How do I get a property of an object in an array of those objects in Objective C?

Question

I have an array of custom objects, but I can't NSLog the property of an individual object in that array because you can store any objects in an array. How would I go about doing this?

Answer 1

Objective-C offers several introspection techniques trough it runtime system.

You can ask a object, if it is from a certain kind, or responses to a certain message.

for (id anObject in array ){
    if([anObject isKindOfClass:[MyClass class]]){
        MyOtherClass *obj = anObject.myProperty ;
        NSLog(@"%@", obj);
    }
}

and

for (id anObject in array ){
    if( [anObject respondsToSelector:@selector(aMethod)] ) {
        NSLog(@"%@",[anObject aMethod]);
    }
}

As properties usually results in synthesized methods, the second way should also work for them.

Also to mention — although not in the scope of this question:

Classes can also be ask, if they fulfill a certain protocol. And as Objects can tell there class, this is also possible:

[[anObject class] conformsToProtocol:@protocol(MyProtocol)];

Answer 2

The above answer is correct for accessing individual fields. Just as a side note, there are also some interesting tricks that you can do with key-value path coding and Collection Operators when you have a collection like an array. For example, if you have a large array of objects with a small number of values for one field:

 NSArray *allObjects;

you can create an array with all of the unique values of one of your fields with the statement:

 NSArray *values=[allObjects valueForKeyPath:@"@distinctUnionOfObjects.myObjectsInterestingProperty"];

or you can find the average value of a field with the statement:

 NSNumber *average=[allObjects valueForKeyPath:@"@avg.myObjectsInterestingProperty"];

All of the other key-value path Collection Operators you can use this way are listed in Apple's Key-Value Programming Guide