The document discusses ReactiveCocoa and its use alongside the MVVM architecture in Objective-C applications. It highlights how ReactiveCocoa simplifies asynchronous programming through streams, futures, and signals, while addressing the challenges of learning and debugging. Additionally, it presents the benefits of MVVM, including separation of concerns, reusability, and improved testability, and illustrates examples of integrating ReactiveCocoa with MVVM patterns.

1. REACTIVE COCOA & MVVM
Николай Касьянов

2. REACTIVE COCOA
•
Objective-C framework for processing and composing streams
•
Unifies async Cocoa patterns: callbacks, delegates, KVO,
notifications
•
Very composable
•
Helps to minimize state
•
Inspired by Reactive Extensions for .NET

3. CALLBACK HELL
void (^completion)(UIImage *) = ...
id token = [self loadImage:url completion:^(NSData *data, NSError *error) {
if (data == nil) {
completion(defaultImage);
}
else {
[self unpackImageFromData:data completion:^(UIImage *image) {
if (image == nil) { // unpacking failed
completion(defaultImage);
}
else {
completion(image);
}
}]
}
}];
!
// client code
[imageLoader cancel:token];
!

4. FUTURES
•
Future can either complete with a value or reject
with an error
•
JavaScript Promises/A+
•
There are some Objective-C implementations
•
RAC can into futures too

5. RACSignal *image =
[[[self rac_imageFromURL:url] flattenMap:^(NSData *data) {
return [self rac_unpackedImageFromData:data];
}]
catchTo:[RACSignal return:defaultImage]];
!
// client code:
RAC(cell.imageView, image) =
[image takeUntil:cell.rac_prepareForReuseSignal];

6. RACSignal
•
A stream of values
•
One can subscribe to new value, error or completion
•
Supports functional constructs: map, filter, flatMap, reduce
etc
•
Сold or hot
•
A monad

7. RACSignal *allPosts =
[RACSignal createSignal:^(id <RACSubscriber> s) {
[httpClient GET:@"/posts.json" success:^(NSArray *posts) {
[s sendNext:posts];
[s sendCompleted];
} failure:^(NSError *error) {
[s sendError:error];
}];
!
!
}];
return [RACDisposable disposableWithBlock:^{
[operation cancel];
}];
RACSignal *posts =
[[allPosts flattenMap:^(NSArray *items) {
return items.rac_signal;
}] filter:^(Post *post) {
return post.hasComments;
}];
// Nothing happened yet
!
[[posts collect] subscribeNext:^(NSArray *items) { NSLog(@"Posts: %@", items) }];
!
RACDisposable *disposable =
[posts subscribeCompleted:^{ NSLog(@"Done"); }];
// Ooops network request performed twice :(
!
[disposable dispose];

8. STREAMS
•
Powerful abstraction
•
Streams for futures is like iterables for scalar values
•
Umbrella concept for asynchronous Cocoa
patterns
•
You can even use signals as values. Yo dawg…

9. RACSignal *searchResults =
[[[[textField.rac_textSignal throttle:1] filter:^(NSString *query) {
return query.length > 2;
}] map:^(NSString *query) {
return [[[networkClient itemsMatchingQuery:query]
doError:^(NSError *error) {
[self displayError:error];
}]
catchTo:[RACSignal empty]];
}] switchToLatest];
!
// Automatically disposes subscription on self deallocation
// cancelation running network request (if any)
RAC(self, items) = searchResults;

10. DEALING WITH IMPERATIVE
API
•
Property binding (one-way and two-way)
•
Selector lifting (-rac_liftSelector:withSignals:)
•
Signals from selector (-rac_signalForSelector:)
•
Operators for injecting side-effects: doNext, doError,
initially, finally

11. RACSignal *range = [[[RACSignal merge:@[
[self rac_signalForSelector:@selector(tableView:willDisplayCell...)]
[self rac_signalForSelector:@selector(tableView:didEndDisplayingCell...)]
]] map:^(RACtuple *args) {
UITableView *tableView = args[0];
NSArray *indexPaths = tableView.indexPathsForVisibleRows;
NSRange range = NSMakeRange([indexPaths[0] row], indexPaths.count);
return [NSValue valueWithRange:range];
}];
!
[self rac_liftSelector:@selector(updateVisibleRange:)
withSignals:range, nil];

12. CONCURRENCY
•
RACScheduler
•
-deliverOn:
and -subscribeOn:
- (RACSignal *)itemsFromDB {
return [RACSignal createSignal:^(id <RACSubscriber> subscriber) {
NSArray *items = [sqliteWrapper fetchItemsFromTable:@"posts"];
!
[subscriber sendNext:items];
[subscriber sendCompleted];
!
}
!
}];
return nil;
[[[[self itemsFromDB]
subscribeOn:[RACScheduler scheduler]]
deliverOn:[RACScheduler mainThreadScheduler]]
subscribeNext:^(NSArray *items) {
self.items = items;
}];

13. ISSUES
•
Steep learning curve
•
Runtime overhead
•
Tricky debugging: crazy call stacks, lots of asynchrony,
retain cycles
•
You’ll need to deal with imperative Cocoa API anyway
•
Losing last bits of type information

14. MVVM

15. View
Model
View
Controller

16. MVVM
•
Model – View – View Model
•
An alternative to MVC
•
MVC is fine, but…
•
Meet UIViewController, The Spaghetti Monster

17. Model
View
Model
View

18. MVVM
•
MVVM knows nothing about a view
•
MVVM uses underlying layers (persistence, web
service clients, cache) to populate view data
•
You can even use it with ncurses

19. WHY MVVM?
•
Clear separation between view and presentation logiс
•
Reusability across different views and even platforms
•
Testability
•
View models are models
•
Persistence can be hidden behind view model

20. YOU ALREADY CLOSER TO
MVVM THAN YOU THINK
Big monolithic view controllers
!
External data sources, data converters & services
!
MVVM

21. @interface UserRepositoryViewModel : NSObject
!
// KVOable properties
@property (nonatomic, copy, readonly) NSArray *items;
@property (nonatomic, strong, readonly) User *selectedUser;
@property (nonatomic, readonly) BOOL isLoading;
!
- (void)refreshItems;
- (void)selectItemAtIndex;
!
@end

22. MVVM + RAC
•
KVO with RACObserve
•
One-way binding: RAC(object,
•
Two-way binding: RACChannel
•
RACCommand
keyPath) = signal;

23. @interface LoginViewModel : NSObject
!
@property (nonatomic, copy) NSString *login;
@property (nonatomic, copy) NSString *password;
!
@property (nonatomic, strong, readonly) RACCommand *login;
!
@end
!
!
// Somewhere in view controller
id viewTerminal = loginField.rac_newTextChannel;
id modelTerminal = RACChannelTo(viewModel, login);
!
[[viewTerminal map:^(NSString *value) {
return [value lowercaseString];
}] subscribe:modelTerminal];
[modelTerminal subscribe:viewTerminal];
!
loginButton.rac_command = viewModel.login;
[self rac_liftSelector:@selector(displayError:)
withSignals:viewModel.login.errors, nil];
!
RAC(spinner, animating) = viewModel.login.executing;

24. RACCommand
•
Runs signal block on -execute: and subscribes to its result
•
Multicasts execution signals to consumers
•
Multicasts inner signal errors to consumers
•
Enabled/disabled state can be controlled by a bool signal
•
Exposes execution state (running/not running)
•
Can be bound to UI control

25. RACSignal *networkReachable = RACObserve(httpClient, reachable);
!
RACCommand *login = [[RACCommand alloc]
initWithEnabled:networkReachable
signalBlock:^(id _) {
// Boolean signal, sends @YES on success
return [self.backendClient loginWithLogin:self.login
password:self.password];
}];
!
RAC(self, loggedIn) = [[login.executionSignals switchToLatest]
startWith:@NO];

26. •
Make no assumptions about a view
•
Expose bindable properties or signals
•
Expose commands to consumers
•
Throttle/unsubscribe signals when view is
inactive

27. ANY QUESTIONS?

28. LINKS
•
https://github.com/ReactiveCocoa/ReactiveCocoa/
•
https://github.com/ReactiveCocoa/ReactiveViewModel
•
http://cocoamanifest.net/articles/2013/10/mvc-mvvm-frpand-building-bridges.html
•
https://rx.codeplex.com
•
http://netflix.github.io/RxJava/javadoc/rx/Observable.html

29. RAC-POWERED LIBRARIES
•
https://github.com/octokit/octokit.objc
•
https://github.com/jonsterling/ReactiveFormlets
•
https://github.com/ReactiveCocoa/
ReactiveCocoaLayout

30. SAMPLE PROJECTS
•
https://github.com/AshFurrow/C-41
•
https://github.com/jspahrsummers/GroceryList/
•
https://github.com/corristo/SoundCloudStream