You don’t need to retain signals in order to process them.

Subscribers

Any objects referenced from those blocks will therefore be retained as part of the subscription.

Finite or Short-Lived Signals

Subscription is automatically terminated upon completion or error, and the subscriber removed.
The RACSignal -> RACSubscriber relationship is torn down as soon as signal finishes, breaking the retain cycle.

Infinite Signals

Disposing of a subscription will remove the associated subscriber, and just generally clean up any resources associated with that subscription.

Signals Derived from self

Any time a signal’s lifetime is tied to the calling scope, you’ll have a much harder cycle to break.
This commonly occurs when using RACObserve() on a key path that’s relative to self, and then applying a block that needs to capture self.

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__weak id weakSelf = self;
[RACObserve(self, username) subscribeNext:^(NSString *username) {
    id strongSelf = weakSelf;
    [strongSelf validateUsername];
}];

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@weakify(self);
[RACObserve(self, username) subscribeNext:^(NSString *username) {
    @strongify(self);
    [self validateUsername];
}];

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RACSignal *validated = [RACObserve(self, username) map:^(NSString *username) {
    // Put validation logic here.
    return @YES;
}];

原始地址： http://sxgfxm.github.io/blog/2017/09/30/reactiveobjc-memory-management/
written by Light  posted at http://sxgfxm.github.io