add new decorator pattern

Author: sensorario

README.md

@@ -11,8 +11,9 @@
 
 ## [Structural](structural)
 
- * [Composite](structural/composite) [:notebook:](http://en.wikipedia.org/wiki/Composite_pattern)
- * [Binary Tree compositions](structural/binary-tree-compositions) [:notebook:](https://en.wikipedia.org/wiki/Binary_tree)
  * [Adapter](structural/adapter) [:notebook:](https://en.wikipedia.org/wiki/Adapter_pattern)
+ * [Binary Tree compositions](structural/binary-tree-compositions) [:notebook:](https://en.wikipedia.org/wiki/Binary_tree)
  * [Bridge](structural/bridge) [:notebook:](https://en.wikipedia.org/wiki/Bridge_pattern)
- * [Proxy](structural/proxy) [:notebook:](https://it.wikipedia.org/wiki/Proxy_pattern)
+ * [Composite](structural/composite) [:notebook:](http://en.wikipedia.org/wiki/Composite_pattern)
+ * [Decorator](structural/decorator) [:notebook:](https://en.wikipedia.org/wiki/Decorator_pattern)
+ * [Proxy](structural/proxy) [:notebook:](https://en.wikipedia.org/wiki/Proxy_pattern)

structural/decorator/README.md

@@ -0,0 +1,40 @@
+# Decorator
+
+The decorator pattern provide a lot of benefits when working with legacy code. In this example I have a legacy code. For example a legacy recipe. A decorator add functionality. The functionality in this example is the method Decorate. In the real world Decorate() is represented by a method that must be decorate.
+
+```go
+type LegacyRecipe struct {
+	Decorate() (string, error)
+}
+
+func (lr *LegacyRecipe) Decorate() (string, error) {
+	return "Legacy recipe with the following ingredients:", nil
+}
+```
+
+All object needed to implement the decorator pattern should implement same interface.
+
+```go
+type Decorable interface {
+	Decorate() (string, error)
+}
+```
+
+Because of we are treating with a legacy recipe. All new ingredient must contains a `Decorable` recipe.
+
+```go
+type NewIngredient struct {
+	recipe Decorable
+}
+
+func (ni *NewIngredient) Decorate() (string, error) {
+	if ni.recipe == nil {
+		return "", errors.New("decorable recipe is needed")
+	}
+	s, err := ni.recipe.Decorate()
+	if err != nil {
+		return "", err
+	}
+	return fmt.Sprintf("%s %s,", s, "new ingredient"), nil
+}
+```

structural/decorator/decorator.go

@@ -0,0 +1,32 @@
+package decorator
+
+import (
+	"errors"
+	"fmt"
+)
+
+type LegacyRecipe struct {
+}
+
+type Decorable interface {
+	Decorate() (string, error)
+}
+
+type NewIngredient struct {
+	recipe Decorable
+}
+
+func (lr *LegacyRecipe) Decorate() (string, error) {
+	return "Legacy recipe with the following ingredients:", nil
+}
+
+func (ni *NewIngredient) Decorate() (string, error) {
+	if ni.recipe == nil {
+		return "", errors.New("decorable recipe is needed")
+	}
+	s, err := ni.recipe.Decorate()
+	if err != nil {
+		return "", err
+	}
+	return fmt.Sprintf("%s %s,", s, "new ingredient"), nil
+}

structural/decorator/decorator_test.go

@@ -0,0 +1,33 @@
+package decorator
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+)
+
+func TestNewIngredientCannotBeInstantiateWithotDecorableObject(t *testing.T) {
+	ni := &NewIngredient{}
+	niResult, err := ni.Decorate()
+	if err == nil {
+		t.Errorf(
+			"Decorator cant decorate "+
+				"without legacy object, "+
+				"not a string with '%s'",
+			niResult,
+		)
+	}
+}
+
+func TestNewIngredientMustReturnDefaultText(t *testing.T) {
+	ni := &NewIngredient{&LegacyRecipe{}}
+	niResult, _ := ni.Decorate()
+	if !strings.Contains(niResult, "new ingredient") {
+		t.Errorf(
+			"Legacy object must contains 'new ingredient', not '%s'",
+			niResult,
+		)
+	}
+
+	fmt.Println(niResult)
+}