DropdownButton Widget

In this tutorial, you will learn how to use the DropdownButton in the flutter.
result
Before Start
Before starting our tutorial, I will create a page to contain our code. 
http://www.w3.org/2000/svg
import "package:flutter/material.dart";
import 'package:flutter_widgets/const/_const.dart';
​
class DropDownButtonPage extends StatefulWidget {
  @override
  _DropDownButtonState createState() => _DropDownButtonState();
}
​
class _DropDownButtonState extends State<DropDownButtonPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(PageName.DROP_DOWN_BUTTON),
      ),
      body: SingleChildScrollView(
        child: Column(
          children: <Widget>[
            //our code.
            SizedBox(height: 600)
          ],
        ),
      ),
    );
  }
}
It will show an empty page and I will put the code in the children.
Simple Use
So let's look at our first simple about the DropDownButton.
http://www.w3.org/2000/svg
 DropdownButton _normalDown() => DropdownButton<String>(
        items: [
          DropdownMenuItem(
            value: "1",
            child: Text(
              "First",
            ),
          ),
          DropdownMenuItem(
            value: "2",
            child: Text(
              "Second",
            ),
          ),
        ],
        onChanged: (value) {
          setState(() {
            _value = value;
          });
        },
        value: _value,
      );
When we click the button, it will show you in the picture as below. It is very useful to our 
Constructor
As common, Let's look at the constructor of the DropdownButton.
http://www.w3.org/2000/svg
DropdownButton({
    Key key,
    @required this.items,
    this.value,
    this.hint,
    this.disabledHint,
    @required this.onChanged,
    this.elevation = 8,
    this.style,
    this.iconSize = 24.0,
    this.isDense = false,
    this.isExpanded = false,
  }) 
So let's look at the parameters step by step.
items
We have used this parameter above, but we don't really know it. When we see its definition, we know it is a list.
http://www.w3.org/2000/svg
  final List<DropdownMenuItem<T>> items;
Then we look at the constructor of the DropdownMenuItem. 
http://www.w3.org/2000/svg
const DropdownMenuItem({
    Key key,
    this.value,
    @required this.child,
  }) 
You need to know that the T is a common class, usually, it is the type of your data. Such as String, double. The value will be this class same as the T. The child will show in the menu. It is a widget, I use a Text in the fore example. In this example, I will use a Row to show you.
http://www.w3.org/2000/svg
 DropdownButton _itemDown() => DropdownButton<String>(
        items: [
          DropdownMenuItem(
            value: "1",
            child: Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: <Widget>[
                Icon(Icons.build),
                SizedBox(width: 10),
                Text(
                  "build",
                ),
              ],
            ),
          ),
          DropdownMenuItem(
            value: "2",
            child: Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: <Widget>[
                Icon(Icons.settings),
                SizedBox(width: 10),
                Text(
                  "Setting",
                ),
              ],
            ),
          ),
        ],
        onChanged: (value) {
          setState(() {
            _value = value;
          });
        },
        value: _value,
        isExpanded: true,
      );
It will show you as follows. You should notice isExpanded == true. It will expand the menu. When you click it, it will show you the right one.
But you should care about that the _value should be initiated with a valid value. In the above code, we have two choices, 1, 2. var _value = "1";If you set it to other value, such _value ="3".It will occur such error. 
http://www.w3.org/2000/svg
I/flutter ( 4147): 'package:flutter/src/material/dropdown.dart': Failed assertion: 'items == null ||
I/flutter ( 4147): value == null || items.where((DropdownMenuItem<T> item) => item.value == value).length == 1': is not
I/flutter ( 4147): true.
hint&disabledHint
If you do not set value, it will show a widget in the default state. Let's look at the code with not setting the value parameter.
http://www.w3.org/2000/svg
  DropdownButton _hintDown() => DropdownButton<String>(
        items: [
          DropdownMenuItem<String>(
            value: "1",
            child: Text(
              "First",
            ),
          ),
          DropdownMenuItem<String>(
            value: "2",
            child: Text(
              "Second",
            ),
          ),
        ],
        onChanged: (value) {
          print("value: $value");
        },
        hint: Text(
          "Please select the number!",
          style: TextStyle(
            color: TEXT_BLACK,
          ),
        ),
      );
It will show you like this.
If we set onChange or items to null, I will show the disabledHint.
http://www.w3.org/2000/svg
 DropdownButton _hint2Down() => DropdownButton<String>(
        items: null,
        onChanged: null,
        disabledHint: Text("You can't select anything."),
      );
style
In this part, I will give an example of many parameters, which are very easy to understand. These parameters are below.
http://www.w3.org/2000/svg
    this.elevation = 8,
    this.style,
    this.iconSize = 24.0,
    this.isDense = false,
So let's look at the example.
http://www.w3.org/2000/svg
 DropdownButton _normal2Down() => DropdownButton<String>(
        items: [
          DropdownMenuItem<String>(
            value: "1",
            child: Text(
              "First",
            ),
          ),
          DropdownMenuItem<String>(
            value: "2",
            child: Text(
              "Second",
            ),
          ),
        ],
        onChanged: (value) {
          setState(() {
            _value = value;
          });
        },
        value: _value,
        elevation: 2,
        style: TextStyle(color: PURPLE, fontSize: 30),
        isDense: true,
        iconSize: 40.0,
      );
The elevation controls the shadow of the menu, we have used it so many time, so I will not tell this again in the other widgets. The style controls the Text in the menu. The isDense will reduce the height of the button. The iconSize controls the size of the  triangle-arrow. The result is below.
Conclusion
Today, we have learned the DropdownButton. It just jumps out a menu to select. It is easy to understand, but need care about the value must be in the range of values of the items.
Thanks for your reading!
The end.
Whole code in GitHub,star to support.
Before Start

Simple Use

Constructor

items

hint&disabledHint

style

Conclusion

Whole code in GitHub,star to support.

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