Now that we have a pretty good understanding of Objective-C, let's discuss what Objective-C code looks like in a project. Unlike the Swift code, Objective-C is written in two different types of files. One of the types is called a header file and it ends in the extension
h. The other type is called an implementation file and ends in the extension
Before we really discuss what the difference between the two is, we first have to discuss code exposure. In Swift, all the code that you write is accessible to all other code in your project. This is not true with Objective-C. In Objective-C, you must explicitly indicate that you want to have access to the code in another file.
Header files are the types of files that can be included ...