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POWhy must my ObjC implementation class #import the _interface_ that implements a protocol?
 

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  1. POWhy must my ObjC implementation class #import the _interface_ that implements a protocol?
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    <p>I don't understand why I have to do the following in Objective-C. I'm interested in discovering why, in a general sense (i.e. why does the language/compiler force me to do it this way), and maybe discover alternatives to the implementation I have come up with:</p> <p>I have a protocol:</p> <pre><code>// (file: MyObserver.h) @class TheObserved; @protocol MyObserver &lt;NSObject&gt; - (void)itWasObserved:(TheObserved *)observedInstance; @end </code></pre> <p>There is an interface, 'TheObserved' that (cyclically) uses the protocol:</p> <pre><code>// (file: TheObserved.h) @protocol MyObserver; @interface TheObserved : NSObject @property id &lt;MyObserver&gt; myObserver; - (void)lookAtThisData:(NSString *)someData withObserver:(id &lt;MyObserver&gt;)observer; @end </code></pre> <p>This is to say, as you can see, the protocol has a message which takes an instance of the interface, and the interface has a method which takes an instance that implements the protocol.</p> <p>In my implementation of the interface:</p> <pre><code>// (file: TheObserved.m) #import "TheObserved.h" #import "MyViewController.h" // &lt;-- note this import @implementation TheObserved @synthesize myObserver; - (void)lookAtThisData:(NSString *)someData withObserver:(id &lt;MyObserver&gt;)observer { self.myObserver = observer; // some asynchronous looking at the data is done here. } - (void)asynchCallback:(NSData *) data { // check if the data is as we expect, if so .. [self.myObserver itWasObserver: self] } @end </code></pre> <p>MyViewController.h implements the protocol:</p> <pre><code>// (file: MyViewController.h) #import &lt;UIKit/UIKit.h&gt; #import "OWLMorphDataObserver.h" @interface MyViewController : UITableViewController &lt;MyObserver&gt; @end </code></pre> <p>And in its implementation:</p> <pre><code>// (file: MyViewController.m) #import "TheObserved.h" @interface MyViewController () { @property TheObserved *observed; @end @implementation MyViewController @synthesize observed; - (void) aMethod:(NSString *)dataString { [self.observed lookAtThisData:dataString withObserver:self] } @end </code></pre> <p>My question is: why do, in my Observed class, I need to #import the "MyViewController.h" which implements the @protocol even though the concrete implementation is never explicitly referenced in the Observed class? If I don't, I get and compilation error: </p> <pre><code>no known instance method for selector 'lookAtThisData:' </code></pre> <p>The issue here is, of course that I want to have multiple different view controllers implement this protocol. So why am I forced to import one of them to get it to compile?</p> <p>Is there another way I can structure this code to get the desired effect without a importing a concrete implementation of the protocol in the class that wants to use it?</p>
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    1. COcan you name the files in which each snippet is contained? e.g. "I have a protocol" add "Defined in foo.h".
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    2. COsure, I did that.
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    3. COThe error message does not match the code you've posted. `-lookAtThisData:` and `-lookAtThisData:withObserver:` are two different methods. Otherwise, the code you've posted is correct. `-lookAtThisData:withObserver:` is declared in TheObserved.h, and that's the header you need to import in order to call that method.
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