I had an issue with and angular-meteor project flickering every time a state using the campaigns subscription, would load. By flickering, I mean the data was there, then it would go away and come back a half second later.
I added this to the resolve property of the state (using ui-router):
campaigns: ($q) => {
var deferred = $q.defer();
Meteor.subscribe('campaigns', {
onReady: deferred.resolve,
onStop: deferred.reject
});
return deferred.promise;
}
The flickering stopped, but I don't really understand this code. Can someone who understand angular break this resolve/defer situation down?
Just not sure why it worked. thanks.
$q is angular's implementation of promises.
in a very itty bitty nutshell, a promise has two callbacks that resolve when data is returned; a resolve function if the call succeeds and a reject if the call fails. whatever data it gets will be passed into these functions (essentially doing deferred.resolve(data) or deferred.reject(error)) . $q.defer() allows us to assign the resolution/rejections later.
meteor's subscribe function takes a few arguments. the string name of the collection, a function that returns an array of arguments to be passed to the collection, and an object/function. the object part of the final argument expects an "onReady" and "onStop" functions, and will execute those functions and pass along any data it gets. we pass in our callbacks here.
finally, we return the promise. resolve.campaigns will be a promise, which we can get the values from using the .then(successCallback, failureCallback) call. meteor handles this behind the scenes.
Related
I'm getting data from the server using $resource like this
service
.factory('rulesService', ['$resource', function ($resource) {
var systems = $resource('url');
return systems;
}]);
controller
$scope.rules= rulesService.query();
console.log($scope.rules);
The output I get is
0: Resource
1: Resource
$promise: Promise
$resolved: true
length: 2
I tried to strip $promise & $resolved using
1) angular.toJson($scope.rules)
2)JSON.stringify($scope.rules, null, 2)
Both these are returning []
Can someone help me on this
After reading your comments above, I think your problem is you use $resource wrong.
$resource will return a empty object or array, and then make the http call in the background and populate the array/object once it is complete.
This means that, at the time you console.log the object, it is actually an empty array, but since the log in the browser is pretty smart, it will update the object in the log as well, once the $resource call is done.
this is why console.log(rules[0]) is undefined, while console.log(rules) says the element exists. It didn't at the time of the log.
If you need to do further processing you have to do something like:
var rules = rulesService.query(function () {
console.log(rules[0])
})
you can also use promises instead of a callback, but either way you need to ensure the data is fully loaded
You should be able to simply iterate over the array (and ignore the extra properties). If you want a clean array you could always use a map or similar.
$scope.new_rules = $scope.rules.map(function (rule){
return rule;
})
Your problem is related to asynchronous execution and race conditions.
You're trying to refer to rules[0] before data actually arrives from the $resource.query() call.
Examples:
var rules = rulesService.query();
console.log(rules[0]); //will print nothing. the GET request hasn't been resolved yet.
rules.$promise.then(function (response) {
console.log(rules[0]); //This WILL work assuming you actually get data from the backend.
console.log(response[0]); //This will also work with the same data.
});
So I wanted to execute 2 http calls to get some groups and questions from my server and have both of these resolved in routeprovider so that the corresponding controller will not load before I have the relevant data.
In my other controllers I always worked with an initalData object to hold the data.
The resolve part:
resolve: {
initialData: ["GroupsService" "QuestionsService", function (GroupsService, QuestionsService) {
return {
questions: QuestionsService.getAll(),
groups: GroupsService.getAll()
}]
}
When I tried to access the data in the controller using initialData.questions and initialData.groups respectively, I however received 2 promises instead of the data, even though the callback from the http got logged before the controller was instantiated.
QuestionsCtrl.$inect = ["DialogsService", "initialData", "QuestionsService"];
function QuestionsCtrl(DialogsService, questions, groups, QuestionsService) {
//Initialdata object which has 2 Promise properties
//This logs AFTER the callback in both http callbacks
console.log('controller initialized', initialData);
When I replaced the code with this (didn't use an initialData object, instead returned two other objects, it did work:
resolve: {
questions: function (QuestionsService) {
//$http call for all questions
return QuestionsService.getAll();
},
groups: function (GroupsService) {
//$http call for all groups
return GroupsService.getAll();
}
}
Does anyone have any logical explanation for why, in the first case I got back promises (despite that the data was actually present in the client), and the second one worked flawlessly?
When you pass resolve to a route it calls $q.all on it implicitly for you. So when you return multiple values in resolve it waits for all of them to finish.
In your example - you just returned an object containing some values that are promises - you didn't wait for them so it got resolved immediately with the promises instead of unwrapping them.
You can of course explicitly wait for them too:
initialData: ["a" "b","$q", function (a, b, $q) {
return $q.all({
questions: a.getAll(),
groups: b.getAll()
});
}]
If you wish resolve to wait, you need to return a promise, in your first case it's not a promise, rather it's just an object, it happens to have 2 objects that are however promises, but angular won't know that... If you wish to return it as a single resolve you can use:
return $q.all({ key1: promise, key2: promise });
and add $q as a dependency
Another thing, promises doesn't turn them self into raw values when data is received from the server, they stay promises, in the case of resolve, angular will dig out the resolved value and provide those instead of the promises. And again we need to go back to that angular needs to know it is dealing with promises.
I'm trying to write what I think is a fairly simple test in protractor, but it would seem that the minute you try to do anything synchronously, Protractor makes life hard for you! Normally, dealing with locator functions (that return a promise) are not an issue, since any expect statement will automatically resolve any promise statement passed to it before testing the assertion. However, what I'm trying to do involves resolving these locator promises before the expect statement so that I can conditionally execute some test logic. Consider (pseudocode):
// Imagine I have a number of possible elements on the page
// and I wish to know which are on the page before continuing with a test.
forEach(elementImLookingFor){
if (elementImLookingFor.isPresent) {
// record the fact that the element is (or isnt) present
}
}
// Now do something for the elements that were not found
However, in my above example, the 'isPresent' call returns a promise, so can't actually be called in that way. Calling it as a promise (i.e. with a then) means that my forEach block exits before I've recorded if the element is present on the page or not.
I'm drawing a blank about how to go about this, has anyone encountered something similar?
I've used bluebird to do the following;
it('element should be present', function(done)
Promise.cast(elementImLookingFor.isPresent)
.then(function(present){
expect(present).toBeTruthy();
})
.nodeify(done);
});
If you have a few elements that you want to check the isPresent on you should be able to do the following;
it('check all elements are present', function(done){
var promises = [element1, element2].map(function(elm){
return elm.isPresent();
});
// wait until all promises resolve
Promise.all(promises)
.then(function(presentValues){
// check that all resolved values is true
expect(presentValues.every(function(present){
return present;
})).toBeTruthy();
})
.nodeify(done);
});
Hope this helps
So elementImLookingFor is a promise returned by element.all, I presume? Or, as stated in the Protractor documentation, an ElementArrayFinder. You can call the method .each() on it and pass it a function that expects things.
When I have multiple promises and resolve them one by one I can easily get notifications from each promise while it is being resolved.
However it seems that when I chain promises or use the all() method then some notifications are getting lost. I am wondering why this is the case.
Here is an example. Lets say I have two async functions that return a promise, do some work and while doing this they call notify one or more times:
function returnsSomePromise1() {...; setTimeout(...); return promise;}
function returnsSomePromise2() {...; setTimeout(...); return promise;}
Now I have three options to resolve the promises:
var promise1 = returnsSomePromise1();
var promise2 = returnsSomePromise2();
//Option 1: Resolve separate
promise1.then(...);
promise2.then(...);
//Option 2: Resolve chained
promise1.then(promise2).then(...);
//Option 3: Resolve with all()
$q.all([promise1, promise2]).then(...);
In each then function I attach a simple function to log the notifications to the console:
.then(..., ..., function(update) { console.log(update); });
Now I find it interesting that all three options yield different results when it comes to notify. The first one prints all notifications of each async operation. The second option only prints the notifications of the first promise, the last option using the all() method does not print any notifications at all.
Can anyone explain what is causing these differences and if it is possible to get notifications when using $q.all()?
Option 1 will fire the promises in parallel, they will be resolved individually. If you want to do something after both have resolved you would need to add extra logic using this option
Option 2 will fire the promises one after the other. This is useful if promise2 depends on something from promise1.
Also, You code should be promise1.then(returnsSomePromise2).then(...); it wants a function in .then() where before it was getting a promise (promise2). The function it calls will return the promise and it will chain it.
Option 3 will fire them in parallel and resolve a function when both (all) have been completed. $q.all() can take an array OR an object. In the case o an array, which you have in your example. The function passed to .then() will be passed an array. The array lines of with the order of the promises passed to $.all().
$q.all([promise1, promise2]).then(function(result){
var promise1Result = result[0];
var promise2Result = result[1];
})
If you pass an object, with named keys, you will get an object back in the function passes to .then(). The keys will line up with the object passed to $q.all()
$q.all({myFirstPromise:promise1, secondPromise:promise2}).then(function(result){
var promise1Result = result.myFirstPromise;
var promise2Result = result.secondPromise;
})
I found this snippet of code that does what I want it to:
var promise = this.model.save();
$.when(promise).then(function() {
console.log(promise.responseText);
});
I want to get back the responseText from my Backbone call to this.model.save(). This code was documented here. But it's not logging anything, even if I pull a raw text string in the console.log() call.
Could someone please explain in layman's terms what a jQuery promise is? I've read about them, but I don't think I quite got what they were. That might help me understand why this code isn't working for me. If I console.log(promise) in between the first and second lines of code, then I get the responseText. So something is happening in either the $.when or the then that is causing this to go wrong.
EDIT:
After reading the article, I discovered I could do this:
var promise = this.model.save();
$.when(promise).then(null, function(obj) {
console.log(obj.responseText);
});
But I don't understand what the null represents. then seems to take two parameters, a success function and a failure function. But wouldn't the success function be first? I get a 200 response from the server.
So first off, I'm pretty sure you don't need the when part; from the jQuery docs:
The jqXHR objects returned by $.ajax() as of jQuery 1.5 implement the
Promise interface, giving them all the properties, methods, and
behavior of a Promise (see Deferred object for more information).
Since Promise has a then method already, you can just do:
this.model.save().then(null, function(obj) {
console.log(obj.responseText);
});
(The fact that the above code almost reads like an English sentence is a major advantage of using Deferreds, for me at least.)
As for your null argument, the docs are again pretty clear. There are three signatures for then (and that's just to cover the different jQuery versions; any given version has less):
deferred.then( doneFilter [, failFilter ] [, progressFilter ] )
deferred.then( doneCallbacks, failCallbacks )
deferred.then( doneCallbacks, failCallbacks [, progressCallbacks ] )
As you can see, all three take the "done" function first, and the failure function second. This does seem to imply that you're getting a failure, which is confusing. One way to avoid the problem is to not use then at all. Instead, try the following:
this.model.save().always(function(obj) {
console.log(obj.responseText);
});
That will make your function get called no matter what happens. However, you probably should figure out what's going on, so you might want to instead add a success and failure callback to do some debugging:
this.model.save().done(function() {
// Success case
}).fail(function() {
// Failure case
});
Because this.model.save returns a promise, you can do the following instead:
this.model.save()
.done(function(response) {
console.log("Success!");
})
.fail(function(response) {
console.log("Error!");
});
(That's easier than the whole $.when bit.)
My guess is that although your response is returning a 200 code, it is still "failing" because the response data type doesn't match up with what you're expecting (what's set in the dataType attribute in the $.ajax call).
I am a big fan of using promise, and I think the promise means very similar things in different packages.
To answer your question, which previous answers didn't, the "then" function is a function of a promise, the "when" function is a fail-save, incase the object is not a promise, a "when(obj)" will make sure it is a promise, so that you can use the elegant xxx.then(success(){},error(){}).
btw, the "deferred" that machineghost said is the package that let you use promise.
For starting to know promise and how to use it. check out this tutorial. It explains every thing very clearly, it is the article that made me so into promises.
http://strongloop.com/strongblog/promises-in-node-js-with-q-an-alternative-to-callbacks/
Now, as machineghost said, it seems your sync call is getting an error, according to a REST API documentation,https://parse.com/docs/rest# (don't know if it is the same as backbone), the server will response a JSON object for a "create" request in this format:
{"createdAt": "2011-08-20T02:06:57.931Z","objectId": "Ed1nuqPvcm"}
My guess is, maybe your server did not respond the request with the correct JSON, so the save() think the operation failed because there was no "createAt" field, event thought your sever did created the item.