go get github.com/golang/mock/gomock
go get github.com/golang/mock/mockgen

gomock_test
├── doer
│ └── doer.go
├── mocks
│ └── mock_doer.go
└── user
  ├── user.go
  └── user_test.go

package doer

type Doer interface {
  DoSomething(int, string) error
  SaySomething(string) string
}

package user

import "gomock_test/doer"

const (
	filtered   = "OK"
	unfiltered = "spam"
	Nice       = "nice"
	Bad        = "bad"
)

type User struct {
	// struct while mocking the doer interface
	Doer doer.Doer
}

// method Use using it
func (u *User) Use() error {
	return u.Doer.DoSomething(123, "Hello GoMock")
}

func (u *User) SaySomething(num int) string {
	if num == 3 {
		return u.Doer.SaySomething(unfiltered)
	}
	return u.Doer.SaySomething(filtered)
}

type Student struct{}

func (s *Student) DoSomething(_ int, _ string) error {
	panic("not implemented") // TODO: Implement
}

func (s *Student) SaySomething(kata string) string {
	if kata == filtered {
		return Nice
	}
	return Bad
}

mockgen -destination=../mocks/mock_doer.go -package=mocks gomock_test/doer Doer

go:generate mockgen -destination=../mocks/mock_doer.go -package=mocks gomock_test/doer Doer

// Code generated by MockGen. DO NOT EDIT.
// Source: github.com/timliudream/go-test/gomock_test/doer (interfaces: Doer)

// Package github.com/timliudream/go-test/gomock_test/mocks is a generated GoMock package.
package mocks

import (
	gomock "github.com/golang/mock/gomock"
	reflect "reflect"
)

// MockDoer is a mock of Doer interface.
type MockDoer struct {
	ctrl     *gomock.Controller
	recorder *MockDoerMockRecorder
}

// MockDoerMockRecorder is the mock recorder for MockDoer.
type MockDoerMockRecorder struct {
	mock *MockDoer
}

// NewMockDoer creates a new mock instance.
func NewMockDoer(ctrl *gomock.Controller) *MockDoer {
	mock := &MockDoer{ctrl: ctrl}
	mock.recorder = &MockDoerMockRecorder{mock}
	return mock
}

// EXPECT returns an object that allows the caller to indicate expected use.
func (m *MockDoer) EXPECT() *MockDoerMockRecorder {
	return m.recorder
}

// DoSomething mocks base method.
func (m *MockDoer) DoSomething(arg0 int, arg1 string) error {
	m.ctrl.T.Helper()
	ret := m.ctrl.Call(m, "DoSomething", arg0, arg1)
	ret0, _ := ret[0].(error)
	return ret0
}

// DoSomething indicates an expected call of DoSomething.
func (mr *MockDoerMockRecorder) DoSomething(arg0, arg1 interface{}) *gomock.Call {
	mr.mock.ctrl.T.Helper()
	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "DoSomething", reflect.TypeOf((*MockDoer)(nil).DoSomething), arg0, arg1)
}

// SaySomething mocks base method.
func (m *MockDoer) SaySomething(arg0 string) string {
	m.ctrl.T.Helper()
	ret := m.ctrl.Call(m, "SaySomething", arg0)
	ret0, _ := ret[0].(string)
	return ret0
}

// SaySomething indicates an expected call of SaySomething.
func (mr *MockDoerMockRecorder) SaySomething(arg0 interface{}) *gomock.Call {
	mr.mock.ctrl.T.Helper()
	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "SaySomething", reflect.TypeOf((*MockDoer)(nil).SaySomething), arg0)
}

//core of gomock
mockCtrl := gomock.NewController(t)
//used to trigger final assertions. if its ignored, mocking assertions will never fail
defer mockCtrl.Finish()
// create a new mock object, passing the controller instance as parameter
// for a newly created mock object it will accept any input and outpuite
// need to define its behavior with the method expect
mockDoer := mocks.NewMockDoer(mockCtrl)

package user

import (
	"fmt"
	"github.com/golang/mock/gomock"
	"gomock_test/mocks"
	"testing"
)

func TestUse(t *testing.T) {
	//core of gomock
	mockCtrl := gomock.NewController(t)
	//used to trigger final assertions. if its ignored, mocking assertions will never fail
	defer mockCtrl.Finish()
	// create a new mock object, passing the controller instance as parameter
	// for a newly created mock object it will accept any input and outpuite
	// need to define its behavior with the method expect
	mockDoer := mocks.NewMockDoer(mockCtrl)
	testUser := &User{Doer: mockDoer}
	//
	// Expect Do to be called once with 123 and "Hello GoMock" as parameters, and return nil from the mocked call.
	mockDoer.EXPECT().DoSomething(123, "Hello GoMock").Return(nil).Times(1)
	fmt.Println(testUser.Use())
	fmt.Println()
}

func TestUser_SaySomething(t *testing.T) {
	mockCtrl := gomock.NewController(t)
	defer mockCtrl.Finish()
	mockDoer := mocks.NewMockDoer(mockCtrl)
	user := User{
		Doer: mockDoer,
	}
	type args struct {
		num int
	}
	tests := []struct {
		name    string
		args    args
		want    string
		expect  func()
		wantErr bool
	}{
		{
			name: "Positive test case 1",
			expect: func() {
				mockDoer.EXPECT().SaySomething("spam").Return("bad")
			},
			args:    args{num: 3},
			wantErr: false,
			want:    "bad",
		},
	}
	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			tt.expect()
			if got := user.SaySomething(tt.args.num); (got != tt.want) != tt.wantErr {
				fmt.Println("gott :", got)
				t.Errorf("User.SaySomething() = %v, want %v", got, tt.want)
			}
		})
	}
}

=== RUN   TestUser_SaySomething
=== RUN   TestUser_SaySomething/Positive_test_case_1
--- PASS: TestUser_SaySomething (0.00s)
    --- PASS: TestUser_SaySomething/Positive_test_case_1 (0.00s)
PASS
ok      github.com/tokopedia/go_learning/udemy/pzn/gomock_test/user     1.100s