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    <p>A delegate is an object that will respond to pre-chosen selectors (function calls) at some point in the future.</p> <p>Suppose I am loading a URL asynchronously (in the background), using an object of class FancyAsynchronousURLGetter. Since it is running in the background, I want to be able to go and do something else while the url is loading, and then be notified when it's ready. By using a delegate on the FancyAsynchronousURLGetter and writing the appropriate code, I can specify an object with a particular selector that will be called when the FancyAsynchronousURLGetter is finished. Something like this:</p> <pre><code>- (void)loadView { ... FancyAsynchronousURLGetter* getter = [[FancyAsynchronousURLGetter alloc] initWithURL:url]; [getter setDelegate:self]; /* getter will call either - (void)fancyAsynchronousURLGetterLoadSucceeded:(FancyAsynchronousURLGetter*)g or - (void)fancyAsynchronousURLGetterLoadFailed:(FancyAsynchronousURLGetter*)g on its delegate, depending on whether load succeeded or failed */ [getter start]; ... } - (void)fancyAsynchronousURLGetterLoadSucceeded:(FancyAsynchronousURLGetter*)g { NSLog(@"Load succeeded."); } - (void)fancyAsynchronousURLGetterLoadFailed:(FancyAsynchronousURLGetter*)g { NSLog(@"Load failed."); } </code></pre> <p>and in the FancyAsynchronousURLGetter itself:</p> <pre><code>- (void)start { [self performSelectorInBackground:@selector(fetchURL) withObject:nil]; } - (void))fetchURL { Fetch the URL synchronously if ( success ) [delegate fancyAsynchronousURLGetterLoadSucceeded:self]; // note: probably want to call this on the main thread else [delegate fancyAsynchronousURLGetterLoadFailed:self]; } </code></pre>
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    1. COI'd replace (function calls) with (method invocations). A method invocation is not the same as directly calling a function. And the method names you use are impossible. Make it one delegate method, called `fancyAsynchronousURLGetter:(id)sender succeeded:(BOOL)success`.
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      1. USRudy Velthuis
 

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