RAC：如何延迟订阅信号，即如果该信号已经或将要被其他订阅者执行，则不触发它的执行？

Question

这是关于 RAC 2.0 的问题，我对它还很陌生：

假设我有一个信号：

- (RACSignal *)signalWithAsyncWorkInIt;

当我的应用启动时，我想做

RACSignal *sig1 = [self.signalWithAsyncWorkInIt subscribeNext:...];

// then somewhere else later:
RACSignal *sig2 = [self.signalWithAsyncWorkInIt subscribeNext:...]; // looking for some option other than subscribeNext:

但是如果 sig1 已经执行或即将执行，sig2 的 subscribeNext: 方法将不会触发 signalWithAsyncWorkInIt 的执行，即这样 sig2 只会重播 signalWithAsyncWorkInIt 的 "latest result" case 如果它存在并等待像 sig1 这样的东西第一次触发 signalWithAsyncWorkInIt 如果它还没有被触发。

Answer 1

我不确定我是否理解正确，但也许您要查找的是 replay、replayLast 或 replayLazily.

通过回放，您可以避免多次执行一个信号。它将结果多播给多个订阅者，而不是对每个订阅者都执行操作。

简单示例：

RACSignal* asyncOperationSignal = [[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
    NSLog(@"execute something here and return with the result");
    [subscriber sendNext:@"42"];
    return nil;
}]
replayLazily]; // call replayLazily to make sure the operation only executes after the first subscription
               // and returns the result for every consecutive subscription

[asyncOperationSignal subscribeNext:^(NSString* result) {
    NSLog(@"Result: %@", result);
}];

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 3 * NSEC_PER_SEC), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // subscribe again later in the code
    [asyncOperationSignal subscribeNext:^(NSString* result) {
        NSLog(@"Result: %@", result);
    }];
});

输出将是：

execute something here and return with the result
42
42

这是一篇关于此主题的文章，写得非常好： http://spin.atomicobject.com/2014/06/29/replay-replaylast-replaylazily/

RAC：如何延迟订阅信号，即如果该信号已经或将要被其他订阅者执行，则不触发它的执行？

RAC: how to subscribe to signal lazily i.e. do not trigger its execution if that signal already was or going to be executed by other subscriber?

reactive-cocoa