I would like to make multiple Ajax calls in a chain. But I also would like to massage the data after each call before making the next call. In the end, when All calls are successful, I would like to run some other code.
I am using Angular $http service for my Ajax calls and would like to stick to that.
Is it possible?
Yes, this is handled very elegantly by AngularJS since its $http service is built around the PromiseAPI. Basically, calls to $http methods return a promise and you can chain promises very easily by using the then method. Here is an example:
$http.get('http://host.com/first')
.then(function(result){
//post-process results and return
return myPostProcess1(result.data);
})
.then(function(resultOfPostProcessing){
return $http.get('http://host.com/second');
})
.then(function(result){
//post-process results of the second call and return
return myPostProcess2(result.data);
})
.then(function(result){
//do something where the last call finished
});
You could also combine post-processing and next $http function as well, it all depends on who is interested in the results.
$http.get('http://host.com/first')
.then(function(result){
//post-process results and return promise from the next call
myPostProcess1(result.data);
return $http.get('http://host.com/second');
})
.then(function(secondCallResult){
//do something where the second (and the last) call finished
});
The accepted answer is good, but it doesn't explain the catch and finally methods which really put the icing on the cake. This great article on promises set me straight. Here's some sample code based on that article:
$scope.spinner.start();
$http.get('/whatever/123456')
.then(function(response) {
$scope.object1 = response.data;
return $http.get('/something_else/?' + $scope.object1.property1);
})
.then(function(response) {
$scope.object2 = response.data;
if ($scope.object2.property88 == "a bad value")
throw "Oh no! Something failed!";
return $http.get('/a_third_thing/654321');
})
.then(function(response) {
$scope.object3 = response.data;
})
.catch(function(error) {
// this catches errors from the $http calls as well as from the explicit throw
console.log("An error occured: " + error);
})
.finally(function() {
$scope.spinner.stop();
});
Related
Is this a bad way to do this? I'm basically chaining promises, where each successful return from server, launches a new http.get() for more information BUT NOT when it errors. No more http.get()s if it causes an errorCallback!
$http.get(...).then(function() {
$http.get(...).then(function(){
$http.get(...).then(function(){}, function(){}),
function(){}); },
mainErrorCallback);
Would it make a difference if it was instead of "$http.get()" it does "ViewsService.loadViews()" inside the
$http.get(...).then( function() { ViewsService.loadViews(); }, function(){ console.log("error"); }).
EDIT: Here's what I mean, synchronously.. it seems like it works, but code needs cleanup/efficency to look a little neater:
http://jsfiddle.net/4n9fao9q/6/
(with delayed http requests): http://jsfiddle.net/4n9fao9q/26
$http.get(...).then((res) => {
//res has data from first http
return $http.get(...);
}).then((res) => {
//res has data from second http
return $http.get(...);
}).then((res) => {
//res has data from third http
}).catch((err) => {
//exploded
});
I think is cleaner. You can replace $http.get with whatever function returns a promise. If ViewsService.loadViews() returns a promise, you can use it.
As asked in the comments.
...
ViewsService.loadViews = function() {
//returns a promise
return $http.get(...);
}
OR
ViewsService.loadViews = function() {
return new Promise((resolve, reject) => {
$http.get(...).then((result) => {
//whatever
return resolve();
})
})
return $http.get(...);
}
With any of this options for loadViews you can do ViewsService.loadViers.then(etc)
Is this a bad way to do this?
Efficiency
Unless you are using the response from the first request as input to the following request then this isn't
a very efficient way to do this, as each request will be blocked until the previous one has returned. A better
way would be to use $.all($http.get(...),$http.get(...))
Style
The nested calls (the pyramid of doom) are difficult to read. As each call has the same failure response you could just chain these calls instead. e.g.
$http.get(..).then
($http.get(..)).then(
($http.get(..)).catch(errHander)
In all my services, I'm just invoking REST services and returning the promises to the controllers. All the error's are handled at controllers using catch like below,
MyService.getData(url).then(getDataSuccess).catch(exception.catcher('Contact Admin : '));
My question here is, Since the real $http calls will be made at service, should I have to write catchers in service or catching in controller is fine?,
Scenario 1:
function getData(url){
return $http.get(url);
}
Scenario 2: (Nested calls to make combined results)
function getOtherData(url){
var defer = $q.defer();
$http.get(url).then(
function(response){
$http.get(nextService).then(
function(res){
defer.resolve('combined data');
}
)
}
);
return defer.promise;
}
Both the service method is not handling any errors. Instead it just returns the promise. Will there be any situation where this kind of exception handling will get failed?
Note: I have created decorators for handling javascript,angular errors and route errors separately. This question is particularly about $http service errors.
Yes what you have can fail triggering your catch because you have no reject().
You are using an anti-pattern creating your own promise and not chaining the nested request properly. Neither of those request rejections will be returned anywhere.
To be able to chain these get rid of the $q.defer() and do:
function getOtherData(url) {
// return beginning of promise chain
return $http.get(url).then(function (response) {
// return next promise
return $http.get(nextService).then(function (res) {
// combine and return the data
return {
d1 : response.data,
d2 : res.data
};
});
});
}
Now walk through the scenarios and each part of chain is intact.
Think of the chain as each then needs a return until the end of the chain
Scenario 2: (Nested calls to make combined results)
Failed Scenario
function getOtherData(url){
var defer = $q.defer();
$http.get(url).then(
function(response){
$http.get(nextService).then(
function(res){
defer.resolve('combined data');
}
)
}
);
return defer.promise;
}
This scenario will fail if the first $http.get has an error. The promise will hang and never get resolved. This is why we recommend avoiding using $q.defer to create promises from services that already return promises.
Instead return data and chain promises.
function getOtherData(url) {
var promise = $http.get(url);
var derivedPromise =
promise.then ( function (response) {
var data = response.data;
var nextPromise = $http.get(nextService);
var derivedNext = nextPromise.then(function(response) {
//return for chaining
return response.data;
});
//return composite for chaining
return $q.all([data, derivedNext]);
});
return derivedPromise;
};
The getOtherData(url) promise will be fulfilled with an array with the data from the two XHRs or it will be rejected with the first error response.
It is possible to create chains of any length and since a promise can be resolved with another promise (which will defer its resolution further), it is possible to pause/defer resolution of the promises at any point in the chain. This makes it possible to implement powerful APIs.1
Chaining error handlers
In an error handler, to convert a rejected resolution to a fulfilled resolution return data. To chain a rejection, throw the error.
For example:
promise = http.get(someUrl);
derivedPromise = promise.catch(function(errorResponse) {
if (fixable) {
fixedPromise = $http.get(fixedUrl);
//return to convert
return fixedPromise;
} else {
//throw to chain rejection
throw errorResponse;
}
};
By chaining error handlers, errors can be handled both by the service and the client of the service.
This makes it possible to implement powerful APIs like $http's response interceptors.1
Building on #georgeawg's answer, if you want to return multiple sets of data then you don't need nested calls.
function getOtherData(url) {
var promise1 = $http.get(url).then ( function (response) {
return response.data;
});
var promise2 = $http.get(nextService).then(function(response) {
return response.data;
});
return $q.all([promise1, promise2]);
};
Now the caller gets a promise that resolves to a list of the 2 data items (or is rejected if either request fails). The only real difference is that both requests are issues in parallel.
This generalises easily to a situation where you could have a list of urls, fetch them all in parallel and get an array of the response.data items.
Because you get back only a single promise that resolves to an array of data you can handle the result in the controller, but you only need one error handler.
MyService.getOtherData(url)
.then(getDataSuccess)
.catch(exception.catcher('Contact Admin : '));
Although the original question doesn't specify, it might be the case that the second url depends on the result from the first. You can handle that case here as well if you remember that you can call .then() multiple times on the same promise:
function getOtherData(url) {
var promise1 = $http.get(url).then ( function (response) {
return response.data;
});
var promise2 = promise1.then(function(response) {
// compute nextService from response.data here...
var nextService = foo(response.data);
return $http.get(nextService).then(function(response) {
return response.data;
});
});
return $q.all([promise1, promise2]);
};
If a Javascript function is designed with callbacks, how does one encapsulate that function within an AngularJS promise?
For example, I am looking at using the following Cordova plugin: cordova.plugins.diagnostic (see https://www.npmjs.com/package/cordova.plugins.diagnostic). Many of its functions are designed with callbacks. Because the requests are engaging the device's OS, it may take a little time before a function completes, so I'm considering whether they should be called within a promise structure. For example, how would one convert the following:
cordova.plugins.diagnostic.isWifiEnabled(function(enabled){
<do something>
}, function(error){
<do something>
});
or really any generic callback structure...
masterFunction(function(enabled){
<do something>
}, function(error){
<do something>
});
to operate within an AngularJS promise? Would it be something like this?
function callMasterFunction() {
var deferred = $q.defer();
masterFunction(function(enabled){
<do something>
deferred.resolve(enabled);
}, function(error){
<do something>
deferred.resolve(error);
});
return deferred.promise;
}
I would think that this would also be a concern when using AngularJS with Cordova and the W3C Geolocation APIs. It seems to me that I may not have a clear picture of how the scope is being managed in these cases.
Ultimately, I can see chaining many of these sorts of calls together. Something like:
var promise = callMasterFunction1()
.then(function(response) { return callMasterFunction2(); })
.then(function(response) { return callMasterFunction3(); })
...
Any help would be appreciated. Thank you for your time.
You can use the promise constructor to create a promise from a callback-based API:
function callMasterFunction() {
return $q(function (resolve, reject) {
cordova.plugins.diagnostic.isWifiEnabled(resolve, reject);
});
}
Now callMasterFunction() returns a promise:
callMasterFunction()
.then(function (enabled) {
console.log('Wifi is ' + (enabled ? '' : 'not ') + 'enabled.');
})
.catch(function (error) {
console.error('Something went wrong: ', error);
});
And when you want to chain them, you can do this:
var promise = callMasterFunction1()
.then(callMasterFunction2)
.then(callMasterFunction3);
I am learning AngularJS after converting from jQuery for a few years. And some bits are much more intuitive. Some not so much :).
I am trying to get my head around the use of promises, particularly $q in use with $http and there does not seem to be too much information around these two combined that I can find.
Why would I use promises in place of the success/error callback? They both make uses of callbacks in reality, so why is a promise considered better? E.g. I could set up a get(...) function like follows:
function get(url, success, error) {
success = success || function () {};
error = error || function () {};
$http.get(url)
.success(function (data) {
success(data);
})
.error(function (error) {
error(error);
});
}
get('http://myservice.com/JSON/',
function () {
// do something with data
},
function () {
// display an error
}
);
Which is good(?) because it gives me complete control over what is happening. If I call get(...) then I can control any success/errors wherever get is called.
If I convert this to use promises, then I get:
function get(url) {
return $http.get(url)
.then(function (data) {
return data;
},
function (error) {
return error;
});
}
get('http://myservice.com/JSON/')
.then(function (data) {
// do something with data
});
// cannot handle my errors?
Which is condensed, I agree; we also do not have to explicitly worry about the success/error callback, but I seem to have lost control over my error callback for a start - because I cannot configure a second callback to handle an error.
Which means that if I use this function in a service which can be used by multiple controllers, then I cannot update the UI to alert the user to an error.
Am I missing something? Is there a reason why promises is preferred? I cannot find an example why.
Usually you'll deal with asynchronous tasks in Javascript with callbacks;
$.get('path/to/data', function(data) {
console.log(data);
});
It works fine, but start to complicate when you go into whats called the 'callback hell';
$.get('path/to/data', function(data) {
$.get('path/to/data2' + data, function(data2) {
$.get('path/to/data3' + data2, function(data3) {
manipulate(data, data2, data3);
}, errorCb);
}, errorCb);
}, errorCb);
The alternative is working with promises and defered object;
Deferreds - representing units of work
Promises - representing data from those Deferreds
Sticking to this agenda can assist to you in every extreme asynctask case:
You have a regular call that need to get data from the server, manipulate it, and return to the scope
You have multiple calls that each is depending on the precious one (cahin strategy)
You want to send multiple (parallel) calls and handle their success in 1 block
You want your code to be orginized (prevent dealing with handling results on controllers)
Your task is the easiest one to handle with $q and $http
function get(url) {
var deferred = $q.defer();
$http.get(url)
.success(function (data) {
deferred.resolve(data);
})
.error(function (error) {
deferred.reject(error);
});
return deferred.promise;
}
And calling the service function is the same
get('http://myservice.com/JSON/')
.then(function (data) {
// do something with data
});
// cannot handle my errors?
You can handle the error like this:
get('http://myservice.com/JSON/')
.then(function (data) {
// do something with data
},
function (error) {
//do something with error
});
But unfortunately since you have already caught the error then the final error won't be triggered. You will also have the same problem with success.
To get that to work you ned to use $q.
function get(url) {
var deferred = $q.defer();
$http.get(url)
.success(function (data) {
deferred.resolve(data);
})
.error(function (error) {
deferred.reject(error);
});
return deferred.promise;
}
Also there is no need to pass in success and error functions because you can use promises instead.
I'm a bit confused how to chain promises in AngularJS. I have a $resource implementation... This is some simplified code to get my point:
myResource
.save({ id: 123 })
.$promise
.then(function(data) {
// redirect to root on success
$location.url("/");
})
.catch(function() {
console.log("Failed");
})
.finally(function() {
// reenable form
$scope.enabled = true;
})
What I'm confused about is whether my .catch() function will be called when my resource will fail saving or will it only catch errors that would originate in .then() function or maybe both?
When chaining these how can I have a single .catch (before .finally) that would catch all errors of calls that happened in the chain proceeding it?
Note: I'm aware I could provide the success and error functions in .save function but I have other parts to handle as well.
First of all, read the docs of Angular's implementation of promises.
Promises can be resolved in two ways - either successful (the resolve method) or not (the reject method).
When resource could not be saved (i.e. server did not respond with 2XX status), then your catch method will be called, as the promise will be rejected.
Also please note, that your catch handler should reject the promise for next possible handlers. When you return non-rejected value from your catch method, it is assumed that the error has been handled and the promise can call subsequent then methods, if any. Therefore, if there is any possibility that the promise is used somewhere else, your code should look like this:
// inject $q
myResource
.save({ id: 123 })
.$promise
.then(function(data) {
// redirect to root on success
$location.url("/");
})
.catch(function() {
console.log("Failed");
return $q.reject();
})
.finally(function() {
// reenable form
$scope.enabled = true;
})
Also notice that you made a typo in the then method - you wrote "ulr" instead of "url".