Merge pull request #69 from ilopX/add-conceptual-proxy-pattern

Add conceptual proxy pattern

Author: ilopX

CHANGELOG.md

@@ -1,3 +1,6 @@
+## 0.31.0
+- Add conceptual proxy pattern.
+
 ## 0.30.0
 - Add strategy pattern: View Strategy.
 

README.md

@@ -28,7 +28,7 @@ It contains **Dart** examples for all classic **GoF** design patterns.
     - [x] **Decorator** - [[Data Source Decoder](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/decorator/data_source_decoder)]
     - [ ] **Facade**
     - [x] **Flyweight** - [[Conceptual](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/flyweight/conceptual)]
-    - [ ] **Proxy**
+    - [x] **Proxy** - [[Conceptual](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/proxy/conceptual)]
 
 ## Requirements
 The examples were written in **Dart 2.17**.

patterns/proxy/conceptual/README.md

@@ -0,0 +1,29 @@
+# Proxy Pattern
+Proxy is a structural design pattern that lets you provide a substitute or placeholder for another 
+object. A proxy controls access to the original object, allowing you to perform something either 
+before or after the request gets through to the original object.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/proxy).
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/175926828-d4fed7c6-ea82-4717-a24b-8ad2b23910ba.png)
+
+### Client code:
+```dart
+void main() async {
+  final subject = Proxy();
+  print(subject.request()); // print "Proxy data"
+
+  print('Wait for 2 seconds...');
+  await Future.delayed(Duration(seconds: 2));
+
+  print(subject.request()); // print "Real data"
+}
+```
+
+### Output:
+```
+Proxy data.
+Wait 2 seconds...
+Real data.
+```

patterns/proxy/conceptual/main.dart

@@ -0,0 +1,16 @@
+import 'pattern/proxy.dart';
+import 'pattern/subject.dart';
+
+void main() async {
+  final subject = Proxy();
+  client(subject); // print "Proxy data"
+
+  print('Wait 2 seconds...');
+  await Future.delayed(Duration(seconds: 2));
+
+  client(subject); // print "Real data"
+}
+
+void client(Subject subject) {
+  print(subject.request());
+}

patterns/proxy/conceptual/pattern/proxy.dart

@@ -0,0 +1,24 @@
+import 'subject.dart';
+import 'real_subject.dart';
+
+class Proxy implements Subject {
+  @override
+  String request() {
+    if (isSubjectLoaded) {
+      return _subject!.request();
+    }
+
+    _load();
+    return 'Proxy data.';
+  }
+
+  bool get isSubjectLoaded => _subject != null;
+
+  void _load() async {
+    Future.delayed(Duration(seconds: 1), () {
+      _subject = RealSubject();
+    });
+  }
+
+  Subject? _subject;
+}

patterns/proxy/conceptual/pattern/real_subject.dart

@@ -0,0 +1,8 @@
+import 'subject.dart';
+
+class RealSubject implements Subject {
+  @override
+  String request() {
+    return 'Real data.';
+  }
+}

patterns/proxy/conceptual/pattern/subject.dart

@@ -0,0 +1,3 @@
+abstract class Subject {
+  String request();
+}

pubspec.yaml

@@ -1,6 +1,6 @@
 name: design_patterns_dart
 description: Dart examples for all classic GoF design patterns.
-version: 0.30.0
+version: 0.31.0
 homepage: https://refactoring.guru/design-patterns
 repository: https://github.com/RefactoringGuru/design-patterns-dart
 issue_tracker: https://github.com/RefactoringGuru/design-patterns-dart/issue