With promise API, how to send two asynchronous request in parallel, and resolve the combined result as the response.
var get = function(id){
var res1, res2;
var deferred = $q.defer();
Db.get(id, "abc")
.then(function (d) {
//deferred.resolve(d));
res1 = d;
}, function (e) {
//error
});
Db.get(id, "def")
.then(function (d) {
//deferred.resolve(d));
res2 = d;
}, function (e) {
//error
});
//?????? how to return {res1:res1 , res2: res2}
return deferred.promise;
};
now, when I call get() like
get(123).then(function(d)){
// d= {res1: res1, res2: res2}
},
...
I need to get the combined result as indicated. How to do this with Angular promise API?
As #Matt said, you need to use $q.all, but the usage isn't quite right. AngularJS doesn't support .done and .fail and they don't work quite like that anyway in that there's no such thing as a promise for multiple values, instead you just have a promise for an array.
If you were writing this using the full Q we would write:
var get = function (id) {
return Q.all([Db.get(id, "abc"), Db.get(id, "def")])
.spread(function (res1, res2) {
return {res1: res1, res2: res2};
});//the error case is handled automatically
};
In this case, .spread acts like .then except that it spreads the results of an array for a promise out over the arguments of its onFulfilled function. To change this to use the promise methods from AngularJS we just need to make do without .spread. This leads to the following solution:
var get = function (id) {
return $q.all([Db.get(id, "abc"), Db.get(id, "def")])
.then(function (res) {
return {res1: res[0], res2: res[1]};
});//the error case is handled automatically
};
The beauty of this is that we are freed from handling all the nitty grity of error propagation and storing the partial results because .then acts as a filter. If you leave out the error handler, it automatically propagates any errors. This means that if either of the input promises are rejected, the result will be rejected. If both promises are fulfilled successfully, res is the array of those resolution values.
I have something to add to #ForbesLindesay answer.
In our case, we wanted partial results: if a request failed (eg. server has an hiccup, we request something deleted by somebody else, etc.), we still want to collect the valid responses, and to report the errors.
I found out that we need to handle success and failure on each promise, returning a value that will be collected by $q.all.
Here is our code, simplified and made generic ('item'...):
var promiseList = _.map(itemList, function(item)
{
return DataService.getISubtems(item.id)
.then(
function(response)
{
var subItems = response.data;
$log.info('Received sub-item list;' + subItems.length + ';items received');
return subItems;
},
function(reason)
{
$log.warn('Sub-item list not received for item;' + item.name + ';' + item.id);
$scope.errorList.push('Sub-item list not received for item "' + item.name + '"');
}
);
});
$q.all(promiseList)
.then(function(itemArray)
{
// We get an array of arrays interleaved with undefined value when an error was raised.
// That's because error handling doesn't return anything, ie. returns undefined.
// We remove these undefined values then put all operations at the same level.
var allOperations = _(operationArray).reject(_.isUndefined).flatten().value();
if ($scope.errorList.length > 0)
{
NotificationService.warning('Items Fetching', 'Errors while getting item list:\n' +
$scope.errorList.join('\n'));
}
$scope._onItemListReceived(allItems);
});
Note that we use Lodash (_.map, _.flatten, _.reject, _.isUndefined) but I think the usage is quite clear (that's the nice point of this library!).
You can use the angular-q-spread library and then use the same code as #ForbesLindesay's first example:
// The module needs $q-spread as a dependency:
// angular.module('…', ['$q-spread']);
var get = function (id) {
return $q.all([Db.get(id, "abc"), Db.get(id, "def")])
.spread(function (res1, res2) {
return {res1: res1, res2: res2};
});
};
Related
I'm new to Node/Express and am trying to use Promises to executive successive API calls to Apple's CloudKit JS API.
I'm unclear on how to put the functions in sequence and pass their respective return values from one function to the next.
Here's what I have so far:
var CloudKit = require('./setup')
//----
var fetchUserRecord = function(emailConfirmationCode){
var query = { ... }
// Execute the query
CloudKit.publicDB.performQuery(query).then(function (response) {
if(response.hasErrors) {
return Promise.reject(response.errors[0])
}else if(response.records.length == 0){
return Promise.reject('Email activation code not found.')
}else{
return Promise.resolve(response.records[0])
}
})
}
//-----
var saveRecord = function(record){
// Update the record (recordChangeTag required to update)
var updatedRecord = { ... }
CloudKit.publicDB.saveRecords(updatedRecord).then(function(response) {
if(response.hasErrors) {
Promise.reject(response.errors[0])
}else{
Promise.resolve()
}
})
}
//----- Start the Promise Chain Here -----
exports.startActivation = function(emailConfirmationCode){
CloudKit.container.setUpAuth() //<-- This returns a promise
.then(fetchUserRecord) //<-- This is the 1st function above
.then(saveRecord(record)) //<-- This is the 2nd function above
Promise.resolve('Success!')
.catch(function(error){
Promise.reject(error)
})
}
I get an error near the end: .then(saveRecord(record)) and it says record isn't defined. I thought it would somehow get returned from the prior promise.
It seems like this should be simpler than I'm making it, but I'm rather confused. How do I get multiple Promises to chain together like this when each has different resolve/reject outcomes?
There are few issues in the code.
First: you have to pass function to .then() but you actually passes result of function invocation:
.then(saveRecord(record))
Besides saveRecord(record) technically may return a function so it's possible to have such a statement valid it does not seem your case. So you need just
.then(saveRecord)
Another issue is returning nothing from inside saveRecord and fetchUserRecord function as well.
And finally you don't need to return wrappers Promise.resolve from inside .then: you may return just transformed data and it will be passed forward through chaining.
var CloudKit = require('./setup')
//----
var fetchUserRecord = function(emailConfirmationCode){
var query = { ... }
// Execute the query
return CloudKit.publicDB.performQuery(query).then(function (response) {
if(response.hasErrors) {
return Promise.reject(response.errors[0]);
}else if(response.records.length == 0){
return Promise.reject('Email activation code not found.');
}else{
return response.records[0];
}
})
}
//-----
var saveRecord = function(record){
// Update the record (recordChangeTag required to update)
var updatedRecord = { ... }
return CloudKit.publicDB.saveRecords(updatedRecord).then(function(response) {
if(response.hasErrors) {
return Promise.reject(response.errors[0]);
}else{
return Promise.resolve();
}
})
}
//----- Start the Promise Chain Here -----
exports.startActivation = function(emailConfirmationCode){
return CloudKit.container.setUpAuth() //<-- This returns a promise
.then(fetchUserRecord) //<-- This is the 1st function above
.then(saveRecord) //<-- This is the 2nd function above
.catch(function(error){});
}
Don't forget returning transformed data or new promise. Otherwise undefined will be returned to next chained functions.
Since #skyboyer helped me figure out what was going on, I'll mark their answer as the correct one.
I had to tweak things a little since I needed to pass the returned values to subsequent functions in my promise chain. Here's where I ended up:
exports.startActivation = function(emailConfirmationCode){
return new Promise((resolve, reject) => {
CloudKit.container.setUpAuth()
.then(() => {
return fetchUserRecord(emailConfirmationCode)
})
.then((record) => {
resolve(saveRecord(record))
}).catch(function(error){
reject(error)
})
})
}
This question already has answers here:
How to know when all Promises are Resolved in a dynamic "iterable" parameter?
(5 answers)
Closed 6 years ago.
I've got an api call that sometimes returns paged responses. I'd like to automatically add these to my promises so I get the callback once all the data has arrived.
This is my attempt. I'd expect the new promise to be added and Promise.all to resolve once that is done.
What actually happens is that Promise.all doesn't wait for the second request. My guess is that Promise.all attaches "listeners" when it's called.
Is there a way to "reintialize" Promise.all()?
function testCase (urls, callback) {
var promises = [];
$.each(urls, function (k, v) {
promises.push(new Promise(function(resolve, reject) {
$.get(v, function(response) {
if (response.meta && response.meta.next) {
promises.push(new Promise(function (resolve, reject) {
$.get(v + '&offset=' + response.meta.next, function (response) {
resolve(response);
});
}));
}
resolve(response);
}).fail(function(e) {reject(e)});
}));
});
Promise.all(promises).then(function (data) {
var response = {resource: []};
$.each(data, function (i, v) {
response.resource = response.resource.concat(v.resource);
});
callback(response);
}).catch(function (e) {
console.log(e);
});
}
Desired flow is something like:
Create a set of promises.
Some of the promises spawn more promises.
Once all the initial promises and spawned promises resolve, call the callback.
It looks like the overall goal is:
For each entry in urls, call $.get and wait for it to complete.
If it returns just a response without "next", keep that one response
If it returns a response with a "next," we want to request the "next" as well and then keep both of them.
Call the callback with response when all of the work is done.
I would change #2 so you just return the promise and fulfill it with response.
A key thing about promises is that then returns a new promise, which will be resolved based on what you return: if you return a non-thenable value, the promise is fulfilled with that value; if you return a thenable, the promise is resolved to the thenable you return. That means that if you have a source of promises ($.get, in this case), you almost never need to use new Promise; just use the promises you create with then. (And catch.)
(If the term "thenable" isn't familiar, or you're not clear on the distinction between "fulfill" and "resolve", I go into promise terminology in this post on my blog.)
See comments:
function testCase(urls) {
// Return a promise that will be settled when the various `$.get` calls are
// done.
return Promise.all(urls.map(function(url) {
// Return a promise for this `$.get`.
return $.get(url)
.then(function(response) {
if (response.meta && response.meta.next) {
// This `$.get` has a "next", so return a promise waiting
// for the "next" which we ultimately fulfill (via `return`)
// with an array with both the original response and the
// "next". Note that by returning a thenable, we resolve the
// promise created by `then` to the thenable we return.
return $.get(url + "&offset=" + response.meta.next)
.then(function(nextResponse) {
return [response, nextResponse];
});
} else {
// This `$.get` didn't have a "next", so resolve this promise
// directly (via `return`) with an array (to be consistent
// with the above) with just the one response in it. Since
// what we're returning isn't thenable, the promise `then`
// returns is resolved with it.
return [response];
}
});
})).then(function(responses) {
// `responses` is now an array of arrays, where some of those will be one
// entry long, and others will be two (original response and next).
// Flatten it, and return it, which will settle he overall promise with
// the flattened array.
var flat = [];
responses.forEach(function(responseArray) {
// Push all promises from `responseArray` into `flat`.
flat.push.apply(flat, responseArray);
});
return flat;
});
}
Note how we never use catch there; we defer error handling to the caller.
Usage:
testCase(["url1", "url2", "etc."])
.then(function(responses) {
// Use `responses` here
})
.catch(function(error) {
// Handle error here
});
The testCase function looks really long, but that's just because of the comments. Here it is without them:
function testCase(urls) {
return Promise.all(urls.map(function(url) {
return $.get(url)
.then(function(response) {
if (response.meta && response.meta.next) {
return $.get(url + "&offset=" + response.meta.next)
.then(function(nextResponse) {
return [response, nextResponse];
});
} else {
return [response];
}
});
})).then(function(responses) {
var flat = [];
responses.forEach(function(responseArray) {
flat.push.apply(flat, responseArray);
});
return flat;
});
}
...and it'd be even more concise if we were using ES2015's arrow functions. :-)
In a comment you've asked:
Could this handle if there was a next next? Like a page 3 of results?
We can do that by encapsulating that logic into a function we use instead of $.get, which we can use recursively:
function getToEnd(url, target, offset) {
// If we don't have a target array to fill in yet, create it
if (!target) {
target = [];
}
return $.get(url + (offset ? "&offset=" + offset : ""))
.then(function(response) {
target.push(response);
if (response.meta && response.meta.next) {
// Keep going, recursively
return getToEnd(url, target, response.meta.next);
} else {
// Done, return the target
return target;
}
});
}
Then our main testCase is simpler:
function testCase(urls) {
return Promise.all(urls.map(function(url) {
return getToEnd(url);
})).then(function(responses) {
var flat = [];
responses.forEach(function(responseArray) {
flat.push.apply(flat, responseArray);
});
return flat;
});
}
Assuming you are using jQuery v3+ you can use the promises returned by $.ajax to pass to Promise.all().
What you are missing is returning the second request as a promise instead of trying to push it to the promises array
Simplified example
var promises = urls.map(function(url) {
// return promise returned by `$.ajax`
return $.get(url).then(function(response) {
if (response.meta) {
// return a new promise
return $.get('special-data.json').then(function(innerResponse) {
// return innerResponse to resolve promise chain
return innerResponse;
});
} else {
// or resolve with first response
return response;
}
});
})
Promise.all(promises).then(function(data) {
console.dir(data)
}).catch(function(e) {
console.log(e);
});
DEMO
How do i chain promises sequentially within for loop, i have seen lot of examples on google to do this but i couldn't implement for my case:
i have gone through this link for sequential chaining of Promises.
What I'm trying to acheive:
Promise1: login();
Promise2: sync();
sync function calls another service complete() for an array of elements. These array of elements must be done sequentially.
ServiceA.login().
then(function(response){
ServiceA.sync()
.then(function(response){
})
})
function sync(){
ServiceB.complete()
.then(function(){
var promises = [];
angular.forEach(response, function (value) {
// The below service call doSomething() must be done sequentially for each "value"
promises.push(doSomething(value));
});
$q.all(promises).then(function () {
});
});
})
}
How do I capture the error occuring in each Promise?
Update:
I have tried the approach suggested by #zaptree with the following code:
ServiceA.login()
.then(function(response){
// you must always return your promise
return ServiceA.sync()
})
// don't nest the .then make them flat like this
.then(function(response){
})
.catch(function(){
// if you made sure to always return your promises this catch will catch any errors throws in your promise chain including errors thrown by doSomething()
});
function sync(){
// you must always return your promise
return ServiceB.complete()
.then(function(){
var result = $q.when();
angular.forEach(response, function (value) {
result = result.then(doSomething(value)); // problem is here that doSomething function is being called before the first call it is resolved
// doSomething is a http call.
});
return result;
})
.then(function(){
// the array of promises has run sequentially and is completed
});
}
function doSomething(data){
return $http({
method: 'POST',
url: '/api/do',
data: data,
headers: {
"Content-Type": "application/json"
}
}).then(function (response) {
}, function (error) {
});
}
If the response in the near the for each loop has 2 values (valuea, valueb) in it, the code is behaving as follows:
1. calling doSomething(valuea)
2. calling doSomething(valueb) before the above promise is resolved.
Expected behaviour:
after the POST method has succesfully completed by the call doSOmething(valuea), then the another POST call should happend i.e., soSomething(valueb).
Here's what I came up with. You'll need to reduce the array into a single promise.
var results = [...];
var sequentialPromise = results.reduce(function(a, b) {
return a.then(function(){
return doSomething(b);
});
}, $q.resolve());
sequentialPromise.then(function(){...});
So here is an example on how you would do the sequential promises with Q, also some improvements on how to do your promises so you can properly catch errors thrown at any point in your promise chain. You must always make sure to return a promise on any method that uses them. Also avoid pyramid code by not nesting the .then to make your code cleaner:
ServiceA.login()
.then(function(response){
// you must always return your promise
return ServiceA.sync()
})
// don't nest the .then make them flat like this
.then(function(response){
})
.catch(function(){
// if you made sure to always return your promises this catch will catch any errors throws in your promise chain including errors thrown by doSomething()
});
function sync(){
// you must always return your promise
return ServiceB.complete()
.then(function(){
var result = $q.when();
angular.forEach(response, function (value) {
result = result.then(doSomething(value));
});
return result;
})
.then(function(){
// the array of promises has run sequentially and is completed
});
}
it is a common pattern that we cascade across a list of sources of data with the first success breaking the chain like this:
var data = getData1();
if (!data) data = getData2();
if (!data) data = getData3();
et cetera. if the getDataN() functions are asynchronous, however, it leads us to 'callback hell':
var data;
getData1(function() {
getData2(function () {
getData3(function () { alert('not found'); })
})
});
where the implementations may look something like:
function getData1(callback) {
$.ajax({
url: '/my/url/1/',
success: function(ret) { data = ret },
error: callback
});
}
...with promises I would expect to write something like this:
$.when(getData1())
.then(function (x) { data = x; })
.fail(function () { return getData2(); })
.then(function (x) { data = x; })
.fail(function () { return getData3(); })
.then(function (x) { data = x; });
where the second .then actually refers to the return value of the first .fail, which is itself a promise, and which I understood was chained in as the input to the succeeding chain step.
clearly I'm wrong but what is the correct way to write this?
In most promise libs, you could chain .fail() or .catch() as in #mido22's answer, but jQuery's .fail() doesn't "handle" an error as such. It is guaranteed always to pass on the input promise (with unaltered state), which would not allow the required "break" of the cascade if/when success happens.
The only jQuery Promise method that can return a promise with a different state (or different value/reason) is .then().
Therefore you could write a chain which continues on error by specifying the next step as a then's error handler at each stage.
function getDataUntilAsyncSuccess() {
return $.Deferred().reject()
.then(null, getData1)
.then(null, getData2)
.then(null, getData3);
}
//The nulls ensure that success at any stage will pass straight through to the first non-null success handler.
getDataUntilAsyncSuccess().then(function (x) {
//"success" data is available here as `x`
}, function (err) {
console.log('not found');
});
But in practice, you might more typically create an array of functions or data objects which are invoked in turn with the help of Array method .reduce().
For example :
var fns = [
getData1,
getData2,
getData3,
getData4,
getData5
];
function getDataUntilAsyncSuccess(data) {
return data.reduce(function(promise, fn) {
return promise.then(null, fn);
}, $.Deferred().reject());// a rejected promise to get the chain started
}
getDataUntilAsyncSuccess(fns).then(function (x) {
//"success" data is available here as `x`
}, function (err) {
console.log('not found');
});
Or, as is probably a better solution here :
var urls = [
'/path/1/',
'/path/2/',
'/path/3/',
'/path/4/',
'/path/5/'
];
function getDataUntilAsyncSuccess(data) {
return data.reduce(function(promise, url) {
return promise.then(null, function() {
return getData(url);// call a generalised `getData()` function that accepts a URL.
});
}, $.Deferred().reject());// a rejected promise to get the chain started
}
getDataUntilAsyncSuccess(urls).then(function (x) {
//"success" data is available here as `x`
}, function (err) {
console.log('not found');
});
As a beginner, stumbling across the same problem, I just realized how much simpler this has become with async and await:
The synchronous pattern
var data = getData1();
if (!data) data = getData2();
if (!data) data = getData3();
can now easily be applied to asynchronous code:
let data = await getData1();
if (!data) data = await getData2();
if (!data) data = await getData3();
Just remember to add an async to the function that this code is used in.
According to this answer, the promise has been created, but the method 'then'( also 'done') will be executed without waiting for the output from subprocess, I need to have a method which is to be called after completely executing all subprocess, how this can be accomplished using bluebird api?
Sample Code
var Promise = require('bluebird')
var exec = require('child_process').exec
// Array with input/output pairs
var data = [
['input1', 'output1'],
['input2', 'output2'],
.....
]
var PROGRAM = 'cat'
Promise.some(data.map(function(v) {
var input = v[0]
var output = v[1]
new Promise(function(yell, cry) {
exec('echo "' + input + '" | ' + PROGRAM, function(err, stdout) {
if(err) return cry(err)
yell(stdout)
})
}).then(function(out) {
if(out !== output) throw new Error('Output did not match!')
})
}),data.length)
.then(function() {
// Send succes to user if all input-output pair matched
}).catch(function() {
// Send failure to the user if any one pair failed to match
})
Here the 'then' function is executed immediately even before the subprocess is completed.
Promise.some() expects an array of promises as its first argument. You are passing the results of data.map() to it, but your callback to data.map() never returns anything so therefore .map() doesn't construct an array of promises and therefore Promise.some() has nothing to wait on so it calls it's .then() handler immediately.
Also, if you're going to wait for all the promises, then you might as well use Promise.all() instead.
This is what I think you want.
Changes:
Switch to Promise.all() since you want to wait for all the promises.
Return the new Promise so .map() will create the array of promises.
Moved the output check into the original promise so it can reject rather than throw and it just seems like that moves all the result checking into one place.
Added all missing semi-colons.
Changed cry and yell to resolve and reject so code would be more familiar to outsiders expecting normal promise names.
Here's the code:
var Promise = require('bluebird');
var exec = require('child_process').exec;
// Array with input/output pairs
var data = [
['input1', 'output1'],
['input2', 'output2']
];
var PROGRAM = 'cat';
Promise.all(data.map(function(v) {
var input = v[0];
var output = v[1];
return new Promise(function(resolve, reject) {
exec('echo "' + input + '" | ' + PROGRAM, function(err, stdout) {
if(err) {
reject(err);
} else if (stdout !== output) {
reject(new Error('Output did not match!'));
} else {
resolve(stdout);
}
});
});
})).then(function() {
// Send succes to user if all input-output pair matched
}).catch(function() {
// Send failure to the user if any one pair failed to match
});