I am learning iOS development in Objective-C, and I have found a lot of code examples.
Some of them, though, say that they are only for Mac OS X (not iOS).
Can someone please explain to me what is the difference between Cocoa and Cocoa Touch (at a library level), so that I know what code will work and what will not work?
Cocoa is commonly referred to as the combination of the Foundation and AppKit frameworks, while Cocoa Touch is the combination of the Foundation and UIKit frameworks.
Cocoa and Cocoa Touch sit on top of other collections of frameworks to create the API stacks. The other layers are Media, Core Services and Core OS.
The main difference between Cocoa and Cocoa touch is that the UI classes and APIs aren't the same as Mac OS X’s and macOS’s, so instead of NSTextField, you have UITextField. Many of the classes share the same functionality and can be ported quite easily by simply changing the class name, though most will require some more changes, but usually nothing too heavy.
There are also some differences between the Foundation frameworks in Cocoa and Cocoa Touch, most commonly missing classes, for example, Cocoa has NSHost and Cocoa Touch doesn't.
You will come to know more of the nuances between the two and will soon be able to instinctively know what will work on an iPhone with little/no modification and what will require some work to port between, but it's not that difficult.
The Migrating from Cocoa Touch section of the iPhone OS Technology Overview provides a good overview of the differences with links to more specific documents for each layer.
The big difference is the UI layer. On the Mac, you have the AppKit framework, on the iPhone, you have UIKit. The smaller differences are in the Foundation framework (linked in the document above).
Cocoa
NeXTSTEP -> Cocoa -> Swift Standard Library
Cocoa is an application environment for iOS and OS X.
application environment is a set of
application environments: - OS X: Cocoa, Carbon - iOS: only Cocoa
Cocoa Touch is iOS layer which contains a set of libraries/frameworks for supporting iOS applications. For example - UIKit, MapKit...
*Usually Cocoa is considered as just a set of libraries/frameworks
Cocoa framework is a framework with descendant classes of NSObject* which run under Objective-C runtime[About]
Frameworks with a Cocoa API - secondary frameworks which work with Cocoa frameworks and supports Cocoa conventions