Elegant way to implement template method pattern in Golang - go

Is there an elegant canonical way to implement template method pattern in Go?
In C++ this looks like this:
#include <iostream>
#include <memory>
class Runner {
public:
void Start() {
// some prepare stuff...
Run();
}
private:
virtual void Run() = 0;
};
class Logger : public Runner {
private:
virtual void Run() override {
std::cout << "Running..." << std::endl;
}
};
int main() {
std::unique_ptr<Runner> l = std::make_unique<Logger>();
l->Start();
return 0;
}
In golang i wrote something like this:
package main
import (
"fmt"
"time"
)
type Runner struct {
doRun func()
needStop bool
}
func (r *Runner) Start() {
go r.doRun()
}
func NewRunner(f func()) *Runner {
return &Runner{f, false}
}
type Logger struct {
*Runner
i int
}
func NewLogger() *Logger {
l := &Logger{}
l.doRun = l.doRunImpl
return l
}
func (l *Logger) doRunImpl() {
time.Sleep(1 * time.Second)
fmt.Println("Running")
}
func main() {
l := NewLogger()
l.Start()
fmt.Println("Hello, playground")
}
But this code fails with runtime null pointer error.
Basic idea is to mix in some functionality from derived classes (go structs) to the base class routine in a way that base class state is available from this mix-in derived routine.

The essence of the template method pattern is it allows you to inject in an implementation of a particular function or functions into the skeleton of an algorithm.
You can achieve this in Go by injecting in a function or an interface into your Runner. To achieve the basic template method pattern you don't really need your Logger struct at all:
package main
import (
"fmt"
)
type Runner struct {
run func()
}
func (r *Runner) Start() {
// some prepare stuff...
r.run()
}
func runLog() {
fmt.Println("Running")
}
func NewLogger() *Runner {
return &Runner{runLog}
}
func main() {
l := NewLogger()
l.Start()
}

Logger embeds a pointer which will be nil when you allocate the struct. That's because embedding does not put everything inside the struct, it actually creates a field (named Runner of type *Runner in your case) and the language gives you some syntactic sugar to access what's inside it. In your case it means that you can access Runner fields in two ways:
l := Logger{}
l.needStop = false
//or
l.Runner.needStop = false
To fix the error you need to allocate Runner field inside the Logger like so:
l := Logger{Runner:&Runner{}}
Or embed by value instead of pointer.

The key to have the Template Method Design Pattern work in Golang is to properly use the embedding feature and the function assignment.
Below, a code snippet which works as expected.
package main
import (
"fmt"
)
type Runner struct {
run func() // 1. this has to get assigned the actual implementation
}
func NewRunner(i func()) *Runner {
return &Runner{i}
}
func (r *Runner) Start() {
r.run()
}
type Logger struct {
Runner
}
func NewLogger() *Logger {
l := Logger{}
l.run = l.loggerRun // 2. the actual version is assigned
return &l
}
func (l *Logger) loggerRun() {
fmt.Println("Logger is running...")
}
func main() {
l := NewLogger() // 3. constructor should be used, to get the assignment working
l.Start()
}
The type Runner defines a func() attribute which is supposed to receive the actual implementation, according to the specific subtype. Start() wraps call to run(), and once invoked on the right receiver (the base one) it is be able to run the right version of run(): this happens iff in the constructor (i.e. NewLogger()) the actual version of the method run() is assigned to the attribute run of the embedded type.
And, output is:
Logger is running...
Program exited.
Here the code can be run, and modified to test any other variant of this design pattern.

Related

Testing my interface in golang with mocks. Specifically test 1 function that calls a sibling function

Let's say I have this code and I want to create a test for Foo()
The important part it Foo makes a call to Bar
package main
type MyInterface interface {
Foo() error
Bar() error
}
type MyStruct struct {
}
func NewMyStruct() MyInterface{
return &MyStruct{}
}
func (m *MyStruct) Foo() error {
// do something
m.Bar()
// do something else
return nil
}
func (m *MyStruct) Bar() error {
// do something
return nil
}
Is it possible to create a test for this where I can mock the behaviour of Bar before I run Foo? or am I doing something fundamentally wrong?
I can see that if I extracted Bar into its own service I would mock it that way but that also doesn't feel right.
Any insights or links to documentation would be great.
You should be able to achieve what you need with a couple of changes. First, let me present the code and then I'll walk you through all of the relevant changes.
main.go file
package main
type MyInterface interface {
Foo() error
Bar() error
}
type MyStruct struct {
DS MyInterface
}
// here you've to depend upon an interface and return a pointer to a struct
func NewMyStruct(ds MyInterface) *MyStruct {
return &MyStruct{
DS: ds,
}
}
func (m *MyStruct) Foo() error {
// do something
m.DS.Bar()
// do something else
return nil
}
func (m *MyStruct) Bar() error {
// do something
return nil
}
func main() {}
Here, I changed a couple of things in order to be able to successfully mock our dependencies. Let me recap them:
The MyStruct has a dependency of type MyInterface
The function NewMyStruct accepts the interface as parameter and returns a pointer to the MyStruct struct
In the Foo method, we're going to rely on the DS field of our pointer receiver type instance
Now, let's switch to the test file.
main_test.go file
package main
import (
"testing"
"github.com/stretchr/testify/assert"
"github.com/stretchr/testify/mock"
)
// 1. declare mock
type myStructMock struct {
mock.Mock
}
// 2. implement the interface
func (m *myStructMock) Foo() error {
args := m.Called()
return args.Error(0)
}
func (m *myStructMock) Bar() error {
args := m.Called()
return args.Error(0)
}
func TestFoo(t *testing.T) {
// 3. instantiate/setup mock
myStructMock := new(myStructMock)
myStructMock.On("Bar").Return(nil).Times(1)
sut := NewMyStruct(myStructMock)
err := sut.Foo()
// 4. check that all expectations were met on the mock
assert.Nil(t, err)
assert.True(t, myStructMock.AssertExpectations(t))
}
Here, I found it best to add comments within the code to give you a chronological order of what's going on. The idea is that the real system tested (e.g. the sut variable) is relying on mock instead of actual implementations. Thanks to the method Times, we make sure that the Bar method is called once.
I always used this approach when it comes to testing production code and I find it pretty flexible and amazing!
Let me know if this helps you or if you still need something else, thanks!

How to Implement Inheritance pattern in Go [duplicate]

How to implement an abstract class in Go? As Go doesn't allow us to have fields in interfaces, that would be a stateless object. So, in other words, is it possible to have some kind of default implementation for a method in Go?
Consider an example:
type Daemon interface {
start(time.Duration)
doWork()
}
func (daemon *Daemon) start(duration time.Duration) {
ticker := time.NewTicker(duration)
// this will call daemon.doWork() periodically
go func() {
for {
<- ticker.C
daemon.doWork()
}
}()
}
type ConcreteDaemonA struct { foo int }
type ConcreteDaemonB struct { bar int }
func (daemon *ConcreteDaemonA) doWork() {
daemon.foo++
fmt.Println("A: ", daemon.foo)
}
func (daemon *ConcreteDaemonB) doWork() {
daemon.bar--
fmt.Println("B: ", daemon.bar)
}
func main() {
dA := new(ConcreteDaemonA)
dB := new(ConcreteDaemonB)
start(dA, 1 * time.Second)
start(dB, 5 * time.Second)
time.Sleep(100 * time.Second)
}
This won't compile as it's not possible to use interface as a receiver.
In fact, I have already answered my question (see the answer below). However, is it an idiomatic way to implement such logic? Are there any reasons not to have a default implementation besides language's simplicity?
The other answers provide an alternative to your problem, however they proposed solution without using abstract classes/struct, and I guess if you were interested in using abstract class like solution, here is very precise solution to your problem:
Go plaground
package main
import (
"fmt"
"time"
)
type Daemon interface {
start(time.Duration)
doWork()
}
type AbstractDaemon struct {
Daemon
}
func (a *AbstractDaemon) start(duration time.Duration) {
ticker := time.NewTicker(duration)
// this will call daemon.doWork() periodically
go func() {
for {
<- ticker.C
a.doWork()
}
}()
}
type ConcreteDaemonA struct {
*AbstractDaemon
foo int
}
func newConcreteDaemonA() *ConcreteDaemonA {
a:=&AbstractDaemon{}
r:=&ConcreteDaemonA{a, 0}
a.Daemon = r
return r
}
type ConcreteDaemonB struct {
*AbstractDaemon
bar int
}
func newConcreteDaemonB() *ConcreteDaemonB {
a:=&AbstractDaemon{}
r:=&ConcreteDaemonB{a, 0}
a.Daemon = r
return r
}
func (a *ConcreteDaemonA) doWork() {
a.foo++
fmt.Println("A: ", a.foo)
}
func (b *ConcreteDaemonB) doWork() {
b.bar--
fmt.Println("B: ", b.bar)
}
func main() {
var dA Daemon = newConcreteDaemonA()
var dB Daemon = newConcreteDaemonB()
dA.start(1 * time.Second)
dB.start(5 * time.Second)
time.Sleep(100 * time.Second)
}
If this is still not obvious how to use abstract classes/multi-inheritance in go-lang here is the post with comprehensive details. Abstract Classes In Go
If you want to provide a "default" implementation (for Daemon.start()), that is not the characteristic of an interface (at least not in Go). That is a characteristic of a concrete (non-interface) type.
So Daemon in your case should be a concrete type, conveniently a struct since you want it to have fields. And the task to be done can be either a value of an interface type, or in a simple case just a function value (a simple case means it would only have one method).
With interface type
Try the complete app on the Go Playground.
type Task interface {
doWork()
}
type Daemon struct {
task Task
}
func (d *Daemon) start(t time.Duration) {
ticker := time.NewTicker(t)
// this will call task.doWork() periodically
go func() {
for {
<-ticker.C
d.task.doWork()
}
}()
}
type MyTask struct{}
func (m MyTask) doWork() {
fmt.Println("Doing my work")
}
func main() {
d := Daemon{task: MyTask{}}
d.start(time.Millisecond*300)
time.Sleep(time.Second * 2)
}
With a function value
In this simple case this one is shorter. Try it on the Go Playground.
type Daemon struct {
task func()
}
func (d *Daemon) start(t time.Duration) {
ticker := time.NewTicker(t)
// this will call task() periodically
go func() {
for {
<-ticker.C
d.task()
}
}()
}
func main() {
d := Daemon{task: func() {
fmt.Println("Doing my work")
}}
d.start(time.Millisecond * 300)
time.Sleep(time.Second * 2)
}
An easy solution is to move daemon *Daemon to the argument list (thus removing start(...) from the interface):
type Daemon interface {
// start(time.Duration)
doWork()
}
func start(daemon Daemon, duration time.Duration) { ... }
func main() {
...
start(dA, 1 * time.Second)
start(dB, 5 * time.Second)
...
}
You can implement abstract class in go.
The definition:
type abstractObject interface{
print()
}
type object struct{
a int
abstractObject
}
Now object is an abstract class, like java's.
You can inherit it and use its members:
type concreteObject struct{
*object
}
(o *concreteObject) print() {
fmt.Println(o.a)
}
func newConcreteObject(o *object) {
obj := &concreteObject{object: o}
o.abstractObject = obj // all magics are in this statement.
}
And use the object with concreteObject's methods:
o := &object{}
newConcereteObject(o)
o.print()
And cast abstract object to concrete object:
concObj := o.abstractObject.(*concreteObject)
Just like other OOP languages.
The solution by Max Malysh would work in some cases if you don't need a factory. However the solution given by Adrian Witas could cause cyclic dependencies issues.
This is the way I achieved implementing an abstract class the easy way respecting cyclic dependencies and good factory patterns.
Let us assume we have the following package structure for our component
component
base
types.go
abstract.go
impl1
impl.go
impl2
impl.go
types.go
factory.go
Define the definition of the component, in this example it will be defined here:
component/types.go
package component
type IComponent interface{
B() int
A() int
Sum() int
Average() int
}
Now let's assume we want to create an abstract class that implements Sum and Average only, but in this abstract implementation we would like to have access to use the values returned by the implemented A and B
To achieve this, we should define another interface for the abstract members of the abstract implementation
component/base/types.go
package base
type IAbstractComponentMembers {
A() int
B() int
}
And then we can proceed to implement the abstract "class"
component/base/abstract.go
package base
type AbstractComponent struct {
IAbstractComponentsMember
}
func (a *AbstractComponent) Sum() int {
return a.A() + a.B()
}
func (a *AbstractComponent) Average() int {
return a.Sum() / 2
}
And now we proceed to the implementations
component/impl1/impl.go // Asume something similar for impl2
package impl1
type ComponentImpl1 struct {
base.AbstractComponent
}
func (c *ComponentImpl1) A() int {
return 2
}
func (c *ComponentImpl1) A() int {
return 4
}
// Here is how we would build this component
func New() *ComponentImpl1 {
impl1 := &ComponentImpl1{}
abs:=&base.AbstractComponent{
IAbstractComponentsMember: impl1,
}
impl1.AbstractComponent = abs
return impl1
}
The reason we use a separate interface for this instead of using Adrian Witas example, is because if we use the same interface in this case, if we import the base package in impl* to use the abstract "class" and also we import the impl* packages in the components package, so the factory can register them, we'll find a circular reference.
So we could have a factory implementation like this
component/factory.go
package component
// Default component implementation to use
const defaultName = "impl1"
var instance *Factory
type Factory struct {
// Map of constructors for the components
ctors map[string]func() IComponent
}
func (f *factory) New() IComponent {
ret, _ := f.Create(defaultName)
return ret
}
func (f *factory) Create(name string) (IComponent, error) {
ctor, ok := f.ctors[name]
if !ok {
return nil, errors.New("component not found")
}
return ctor(), nil
}
func (f *factory) Register(name string, constructor func() IComponent) {
f.ctors[name] = constructor
}
func Factory() *Factory {
if instance == nil {
instance = &factory{ctors: map[string]func() IComponent{}}
}
return instance
}
// Here we register the implementations in the factory
func init() {
Factory().Register("impl1", func() IComponent { return impl1.New() })
Factory().Register("impl2", func() IComponent { return impl2.New() })
}
The functionality of abstract class has below requirements
1. It should not be possible to create direct instance of abstract class
2. It should provide default fields and methods.
A combination of interface and struct can be used to fulfill above two requirements. For example we can see below
package main
import "fmt"
//Abstract Interface
type iAlpha interface {
work()
common(iAlpha)
}
//Abstract Concrete Type
type alpha struct {
name string
}
func (a *alpha) common(i iAlpha) {
fmt.Println("common called")
i.work()
}
//Implementing Type
type beta struct {
alpha
}
func (b *beta) work() {
fmt.Println("work called")
fmt.Printf("Name is %s\n", b.name)
}
func main() {
a := alpha{name: "test"}
b := &beta{alpha: a}
b.common(b)
}
Output:
common called
work called
Name is test
One important point to mention here is that all default method should have iAlpha as first argument, and if default method needs to call any unimplemented method they they will call on this interface. This is same as we did in common method above - i.work().
Source: https://golangbyexample.com/go-abstract-class/

Testify Mock a function return inside a function

I would like to Mock the response of a function. But this function is located or called inside another function. Let say I have this function
// main.go
func TheFunction() int {
// Some code
val := ToMockResponse()
return val
}
func ToMockResponse() int {
return 123
}
Now on my test file
// main_test.go
func TestTheFunction(t *testing.T) {
mockInstance = new(randomMock)
mockInstance.On("ToMockResponse").Return(456)
returned := TheFunction()
assert.Equal(t, 456, returned)
}
As you can see within function TheFunction() a call to function ToMockResponse is made. Now I would like to test TheFunction but I would like to Mock the response of ToMockResponse how can I achieve this?
You should consider passing in the second function is as an parameter to the first.
You have a few options for doing that. You could simply pass it as a parameter.
func DoX(doY func()) {
doY()
}
That’s simple but doesn’t work well as the core get more complex. The better alternative is often to make the functions methods on a struct.
type X struct {
Y Y
}
type Y interface {
Do()
}
func (x *X) Do() {
x.Y.Do()
}
Then Y can be replaced with a mock.
Let's consider we have two functions, executor() and process() where executor function calls process function,
We need to write both the functions as shown below with structures and interfaces:
main.go is as shown below:
package main
import "fmt"
// A structure defined which will have a value of type processHandler(interface)
// This is helpful to pass a structure instance of processData later(see in main function)
// This is required to pass mock instance also
type processDataHandler struct{ procData processHandler }
// Interface defined for process method
type processHandler interface {
process() (int, string)
}
// structure on top of which executor method is getting called
type processData struct{}
// process method called by executor method
func (p processData) process() (int, string) {
return 23, "test2"
}
// executor method which has "processDataHandler" as argument
func (e processDataHandler) executor() (int, string) {
// Some code
//process function call
val, str_val := e.procData.process()
if val == 23 {
return 40, "success"
}
return 45, str_val
}
func main() {
procData := processData{}
dataHandle := processDataHandler{procData: procData}
val1, val2 := dataHandle.executor()
fmt.Println(val1)
fmt.Println(val2)
}
main_test.go is as shown below:
package main
import (
"testing"
"github.com/stretchr/testify/assert"
"github.com/stretchr/testify/mock"
)
//Define a mock insatnce
type processDataMock struct {
mock.Mock
}
func (m *processDataMock) process() (int, string) {
// this records that the method was called and passes in the value
// it was called with
args := m.Called()
// it then returns whatever we tell it to return
// in this case we are returing a integer type followed by a string type
return args.Int(0), args.String(1)
}
//Test function for executor method
func TestExecutor(t *testing.T) {
//define mock instance
procMock := new(processDataMock)
//Make process function to return 1 and "test" via mock
procMock.On("process").Return(1, "test")
//create a processDatahandler with mock instance
handler := processDataHandler{procData: procMock}
//call executor function which inturn mocks process via "procMock.On" above
a, b := handler.executor()
assert.Equal(t, 45, a)
assert.Equal(t, "test", b)
}

Polymorphism in Golang?

I am fairly new to Go and I have been doing OOP for a long time. Now, I understand that inheritance is done via composition but ... I'd like to send a specialization in a function expecting a generalization as such:
package main
import (
"fmt"
)
type A struct {
ONE int
}
type B struct {
A
ANOTHER int
}
func main() {
fmt.Println("Hello, playground")
a := A{1}
b := B{a, 2}
fmt.Println(a.ONE)
fmt.Println(b.ANOTHER)
takeA(&b)
}
func takeA(a *A) {
fmt.Println(a.ONE)
}
Sure, I can do takeA(&b.A) but it defeats the hierarchy I am trying to set up as some A fields/method could be shadowed in B.
Is there a way around this? Or should I create a dumb interface such as:
package main
import (
"fmt"
)
type A struct {
ONE int
}
func (a A) getOne() int{
return a.ONE
}
type B struct {
A
ANOTHER int
}
type HasOne interface {
getOne() int
}
func main() {
fmt.Println("Hello, playground")
a := A{1}
b := B{a, 2}
fmt.Println(a.ONE)
fmt.Println(b.ANOTHER)
takeA(b)
}
func takeA(a HasOne) {
fmt.Println(a.getOne())
}
There is no inheritance in Go. Composition doesn't give you dynamic dispatch (which is usually implied when 'polymorphism' and 'inheritance' are used in the same paragraph): the embedded method call always happens on the embedded field as a receiver, not on the container type it is embedded into.

How to change function reference field within method?

type FuncPtr func(int) int
func Foo(i int) { return i * i }
type Events struct {
SomeFunc FuncPtr
}
type Top struct {
events Events
}
func (self *Top) initEvents() {
// This change works within this function, but
// doesn't stick around after this method returns.
self.events.SomeFunc = Foo
}
func main() {
var t := Top{}
t.initEvents()
t.events.SomeFunc == nil // True: Change in initEvents() doesn't stick
}
How can I make the change inside initEvents() method persist? That is, I want to change the value of the function reference Top::Events::SomeFunc within the initEvents() method and for that change to stick once the initEvents() method returns.
The code you have works with some minor corrections to make it compileable.
There's no package declaration at the top
You're not specifying an integer return type for your function Foo
I've included a full example of the code below for convenience and if you want to run this yourself you can do it at: https://play.golang.org/p/Ngu8FFiGrI
package main
import(
"fmt"
)
type FuncPtr func(int) int
func Foo(i int) int {
return i*i
}
type Events struct {
SomeFunc FuncPtr
}
type Top struct {
events Events
}
func (self *Top) initEvents() {
// This change works within this function, but
// doesn't stick around after this method returns.
self.events.SomeFunc = Foo
}
func main() {
var t = Top{}
t.initEvents()
if t.events.SomeFunc == nil {
fmt.Println("SomeFunc not set")
}
fmt.Println("6^2 =",t.events.SomeFunc(6))
}

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