adds code documentation and license

lib/src/flutter/drop_down_button.dart

@@ -21,22 +21,68 @@ class VxDropDownButton extends VxWidgetBuilder<StatefulBuilder> {
  @required this.valueHolder,
  }) : assert(valueHolder != null);
 
+ /// The List<String> used to create [DropdownMenuItem].
+ ///
+ /// _items can't be null.
  final List<String> _items;
+
+ /// The value used to define currently selected value of [VxDropDownButton].
  String valueHolder;
+
+ /// Defines the [TextStyle] for [DropdownMenuItem].
+ ///
+ /// By default the value of `_textStyle` is [TextTheme.subhead]
+ /// value of the current [ThemeData.textTheme].
  TextStyle _textStyle;
+
+ /// Defines the widget used to draw underline for [VxDropDownButton]
  Widget _underLine;
+
+ /// Defines the elevation for menu when drop down is open.
+ ///
+ /// By default, the value of `_elevation` is 8.
  int _elevation;
+
+ /// Defines the size of [VxDropDownButton] icon.
+ ///
+ /// By default, the value of `_iconSize` is 24.
  double _iconSize;
+
+ /// The [Icon] used for [VxDropDownButton].
+ ///
+ /// By default, the icon is [Icons.arrow_drop_down]
  Icon _dropDownIcon;
+
+ /// Defines the drop down button icon color when button is disabled.
+ ///
+ /// By default, the color is `Colors.grey.shade400`.
  Color _disabledIconColor;
+
+ /// Defines the drop down button icon color when button is enabled.
+ ///
+ /// By default, the color is `Colors.grey.shade700` when the theme's [ThemeData.brightness]
+ /// is [Brightness.light] and to [Colors.white70] when it is [Brightness.dark].
  Color _enabledIconColor;
+
+ /// The color for the button's Material when it has the input focus.
  Color _focusColor;
+
+ /// If the `_autoFocus` is true, [VxDropDownButton] will be selected as the initial focus when no other node
+ /// in its scope is currently focused.
  bool _autoFocus = false;
+
+ /// Set the dropdown's inner contents to horizontally fill its parent.
  bool _isExpanded = false;
+
+ /// Reduce the button's height.
+ ///
+ /// By default this button's height is the same as its menu items' heights.
  bool _isDense = false;
 
+ /// Called when user selects a value from drop down menu.
  ValueChanged<String> _onValueChange;
 
+
  VxDropDownButton textStyle(TextStyle style) => this.._textStyle = style;
 
  VxDropDownButton underLine(Widget widget) => this.._underLine = widget;

test/flutter/drop_down_button_test.dart

@@ -1,3 +1,16 @@
+/*
+ * Copyright 2020 Pawan Kumar. All rights reserved.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:velocity_x/velocity_x.dart';