Protocol Buffer runtime library and 'protoc' plugin

lib/protobuf/runtime/ExtensionRegistry.dart

+// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/**
+ * A collection of [Extension] objects, organized by the message type they
+ * extend.
+ */
+class ExtensionRegistry {
+
+  Map<String, Map<int, Extension>> _extensions;
+
+  ExtensionRegistry() : _extensions =
+      new Map<String, Map<int, Extension>>();
+
+  static ExtensionRegistry _EMPTY_REGISTRY;
+
+  static ExtensionRegistry get EMPTY_REGISTRY() {
+    if (_EMPTY_REGISTRY == null) {
+      _EMPTY_REGISTRY = new _EmptyExtensionRegistry();
+    }
+    return _EMPTY_REGISTRY;
+  }
+
+  /**
+   * Store an extension in the registry.
+   */
+  void add(Extension extension) {
+    Map<int, Extension> map = _extensions[extension.extendee];
+    if (map == null) {
+      map = new Map<int, Extension>();
+      _extensions[extension.extendee] = map;
+    }
+    map[extension.tagNumber] = extension;
+  }
+
+  /**
+   * Retrieve an extension from the registry that adds the given tag
+   * number to the given message type.
+   */
+  Extension getExtension(String messageName, int tagNumber) {
+    Map<int, Extension> map = _extensions[messageName];
+    if (map != null) {
+      return map[tagNumber];
+    }
+    return null;
+  }
+}
+
+class _EmptyExtensionRegistry extends ExtensionRegistry {
+  void add(Extension extension) {
+    throw new UnsupportedOperationException("Immutable ExtensionRegistry");
+  }
+
+  Extension getExtension(String messageName, int tagNumber) => null;
+}