RAC的map:方法

RACSignal *signal = [self.accountField.rac_textSignal map:^id _Nullable(NSString * _Nullable value) {
  RACSignal *newSignal = [RACSignal createSignal:^RACDisposable * _Nullable(id<RACSubscriber>  _Nonnull subscriber) {
    [subscriber sendNext:[NSString stringWithFormat:@"map后的字符串+%@",value]];
    return nil;
  }];
  return newSignal;
}];

[signal subscribeNext:^(id  _Nullable x) {
  //这里的x变量是上面的newSignal
}];

上面代码相当于：

id (^firstBlock)(id value) = ^id _Nullable(NSString * _Nullable value) {
    RACSignal *newSignal = [RACSignal createSignal:^RACDisposable * _Nullable(id<RACSubscriber>  _Nonnull subscriber) {
    [subscriber sendNext:[NSString stringWithFormat:@"map后的字符串+%@",value]];
    return nil;
  }];
  return newSignal;
}

RACSignal *signal = [self.accountField.rac_textSignal map:firstBlock]; //map:firstBlock返回的是newSignal

[sig subscribeNext:^(id  _Nullable x) {
  //这里的x变量是上面的newSignal
}];

//map:方法实现
- (__kindof RACStream *)map:(id (^)(id value))firstBlock {
    NSCParameterAssert(firstBlock != nil);

    Class class = self.class;
    
    return [[self flattenMap:^(id value) {
        return [class return:firstBlock(value)]; //这里返回的是一个RACReturnSignal，里面的value属性保存的是firstBlock(value)，也就是newSignal
    }] setNameWithFormat:@"[%@] -map:", self.name];
}


//map:方法改写
- (__kindof RACStream *)map:(id (^)(id value))firstBlock {
    NSCParameterAssert(firstBlock != nil);

    Class class = self.class;
        
        //secondBlock返回的是一个RACReturnSignal对象，里面的value属性保存的是firstBlock(value)，firstBlock(value)就是newSignal
        RACStream * (^secondBlock)(id value)) = ^(id value) {
        return [class return:firstBlock(value)]; 
    };
    
    return [[self flattenMap:secondBlock] setNameWithFormat:@"[%@] -map:", self.name];
}

//flattenMap:方法实现
- (__kindof RACStream *)flattenMap:(__kindof RACStream * (^)(id value))secondBlock {
    Class class = self.class;

    return [[self bind:^{
        return ^(id value, BOOL *stop) {
            id stream = secondBlock(value) ?: [class empty];  //这里secondBlock(value)就是上面说的RACReturnSignal对象，里面的value属性保存的是firstBlock(value)，firstBlock(value)就是newSignal
            NSCAssert([stream isKindOfClass:RACStream.class], @"Value returned from -flattenMap: is not a stream: %@", stream);

            return stream;
        };
    }] setNameWithFormat:@"[%@] -flattenMap:", self.name];
}

//flattenMap:方法改写
- (__kindof RACStream *)flattenMap:(__kindof RACStream * (^)(id value))secondBlock {
    Class class = self.class;
        
        //这里bindingBlock调用后返回的就是stream = secondBlock(value)
    RACSignalBindBlock bindingBlock =  ^RACSignal* (id value, BOOL *stop) {
        id stream = secondBlock(value) ?: [class empty];  //这里secondBlock(value)就是上面说的RACReturnSignal对象，里面的value属性保存的是firstBlock(value)，firstBlock(value)就是newSignal
        NSCAssert([stream isKindOfClass:RACStream.class], @"Value returned from -flattenMap: is not a stream: %@", stream);

        return stream;
    };
    return [[self bind:bindingBlock] setNameWithFormat:@"[%@] -flattenMap:", self.name];
}

bind:方法内部：
RACSignal *signal = createSignal:block
在block里面做了下面的事情
调用 id signalA = bindingBlock() //得到的signalA就是上面的stream，也就是secondBlock(value) ，secondBlock(value) 返回的是一个RACReturnSignal对象，里面保存了value = firstBlock(value)
signalA suscribeNext:blockA //因为signalA是一个RACReturnSignal对象，该对象suscribeNext:内signal的订阅者会调用sendNext:方法将value属性（保存的firstBlock(value)，也就是newSignal）当做参数传递
[sig subscribeNext:^(id _Nullable x) {
这里收到的就是newSignal
}];
return signal