如何在Swift中编写编译时代码？

Question

I'm wanting to write some compile-time code (ie code that is run when the project is compiled). This is used in other languages (eg Java) to write code generators (eg Dagger 2).

Is this possible in Swift? If so how? It is rather important for what I'm attempting to do.

Answer 1

Your question is very vague. There are multiple reasons for writing compile-time code, and it all depends on what you want to do.

If you want to write conditional code that only runs based on certain compile-time directives, use

#if build configuration
    statements
#else
    statements
#endif

If you want to generate code like a preprocessor would do in macros to reduce code duplication, then you should simply change your mindset and use swift functions and generics that will achieve the same result, only better. (Simple macros like #define xxx ... are replaced by let xxx = ... ).

Finally, if you want to generate huge amounts of code then Swift isn't the language for you, you should use a dedicated code generator or some scripting language. For example, if you're looking to generate code for a state machine, use dedicated tools for that task that take as input UML. Obviously you'll have a hard time today finding tools that output Swift code (least of which being that Swift itself is a moving target), but that'll change over time.