Suppose I use Nodejs to try to run two async calls to get some answers. I know there's an async package, where you can just pass two functions, and an optional callback.
async.parallel([fun1(){callback(null,1);},
fun2(){callback(null,2);}],
function(err, results) {
});
But suppose I have a priority now, if fun1 returns a value, then I do not need fun2's answer, only if fun1 returns null, then I wait for fun2. So I don't want to use the callback function, because the callback waits for both functions to finish, and fun2 may take very long.
Right now I just use a very exhaustive way by creating a callback function for both async calls.
function(){
var theAnswer,FromFun1,FromFun2;
var reply1,reply2;
fun1(reply1="answered";FromFun1=1;complete());
fun2(reply2="answered";FromFun2=2;complete());
function complete(answer){
if(reply1=="answered"){
theAnswer=FromFun1;
}else if(reply1==null){
// Don't do anything because fun1 is not finished running.
}else if(reply2=="answered"){
theAnswer=FromFun2;
}else{
// Both have no answer, err.
}
}
}
Is there a better way for this?
Its better to use waterfall instead of parallel execution coz you can pass the result form your previous function to next one as argument.
async.waterfall([
function(callback){
callback(null, 'one');
},
function(arg1, callback){
// arg1 now equals 'one'
if(arg1==null){
// it will not take time it returns immediately
callback(null, 'done');
}else{
// do your another stuff here fun2 logic here
// it will not be executed and so it dont take time
}
}
], function (err, result) {
// result now equals 'done'
});
The trick I've used for this scenario is to return "done" in first argument of the callback:
async.parallel([
function(callback){
callback("done",1);
},
function(callback){
callback(null,2);
}
], function(err, results) {
console.log(err, results); // done [ 1 ] or done [ 1 , 2 ]
});
Sounds like a hack and I don't usually do it but in some rare cases like this one, it actually keeps the code clean... just document it so that others know what you intention is.