Where can I learn more about annotations? I'm read...

# getting-started

[Mike R]

Where can I learn more about annotations? I'm reading through the docs on the Kotlin website but not finding much that's useful to me. Here's what I'm trying to do: I want to annotate functions that belong to a class. Those annotated functions when called should, instead of being executed right away, be added to a BlockingQueue that belongs to the containing class.

So far I've figured out that I can make a annotation class Queueable with a Target of AnnotationTarget.FUNCTION.

I'm not finding any documentation anywhere on how I can accomplish the rest of what I'd like to do though...

In Python, for example, I can annotate a function and access it's arguments and other properties about it since the annotation wraps the function. Am I thinking about annotations incorrectly in Kotlin?

[Amirul Zin]

Kotlin use annotations just like Java, that is as a metadata.

Thus, there are some things you need to be aware of if you don’t want to resort into creating your own annotation processor:

1. If you need the annotation at runtime, you need to retain the annotation class under @Retention(RetentionPolicy.RUNTIME)

2. You can then access this via reflection.

I attached a simple snippet below in Java. How you do it in Kotlin and finally achieve what you want will be left as an exercise 😄

Untitled

[karelpeeters]

What you do with annotations in Python you'd just do with normal code in Kotlin:

class Test {
    val queue = ArrayDeque<() -> Unit>()
    private fun wrap(block: () -> Unit) {
        queue.add(block)
    }

    fun foo(arg: String) = wrap {
        println("foo $arg")
    }
}

Because you seem to be used to Python I'll note that the = in foo is just an expression body, you don't actually assign foo to something.