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    <p><strong>Iterative algorithm</strong>:</p> <ul> <li>Create a <code>__block</code> variable (<code>int urlNum</code>) to keep track of the current URL (inside an <code>NSArray</code> of them).</li> <li>Have the onUrlComplete block fire off the next request until all URLs have been loaded.</li> <li>Fire the first request.</li> <li>When all URLs have been loaded, do the "//success!" dance.</li> </ul> <p>Code written without the aid of XCode (meaning, there may be compiler errors -- will fix if necessary):</p> <pre><code>- (void)loadUrlsAsynchronouslyIterative:(NSArray *)urls { __block int urlNum = 0; void(^onUrlComplete)(int) = nil; //I don't remember if you can call a block from inside itself. onUrlComplete = ^(int status) { if (urlNum &lt; urls.count) { id nextUrl = urls[urlNum++]; [remoteAPIWithURL:nextUrl success:onUrlComplete]; } else { //success! } } onUrlComplete(0); //fire first request } </code></pre> <p><strong>Recursive algorithm</strong>:</p> <ul> <li>Create a method to load all the remaining URLs.</li> <li>When remaining URLs is empty, fire "onSuccess".</li> <li>Otherwise, fire request for the next URL and provide a completion block that recursively calls the method with all but the first remaining URLs.</li> <li>Complications: we declared the "onSuccess" block to accept an <code>int status</code> parameter, so we pass the last status variable down (including a "default" value).</li> </ul> <p>Code written without the aid of XCode (<em>bug disclaimer here</em>):</p> <pre><code>- (void)loadUrlsAsynchronouslyRecursive:(NSArray *)remainingUrls onSuccess:(void(^)(int status))onSuccess lastStatus:(int)lastStatus { if (remainingUrls.count == 0) { onSuccess(lastStatus); return; } id nextUrl = remainingUrls[0]; remainingUrls = [remainingUrls subarrayWithRange:NSMakeRange(1, remainingUrls.count-1)]; [remoteAPIWithUrl:nextUrl onSuccess:^(int status) { [self loadUrlsAsynchronouslyRecursive:remainingUrls onSuccess:onSuccess lastStatus:status]; }]; } //fire first request: [self loadUrlsAsynchronouslyRecursive:urls onSuccess:^(int status) { //success here! } lastStatus:0]; </code></pre> <p><strong>Which is better</strong>?</p> <ul> <li>The iterative algorithm is simple and concise -- if you're comfortable playing games with <code>__block</code> variables and scopes.</li> <li>Alternatively, the recursive algorithm doesn't require <code>__block</code> variables and is fairly simple, as recursive algorithms go.</li> <li>The recursive implementation is more re-usable that the iterative one (as implemented).</li> <li>The recursive algorithm might leak (it requires a reference to <code>self</code>), but there are several ways to fix that: make it a function, use <code>__weak id weakSelf = self;</code>, etc.</li> </ul> <p><strong>How easy would it be to add error-handling</strong>?</p> <ul> <li>The iterative implementation can easily be extended to check the value of <code>status</code>, at the cost of the <code>onUrlComplete</code> block becoming more complex.</li> <li>The recursive implementation is perhaps not as straight-forward to extend -- primarily because it is re-usable. Do you want to cancel loading more URLs when the status is such-and-such? Then pass down a status-checking/error-handling block that accepts <code>int status</code> and returns <code>BOOL</code> (for example <code>YES</code> to continue, <code>NO</code> to cancel). Or perhaps modify <code>onSuccess</code> to accept both <code>int status</code> and <code>NSArray *remainingUrls</code> -- but you'll need to call <code>loadUrlsAsynchronouslyRecursive...</code> in your <code>onSuccess</code> block implementation.</li> </ul>
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    1. COAlthough I like the way you split it into 2 ways. I still see this becoming very unreadable very quickly with either method. Just collecting 4 urls is one of the more simple examples, and the code already looks inside out. Just imagine a slightly more complicated goal, and the blocks, callbacks, state and parameters which need to be passed around grow quickly out of control
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