How to detect a property return type in Objective-C

Question

I have an object in objective-c at runtime, from which I only know the KVC key and I need to detect the return value type (e.g. I need to know if its an NSArray or NSMutableArray) of this property, how can I do that?

Answer 1

You're talking about runtime property introspection, which happens to be something that Objective-C is very good at.

In the case you describe, I'm assuming you have a class like this:

@interface MyClass
{
    NSArray * stuff;
}
@property (retain) NSArray * stuff;
@end

Which gets encoded in XML something like this:

<class>
    <name>MyClass</name>
    <key>stuff</key>
</class>

From this information, you want to recreate the class and also give it an appropriate value for stuff.

Here's how it might look:

#import <objc/runtime.h>

// ...

Class objectClass;       // read from XML (equal to MyClass)
NSString * accessorKey;  // read from XML (equals @"stuff")

objc_property_t theProperty =
    class_getProperty(objectClass, accessorKey.UTF8String);

const char * propertyAttrs = property_getAttributes(theProperty);
// at this point, propertyAttrs is equal to: T@"NSArray",&,Vstuff
// thanks to Jason Coco for providing the correct string

// ... code to assign the property based on this information

Apple's documentation (linked above) has all of the dirty details about what you can expect to see in propertyAttrs.