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    <p>So, there are two major things going on here:</p> <ol> <li>You want to share some side effects (in this case, fetching a token) without re-triggering them every time there's a new subscriber.</li> <li>You want anyone subscribing to <code>-getToken</code> to get the same values no matter what.</li> </ol> <p>In order to share side effects (#1 above), we'll use <a href="https://github.com/github/ReactiveCocoa/blob/83e23a96e726c99a444e83f306271d0e7d981379/ReactiveCocoaFramework/ReactiveCocoa/RACMulticastConnection.h" rel="noreferrer">RACMulticastConnection</a>. Like the documentation says:</p> <blockquote> <p>A multicast connection encapsulates the idea of sharing one subscription to a signal to many subscribers. This is most often needed if the subscription to the underlying signal involves side-effects or shouldn't be called more than once.</p> </blockquote> <p>Let's add one of those as a private property on the API client class:</p> <pre><code>@interface APIClient () @property (nonatomic, strong, readonly) RACMulticastConnection *tokenConnection; @end </code></pre> <p>Now, this will solve the case of N current subscribers that all need the same future result (API calls waiting on the request token being in-flight), but we still need something else to ensure that <em>future</em> subscribers get the same result (the already-fetched token), no matter when they subscribe.</p> <p>This is what <a href="https://github.com/github/ReactiveCocoa/blob/83e23a96e726c99a444e83f306271d0e7d981379/ReactiveCocoaFramework/ReactiveCocoa/RACReplaySubject.h" rel="noreferrer">RACReplaySubject</a> is for:</p> <blockquote> <p>A replay subject saves the values it is sent (up to its defined capacity) and resends those to new subscribers. It will also replay an error or completion.</p> </blockquote> <p>To tie these two concepts together, we can use <a href="https://github.com/github/ReactiveCocoa/blob/83e23a96e726c99a444e83f306271d0e7d981379/ReactiveCocoaFramework/ReactiveCocoa/RACSignal%2BOperations.h#L238-L241" rel="noreferrer">RACSignal's -multicast: method</a>, which turns a normal signal into a connection <em>by using</em> a specific kind of subject.</p> <p>We can hook up most of the behaviors at initialization time:</p> <pre><code>- (id)init { self = [super init]; if (self == nil) return nil; // Defer the invocation of -reallyGetToken until it's actually needed. // The -defer: is only necessary if -reallyGetToken might kick off // a request immediately. RACSignal *deferredToken = [RACSignal defer:^{ return [self reallyGetToken]; }]; // Create a connection which only kicks off -reallyGetToken when // -connect is invoked, shares the result with all subscribers, and // pushes all results to a replay subject (so new subscribers get the // retrieved value too). _tokenConnection = [deferredToken multicast:[RACReplaySubject subject]]; return self; } </code></pre> <p>Then, we implement <code>-getToken</code> to trigger the fetch lazily:</p> <pre><code>- (RACSignal *)getToken { // Performs the actual fetch if it hasn't started yet. [self.tokenConnection connect]; return self.tokenConnection.signal; } </code></pre> <p>Afterwards, anything that subscribes to the result of <code>-getToken</code> (like <code>-requestSignalWithURLRequest:</code>) will get the token if it hasn't been fetched yet, start fetching it if necessary, or wait for an in-flight request if there is one.</p>
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    1. POHow to using ReactiveCocoa to transparently authenticate before making API calls?
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    1. USNick Forge
    1. USJustin Spahr-Summers
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    1. POHow to using ReactiveCocoa to transparently authenticate before making API calls?
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    1. COAwesome explanation. Thank you!
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      1. USbvanderveen
    2. COHow would you handle log out? Or multiple accounts?
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      1. USColin Barrett
    3. CO@ColinBarrett Each of those would require a detailed answer in their own right – this was just the simplest solution to the problem laid out above. Supporting logout might involve putting tokenSignal into a RACReplaySubject of capacity 1, so that you can push a new signal onto it at will. Multiple accounts would be a much bigger change, because assumably the request API would need to be updated too. I'd be happy to answer either in more detail in a new question on SO or an issue on GitHub.
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      1. USJustin Spahr-Summers
 

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