How to copy superclass properties to a subclass? - iphone

Question

Let's say I have two Classes like so:
Car
{
NSInteger wheels;
NSInteger bumpers;
}
+ (Car *)carWithData:(NSDictionary *)carData;
Lexus : Car
{
GPS *navigation;
}
+ (Lexus *)carWithData:(NSDictionary *)carData;
carWithData: is a simple helper method that creates an instance of Car populated with variables from carData. Lexus' version would also set the navigation data.
How would Lexus' carWithData look like without duplicating code from Car?

Answer 1

This is accomplished by calling super's implementation of init… in the init method:
//Car.m:
- (id)initWithData:(NSDictionary *)carData {
self = [super init];
if (self) {
//setup generic car properties:
self.wheels = [carData objectForKey:#"wheels"]; //example
self.bumpers = [carData objectForKey:#"bumpers"]; //example
}
return self;
}
+ (id)carWithData:(NSDictionary *)carData {
return [[[self alloc] initWithData:carData] autorelease];
}
//Lexus.m:
- (id)initWithData:(NSDictionary *)carData {
//this call to super is where the car's generic properties get initialized:
self = [super initWithWithData:carData];
if (self) {
//setup lexus car properties:
self.navigation = [carData objectForKey:#"navigation"]; //example
}
return self;
}
//there is no need to override super's [carWithData:] method as it's only a wrapper anyway.
Also note that both the initWith… and carWith… methods return id, not Car or Lexus.
The way your code is set up you end up with casting problems, where [Lexus carWithData:dataDict] does return an object of class Lexus, but the compiler doesn't know about it, as it expects a Car.

Answer 2

You would not define the methods with different signatures like:
+ (Car *)carWithData:(NSDictionary *)carData;
+ (Lexus *)carWithData:(NSDictionary *)carData;
you should instead use
+ (id)carWithData:(NSDictionary *)carData;
The implementation of the subclass would then look like
- (id)initWithData:(NSDictionary *)carData;
{
self = [super initWithData:carData];
if (self) {
_navigation = [carData valueForKey:#"navigation"];
}
return self;
}
+ (id)carWithData:(NSDictionary *)carData;
{
return [[[self alloc] initWithCarData:carData] autorelease];
}

Answer 3

Here would be my solution:
// interface
-(id) initWithCarData:(NSDictionary *) carData;
+(Car *) carWithCarData:(NSDictionary *) carData;
// car implementation
-(id) initWithCarData:(NSDictionary *) carData
{
if (self = [super init])
{
// initialize car data
}
return self;
}
+(Car *) carWithCarData:(NSDictionary *) carData
{
// note that 'self' here is the current class,
// there is no need to overwrite this method in the subclass
return [[self alloc] initWithCarData:carData];
}
// lexus implementation
-(id) initWithCarData:(NSDictionary *) carData
{
// initialize the variables that the superclass recognizes
if (self = [super initWithCarData:carData])
{
// initialize the lexus data
}
return self;
}
So, when you call [Lexus carWithCarData:myData] it ends up calling the Lexus's init method, not the Car's.

Answer 4

There's another way, which is more generic, by using the NSCoding protocol and NSKeyedArchiver.
If the object you want to copy into a subclass of yours implements the NSCoding protocol, which is the case for many NS... based classes, the following totally legal and safe trick can be used:
// Assumptions:
// copyFrom is an object of ClassA
// We have a ClassB that is a subclass of ClassA
NSMutableData *data = [NSMutableData data];
NSKeyedArchiver *arch = [[NSKeyedArchiver alloc] initForWritingWithMutableData:data];
[copyFrom encodeWithCoder:arch]; // this archives its properties
[arch finishEncoding];
NSKeyedUnarchiver *ua = [[NSKeyedUnarchiver alloc] initForReadingWithData:data];
ClassB *ourCopy = [[ClassB alloc] initWithCoder:ua]; // this restores the properties into our new object