Question

I need to determine an object’s property (passed by name) type in order to perform deserialization from XML. I have some general pseudo-code (however I am unsure of how to perform these comparisons in Objective-C):

id object = [[[Record alloc] init] autorelease];

NSString *element = @"date";
NSString *data = @"2010-10-16";

objc_property_t property = class_getProperty([object class], [element UTF8String]);
const char *attributes = property_getAttributes(property);

char buffer[strlen(attributes) + 1];
strcpy(buffer, attributes);

char *attribute = strtok(buffer, ",");
if (*attribute == 'T') attribute++; else attribute = NULL;

if (attribute == NULL);
else if (strcmp(attribute, "@\"NSDate\"") == 0) [object setValue:[NSDate convertToDate:self.value] forKey:element];
else if (strcmp(attribute, "@\"NSString\"") == 0) [object setValue:[NSString convertToString:self.value] forKey:element];
else if (strcmp(attribute, "@\"NSNumber\"") == 0) [object setValue:[NSNumber convertToNumber:self.value] forKey:element];

I have looked through the class_getProperty and property_getAttributes, however I am still not sure how to do the above comparisons.

Answer 1

@Ahruman’s answer is correct, if you’re dealing with objects. Let me suggest some alternatives:

1. valueForKey:: If you use [myObject valueForKey:@"myPropertyName"], it will return an object. If the property corresponds to some sort of primitive (int, float, CGRect, etc), then it will be boxed for you into an NSNumber or NSValue (as appropriate). If it comes back as an NSNumber, you can then easily extract a double representation (doubleValue) and use that as an NSTimeInterval to create an NSDate. I would probably recommend this approach.

2. Special case each type. property_getAttributes() returns a char* representing all of the attributes of the property, and you can extract the type by doing this:

   const char * type = property_getAttributes(class_getProperty([self class], "myPropertyName"));
   NSString * typeString = [NSString stringWithUTF8String:type];
   NSArray * attributes = [typeString componentsSeparatedByString:@","];
   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 ....

   This is pedantically more correct than Louis’s answer, because while most types have a single-character encoding, they don’t have to. (his suggestion assumes a single-character encoding)

3. Finally, if you’re doing this on a subclass of NSManagedObject, then I would encourage checking out NSPropertyDescription.

From these alternatives, you can probably see that letting the runtime box the value for you is probably simplest.

edit extracting the type:

From the code above, you can extract the class name like so:

if ([typeAttribute hasPrefix:@"T@"] && [typeAttribute length] > 1) {
  NSString * typeClassName = [typeAttribute substringWithRange:NSMakeRange(3, [typeAttribute length]-4)];  //turns @"NSDate" into NSDate
  Class typeClass = NSClassFromString(typeClassName);
  if (typeClass != nil) {
    [object setValue:[self convertValue:self.value toType:typeClass] forKey:element];
  }
}

And then instead of using class method categories to do the conversion (ie, [NSDate convertToDate:]), make a method on self that does that for you and accepts the desired type as a parameter. You could (for now) do it like:

- (id) convertValue:(id)value toType:(Class)type {
  if (type == [NSDate class]) {
    return [NSDate convertToDate:value];
  } else if (type == [NSString class]) {
    return [NSString convertToString:value];
  } ...
}

Part of me is wondering, though: why on earth are you needing to do things this way? What are you making?