added coffee decorator example to decorator

Author: AdelDaniel

structural_design_pattern/decorator/README.md

@@ -3,23 +3,25 @@ also Known as `wrapper`
  > - structural pattern.
  > - allows a user to add new functionality to an existing object without altering (Changing) its structure.
 
-<table> 
+<table align="center"> 
     <tr>
-        <td><img src = "assets/Decorator_UML_class_diagram.png" ></td>
+        <td><img style="background-color:#33475b" src = "assets/Decorator_UML_class_diagram.png" ></td>
         <td><img src = "assets/decorator_structure.png" ></td>
     </tr>
 </table>
 
 ## Sections
 - [Definitions](#Definitions)
-- [What problems can it solve?](#What-problems-can-it-solve?)
-- [What solution does it describe?](#What-solution-does-it-describe?)
+- [What problems can it solve](#What-problems-can-it-solve)
+- [What solution does it describe](#What-solution-does-it-describe)
 - [Examples](#Examples)
     - [Shape Decorator Example](#Shape-Decorator-Example)
     - [Windows Decorator Example](#Windows-Decorator-Example)
     - [Email Notifier Example](#Email-Notifier-Example)
+    - [Coffee Decorator Example](#Coffee-Decorator-Example)
 - [TO DO](#TO-DO)
 - [Summery](#Summery)
+- [Sources](#Sources)
 
 
 ## Definitions
@@ -44,23 +46,23 @@ also Known as `wrapper`
         - because an object's behavior can be augmented without defining an entirely new object.
 
 
-## What problems can it solve?
+## What problems can it solve
 1. Responsibilities should be added to (and removed from) an object dynamically at run-time.
 2. A flexible alternative (Changing) to subclassing for extending functionality should be provided.
     - When using subclassing, different subclasses extend a class in different ways.
     - `But an extension is bound (restricted) to the class at compile-time and can't be changed at run-time.`
 
-## What solution does it describe?
+## What solution does it describe
 1. implement the interface of the extended (decorated) object (Component) transparently by forwarding all requests to it
 perform additional functionality before/after forwarding a request.
 2. This allows working with different Decorator objects to extend the functionality of an object dynamically at run-time.
 
 > See also the UML class and sequence diagram below.
-<center><img src = "assets/Decorator_Design_Pattern_UML.jpg"></center>
+<p align="center" ><img  src = "assets/Decorator_Design_Pattern_UML.jpg"></p>
 
 ## Examples 
 
-### Shape Decorator 
+### Shape Decorator Example 
 > in this example we will decorate a shape with some color
 > without alter (Change) shape class.
 
@@ -278,6 +280,9 @@ void main(List<String> args) {
 ### Email Notifier Example
 - Example in dart: <a href="email_notifier_example/" target="_blank"> click here </a>
 
+### Coffee Decorator Example
+- Example in dart: <a href="coffee_decorator_example.dart" target="_blank"> click here </a>
+- Example Source: https://github.com/scottt2/design-patterns-in-dart/tree/master/decorator
 
 ## TO Do
 https://refactoring.guru/design-patterns/decorator

structural_design_pattern/decorator/coffee_decorator_example.dart

@@ -0,0 +1,60 @@
+abstract class Beverage {
+  double get cost;
+  String get ingredients;
+}
+
+class Ingredient {
+  double cost;
+  String name;
+
+  Ingredient(this.name, this.cost);
+
+  @override
+  String toString() => name;
+}
+
+var coffee = Ingredient("coffee", .25);
+var milk = Ingredient("milk", .5);
+var sugar = Ingredient("sugar", .1);
+
+class Coffee implements Beverage {
+  final Set<Ingredient> _ingredients = {coffee, milk, sugar};
+
+  @override
+  double get cost =>
+      _ingredients.fold(0, (total, ingredient) => total + ingredient.cost);
+
+  @override
+  String get ingredients {
+    var stringIngredients = _ingredients.fold<String>("", (total, ingredient) {
+      if (_ingredients.last.name == ingredient.name) {
+        return total + "and ${ingredient.name}";
+      }
+      return total + "${ingredient.name}, ";
+    });
+
+    return stringIngredients;
+  }
+}
+
+class StarbucksCoffeeDecorator implements Beverage {
+  final Beverage _coffee = Coffee();
+
+  @override
+  double get cost => _coffee.cost * 5;
+
+  @override
+  String get ingredients => _coffee.ingredients;
+}
+
+void main() {
+  var coffee = Coffee();
+  var starbucksCoffee = StarbucksCoffeeDecorator();
+
+  print("Coffee contains ${coffee.ingredients}. It costs \$${coffee.cost}");
+  print(
+      "Starbucks coffee contains ${starbucksCoffee.ingredients}. It costs \{starbucksCoffee.cost}");
+
+  // Coffee contains coffee, milk, and sugar. It costs $0.85
+  // Starbucks coffee contains coffee, milk, and sugar. It costs $4.25
+}