How can i implement the promise pattern with ReactiveCocoa? - cocoa-touch

Question

I am new to iOS development coming from a JS background with EmberJS. I want to port my EmberJS App to an iOS App. Therefore i would like to use similiar structures in my iOS App. As EmberJS makes heavy use of promises i searched for something similar for iOS and stumbled upon ReactiveCocoa. It is said in the introduction of ReactiveCocoa that this framework can be used to implement Promises. I tried it but it does not work properly. I wanted to start with a quite simple example:
Make an asynchronous network request (to fill a UITableViewController). Return a promise from this method.
Subscribe to this promise and reload the TableView when it is finished.
I want to do it this way, because i will have to perform several things after the data has been loaded successfully. My approach works basically but i am experiencing the following issues:
My TableView does not reload immediately after the request has been finished.
I am seeing the Log Statements in my subscribeCompleted immediately after the request finished. But the TableView stays blank.
The TableView loads the data after a few seconds of waiting.
If i start scrolling the TableView after i have seen the Log output, the TableView is suddenly loaded.
I suspect this may happen because i am fetching the data in a background thread. I think the resolve of the promise (subscribeCompleted) may happen in the background thread too and Cocoa Touch may not like this. Am i right? But if this is the case, how am i supposed implement a promise?
I hope you can help me getting started with ReactiveCocoa. Thx! :-)
UPDATE:
I managed to fix it by wrapping the to reloadData in a dispatch_async(dispatch_get_main_queue(), ^{... But still i am not sure wether this is the best way to go or what is recommended by ReactiveCocoa. So i am still keen on hearing some answers :-)
// this method wants to use the promise
- (void) loadDataAndPerformActionsAfterwards{
RACSignal *signal = [self fetchObjects];
[signal subscribeCompleted:^{
NSLog(#"Entered subscribeCompleted block signal!");
NSLog(#"Number of objects: %i", self.objects.count);
[self.tableView reloadData];
}];
}
// this method returns a promise. I omitted some parts but it shows basically how i go about resolving the promise.
- (RACSignal*) fetchMoviesForCurrentFormState{
return [RACSignal createSignal:^RACDisposable*(id<RACSubscriber> subscriber) {
NSLog(#"RAC createSignal Block called");
NSString *requestURL = #"...";
NSURL *urlObj = [NSURL URLWithString: requestURL];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSData* data = [NSData dataWithContentsOfURL: urlObj];
if(data){
[self performSelectorOnMainThread:#selector(fetchedData:)
withObject:data waitUntilDone:YES];
[subscriber sendCompleted];
}else{
// Not implemented yet: handle the error case
[subscriber sendCompleted];
}
});
// actually i do not know yet what i should return here. Copied from a basic example.
return nil;
}];
}

Answer 1

You're right that this is an issue with threading. However, you don't need to drop down to the level of GCD.
Signals can be "delivered" onto another thread, which just invokes any subscription callbacks there:
- (void) loadDataAndPerformActionsAfterwards {
[[[self
fetchObjects]
deliverOn:RACScheduler.mainThreadScheduler]
subscribeCompleted:^{
NSLog(#"Entered subscribeCompleted block signal!");
NSLog(#"Number of objects: %i", self.objects.count);
[self.tableView reloadData];
}];
}

Answer 2

You may take a look into RXPromise. It's an Objective-C implementation of the Promises/A+ specification with a couple more features. (I'm the author).
A solution utilizing the RXPromise library would look as follows:
- (void) loadDataAndPerformActionsAfterwards {
[self fetchMovie]
.thenOn(dispatch_queue_get_main(), ^id(id fetchedMovie) {
self.model = fetchedObjects;
[self.tableView reloadData];
}, nil);
}
This assumes, method fetchMovie returns a Promise.
How do you get this? Well, you can easily wrap any asynchronous method or operation into one that returns a Promise. This works for any signal approach: completion blocks, callback functions, delegates, KVO, Notification, etc.
For example, a simplified implementation for NSURLConnection's async convenience class method (in practice you should check the response and do better error handling):
- (RXPromise*) fetchMovie {
RXPromise* promise = [[RXPromise alloc] init];
NSMutableRequest* request = ...;
[NSURLConnection sendAsynchronousRequest:request
queue:networkQueue
completionHandler:^(NSURLResponse* response, NSData* data, NSError* error){
if (error) {
[promise rejectWithReason:error];
}
else {
[promise fulfillWithValue:data];
}
}];
return promise;
}
You might want to use an approach using the NSURLConnection delegates, or an approach utilizing a NSOperation subclass. This enables you to implement cancellation:
- (RXPromise*) fetchObjects {
RXPromise* promise = [[RXPromise alloc] init];
NSMutableRequest* request = ...;
HTTPOperation* op =
[[HTTPOperation alloc] initWithRequest:request
queue:networkQueue
completionHandler:^(NSURLResponse* response, NSData* data, NSError* error){
if (error) {
[promise rejectWithReason:error];
}
else {
[promise fulfillWithValue:data];
}
}];
promise.then(nil, ^id(NSError* error){
[op cancel];
return nil;
});
[op start];
return promise;
}
Here, the HTTPOperation object will listen to its own promise for an error signal. If it receives one, for example a cancel message send from another object to the promise, the handler then "forwards" the cancel message to the operation.
A View Controller for example can now cancel a running HTTPOperation as follows:
- (void) viewWillDisappear:(BOOL)animate {
[super viewWillDisappear:animate];
[self.fetchObjectsPromise cancel];
self.fetchObjectPromise = nil;
}