Impl conceptual proxy pattern.

Author: ilopX

patterns/proxy/conceptual/README.md

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+# 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/8049534/175924643-8d4f9ee4-80fc-4377-864c-d115a94a720c.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

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+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

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+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

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

patterns/proxy/conceptual/pattern/subject.dart

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+abstract class Subject {
+  String request();
+}