How Can I Dynamically Invoke Methods on Interfaces in Go, Handling Both Value and Pointer Receivers?

Dynamically Invoking Methods on Interfaces

Challenge

Templating systems heavily rely on the reflect package in Go. When working with this package, one might encounter difficulties in dynamically invoking methods on interfaces. This issue becomes apparent when the data type is stored as an interface{}.

Conceptual Understanding

Understanding the dynamics of method invocation for interfaces is crucial. There are four scenarios to consider:

• Interface holding a value with a value receiver method
• Interface holding a pointer with a value receiver method
• Interface holding a value with a pointer receiver method
• Interface holding a pointer with a pointer receiver method

Reflection can help determine the underlying data value of an interface. With this information, one can generate the alternate data type and differentiate between value and pointer receiver methods.

Solution

To resolve the issue, it is necessary to create both a value and a pointer representation of the data:

value := reflect.ValueOf(data)
if value.Type().Kind() == reflect.Ptr {
    ptr = value
    value = ptr.Elem() // acquire value referenced by pointer
} else {
    ptr = reflect.New(reflect.TypeOf(i)) // create new pointer
    temp := ptr.Elem() // create variable to value of pointer
    temp.Set(value) // set value of variable to our passed in value
}

With both data types available, checking for the presence of a method becomes straightforward:

var finalMethod reflect.Value
method := value.MethodByName(methodName)
if method.IsValid() {
    finalMethod = method
}
// check for method on pointer
method = ptr.MethodByName(methodName)
if method.IsValid() {
    finalMethod = method
}

if (finalMethod.IsValid()) {
    return finalMethod.Call([]reflect.Value{})[0].String()
}

By embracing this approach, it becomes possible to effectively invoke any method dynamically, regardless of whether it is defined as a value or pointer receiver.

Demonstration

package main

import (
    "fmt"
    "reflect"
)

type Test struct {
    Start string
}

// value receiver
func (t Test) Finish() string {
    return t.Start + "finish"
}

// pointer receiver
func (t *Test) Another() string {
    return t.Start + "another"
}

func CallMethod(i interface{}, methodName string) interface{} {
    var ptr reflect.Value
    var value reflect.Value
    var finalMethod reflect.Value

    value = reflect.ValueOf(i)

    // if we start with a pointer, we need to get value pointed to
    // if we start with a value, we need to get a pointer to that value
    if value.Type().Kind() == reflect.Ptr {
        ptr = value
        value = ptr.Elem()
    } else {
        ptr = reflect.New(reflect.TypeOf(i))
        temp := ptr.Elem()
        temp.Set(value)
    }

    // check for method on value
    method := value.MethodByName(methodName)
    if method.IsValid() {
        finalMethod = method
    }
    // check for method on pointer
    method = ptr.MethodByName(methodName)
    if method.IsValid() {
        finalMethod = method
    }

    if (finalMethod.IsValid()) {
        return finalMethod.Call([]reflect.Value{})[0].Interface()
    }

    // return or panic, method not found of either type
    return ""
}

func main() {
    i := Test{Start: "start"}
    j := Test{Start: "start2"}

    fmt.Println(CallMethod(i, "Finish"))
    fmt.Println(CallMethod(&i, "Finish"))
    fmt.Println(CallMethod(i, "Another"))
    fmt.Println(CallMethod(&i, "Another"))
    fmt.Println(CallMethod(j, "Finish"))
    fmt.Println(CallMethod(&j, "Finish"))
    fmt.Println(CallMethod(j, "Another"))
    fmt.Println(CallMethod(&j, "Another"))
}

Output:

startfinish
startfinish
<nil>
startanother
startfinish
startfinish
<nil>
startanother

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