The idiomatic way of mocking an object in Go is to make it explicit. A healthy interface should be testable by itself. So if we have something like this:
type customType struct {
value int
}
func (c customType) doFunctionOne() {
fmt.Println("Method #1")
}
func (c customType) doFunctionTwo() {
fmt.Println("Method #2")
}
func (c customType) someMethod() {
if c.value <= 0 {
c.doFunctionOne()
} else {
c.doFunctionTwo()
}
}
We have to provide a way to change the implementation of doFunctionOne and doFunctionTwo explicitly. We can generalize the someMethod behavior using interfaces:
type customType struct {
myValue int
}
func (c customType) doFunctionOne() {
fmt.Println("Method #1")
}
func (c customType) doFunctionTwo() {
fmt.Println("Method #2")
}
func (c customType) value() int {
return c.myValue
}
type Interface interface {
value() int
doFunctionOne()
doFunctionTwo()
}
func someMethod(i Interface) {
if i.value() <= 0 {
i.doFunctionOne()
} else {
i.doFunctionTwo()
}
}
type customTestingType struct {
t *testing.T
}
func (c customTestingType) doFunctionOne() {
c.t.Log("Working")
}
func (c customTestingType) doFunctionTwo() {
c.t.Error("Not working")
}
func (c customTestingType) value() int {
return 0
}
func TestInterface(t *testing.T) {
someMethod(customTestingType{t})
}
Surely there will be more ways to provide this behavior but it depends on the particular declaration of your type. As an example, you can look at httptest package. That said, if you really want to mock your type in that way (nonidiomatic), you can some unsafe monkey patching:
package main
import (
"fmt"
"reflect"
"github.com/bouk/monkey"
)
type customType struct {
myValue int
}
func (c customType) doFunctionOne() {
fmt.Println("Method #1")
}
func (c customType) doFunctionTwo() {
fmt.Println("Method #2")
}
func (c customType) someMethod() {
if c.myValue <= 0 {
c.doFunctionOne()
} else {
c.doFunctionTwo()
}
}
func main() {
c := customType{0}
monkey.PatchInstanceMethod(reflect.TypeOf(c), "doFunctionOne",
func(c customType) {
fmt.Println("Method #1, but patched")
})
monkey.PatchInstanceMethod(reflect.TypeOf(c), "doFunctionTwo",
func(c customType) {
fmt.Println("Method #2, but patched")
})
c.someMethod()
}
