@myron-lee 2017-03-22T12:34:03.000000Z 字数 6738 阅读 1341

RAC扩展──异步filter、map

iOS Blog

一、前言

RAC有非常丰富的Operation，可以让我们的开发事半功倍。常用的比如：filter、map。但是原生的filter、map要求我们在Block同步返回结果。但是，有的时候，我们只能异步返回结果。比如我们需要根据用户的输入进行filter（二次确认）；比如我们需要先请求网络才能进行map。

所以，我对原有的Operation进行了扩展，开发了asyncFilter、asyncMap两个新的Operation。

二、使用方法

asyncFilter、asyncMap的入参类型是void (^)(id value, id<RACSubscriber> subscriber)的Block，与原始filter、map直接return结果不同，asyncFilter、asyncMap通过subscriber将结果发送出来。asyncFilter在block中[subscriber sendNext:@(YES)]; / [subscriber sendNext:@(NO)];，asyncMap在block中[subscriber sendNext:yourMappedValue];。注意发送error事件会导致整个observe结束，我并没有做防护，因为考虑到调用方在遇到异常情况的时候，可能需要直接结束整个observe。

三、实现思路

因为异步了，filter、map Block都不会直接返回结果了，结果将会通过一个subscriber返回（这个subscriber是Block的入参，Operation是会塞给Block），此时这个Block也就变成了一个数据源。

内部是把原Signal flattenMap成新的Signal，新的Signal的数据源就是这个Block（把这个Block罩个壳就是新Signal）。

我们知道，flattenMap会把原Signal的每一个value都map成一个新的Signal，在我们的场景里，新的Signal一定只有一个value，所以我asyncFilter、asyncMap两个Operation接收到一个值的时候会自动complete，调用方在发送@(YES)/@(NO)/@(mappedValue)之后不必再手动发送一个complete事件。及时complete掉Signal，dispose掉Signal，提升效率。

1. asyncFilter

主体是flattenMap，构建新的Signal，新的Signal的数据源就是filterBlock，然后对新的Signal就行变换——先filter掉为NO的值，之后map回原来的值。

2. asyncMap

主体是flattenMap，返回新的Signal，新的Signal的数据源就是mapBlock，直接把值传递下去。

四、使用示例

设想一个业务场景: 选中一批订单=>选择一个用户=>二次确认是否要把订单移交给这个用户。选择一个用户这一步骤使用了asyncMap，二次确认使用了asyncFilter。

-(RACSignal *)transferOrdersSignal{
    return [[[[[self selectedOrdersSignal]
             st_asyncMap:^(NSDictionary *selectedOrdersParams, id<RACSubscriber> subscriber) {
                 [[self selectedCustomerSignal]
                          subscribeNext:^(NSString *selectedCustomer) {
                              if (selectedCustomer == nil) {
                                  [subscriber sendCompleted];
                              } else {
                                  NSMutableDictionary *parmas = [selectedOrdersParams mutableCopy];
                                  [parmas setValue:selectedCustomer forKey:@"targetUserId"];
                                  [subscriber sendNext:[parmas copy]];
                              }
                          }];
             }]
             st_asyncFilter:^(NSDictionary *params, id<RACSubscriber> subscriber) {
                 NSNumber *orderCount = params[@"orderCount"];
                 AlertView *alertView = [[AlertView alloc]
                                           initWithmessage:[NSString stringWithFormat:@"是否确定移交此%@笔订单", orderCount]
                                           cancelTitle:@"取消"
                                           cancleColor:[UIColor whiteColor]
                                           otherTitle:@"确定"
                                           otherColor:[UIColor colorWithRGB:0xff9800]
                                           clickIndexHandle:^(NSInteger index) {
                                               if(index == 0){
                                                   [subscriber sendNext:@(NO)];
                                               } else {
                                                   [subscriber sendNext:@(YES)];
                                               }
                                           }];
                 [alertView show];
             }]
             flattenMap:^RACStream *(NSDictionary *params) {
                 //web request
             }]
             doError];
}

五、源码

- (instancetype)st_asyncFilter:(void (^)(id value, id<RACSubscriber> subscriber))block{
    return [self flattenMap:^RACStream *(id actualValue) {
            return [[[[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
                                                block(actualValue, subscriber);
                                                return nil;
                                            }] take:1
                                             ] filter:^BOOL(id filterFlagValue) {
                                                return [filterFlagValue boolValue];
                                            }] map:^id(id value) {
                                                return actualValue;
                                            }];
    }];
}
- (instancetype)st_asyncMap:(void (^)(id value, id<RACSubscriber> subscriber))block{
    return [self flattenMap:^RACStream *(id value) {
        return [[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
                    block(value, subscriber);
                    return nil;
                }] take:1];;
    }];
}

六、Update 2017-2-8

内部换了一种实现。采用了自定义的一次性信号── STOneTimeSignal。

@implementation RACSignal (STExtension)
- (instancetype)st_asyncFilter:(void (^)(id value, id<RACSubscriber> subscriber))block{
    return [self flattenMap:^RACStream *(id value) {
        return [[STOneTimeSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
                                                block(value, subscriber);
                                                return nil;
                                            }]
                                   flattenMap:^RACStream *(id flag) {
                                       if ([flag boolValue]) {
                                           return [RACSignal return:value];
                                       } else {
                                           return [RACSignal empty];
                                       }
                                   }];
    }];
}
- (instancetype)st_asyncMap:(void (^)(id value, id<RACSubscriber> subscriber))block{
    return [self flattenMap:^RACStream *(id value) {
        return [STOneTimeSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
                                block(value, subscriber);
                                return nil;
                            }];
    }];
}

一次性信号── STOneTimeSignal的实现。STOneTimeSignal主要依赖STOneTimeSubscriber。

@interface STOneTimeSignal : RACDynamicSignal
@end
@implementation STOneTimeSignal
//Override subscribe method, use STOneTimeSubscriber to replace RACSubscriber
//And make sure custom subscribe method get called
- (RACDisposable *)subscribe:(id<RACSubscriber>)subscriber {
    NSCAssert(NO, @"This method is not implemented yet");
    return nil;
}
- (RACDisposable *)subscribeNext:(void (^)(id x))nextBlock {
    NSCParameterAssert(nextBlock != NULL);
    //把外部Subscriber转化为OneTimeSubscriber
    STOneTimeSubscriber *o = [STOneTimeSubscriber subscriberWithNext:nextBlock error:NULL completed:NULL];
    return [super subscribe:o];
}
- (RACDisposable *)subscribeNext:(void (^)(id x))nextBlock completed:(void (^)(void))completedBlock {
    NSCParameterAssert(nextBlock != NULL);
    NSCParameterAssert(completedBlock != NULL);
    //把外部Subscriber转化为OneTimeSubscriber
    STOneTimeSubscriber *o = [STOneTimeSubscriber subscriberWithNext:nextBlock error:NULL completed:completedBlock];
    return [super subscribe:o];
}
- (RACDisposable *)subscribeNext:(void (^)(id x))nextBlock error:(void (^)(NSError *error))errorBlock completed:(void (^)(void))completedBlock {
    NSCParameterAssert(nextBlock != NULL);
    NSCParameterAssert(errorBlock != NULL);
    NSCParameterAssert(completedBlock != NULL);
    //把外部Subscriber转化为OneTimeSubscriber
    STOneTimeSubscriber *o = [STOneTimeSubscriber subscriberWithNext:nextBlock error:errorBlock completed:completedBlock];
    return [super subscribe:o];
}
- (RACDisposable *)subscribeError:(void (^)(NSError *error))errorBlock {
    NSCParameterAssert(errorBlock != NULL);
    //把外部Subscriber转化为OneTimeSubscriber
    STOneTimeSubscriber *o = [STOneTimeSubscriber subscriberWithNext:NULL error:errorBlock completed:NULL];
    return [super subscribe:o];
}
- (RACDisposable *)subscribeCompleted:(void (^)(void))completedBlock {
    NSCParameterAssert(completedBlock != NULL);
    //把外部Subscriber转化为OneTimeSubscriber
    STOneTimeSubscriber *o = [STOneTimeSubscriber subscriberWithNext:NULL error:NULL completed:completedBlock];
    return [super subscribe:o];
}
- (RACDisposable *)subscribeNext:(void (^)(id x))nextBlock error:(void (^)(NSError *error))errorBlock {
    NSCParameterAssert(nextBlock != NULL);
    NSCParameterAssert(errorBlock != NULL);
    //把外部Subscriber转化为OneTimeSubscriber
    STOneTimeSubscriber *o = [STOneTimeSubscriber subscriberWithNext:nextBlock error:errorBlock completed:NULL];
    return [super subscribe:o];
}
- (RACDisposable *)subscribeError:(void (^)(NSError *))errorBlock completed:(void (^)(void))completedBlock {
    NSCParameterAssert(completedBlock != NULL);
    NSCParameterAssert(errorBlock != NULL);
    //把外部Subscriber转化为OneTimeSubscriber
    STOneTimeSubscriber *o = [STOneTimeSubscriber subscriberWithNext:NULL error:errorBlock completed:completedBlock];
    return [super subscribe:o];
}
@end

STOneTimeSubscriber的实现。


@interface STOneTimeSubscriber : NSObject <RACSubscriber>
+ (instancetype)subscriberWithNext:(void (^)(id x))next error:(void (^)(NSError *error))error completed:(void (^)(void))completed;
@end
@interface STOneTimeSubscriber ()
//...
@end
@implementation STOneTimeSubscriber
//...
#pragma mark RACSubscriber
- (void)sendNext:(id)value {
    @synchronized (self) {
        void (^nextBlock)(id) = [self.next copy];
        if (nextBlock == nil) return;
        nextBlock(value);
        //complete automatically when receive value, that's why this's called one-time subscriber.
        //you can accomplish the same function using `take` operation. but that's more expensive
        [self sendCompleted];
    }
}
//...
@end