Meteor with Promises
I tried to get used to using promises but ran into problems when trying to use them in server-side code in the context of Meteor. This is the problem:
if (Meteor.isServer) {
Meteor.startup(function () {
// code to run on server at startup
p = function(){
return new Promise(function(res,rej) {
res("asd");
});
};
p().then(function(asd){
console.log("asd is " + asd);
return "zxc"
}).then(Meteor.bindEnvironment(function(zxc){
console.log("zxc is " + zxc);
return "qwe"
})).then(function(qwe){
console.log("qwe is " + qwe);
});
});
}
mvrx:bluebird
package installed
the code is also available on GitHub
Expected Result:
asd is asd
zxc is zxc
qwe is qwe
Actual output:
asd is asd
zxc is zxc
qwe is undefined
Removing the wrapper Meteor.bindEnvironment
fixes the problem, but I need it to be able to use collections inside callbacks
So what am I missing here? is it just not possible to use promises + Meteor this way or is there a bug?
What I am actually trying to accomplish is parallel pipelines, which have important partial results, but need synchronized completion. Something like that.
if (Meteor.isServer) {
Meteor.startup(function () {
// code to run on server at startup
promises = [];
step1 = function(input){
return new Promise(function(res, rej){
console.log(input + ":Step 1");
res(input);
});
};
step2 = function(input){
return new Promise(function(res, rej){
console.log(input + ":Step 2");
res(input);
});
};
step3 = function(input){
return new Promise(function(res, rej){
console.log(input + ":Step 3");
res(input);
});
};
slowIO = function(input){
var inp = input;
return new Promise( function(res,rej){
setTimeout(function(){
console.log(inp + ":SlowIO");
res(inp);
},Math.random()*20000);
});
};
end = function(input){
return new Promise(function(res,rej){
console.log(input + ": done, commiting to database");
res()
});
};
for (var i = 0; i < 100; ++i) {
promises.push(step1("pipeline-" + i).then(step2).then(slowIO).then(step3).then(end));
};
Promise.all(promises).then(function(){
console.log("All complete")
});
});
}
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(Update: I have filed the issue on github to see if it can be resolved. )
It looks like there is a problem with Meteor.bindEnvironment
using this method.
If it called a from outside Fiber
, it will not return its value. Note the missing return
beforeFiber(runWithEnvironment).run()
The simple solution at the moment is to return a Promise instead of a result:
// when passed as a callback to `Promise#then`
// allows it to resolve asynchronously
var asyncThen = function(fn){
return function(arg){
return new Promise(function(resolve, reject){
fn(arg, resolve, reject);
})
};
};
Promise.resolve("asd").then(function(asd){
console.log("asd is " + asd);
return "zxc"
}).then(
asyncThen(
Meteor.bindEnvironment(function(zxc, resolve, reject){
console.log("zxc is", zxc);
resolve("qwe");
})
)
).then(function(qwe){
console.log("qwe is " + qwe);
});
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A promise is just a way to write asynchronous code synchronously. If it's all after, why not use it Meteor.wrapAsync()
? In your case, you have a zxc
riding cowboy in his own fiber and who knows when he will be back. Bluebird works great and is very fast on the client, but I think the code is much cleaner using what Meteor gives you:
//*UNTESTED*//
asd = function() { return 'foo';};
asdSync = Meteor.wrapAsync(asd);
asdResult = asdSync();
qwe = function(input) {return input.reverse()};
qweSync = Meteor.wrapAsync(qwe);
qweResult = qweSync(asdResult); //should return 'oof'
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