How could I check for a particular property at runtime, along with its return type? - objective-c

Question

Since property named "age" would always have a selector named "age" as well, I could use respondsToSelector as this question suggests and that will tell me if a particular selector exists at runtime in any given object.
If a property named "age" exists, I can verify that. How could I know if that selector (the read method for that property) returns an object (id) or non-object (int)?
Is such type determination possible at runtime, or is the Objective-C way to always assume that someone implemented that method using the type I'm hoping it used, or can I also verify the return type?
This is using the latest Objective-C version (LLVM 4.1) in XCode 4.5.
Update: This is the utility-category-on-NSObject that I came up with:
- (NSString*) propertyType: (NSString*)propname
{
objc_property_t aproperty = class_getProperty([self class], [propname cStringUsingEncoding:NSASCIIStringEncoding] ); // how to get a specific one by name.
if (aproperty)
{
char * property_type_attribute = property_copyAttributeValue(aproperty, "T");
NSString *result = [NSString stringWithUTF8String:property_type_attribute];
free(property_type_attribute);
return result;
}
else
return nil;
}
While looking into this question I also wrote this handy-dandy utility method that
can list all the properties on this object:
- (NSArray*) properties;
{
NSMutableArray *results = [NSMutableArray array];
#autoreleasepool {
unsigned int outCount, i;
objc_property_t *properties = class_copyPropertyList([self class], &outCount);
for (i = 0; i < outCount; i++) {
objc_property_t property = properties[i];
const char * aname=property_getName(property);
[results addObject:[NSString stringWithUTF8String:aname]];
//const char * attr= property_getAttributes(property);
//[results addObject:[NSString stringWithUTF8String:attr]];
}
if (properties) {
free(properties);
}
} // end of autorelease pool.
return results;
}

Answer 1

You could use class_copyPropertyList to get a list of properties declared in a class.
class_copyPropertyList
Describes the properties declared by a class.
And then property_getAttributes:
property_getAttributes
Returns the attribute string of an property.
Here you can find some more concrete hints and examples.
As a side note, the following statement:
Since property named "age" would always have a selector named "age" as well
is not correct, since a property can have custom getter and/or setter:
#property (nonatomic, getter=isImmediate) BOOL immediate;
EDIT:
Some sample code I found in another S.O. post:
const char * type = property_getAttributes(class_getProperty([self class], "myPropertyName"));
NSString * typeString = [NSString stringWithUTF8String:type];
NSArray * attributes = [typeString components separatedByString:#","];
NSString * typeAttribute = [attributes objectAtIndex:0];
NSString * propertyType = [typeAttribute substringFromIndex:1];
const char * rawPropertyType = [propertyType UTF8String];
if (strcmp(rawPropertyType, #encode(float)) == 0) {
//it's a float
} else if (strcmp(rawPropertyType, #encode(int)) == 0) {
//it's an int
} else if (strcmp(rawPropertyType, #encode(id)) == 0) {
//it's some sort of object
} else ....

Answer 2

One approach you can take, assuming you know the property name already, is to use the class_getProperty function. You can also use the property_copyAttributeValue() function to get just a particular attribute by name:
objc_property_t number_property = class_getProperty([MyClass class], "number");
char *number_property_type_attribute = property_copyAttributeValue(number_property, "T");
NSLog(#"number property type attribute = %s", number_property_type_attribute);
Will log:
2013-01-14 14:45:37.382 RuntimeFun[61304:c07] number property type
attribute = i
Assuming MyClass looks something like:
#interface MyClass : NSObject
#property (nonatomic) int number;
#end
#implementation MyClass
#end
One you have your type attribute string, you can then compare it to the various Objective-C type encodings. Once you're done with your comparison, be sure to call free() on your attribute string.