74 Function interception

FunctionInterceptor wraps the declared greet call. around calls proceed() once, so the original function still supplies the greeting.

import std.annotation.FunctionInterceptor
import std.annotation.SourceRetention

annotation Trace implements FunctionInterceptor, SourceRetention {
  R around<R>(FunctionInvocation<R> invocation) {
    printLine("enter")
    R result = invocation.proceed()
    printLine("leave")
    return result
  }
}

@Trace()
String greet(String name) { return "Hello, " + name }

main() { printLine(greet(name: "Norm")) }

Expected output:

enter
leave
Hello, Norm

Try it: Remove proceed() and inspect how the required return value must be supplied.

Precise rules: Reference.