Using runtime functions to match up object types safely when setting properties

[Graham Cox]

In my app, I have an interface and code that allows the user to set up  
different values that can be set as properties of an object. The user  
is presented with a list of properties that can be set (suitably  
converted for display in the UI) and then they can set up the values  
of these properties, which can be one of five distinct types (a custom  
type, an image, colour, string or numeric value).

I'd like to be able to gently prevent the user from making an  
inappropriate choice. For example if the given property expects a  
string, they should not be able to choose an image object.

Internally, I use KVC, specifically -setValue:forKey: to apply the  
chosen object to the chosen property.

So I'm looking for a way to safely and reliably determine whether a  
property will accept an object of a given type. I wrote this category  
on NSObject as a simple wrapper on the runtime to attempt this, but it  
doesn't work quite how I expected - any property that returns an  
object simply returns '@' as its return type - I can't tell what class  
it is. Can anyone point me in the right direction?

@implementation NSObject (RuntimeEnhancements)



- (NSString*)           returnTypeOfPropertyNamed:(NSString*) aProperty
{
        NSString* result = nil;
	Method	meth = class_getInstanceMethod([self class],  
NSSelectorFromString(aProperty));
        
        if( meth )
        {
                char* methType = method_copyReturnType( meth );
                
                result = [NSString stringWithUTF8String:methType];
                free( methType );
        }
        
        NSLog(@"object %@ (%p) return type: %@", self, self, result );
        
        return result;
}


- (NSString*)		typeOfArgumentAtIndex:(unsigned) indx forMethodNamed: 
(NSString*) aMethodName
{
        NSString* result = nil;
	Method	meth = class_getInstanceMethod([self class],  
NSSelectorFromString(aMethodName));
        
        if( meth )
        {
                char* argType = method_copyArgumentType( meth, indx );
                
                result = [NSString stringWithUTF8String:argType];
                free( argType );
        }
        
	NSLog(@"object %@ (%p) argument at index %d: %@", self, self, indx,  
result );

        return result;
}



- (BOOL)		propertyNamed:(NSString*) aProperty returnsObjectOfClass: 
(Class) cl
{
	// tests whether the return type of the property named is the same as  
the class <cl>
        
        NSString* propType = [self returnTypeOfPropertyNamed:aProperty];
        return [propType isEqualToString:NSStringFromClass(cl)];
}

@end


thanks, Graham




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