gem-hunter

ReactiveObjC

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Introduction

// When self.username changes, logs the new name to the console.
//
// RACObserve(self, username) creates a new RACSignal that sends the current
// value of self.username, then the new value whenever it changes.
// -subscribeNext: will execute the block whenever the signal sends a value.
[RACObserve(self, username) subscribeNext:^(NSString *newName) {
	NSLog(@"%@", newName);
}];

// Only logs names that starts with "j".
//
// -filter returns a new RACSignal that only sends a new value when its block
// returns YES.
[[RACObserve(self, username)
	filter:^(NSString *newName) {
		return [newName hasPrefix:@"j"];
	}]
	subscribeNext:^(NSString *newName) {
		NSLog(@"%@", newName);
	}];

// Creates a one-way binding so that self.createEnabled will be
// true whenever self.password and self.passwordConfirmation
// are equal.
//
// RAC() is a macro that makes the binding look nicer.
//
// +combineLatest:reduce: takes an array of signals, executes the block with the
// latest value from each signal whenever any of them changes, and returns a new
// RACSignal that sends the return value of that block as values.
RAC(self, createEnabled) = [RACSignal
	combineLatest:@[ RACObserve(self, password), RACObserve(self, passwordConfirmation) ]
	reduce:^(NSString *password, NSString *passwordConfirm) {
		return @([passwordConfirm isEqualToString:password]);
	}];

// Logs a message whenever the button is pressed.
//
// RACCommand creates signals to represent UI actions. Each signal can
// represent a button press, for example, and have additional work associated
// with it.
//
// -rac_command is an addition to NSButton. The button will send itself on that
// command whenever it's pressed.
self.button.rac_command = [[RACCommand alloc] initWithSignalBlock:^(id _) {
	NSLog(@"button was pressed!");
	return [RACSignal empty];
}];

// Hooks up a "Log in" button to log in over the network.
//
// This block will be run whenever the login command is executed, starting
// the login process.
self.loginCommand = [[RACCommand alloc] initWithSignalBlock:^(id sender) {
	// The hypothetical -logIn method returns a signal that sends a value when
	// the network request finishes.
	return [client logIn];
}];

// -executionSignals returns a signal that includes the signals returned from
// the above block, one for each time the command is executed.
[self.loginCommand.executionSignals subscribeNext:^(RACSignal *loginSignal) {
	// Log a message whenever we log in successfully.
	[loginSignal subscribeCompleted:^{
		NSLog(@"Logged in successfully!");
	}];
}];

// Executes the login command when the button is pressed.
self.loginButton.rac_command = self.loginCommand;

// Performs 2 network operations and logs a message to the console when they are
// both completed.
//
// +merge: takes an array of signals and returns a new RACSignal that passes
// through the values of all of the signals and completes when all of the
// signals complete.
//
// -subscribeCompleted: will execute the block when the signal completes.
[[RACSignal
	merge:@[ [client fetchUserRepos], [client fetchOrgRepos] ]]
	subscribeCompleted:^{
		NSLog(@"They're both done!");
	}];

// Logs in the user, then loads any cached messages, then fetches the remaining
// messages from the server. After that's all done, logs a message to the
// console.
//
// The hypothetical -logInUser methods returns a signal that completes after
// logging in.
//
// -flattenMap: will execute its block whenever the signal sends a value, and
// returns a new RACSignal that merges all of the signals returned from the block
// into a single signal.
[[[[client
	logInUser]
	flattenMap:^(User *user) {
		// Return a signal that loads cached messages for the user.
		return [client loadCachedMessagesForUser:user];
	}]
	flattenMap:^(NSArray *messages) {
		// Return a signal that fetches any remaining messages.
		return [client fetchMessagesAfterMessage:messages.lastObject];
	}]
	subscribeNext:^(NSArray *newMessages) {
		NSLog(@"New messages: %@", newMessages);
	} completed:^{
		NSLog(@"Fetched all messages.");
	}];

// Creates a one-way binding so that self.imageView.image will be set as the user's
// avatar as soon as it's downloaded.
//
// The hypothetical -fetchUserWithUsername: method returns a signal which sends
// the user.
//
// -deliverOn: creates new signals that will do their work on other queues. In
// this example, it's used to move work to a background queue and then back to the main thread.
//
// -map: calls its block with each user that's fetched and returns a new
// RACSignal that sends values returned from the block.
RAC(self.imageView, image) = [[[[client
	fetchUserWithUsername:@"joshaber"]
	deliverOn:[RACScheduler scheduler]]
	map:^(User *user) {
		// Download the avatar (this is done on a background queue).
		return [[NSImage alloc] initWithContentsOfURL:user.avatarURL];
	}]
	// Now the assignment will be done on the main thread.
	deliverOn:RACScheduler.mainThreadScheduler];

When to use ReactiveObjC

Handling asynchronous or event-driven data sources

static void *ObservationContext = &ObservationContext;

- (void)viewDidLoad {
	[super viewDidLoad];

	[LoginManager.sharedManager addObserver:self forKeyPath:@"loggingIn" options:NSKeyValueObservingOptionInitial context:&ObservationContext];
	[NSNotificationCenter.defaultCenter addObserver:self selector:@selector(loggedOut:) name:UserDidLogOutNotification object:LoginManager.sharedManager];

	[self.usernameTextField addTarget:self action:@selector(updateLogInButton) forControlEvents:UIControlEventEditingChanged];
	[self.passwordTextField addTarget:self action:@selector(updateLogInButton) forControlEvents:UIControlEventEditingChanged];
	[self.logInButton addTarget:self action:@selector(logInPressed:) forControlEvents:UIControlEventTouchUpInside];
}

- (void)dealloc {
	[LoginManager.sharedManager removeObserver:self forKeyPath:@"loggingIn" context:ObservationContext];
	[NSNotificationCenter.defaultCenter removeObserver:self];
}

- (void)updateLogInButton {
	BOOL textFieldsNonEmpty = self.usernameTextField.text.length > 0 && self.passwordTextField.text.length > 0;
	BOOL readyToLogIn = !LoginManager.sharedManager.isLoggingIn && !self.loggedIn;
	self.logInButton.enabled = textFieldsNonEmpty && readyToLogIn;
}

- (IBAction)logInPressed:(UIButton *)sender {
	[[LoginManager sharedManager]
		logInWithUsername:self.usernameTextField.text
		password:self.passwordTextField.text
		success:^{
			self.loggedIn = YES;
		} failure:^(NSError *error) {
			[self presentError:error];
		}];
}

- (void)loggedOut:(NSNotification *)notification {
	self.loggedIn = NO;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
	if (context == ObservationContext) {
		[self updateLogInButton];
	} else {
		[super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
	}
}

- (void)viewDidLoad {
	[super viewDidLoad];

	@weakify(self);

	RAC(self.logInButton, enabled) = [RACSignal
		combineLatest:@[
			self.usernameTextField.rac_textSignal,
			self.passwordTextField.rac_textSignal,
			RACObserve(LoginManager.sharedManager, loggingIn),
			RACObserve(self, loggedIn)
		] reduce:^(NSString *username, NSString *password, NSNumber *loggingIn, NSNumber *loggedIn) {
			return @(username.length > 0 && password.length > 0 && !loggingIn.boolValue && !loggedIn.boolValue);
		}];

	[[self.logInButton rac_signalForControlEvents:UIControlEventTouchUpInside] subscribeNext:^(UIButton *sender) {
		@strongify(self);

		RACSignal *loginSignal = [LoginManager.sharedManager
			logInWithUsername:self.usernameTextField.text
			password:self.passwordTextField.text];

			[loginSignal subscribeError:^(NSError *error) {
				@strongify(self);
				[self presentError:error];
			} completed:^{
				@strongify(self);
				self.loggedIn = YES;
			}];
	}];

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

Chaining dependent operations

[client logInWithSuccess:^{
	[client loadCachedMessagesWithSuccess:^(NSArray *messages) {
		[client fetchMessagesAfterMessage:messages.lastObject success:^(NSArray *nextMessages) {
			NSLog(@"Fetched all messages.");
		} failure:^(NSError *error) {
			[self presentError:error];
		}];
	} failure:^(NSError *error) {
		[self presentError:error];
	}];
} failure:^(NSError *error) {
	[self presentError:error];
}];

[[[[client logIn]
	then:^{
		return [client loadCachedMessages];
	}]
	flattenMap:^(NSArray *messages) {
		return [client fetchMessagesAfterMessage:messages.lastObject];
	}]
	subscribeError:^(NSError *error) {
		[self presentError:error];
	} completed:^{
		NSLog(@"Fetched all messages.");
	}];

Parallelizing independent work

__block NSArray *databaseObjects;
__block NSArray *fileContents;

NSOperationQueue *backgroundQueue = [[NSOperationQueue alloc] init];
NSBlockOperation *databaseOperation = [NSBlockOperation blockOperationWithBlock:^{
	databaseObjects = [databaseClient fetchObjectsMatchingPredicate:predicate];
}];

NSBlockOperation *filesOperation = [NSBlockOperation blockOperationWithBlock:^{
	NSMutableArray *filesInProgress = [NSMutableArray array];
	for (NSString *path in files) {
		[filesInProgress addObject:[NSData dataWithContentsOfFile:path]];
	}

	fileContents = [filesInProgress copy];
}];

NSBlockOperation *finishOperation = [NSBlockOperation blockOperationWithBlock:^{
	[self finishProcessingDatabaseObjects:databaseObjects fileContents:fileContents];
	NSLog(@"Done processing");
}];

[finishOperation addDependency:databaseOperation];
[finishOperation addDependency:filesOperation];
[backgroundQueue addOperation:databaseOperation];
[backgroundQueue addOperation:filesOperation];
[backgroundQueue addOperation:finishOperation];

RACSignal *databaseSignal = [[databaseClient
	fetchObjectsMatchingPredicate:predicate]
	subscribeOn:[RACScheduler scheduler]];

RACSignal *fileSignal = [RACSignal startEagerlyWithScheduler:[RACScheduler scheduler] block:^(id<RACSubscriber> subscriber) {
	NSMutableArray *filesInProgress = [NSMutableArray array];
	for (NSString *path in files) {
		[filesInProgress addObject:[NSData dataWithContentsOfFile:path]];
	}

	[subscriber sendNext:[filesInProgress copy]];
	[subscriber sendCompleted];
}];

[[RACSignal
	combineLatest:@[ databaseSignal, fileSignal ]
	reduce:^ id (NSArray *databaseObjects, NSArray *fileContents) {
		[self finishProcessingDatabaseObjects:databaseObjects fileContents:fileContents];
		return nil;
	}]
	subscribeCompleted:^{
		NSLog(@"Done processing");
	}];

Simplifying collection transformations

NSMutableArray *results = [NSMutableArray array];
for (NSString *str in strings) {
	if (str.length < 2) {
		continue;
	}

	NSString *newString = [str stringByAppendingString:@"foobar"];
	[results addObject:newString];
}

RACSequence *results = [[strings.rac_sequence
	filter:^ BOOL (NSString *str) {
		return str.length >= 2;
	}]
	map:^(NSString *str) {
		return [str stringByAppendingString:@"foobar"];
	}];

System Requirements

Importing ReactiveObjC

More Info