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POCircular dependencies in Cocoa bindings
 

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  1. POCircular dependencies in Cocoa bindings
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    <p>This is doing my head in...</p> <p>Simplified version: I have two text fields - Field A and Field B. Field B can be derived from field A and similarly Field B can be derived from Field A.</p> <p>(There's couple of other fields that in combination with either A or B produce data for multiple TextLabels)</p> <p>What I want to do is: when user changes Field A, Field B is updated, and vice versa.</p> <p>So I created two methods that do A to B and B to A. And defined dependencies like:</p> <pre><code>+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key { NSSet *keyPaths = [super keyPathsForValuesAffectingValueForKey:key]; if ([key isEqualToString:@"calculatedFieldA"]) { NSSet *dependentKeys = [NSSet setWithObjects:@"valueOfFieldB", nil]; keyPaths = [keyPaths setByAddingObjectsFromSet:dependentKeys]; } if ([key isEqualToString:@"calculatedFieldB"]) { NSSet *dependentKeys = [NSSet setWithObjects:@"valueOfFieldA", nil]; keyPaths = [keyPaths setByAddingObjectsFromSet:dependentKeys]; } return keyPaths; } </code></pre> <p>Where <code>calculatedFieldA</code> and <code>calculatedFieldB</code> are the methods that do the conversion and <code>valueOfFieldA</code> and <code>valueOfFieldB</code> are NSString's that are bound to the two text fields.</p> <p>It works (but only one way, B is updated whenever A changes) if I remove the second <code>if</code> statement. When the second <code>if</code> is defined, it just bombs out, because (I think) it sees A updated so goes and updates B, then because B is updated, goes and updates A again, etc, etc...</p> <p>What is the best way to achieve this circular dependency? Is it a time to start reading about <a href="http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ValueTransformers/ValueTransformers.html#//apple_ref/doc/uid/10000175i" rel="nofollow">ValueTransformers</a>?</p> <p>PS. I'm a Cocoa newbie so please bear with me and don't punch too hard if this is a very trivial question...</p> <p><strong>EDIT:</strong></p> <p>I probably need to clarify few points:</p> <p><code>calculatedFieldA</code> accepts B value and returns A, also updates (via setter method) <code>valueOfFieldA</code>. Similarly <code>calculatedFieldB</code> accepts A value and returns B, also updates (via setter method) <code>valueOfFieldB</code>.</p> <p>This is on Lion, with Xcode 4.1.</p>
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