partial class example

gistfile1.txt

// Started here
class Reader : NSObject {}

class PDFReader_Shared : Reader {}
class PDFReader_iPad : PDFReader_Shared {}
class PDFReader_iPhone : PDFReader_Shared {}

class TextReader_Shared : Reader {}
class TextReader_iPad : TextReader_Shared {}
class TextReader_iPhone : TextReader_Shared {}

/////
Now I want Reader to have iPad/iPhone specifics. In the WPF/Silverlight land I could solve this
with partial classes. To be fair it was all solved at compile time. Basically have 3 directores: 
Shared, WPF, Silverlight. Then one build would do the partial classes of Shared+WPF and the 
other was Shared+Silverlight.

For iOS this would need to be a runtime thing. Picking up the right Reader base class.
////
class Reader_Shared : NSObject {}
class Reader_iPad : Reader_Shared {}
class Reader_iPhone : Reader_iPad {}

Now what would the PDFReader_Shared and TextReader_Shared derive from? It would need to be change 
at runtime I guess which is scary (not sure if it's possible?)

You could implement a class cluster. You’d only expose the @interface for the main class in the public header. The implementation file would look like this:

@interface iPhoneClass : MyClass

// iPhone methods defined here

@end

@implementation iPhoneClass

// iPhone methods defined here

@end

@interface iPadClass : MyClass

// iPad methods defined here

@end

@implementation iPadClass

// iPad methods implemented here

@end


@implementation MyClass

+ (id)alloc
{
    if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPad) {
        return [iPadClass alloc];
    }
    else {
        return [iPhoneClass alloc];
    }
}

// Shared methods go here

@end

If you don’t add any instance variables to iPhoneClass or iPadClass, you can skip implementing alloc, as their memory layout will be equivalent.