How to detect a property return type in Objective-C - 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.

Answer 2

Cheap answer: use the NSObject+Properties source here.
It implements the same methodology described above.

Answer 3

The preferred way is to use the methods defined in the NSObject Protocol.
Specifically, to determine if something is either an instance of a class or of a subclass of that class, you use -isKindOfClass:. To determine if something is an instance of a particular class, and only that class (ie: not a subclass), use -isMemberOfClass:
So, for your case, you'd want to do something like this:
// Using -isKindOfClass since NSMutableArray subclasses should probably
// be handled by the NSMutableArray code, not the NSArray code
if ([anObject isKindOfClass:NSMutableArray.class]) {
// Stuff for NSMutableArray here
} else if ([anObject isKindOfClass:NSArray.class]) {
// Stuff for NSArray here
// If you know for certain that anObject can only be
// an NSArray or NSMutableArray, you could of course
// just make this an else statement.
}

Answer 4

This is really a comment addressing an issue raised by Greg Maletic in response to answer provided by e.James 21APR09.
Agreed that Objective-C could use a better implementation for getting these attributes.
Below is a method I quickly threw together to retrieve attributes of a single object property:
- (NSArray*) attributesOfProp:(NSString*)propName ofObj:(id)obj{
objc_property_t prop = class_getProperty(obj.class, propName.UTF8String);
if (!prop) {
// doesn't exist for object
return nil;
}
const char * propAttr = property_getAttributes(prop);
NSString *propString = [NSString stringWithUTF8String:propAttr];
NSArray *attrArray = [propString componentsSeparatedByString:#","];
return attrArray;
}
Partial list of attribute keys:
R Read-only
C Copy of last value assigned
& Reference to last value assigned
N Nonatomic property
W Weak reference
Full list at Apple

Answer 5

You can use isKindOfClass message
if([something isKindOfClass:NSArray.class])
[somethingElse action];

Answer 6

If you know that the property is defined :
id vfk = [object valueForKey:propertyName];
Class vfkClass = vfk.class;
And compare with isKindOfClass, isSubClass, etc.