iOS RAC - 映射

在之前的集合文章中我们使用到了map，他是RAC中主要的两个映射方法之一
还有一个是flattenMap，这两个方法是将我们的信号源的内容映射成为一个新的信号。

先看看我们平时的用法：

    RACSubject * subject = [RACSubject subject];
    
    [subject subscribeNext:^(id  _Nullable x) {
        NSLog(@"%@",x);
    }];
    
    [subject sendNext:@"what happend？"];

然后先看看flattenMap
其实flattenMap内部也是在bind，如下图所示

之前bind的用法我们已经了解过了，照葫芦画瓢

    RACSubject * subject = [RACSubject subject];
    
    RACSignal * signal = [subject flattenMap:^__kindof RACSignal * _Nullable(id  _Nullable value) {
        return [RACReturnSignal return:value];
    }];
    
    [signal subscribeNext:^(id  _Nullable x) {
        NSLog(@"%@",x);
    }];
    
    [subject sendNext:@"what happend？"];

现在我们把发送的数据稍微处理一下

    RACSubject * subject = [RACSubject subject];
    
    [[subject flattenMap:^__kindof RACSignal * _Nullable(id  _Nullable value) {
        value = [NSString stringWithFormat:@"%@ 你别问我，我也不知道！",value];
        return [RACReturnSignal return:value];
    }] subscribeNext:^(id  _Nullable x) {
        NSLog(@"%@",x);
    }];
    
    [subject sendNext:@"what happend？"];

然后我们在使用map完成上面的功能

    RACSubject * subject = [RACSubject subject];
    
    [[subject map:^id _Nullable(id  _Nullable value) {
        return [NSString stringWithFormat:@"%@ 你别问我，我也不知道！",value];
    }] subscribeNext:^(id  _Nullable x) {
        NSLog(@"%@",x);
    }];
    
    [subject sendNext:@"what happend？"];

然后在多发几次数据看看会发生什么？

两者对比，是不是map要更简洁呢？既然map要比flattenMap更好用，那么为什么还有flattenMap呢？
flattenMap当然是有他独特的地方啦！在开发过程中，他更适合用于处理信号中的信号

    RACSubject * subjectOfSignal = [RACSubject subject];
    
    RACSubject * subject1 = [RACSubject subject];
    
    [subjectOfSignal sendNext:subject1];
    
    [subject1 sendNext:@"弄啥嘞"];

之前我们的做法有两种：
第一种：双重订阅

    RACSubject * subjectOfSignal = [RACSubject subject];
    
    RACSubject * subject1 = [RACSubject subject];
    
    [subjectOfSignal subscribeNext:^(id  _Nullable x) {
        [x subscribeNext:^(id  _Nullable x) {
            NSLog(@"%@",x);
        }];
    }];
    
    
    [subjectOfSignal sendNext:subject1];
    
    [subject1 sendNext:@"弄啥嘞"];

第二种：订阅最新的信号

    RACSubject * subjectOfSignal = [RACSubject subject];
    
    RACSubject * subject1 = [RACSubject subject];
    
    [subjectOfSignal.switchToLatest subscribeNext:^(id  _Nullable x) {
        NSLog(@"%@",x);
    }];
    
    [subjectOfSignal sendNext:subject1];
    
    [subject1 sendNext:@"弄啥嘞"];

现在又多一种:

    RACSubject * subjectOfSignal = [RACSubject subject];
    
    RACSubject * subject1 = [RACSubject subject];
    
    [[subjectOfSignal flattenMap:^__kindof RACSignal * _Nullable(id  _Nullable value) {
        return value;
    }] subscribeNext:^(id  _Nullable x) {
        NSLog(@"%@",x);
    }];
    
    [subjectOfSignal sendNext:subject1];
    
    [subject1 sendNext:@"弄啥嘞"];