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  1. PO
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    <p><code>NSPredicate</code> <em>does override</em> <code>isEqual</code>, but this seems to be useful only in very simple cases:</p> <pre><code>NSPredicate *pa = [NSPredicate predicateWithFormat:@"foo = 'bar'"]; NSPredicate *pb = [NSPredicate predicateWithFormat:@"foo = 'bar'"]; BOOL b = [pa isEqual:pb]; // --&gt; YES </code></pre> <p>In this case you could put an object into a dictionary using key <code>pa</code> and get the object back using key <code>pb</code>.</p> <p>But this does not work with block-based predicates at all, even if they reference the <em>same block</em>:</p> <pre><code>BOOL (^block)(id evaluatedObject, NSDictionary *bindings) = ^BOOL(id evaluatedObject, NSDictionary *bindings) { return [evaluatedObject isKindOfClass:[NSString class]]; }; NSPredicate *pa = [NSPredicate predicateWithBlock:block]; NSPredicate *pb = [NSPredicate predicateWithBlock:block]; BOOL b = [pa isEqual:pb]; // --&gt; NO </code></pre> <p>And even if that would work, it would require in your case that the two <em>blocks</em> in the predicate are recognized as equal, which is not the case:</p> <pre><code>BOOL (^block1)(id evaluatedObject, NSDictionary *bindings) = ^BOOL(id evaluatedObject, NSDictionary *bindings) { return [evaluatedObject isKindOfClass:[NSString class]]; }; BOOL (^block2)(id evaluatedObject, NSDictionary *bindings) = ^BOOL(id evaluatedObject, NSDictionary *bindings) { return [evaluatedObject isKindOfClass:NSClassFromString(@"NSString")]; }; BOOL b = [block1 isEqual:block2]; // --&gt; NO </code></pre> <p>Finally it turns out that two <em>blocks with the same body</em> are not equal:</p> <pre><code>BOOL (^block1)(id evaluatedObject, NSDictionary *bindings) = ^BOOL(id evaluatedObject, NSDictionary *bindings) { return [evaluatedObject isKindOfClass:[NSString class]]; }; BOOL (^block2)(id evaluatedObject, NSDictionary *bindings) = ^BOOL(id evaluatedObject, NSDictionary *bindings) { return [evaluatedObject isKindOfClass:[NSString class]]; }; BOOL b = [block1 isEqual:block2]; // --&gt; NO </code></pre> <p>So you can use a predicates as a key in a dictionary, but at least for block-based predicates this is pretty useless.</p>
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    1. POCan I use NSPredicate as key in a NSDictionary
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    1. USMartin R
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    1. POCan I use NSPredicate as key in a NSDictionary
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    1. COYour test is much more comprehensive than mine. Thanks!
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    2. CO@yulan6248: You are welcome. - It was an interesting problem!
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