Add three_state project.

Author: ilopX

patterns/state/three_state/main.dart

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+import 'pattern/switcher.dart';
+import 'states/one.dart';
+
+void main() {
+  final switcher = Switcher(
+    initState: One(),
+  );
+
+  switcher.call(); // call(1): One
+  switcher.call(); // call(2): Two
+  switcher.call(); // call(3): Three
+  switcher.call(); // call:(4) One
+}

patterns/state/three_state/pattern/state.dart

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+part of switcher;
+
+abstract class State {
+  Switcher get context => _context;
+
+  void call();
+
+  late Switcher _context;
+}

patterns/state/three_state/pattern/switcher.dart

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+library switcher;
+
+part 'state.dart';
+
+class Switcher {
+  Switcher({required State initState}) {
+    changeState(initState);
+  }
+
+  int get calls => _calls;
+
+  void call() {
+    _calls++;
+    _state.call();
+  }
+
+  void changeState(State newState) {
+    _state = newState;
+    _state._context = this;
+  }
+
+  late State _state;
+  int _calls = 0;
+}

patterns/state/three_state/states/one.dart

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+import '../pattern/switcher.dart';
+import 'two.dart';
+
+class One extends State {
+  @override
+  void call() {
+    print('call(${context.calls}): One');
+    context.changeState(Two());
+  }
+}

patterns/state/three_state/states/three.dart

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+import '../pattern/switcher.dart';
+import 'one.dart';
+
+class Three extends State {
+  @override
+  void call() {
+    print('call(${context.calls}): Three');
+    context.changeState(One());
+  }
+}

patterns/state/three_state/states/two.dart

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+import '../pattern/switcher.dart';
+import 'three.dart';
+
+class Two extends State {
+  @override
+  void call() {
+    print('call(${context.calls}): Two');
+    context.changeState(Three());
+  }
+}