How to Dynamically Call Methods on an interface{} in Go, Regardless of Receiver Type?

Dynamically Calling Methods on an Interface{} Regardless of Receiver Type

In the realm of template systems in Go, reflection plays a crucial role. A challenge arises when dynamically calling methods on an interface{} with varying receiver types. While this works seamlessly with known types, it fails when the data is wrapped within an interface{}.

Problem Statement

Consider the following code:

type Test struct {
    Start string
}

func (t *Test) Finish() string {
    return t.Start + "finish"
}

func Pass(i interface{}) {
    _, ok := reflect.TypeOf(&i).MethodByName("Finish")
    if ok {
        fmt.Println(reflect.ValueOf(&i).MethodByName("Finish").Call([]reflect.Value{})[0])
    } else {
        fmt.Println("Pass() fail")
    }
}

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

    Pass(i)
    _, ok := reflect.TypeOf(&i).MethodByName("Finish")
    if ok {
        fmt.Println(reflect.ValueOf(&i).MethodByName("Finish").Call([]reflect.Value{})[0])
    } else {
        fmt.Println("main() fail")
    }
}

Observations

• When the data is a known type (main()), the dynamic method call succeeds.
• When the data is wrapped in an interface{} (Pass()), it fails, returning "Pass() fail" on the first call.

The Problem

The problem lies in accessing the address of the data when it is wrapped in an interface{}. Using &i, which usually points to a pointer, does not work in this scenario.

Solution

To address this, we need to handle all possible scenarios:

1. interface{} data as a value and method receiver as a value:

   • If the data is a value, create a pointer to it.

2. interface{} data as a value and method receiver as a pointer:

   • As above, create a pointer to the data.

3. interface{} data as a pointer and method receiver as a value:

   • If the data is a pointer, retrieve the value it points to.

4. interface{} data as a pointer and method receiver as a pointer:

   • Use the pointer directly.

Implementation

Based on this logic, we can create a generalized function to call methods dynamically:

func CallMethod(i interface{}, methodName string) interface{} {
    // Handle all scenarios
    var ptr, value, finalMethod reflect.Value
    value = reflect.ValueOf(i)
    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 and pointer
    method := value.MethodByName(methodName)
    if method.IsValid() {
        finalMethod = method
    }
    method = ptr.MethodByName(methodName)
    if method.IsValid() {
        finalMethod = method
    }

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

    // Method not found
    return ""
}

With this function, we can now call methods dynamically regardless of the receiver type and interface{} wrapper:

i := Test{Start: "start"}
j := Test{Start: "start2"}

fmt.Println(CallMethod(i, "Finish"))
// Output: startfinish
fmt.Println(CallMethod(&i, "Finish"))
// Output: startfinish
fmt.Println(CallMethod(i, "Another"))
// Output:
fmt.Println(CallMethod(&i, "Another"))
// Output: start2another
fmt.Println(CallMethod(j, "Finish"))
// Output: startfinish
fmt.Println(CallMethod(&j, "Finish"))
// Output: start2finish
fmt.Println(CallMethod(j, "Another"))
// Output:
fmt.Println(CallMethod(&j, "Another"))
// Output: start2another

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