Question

I’m trying to create a factory (design pattern) in Objective C
So I’m doing something like:

+ (Car *)createCar:(int)color {
    if (color == 1) {
        return [CarFactory createBlueCar];
    } else if (color == 2) {
        return [CarFactory createRedCar];
    } else {
        return nil;
    }
}

+ (Car *)createBlueCar {
    // ...
}

+(Car*)createRedCar{
    // ...
}

However I don’t want the createBlueCar and the createRedCar to be available to public and if I don’t define them in the .h file then I get a warning of the missing definition.

I’m a former Java developer and a newbie in Objective-C — So it might be just bad practice If so what the be good practice of doing so.

Answer 1

The best way to do this is with a Class Extension.

.h

@interface MyClass : NSObject
@property(readonly) BOOL publiclyReadOnlyPrivatelyWritableFlag;
+ (id) myExposedFactoryMethod;
@end

.m

#import "MyClass.h"

@interface MyClass ()
@property(readwrite) BOOL publiclyReadOnlyPrivatelyWritableFlag;
+ (id) privateMethod1;
+ (id) privateMEthod2;
@end

@implementation MyClass
@synthesize publiclyReadOnlyPrivatelyWritableFlag; // no ivar -- only works in 64 bit and iPhone
+ (id) myExposedFactoryMethod
{
    ...
    [self privateMethod1];
    ...
}

+ (id) privateMethod1;
{
    return ...
}

+ (id) privateMEthod2;
{
    return ...
}
@end

A class extension is a better solution because it is a true extension of the class’s interface whereas a category (without a corresponding implementation) is merely a suggestion that the class implement the methods. That is, the compiler will warn if you don’t implement the interface — the methods — declared in the class extension, but will not warn if you did the same with a named category.

Note also that a class extension allows you to upgrade a readonly property to a readwrite property as per the above example.