I want to use a callback function in a while loop like this:
do {
Methodwithcallback(..function () {
//do something
});
}
while ();
function() will not be called, but it is working without a loop.
The Method take some time to execute and i want to repeat the loop after the Method is finished.
Why is the Method being ignored in the loop? I'm using Node.js
You can't use loops with asynchronous functions like that - the loop wants to execute "now", and the callback executes "later"..
Try with something like this:
function callback() {
// do whatever else is needed
methodWithCallback(callback); // start the next one
}
methodWithCallback(callback); // start the first one
You can use a recursive callback handler that knows when to stop based on a base case, in the below code it executes once for each item in arr. Also you can use async.series() which is a pre-made library to handle this.
const arr = [...items]
let count = 0
const blockingHandler = () => {
// Keep track of times invoked
count++
// Do something
// If there is another item to execute, then do it
if (count < arr.length) {
MethodwithCallback(blockingHandler))
}
}
MethodwithCallback(blockingHandler)
If you want to use Promises, bluebird has mapSeries() which will serially iterate over a collection with the given async function.
const Promise = require('bluebird')
const arr = [...items]
Promise.mapSeries(arr, i => {
MethodwithCallback(() => {
/*
* If there was some error handling you could check for an error
* if there is one, then return a rejected Promise
* if (err) {
* return Promise.reject(err)
* }
*/
// do something
return Promise.resolve()
})
})
.then(() => console.log('All Promises Complete'))
.catch(err => console.log('An error occurred', err))
If MethodwithCallback() can be promisified, then we can shorten this code a little. Promisification is now built into Node as well via util.promisify()
const Promise = require('bluebird')
const asyncMethod = Promise.promisify(MethodwithCallback)
const arr = [...items]
Promise.mapSeries(arr, i => {
return asyncMethod()
.then(() => {
// do something specific for this item
})
})
.then(() => {
// do something when all items done
})
.catch(err => {
// An error occurred during mapSeries
console.log('An error occurred', err)
})
Methodwithcallbackasynchronous ?