Objective-C
Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. It was the primary language used by Apple for macOS and iOS development before the introduction of Swift. Objective-C is known for its dynamic runtime and extensive use of square brackets for method calls.
Developers should learn Objective-C for maintaining and updating legacy Apple applications, as many existing macOS and iOS apps are still written in it. It's also useful for understanding the foundations of Apple's frameworks like Cocoa and Cocoa Touch, and for working with libraries or projects that haven't migrated to Swift yet. Knowledge of Objective-C can be essential for debugging or integrating with older codebases in the Apple ecosystem.