Merge pull request #9 from sensorario/memento

start studying memento pattern

Author: sensorario

README.md

@@ -10,6 +10,7 @@
  * [Strategy](behavioral/strategy) [:notebook:](https://en.wikipedia.org/wiki/Strategy_pattern)
  * [Command](behavioral/command) [:notebook:](https://en.wikipedia.org/wiki/Command_pattern)
  * [Template](behavioral/template) [:notebook:](https://en.wikipedia.org/wiki/Template_pattern)
+ * [Memento](behavioral/memento) [:notebook:](https://en.wikipedia.org/wiki/Memento_pattern)
 
 ## [Creational](creational)
 

behavioral/README.md

@@ -8,3 +8,4 @@ We are going to deal with behaviors instead of define structures or encapsulate
  * [Chain of Responsibility](chain) [:notebook:](https://en.wikipedia.org/wiki/Chain-of-responsibility_pattern)
  * [Command](command) [:notebook:](https://en.wikipedia.org/wiki/Command_pattern)
  * [Template](template) [:notebook:](https://en.wikipedia.org/wiki/Template_pattern)
+ * [Memento](memento) [:notebook:](https://en.wikipedia.org/wiki/Memento_pattern)

behavioral/memento/README.md

@@ -0,0 +1,48 @@
+# Behavioral » Memento
+
+## Description
+
+The memento pattern provides the ability to restore an object to of its
+previous state (undo via rollback). It is implemented with three elements:
+
+ - memento
+ - originator
+ - caretaker
+
+The `memento` is an extra-layer not yet saved. The `originator` is a type in
+charge to build/create the mementos. Finally, the `caretaker` is the type that
+stores the lists of mementos.
+
+## Implementation
+
+Every time we create new memento object, its state is taken from the originator
+element of the design pattern.
+
+```go
+func TestNewMementoTakeOriginatorState(t *testing.T) {
+	firstState := originator{}
+	firstState.state = State{Description: "Idle"}
+	mem := firstState.NewMemento()
+	if mem.state.Description != "Idle" {
+		t.Error("Expected state was not found")
+	}
+}
+
+func (o *originator) NewMemento() memento {
+	return memento{state: o.state}
+}
+```
+In the following test state is changed to "working" to ensure that new state
+will be written.
+
+```go
+func TestStoresIdleState(t *testing.T) {
+	originator := originator{state: State{"Idle"}}
+	idleMemento := originator.NewMemento()
+	originator.state.Description = "Working"
+	originator.ExtractAndStoreState(idleMemento)
+	if originator.state.Description != "Idle" {
+		t.Error("Unexpected state found")
+	}
+}
+```

behavioral/memento/memento.go

@@ -0,0 +1,40 @@
+package main
+
+import (
+	"fmt"
+)
+
+type State struct {
+	Description string
+}
+
+type memento struct {
+	state State
+}
+
+type originator struct {
+	state State
+}
+
+func (o *originator) NewMemento() memento {
+	return memento{state: o.state}
+}
+
+func (o *originator) ExtractAndStoreState(m memento) {
+	o.state = m.state
+}
+
+type careTaker struct {
+	mementoList []memento
+}
+
+func (c *careTaker) Add(m memento) {
+	c.mementoList = append(c.mementoList, m)
+}
+
+func (c *careTaker) Memento(i int) (memento, error) {
+	if i < 0 {
+		return memento{}, fmt.Errorf("Index not found")
+	}
+	return c.mementoList[i], nil
+}

behavioral/memento/memento_test.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestNewMementoTakeOriginatorState(t *testing.T) {
+	firstState := originator{}
+	firstState.state = State{Description: "Idle"}
+	mem := firstState.NewMemento()
+	if mem.state.Description != "Idle" {
+		t.Error("Expected state was not found")
+	}
+}
+
+func TestCareTakerKeepAlMementoInsideAList(t *testing.T) {
+	firstState := originator{}
+	firstState.state = State{Description: "Idle"}
+	mem := firstState.NewMemento()
+	careTaker := careTaker{}
+	lenBeforeAddedMemento := len(careTaker.mementoList)
+	careTaker.Add(mem)
+	if len(careTaker.mementoList) != lenBeforeAddedMemento+1 {
+		t.Errorf("No new elements were added on the list")
+	}
+}
+
+func TestCareTaker(t *testing.T) {
+	originator := originator{}
+	careTaker := careTaker{}
+	originator.state = State{"foo"}
+	careTaker.Add(originator.NewMemento())
+	mem, err := careTaker.Memento(0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if mem.state.Description != "foo" {
+		t.Error("Unexpected state")
+	}
+}
+
+func TestCareTakerDetectIfNonExistentIndexIsRequested(t *testing.T) {
+	originator := originator{}
+	careTaker := careTaker{}
+	originator.state = State{"foo"}
+	careTaker.Add(originator.NewMemento())
+	_, err := careTaker.Memento(0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = careTaker.Memento(-1)
+	if err == nil {
+		t.Fatal("An error is expected")
+	}
+}
+
+func TestStoresIdleState(t *testing.T) {
+	originator := originator{state: State{"Idle"}}
+	idleMemento := originator.NewMemento()
+	originator.state.Description = "Working"
+	originator.ExtractAndStoreState(idleMemento)
+	if originator.state.Description != "Idle" {
+		t.Error("Unexpected state found")
+	}
+}