log( resp );
        return resp;
    }
    catch (err) {
        // err == "Aborting!"
    }
}

main( token.signal, "http://some.tld/other" )
.catch( console.log );   // "Aborting!"

token.abort( "Aborting!" );

Note: As discussed earlier, the ajax(..) call itself is not cancelation aware, and is thus not being aborted here. But we are aborting our waiting on the ajax(..) call. When signal.pr wins the Promise.race(..) race and creates an exception from its promise rejection, flow control jumps straight to the catch (err) { .. } clause.

Beware Of Token Reuse

Beware of creating a single cancelation token that is reused for separate chains of function calls. Unexpected results are likely, and they can be extremely difficult to debug.

As illustrated earlier, it's totally OK and intended that a single cancelation token signal be shared across all the functions in one chain of calls (A -> B -> C). But reusing the same token across two or more chains of calls (A -> B -> C and D -> E -> F) is asking for trouble.

Imagine a scenario where you make two separate fetch(..) calls, one after the other, and the second one runs too long so you cancel it via a timeout:

var one = CAF( function *one(signal){
    signal.pr.catch( reason => {
        console.log( `one: ${reason}` );
    } );

    return yield fetch( "http://some.tld/", {signal} );
} );

var two = CAF( function *two(signal,v){
    signal.pr.catch( reason => {
        console.log( `two: ${reason}` );
    } );

    return yield fetch( `http://other.tld/?v=${v}`, {signal} );
} );

var token = CAF.cancelToken();

one( token.signal )
.then( function(v){
    // only wait 3 seconds for this request
    setTimeout( function(){
        token.abort( "Second response too slow." );
    }, 3000 );

    return two( token.signal, v );
} )
.then( console.log, console.error );

// one: Second response too slow.   <-- Oops!
// two: Second response too slow.
// Second response too slow.

When you call token.abort(..) to cancel the second fetch(..) call in two(..), the signal.pr.catch(..) handler in one(..) still gets called, even though one(..) is already finished. That's why "one: Second response too slow." prints unexpectedly.

The underlying gotcha is that a cancelation token's signal has a single pr promise associated with it, and there's no way to reset a promise or "unregister" then(..) / catch(..) handlers attached to it once you don't need them anymore. So if you reuse the token, you're reusing the pr promise, and all registered promise handlers will be fired, even old ones you likely don't intend.

The above snippet illustrates this problem with signal.pr.catch(..), but any of the other ways of listening to a promise -- such as yield / await, Promise.all(..) / Promise.race(..), etc -- are all susceptible to the unexpected behavior.

The safe and proper approach is to always create a new cancelation token for each chain of CAF-wrapped function calls. For good measure, always unset any references to a token once it's no longer needed; thus, you won't accidentally reuse it, and the JS engine can properly garbage collect it.

npm Package

Because of a naming conflict, this utility's npm package name is async-caf, not caf. So, to install it:

npm install async-caf

And to require it in a node script:

var CAF = require("async-caf");

Builds

The distribution library file (dist/caf.js) comes pre-built with the npm package distribution, so you shouldn't need to rebuild it under normal circumstances.

However, if you download this repository via Git:

1. The included build utility (scripts/build-core.js) builds (and minifies) dist/caf.js from source. Note: Minification is currently disabled. The build utility expects Node.js version 6+.

2. To install the build and test dependencies, run npm install from the project root directory.

   • Note: This npm install has the effect of running the build for you, so no further action should be needed on your part.

3. To manually run the build utility with npm:

   npm run build
   

4. To run the build utility directly without npm:

   node scripts/build-core.js
   

Tests

A comprehensive test suite is included in this repository, as well as the npm package distribution. The default test behavior runs the test suite using src/caf.src.js.

1. You can run the tests in a browser by opening up tests/index.html (requires ES6+ browser environment).

2. The included Node.js test utility (scripts/node-tests.js) runs the test suite. This test utility expects Node.js version 6+.

3. Ensure the test dependencies are installed by running npm install from the project root directory.

   • Note: Starting with npm v5, the test utility is not run automatically during this npm install. With npm v4, the test utility automatically runs at this point.

4. To run the test utility with npm:

   npm test
   

   Other npm test scripts:

   • npm run test:dist will run the test suite against dist/caf.js instead of the default of src/caf.src.js.

   • npm run test:package will run the test suite as if the package had just been installed via npm. This ensures package.json:main properly references dist/caf.js for inclusion.

   • npm run test:all will run all three modes of the test suite.

5. To run the test utility directly without npm:

   node scripts/node-tests.js
   

Test Coverage

If you have Istanbul already installed on your system (requires v1.0+), you can use it to check the test coverage:

npm run coverage

Then open up coverage/lcov-report/index.html in a browser to view the report.

To run Istanbul directly without npm:

istanbul cover scripts/node-tests.js

Note: The npm script coverage:report is only intended for use by project maintainers. It sends coverage reports to Coveralls.

License

All code and documentation are (c) 2018 Kyle Simpson and released under the MIT License. A copy of the MIT License is also included.