Objective-C: How to get the value of a `objc_property_t` property?

Question

I use the following code to iterate through all the properties of an object. I successfully retrieve the property name as char but I have no idea how to get the property value which is of id type. Any ideas on how can I achieve this?

objc_property_t *allProperties = class_copyPropertyList([currentObject class], &allPropertyCount);

for (unsigned int i = 0; i < allPropertyCount; i++) {

      objc_property_t property = allProperties[i];
      const char * propertyName = property_getName(property);

}

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EDIT: Thank you all for the great comments and answers. Some of you asked why do I need this. Well, here is the reason:

I have several objects of the same class. Let's say the class is Person and its instances are Mary, John and David. The properties of each object are set as follows:

mary.age = [NSNumber numberWithInt:20];
john.age = [NSNumber numberWithInt:45];
david.age = [NSNumber numberWithInt:20];

mary.gender = @"female";
john.gender = @"male";
david.gender = @"male";

My purpose is to find a generic way to group the objects based on a given property name Eg. this will create 2 groups [david and mary] and [john]:

[self groupBaseDataObjects:self.persons withPropertyName:"age"];

and this:

[self groupBaseDataObjects:self.persons withPropertyName:"gender"];

will also create 2 groups [john and david] and [mary]

What are you trying to do? Down this path generally lies madness (save for in a handful of outlying esoteric situations that are pretty much crazy town, but still possible). — bbum, Jul 22 '11 at 02:53
Please see the updated description for the idea behind this. — Rad'Val, Jul 22 '11 at 12:45
This related question has what you need: [object attribute list](http://stackoverflow.com/questions/754824/get-an-object-attributes-list-in-objective-c) — Oscar Gomez, Jul 22 '11 at 01:08
`property_getName` returns the name of the property (eg for an object with a property like this `obj.prop` it will return "prop") — Rad'Val, Jul 22 '11 at 01:15

score 5 · Accepted Answer · answered Jul 22 '11 at 16:34

OK -- there appears to be no need to do any funky runtime gymnastics to solve your problem (don't get me wrong -- funky runtime gymnastics can be fun.... but it is probably just a waste of time here).

To restate the question:

You have a single class (MyClass) with many instances of that class. The class provides several attributes with instances of the class having many different values for that attribute. You have a collection of instances and you want to easily grab a collection of multiple subsets where the objects in each subset have the same value for a particular attribute.

To that end, I'd just loop over the original question and do something like (this code has never seen a compiler; it'll probably not compile):

// implement this on MyClass
+ (NSDictionary*)subdivideArrayOfMyClass:(NSArray*) aCollection byAttribute:(NSString*) anAttribute
{
    NSMutableDictionary *collector = [NSMutableDictionary dictionary];
    for (MyClass *anObject in aCollection) {
        id value = [anObject valueForKey: anAttribute];
        // you'd have to special case for nil here if you wanted.
        NSMutableArray *sameValueCollector = [collector objectForKey: value];
        if (!sameValueCollector) {
             sameValueCollector = [NSMutableArray array];
             [collector setObject: sameValueCollector forKey:value];
        }

        [sameValueCollector addObject:anObject];
    }
    return collector; // no need to turn it into an immutable dict
}

This does assume that all the values are safe for use as keys in a dictionary. That would be true for the examples you gave.

If you need all objects with a single value, you could do valueForKey: on the array. If you wanted, you could do some funky set arithmetic to sub-divide the objects. However, that'll likely incur multiple passes over the collection whereas the above does the work with one pass (though a potentially large quantity of hash lookups in the collector). Go with the straightforward until performance analysis indicates that it is too slow.

Ok, so basically you can access `mary.age` like this: `[mary valueForKey: @"age"];`? That's awesome, didn't knew that... — Rad'Val, Jul 22 '11 at 17:56
OK, I've tested the code and indeed it works with `NSObjects` but for custom objects I get `-[Person objectForKey:]: unrecognized selector sent to instance 0x5f41730` I'll do some more digging, worst case scenario I'll implement the `objectForKey` method in Person class myself. Anyway is better than runtime gymnastics :D — Rad'Val, Jul 22 '11 at 18:23
Yup -- been there done exactly that... scratchscratchscratchDERP. :) — bbum, Jul 22 '11 at 21:52

Deepak Danduprolu · Answer 2 · 2011-07-22T13:08:16.130

3

Use valueForKey:.

id object = [customObject valueForKey:[NSString stringWithUTF8String:propertyName]];

You can use isEqual: to compare. You will get an NSNumber or NSValue where appropriate. Read the KVC guide for more.

Earlier Answer

~~Since the getter method will match the property name, you can generate the selector of the property name and call performSelector on the object.~~

[customObject performSelector:NSSelectorFromString([NSString stringWithUTF8String:propertyName])];

edited Jul 22 '11 at 13:08

answered Jul 22 '11 at 01:17

Deepak Danduprolu

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Cool. If you find `NSSelectorFromString([NSString stringWithUTF8String:propertyName])` verbose, you should be able to replace it with `sel_getUid(propertyName)`. – Deepak Danduprolu Jul 22 '11 at 01:36
what about `@property(getter=boogerHead) BOOL isNasty;`? – bbum Jul 22 '11 at 02:52
@mindnoise This is far from being a proper answer. You might want to unmark it as the answer. The problem is clearly indicated indicated in the example above. @bbum You are right. I overlooked the `getter` part but even if we figure it out using the attributes provided by `property_getAttributes` function it is not as trivial as using `performSelector` which is not the right for primitives i.e. `BOOL`. We will have to make the proper calls based on the return type. Do you know of a better way to approach this? – Deepak Danduprolu Jul 22 '11 at 03:36
I see your point bbum, however, i can control the object's properties and the way they are declared and will avoid that situation. – Rad'Val Jul 22 '11 at 12:13
@deepak this serves well my purpose (the properties are never primitives and I don't use the 'getter' part) However, i'll uncheck the answer as being correct to see if someone can come up with a better way to do it. Personally I prefer to do it this way if I can control the object's property, it's simpler than parsing 'property_getAttributes' – Rad'Val Jul 22 '11 at 12:21
@mindnoise Please do. However, I would also like to know what is the idea behind doing this. Probably there is a better approach there. – Deepak Danduprolu Jul 22 '11 at 12:26

Objective-C: How to get the value of a `objc_property_t` property?

2 Answers2