You could use async.mapSeries (https://github.com/caolan/async) with an array
of functions and the iterator would make the check to execute the function,
e.g.:
var async = require('async');
var cancelled = false;
var cnt = 1;
async.mapSeries([
function step1() {
console.log('step 1');
},
function step2() {
console.log('step 2');
cancelled = true;
},
function step3() {
console.log('step 3');
}],
// iterator
function iter(fn, cb) {
// check if the function should run
if (cancelled) {
cb(null, 'skipped '+cnt++);
} else {
fn();
cb(null, 'step '+cnt++);
}
},
// result
function(err, results) {
// check for error
if (err) {
// do something
console.log(err);
}
// process results, if any
if (results) {
console.log(results);
}
}
);
-Edmond
On Dec 8, 2013, at 3:12 PM, Dave Horton <d...@dchorton.com> wrote:
> I'm trying to figure the cleanest way to deal with a scenario where you have
> a long chain of callbacks that are executing, and then something in your
> environment/program changes in such a way that negates the need for that
> work, and could even lead to invalid results if it is allowed to progress.
>
> An example would probably help. Say I have an app that takes a connection
> from client, and then starts some work on their behalf. The work entails
> marshalling the resources of a bunch of network resources on their behalf, so
> that work gets kicked off. Then the client disconnects or otherwise signals
> he wants to cancel the original request. It might look something like this:
>
> var app = require('my-app-thingy')() ;
> app.onRequest( function( req, res ){
> //1: we just got a request from a client
>
> writeTransactionStartTimeToDatabase() ; //write a record to database
> to record start time
>
> req.on('canceled', function() {
> //3: we just got notified that the client has canceled the
> request
> writeTransactionEndTimeToDatabase()
> }) ;
>
> //2: start some long running activity
> acquireNetworkResource( function(err, resource) {
>
> //4: we got the requested resource
>
> doSomeOtherAsynchronousActivity( function( err ) {
> //5: all done
> writeTransactionEndTimeToDatabase() ; //just wrote
> wrong time to database
> }) ;
> }) ;
> })
>
> At time 1, I get a request from a client, and immediately (at time 2) launch
> some work on their behalf. At time 3, however, the request is canceled. Now
> I need to shut down that work. At time 4, part of the long running task
> returns, and since I am not checking the status of the request I launch
> another activity -- at time 5, that returns and I now write a second record
> to the database recording the incorrect end time -- the proper end time was
> written earlier (time 2) and is now overwritten.
>
> Obviously, I could just check the status of the request at every callback
> stage, but that quickly gets ugly (what if there are multiple conditions that
> needed to be checked).
>
> Is there a pattern for writing code to handle this type of scenario simply
> and cleanly?
>
> --
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