介紹

1、RAC是受函數響應式編程的啟發而設計開發。
2、RAC不使用可變變量開發，而是提供捕獲當前值和未來值的信號(由RACSignal表示)。
3、通過操作信號進行鏈式、組合、響應，程序可以被聲明式的實現，代碼不需要對值持續地觀察和更新。
4、信號支持異步操作，可以簡化包括網絡操作在內的異步場景。非常類似于
futures and promises.
5、RAC的一個優勢是：它把多種異步機制統一起來，諸如delegate,notifications,callback blocks, target-action, kvo。

例子

1、最簡單的case，self的屬性username 只要變化，就會打印出來新值。
不用在self的類中重寫observeValueForKeyPath:ofObject:change:context:

[RACObserve(self, username) subscribeNext:^(NSString *newName) {
    NSLog(@"%@", newName);
}];

2、鏈式調用
只有新值的首個字符是"j"時，才會打印。filter也會返回一個RACSingnal,它只會在blk中返回yes的時候，才會發送新值。

[[RACObserve(self, username)
    filter:^(NSString *newName) {
        return [newName hasPrefix:@"j"];
    }]
    subscribeNext:^(NSString *newName) {
        NSLog(@"%@", newName);
    }];

3、狀態傳遞。一個屬性值的改變可能依賴多個值的改變，并互相作用。RAC提供了一個使用信號組及操作組來解決的策略。

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

createEnabled的值是否改變依賴于 password和passwordConfirm兩個值是否相等，而這兩個值可能都在變，這時就需要任何一個最新的變化都觸發一次對比。 +combineLatest:reduce: 剛好支持這樣的操作，第一個參數是個信號數組，第二個參數是判定的條件。
self.createEnabled這個變量的值會跟隨這個綁定的信號組發生改變時，值是否相等而定。

4、信號的值流依賴。它可以建立在一段時間的任何值流上，而非只是KVO.
如按鈕點擊相應

self.button.rac_command = [[RACCommand alloc] initWithSignalBlock:^(id _) {
    NSLog(@"button was pressed!");
    return [RACSignal empty];
}];

4、點擊按鈕，登錄操作

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];
}];

//login command每次執行的時候，都會調用上方的那個block,操作由上方blk信號組成的信號組的信號，就是executionSignals對應的信號
[self.loginCommand.executionSignals subscribeNext:^(RACSignal *loginSignal) {
    // Log a message whenever we log in successfully.
    [loginSignal subscribeCompleted:^{
        NSLog(@"Logged in successfully!");
    }];
}];

self.loginButton.rac_command = self.loginCommand; //點擊按鈕執行login command.

5、信號也可以表示timer、ui事件、以及一切隨著事件改變的事物

6、復雜的組操作

[[RACSignal
    merge:@[ [client fetchUserRepos], [client fetchOrgRepos] ]]
    subscribeCompleted:^{
        NSLog(@"They're both done!");
    }];

兩個網絡操作都完成時，會打印出They're both done!

7、避免block嵌套

[[[[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.");
    }];

登錄 --> 用戶 --> 緩存的消息 --> 最后一條消息獲取最新消息列表 --> 打印新消息列表 --> 所有結束后打印結束。

8、線程間通信

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];

獲取用戶信息 --> 子線程中下載用戶頭像 -->主線程中將圖片設置到imageview上。

RAC擅長

1、異步
2、事件驅動數據源

- (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];
}

3、操作鏈式依賴
普通代碼

[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];
}];

rac

[[[[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.");
    }];

4、并行獨立工作
傳統

__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];

rac

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");
    }];

5、簡化集合轉換

傳統

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

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

rac

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