Can I use NSPredicate as key in a NSDictionary

Question

Say I have two NSPredicates

NSPredicate *pa = [NSPredicate predicateWithBlock:^(BOOL)(id evaluatedObject, NSDictionary *bindings) {
    return [evaluatedObject isKindOfClass:[TestClass class]];
}];

NSPredicate *pb = [NSPredicate predicateWithBlock:^(BOOL)(id evaluatedObject, NSDictionary *bindings) {
    return [evaluatedObject isKindOfClass:NSClassFromString(@"TestClass")];
}];

What I want is that when I put pa in a NSDictionary and associate it with another object, say obj, later when I check in the dictionary using pb I get back obj.

Is that how it works?

I see that NSPredicate implements NSCopying, so I'm hoping that it works as a key. But I'm not sure about the case I outlined above.

Answer 1

NSPredicate does override isEqual, but this seems to be useful only in very simple cases:

NSPredicate *pa = [NSPredicate predicateWithFormat:@"foo = 'bar'"];
NSPredicate *pb = [NSPredicate predicateWithFormat:@"foo = 'bar'"];
BOOL b = [pa isEqual:pb]; // --> YES

In this case you could put an object into a dictionary using key pa and get the object back using key pb.

But this does not work with block-based predicates at all, even if they reference the same block:

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]; // --> NO

And even if that would work, it would require in your case that the two blocks in the predicate are recognized as equal, which is not the case:

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]; // --> NO

Finally it turns out that two blocks with the same body are not equal:

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]; // --> NO

So you can use a predicates as a key in a dictionary, but at least for block-based predicates this is pretty useless.