wrap_method.go

package main

import (
  "reflect"
)

func WrapMethod(method, target interface{}) interface{} {
  // 1. Inspect the method signature so that a
  //    replica can be returned
  methodType := reflect.TypeOf(method)

  // Collect method's input parameters, ignoring the first
  // which is the receiver.
  inTypes := []reflect.Type{}
  for i := 1; i < methodType.NumIn(); i++ {
    inTypes = append(inTypes, methodType.In(i))
  }

  // Collect method's output parameters
  outTypes := []reflect.Type{}
  for i := 0; i < methodType.NumOut(); i++ {
    outTypes = append(outTypes, methodType.Out(i))
  }

  // 2. Return a new replica method which will
  //    use the value at the target address as
  //    the receiver when called
  evalMethodType := reflect.FuncOf(inTypes, outTypes, methodType.IsVariadic())
  return reflect.MakeFunc(evalMethodType, func(in []reflect.Value) []reflect.Value {
    method := reflect.ValueOf(method)

    // Set the first input arg as the receiver at the target address
    args := append([]reflect.Value{reflect.ValueOf(target).Elem()}, in...)

    // Call the wrapped method using the variadic form
    // if the method is variadic
    if methodType.IsVariadic() {
      return method.CallSlice(args)
    } else {
      return method.Call(args)
    }
  }).Interface()
}