In this specific case, what you're trying to determine is the proper semantics for the code in question to be mappable to to Swift semantics. This is a formal way of saying "take this AST and figure out what its meaning is." Exactly how you perform semantic analysis depends a lot on the specifics of how your AST works and what you're trying to do. Once you have your AST, the next step is semantic analysis. Hopefully you have a strong grasp of trees and recursion, because compiler work uses them everywhere.
You can run your parser over Objective-C code and get an Abstract Syntax Tree (AST) out. Once you have a parser, a lot of the hard work is done, but there's still plenty of interesting challenge ahead for you. Until you've got a lot of experience with compiler work, you'd be better off finding an existing parser than trying to build your own. This makes creating a proper parser for them much more difficult. C and Objective-C are level 1, context-sensitive languages, because of the way typedefs work: the same string of code can parse in two different ways, with two different meanings, depending on whether or not the symbol it's reading is a type. It defines four levels of language complexity from 0 (very difficult to parse) to 3 (very easy to parse.) Most modern programming languages are level 2, context-free languages, because it's difficult to create a useful language without a recursive grammar, which is what differentiates level 2 from level 3. From the vocabulary you're using, you may already know a few things about parsing theory and the Chomsky hierarchy. Unfortunately, C (and by extension any superset of the language, such as Objective-C) is difficult to parse. You're right in that your overall task is a source-to-source compiler, and that you need to start with a parser.
We start by breaking it down into smaller and smaller tasks until we get them down to the level of a problem we know how to solve. But then again, so are most things worth doing in programming.
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