How to Mock Struct Method Calls in Go Tests Without Interfaces?

Mocking Method Calls of Structs in Go Test Cases

Problem:

How to mock a method call of a struct in a Go test case without introducing interfaces in the source code?

Code Example:

type A struct {}

func (a *A) perfom(string){
...
...
..
} 

var s := A{}
func invoke(url string){
   out := s.perfom(url)
   ...
   ...
} 

Answer:

To mock a method call of a struct, one approach is to use a mock object.

Solution with Mock Object:

1. Create a Performer interface that defines the method to be mocked.
2. Create a real implementation A of the Performer interface.
3. Create a mock implementation AMock of the Performer interface.
4. Pass the mock implementation to the invoke function in the test case.

Example Code:

type Performer interface {
    perform()
}

type A struct {}

func (a *A) perform() {
    fmt.Println("real method")
}

type AMock struct {}

func (a *AMock) perform () {
    fmt.Println("mocked method")
}

func caller(p Performer) {
    p.perform()
}

In the test case, inject the mock implementation into the invoke function:

func TestCallerMock(t *testing.T) {
    mock := &AMock{}
    caller(mock)
}

In the real code, inject the real implementation into the invoke function:

func RealInvoke(url string) {
    a := &A{}
    out := a.perform(url)
}

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