


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:
-
interface{} data as a value and method receiver as a value:
- If the data is a value, create a pointer to it.
-
interface{} data as a value and method receiver as a pointer:
- As above, create a pointer to the data.
-
interface{} data as a pointer and method receiver as a value:
- If the data is a pointer, retrieve the value it points to.
-
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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