Donner le goût du ReactiveCocoa. L’objectif de la présentation étant de nous expliquer comment migrer du code lié au KVO vers une implémentation ReactiveCocoa. Cette approche du KVO par ReactiveCocoa permet de comprendre très facilement les bases et l’intérêt de ce nouveau framework et de s’y mettre facilement et progressivement.

1. KVO with ReactiveCocoa
An easy way to start with RAC
CocoaHeads Paris
Jeudi 09 septembre 2015
Philippe Converset

2. Introduction
ReactiveCocoa helps solving asynchronous
problems
But some concepts are hard to grasp
And many methods are available

3. How to start with RAC?
Use ReactiveCocoa has a KVO alternative
Learn little by little its features and concepts

4. RAC KVO Code
NSString *const kCurrentUserKey = @“currentUser";
[self removeObserver:self forKeyPath:kCurrentUserKey context:&kMyContext];
…
[self addObserver:self forKeyPath:kCurrentUserKey options:NSKeyValueObservingOptionNew context:&kMyContext]
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void
*)context
{
if([keyPath isEqualToString:kCurrentUserKey])
{
…
}
}
- (void)setCurrentUser:(User *)user
{
if(_currentUser != user)
{
[self willChangeValueForKey:kCurrentUserKey];
_currentUser = user;
[self didChangeValueForKey:kCurrentUserKey];
}
}
RAC(self, userNameLabel) = [RACObserve(self, userManager.currentUser.name) deliverOnMainThread];

5. Basic usages

6. Simple binding
Typical binding
RAC(self, userName) = RACObserve(self, userManager.currentUser.lastName);
userName is bound to userManager.currentUser.lastName.
But what if userManager or currentUser is nil?
RAC(self, userName, @“none”) = RACObserve(self, userManager.currentUser.lastName);
Bonus: Don’t care whether objects in observed
keypath are already allocated

7. Observing changes only
distinctUntilChanged
RAC(self, userName) = [RACObserve(self,
userManager.currentUser.lastName) distinctUntilChanged];

8. Transforming
map:
RAC(self, userName) = [RACObserve(self, userManager.currentUser)
map:^id(User *user) {
return user.lastName;
}];
RAC(self, userName) = [RACObserve(self, userManager.currentUser)
map:^id(User *user) {
return (user == nil ? nil : [NSString stringWithFormat:@"%@ %@",
user.firstName, user.lastName]);
}];

9. Calling some code
subscribeNext:
[RACObserve(self, userManager.currentUser) subscribeNext:^(User *user) {
if(user)
{
NSLog(@"%@", user.lastName);
}
}];

10. Filtering
filter:
[[RACObserve(self, userManager.currentUser) filter:^BOOL(User *user) {
return (user != nil) && user.admin;
}] subscribeNext:^(User *user) {
NSLog(@"%@ is admin", user.lastName);
}];

11. Specifying thread
deliverOnMainThread
RAC(self, userNameLabel.text) = [RACObserve(self,
modelView.userName) deliverOnMainThread];
deliverOn:
[[RACObserve(self, modelView.userName) deliverOn:self.scheduler]
subscribeNext:^(User *user) {
NSLog(@"I'm executed in a specific dispatch queue");
}];

12. Notification
rac_addObserverForName:
RAC(self, loggedIn) = [NSNotificationCenter.defaultCenter
rac_addObserverForName:kUserDidLogOutNotification object:nil]
mapReplace:@NO];

13. Combining
combineLatest & reduce
RAC(self, createEnabled) = [RACSignal
combineLatest:@[RACObserve(self, password), RACObserve(self, passwordConfirmation)]
reduce:^(NSString *password, NSString *passwordConfirm) {
return @([passwordConfirm isEqualToString:password]);
}];

14. Generalities

15. Parameters as id
Most RAC methods return RACSignal and use
id as parameters
[RACObserve(self, userManager.currentUser) subscribeNext:^(id x) {
NSLog(@"%@", ((User *)x).lastName);
}];
☟
[RACObserve(self, userManager.currentUser) subscribeNext:^(User *user) {
NSLog(@"%@", user.lastName);
}];

16. Weakify/Strongify
@weakify(self)
[RACObserve(self, userManager.currentUser) subscribeNext:^(User *user) {
@strongify(self)
[self userChanged:user];
}];
@weakify creates a new weakly referenced variable of the same type you
pass in and assigns the original value to it
@strongify creates a variable that matches the original variable but it exists
in the local scope and assigns to it the variable created by @weakify

17. Weakify/Strongify
_weak id weakSelf = self;
[RACObserve(self, userManager.currentUser) subscribeNext:^(User *user) {
id strongSelf = weakSelf;
[strongSelf userChanged:user];
}];
@strongify in each async block
No need for several @weakify(self)

18. Pros and Cons

19. Pros
Get rid of boilerplate code
Less error prone
(missing automaticallyNotifiesObserversOf<Property>?)
Compatible with property refactoring (no more
reference to keypath as string)
If you change a property name manually, RAC code won’t compile
anymore.
Configuration is much easier, especially with multilevel
keypath
Memory management is rightly done out of the box
Start to think functional, prepare for Swift

20. Cons
All values are object, BOOL are passed as
NSNumber
=> isAdmin.boolValue 😢
Don’t forget deliverOnMainThread for UIKit
components

21. Warnings
Don’t use RAC on intensive property change
for example on Player.currentTime
Don’t look at the stack!

22. Warnings
Don’t use RAC on intensive property change
for example on Player.currentTime
Don’t look at the stack!

24. CocoaHeads Paris
KVO with ReactiveCocoa
Philippe Converset
pconverset@autresphere.com
@autresphere
www.autresphere.com
www.kidzik.com
github.com/autresphere