В докладе расскарывается тема использования функционально-реактивного подхода для разработки iOS- и Mac-приложений, его достоинства и недостатки. Также рассказано об использовании паттерна Model-View-View Model для улучшения архитектуры и повышения тестируемости GUI-кода.

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