dart - How to Create DropdownButton with a list of JSON Data and I want it to populate my DropDownButton in Flutter

Dart - How to Create DropdownButton with a list of JSON Data and I want it to populate my DropDownButton in Flutter

To create a DropdownButton with a list of JSON data in Flutter, you first need to parse the JSON data into a list of objects that represent the items for the dropdown. Then, you can use this list to populate the DropdownButton items. Here's a step-by-step guide:

  1. Parse the JSON data into a list of objects.
  2. Create a DropdownButton widget and pass the list of objects as items.
  3. Use a DropdownMenuItem for each item in the list.

Here's an example assuming you have JSON data representing a list of countries with name and code properties:

import 'dart:convert'; import 'package:flutter/material.dart'; void main() { runApp(MyApp()); } class Country { final String name; final String code; Country(this.name, this.code); } class MyApp extends StatelessWidget { @override Widget build(BuildContext context) { return MaterialApp( home: Scaffold( appBar: AppBar( title: Text('DropdownButton with JSON Data Example'), ), body: Center( child: DropdownButtonExample(), ), ), ); } } class DropdownButtonExample extends StatefulWidget { @override _DropdownButtonExampleState createState() => _DropdownButtonExampleState(); } class _DropdownButtonExampleState extends State<DropdownButtonExample> { List<Country> countries = []; // List to hold parsed JSON data Country? _selectedCountry; // Currently selected country @override void initState() { super.initState(); // Load JSON data String jsonStr = '[{"name":"USA","code":"US"},{"name":"Canada","code":"CA"},{"name":"UK","code":"GB"}]'; List<dynamic> jsonList = jsonDecode(jsonStr); countries = jsonList.map((country) => Country(country['name'], country['code'])).toList(); } @override Widget build(BuildContext context) { return DropdownButton<Country>( hint: Text('Select Country'), // Placeholder text when no item is selected value: _selectedCountry, // Currently selected country onChanged: (Country? newValue) { setState(() { _selectedCountry = newValue; // Update selected country when user selects a new item }); }, items: countries.map((Country country) { return DropdownMenuItem<Country>( value: country, child: Text(country.name), // Display the country name ); }).toList(), ); } }

In this example:

  • We define a Country class to represent a country with name and code properties.
  • In the initState method, we parse the JSON data into a list of Country objects and populate the countries list.
  • In the build method, we create a DropdownButton with a list of DropdownMenuItem widgets. Each DropdownMenuItem represents a country from the countries list, and its child is the country name.
  • When the user selects a country, the onChanged callback is triggered, updating the _selectedCountry variable with the newly selected country.

You can replace the JSON data loading logic with your own method of fetching JSON data, such as loading it from a file, making an HTTP request, or using a package like http or dio to fetch data from an API.

Examples

  1. Query: "Flutter DropdownButton with JSON data"

    • Description: This query discusses how to populate a DropdownButton with a list of items from a JSON data source in Flutter.
    • Code:
      import 'dart:convert'; import 'package:flutter/material.dart'; String jsonData = ''' [ {"id": 1, "name": "Item 1"}, {"id": 2, "name": "Item 2"}, {"id": 3, "name": "Item 3"} ] '''; List<dynamic> items = json.decode(jsonData); String? selectedItem; @override Widget build(BuildContext context) { return DropdownButton<String>( value: selectedItem, onChanged: (newValue) { setState(() { selectedItem = newValue; }); }, items: items.map((item) { return DropdownMenuItem<String>( value: item['name'], child: Text(item['name']), ); }).toList(), ); }

  2. Query: "Flutter DropdownButton with async JSON data"

    • Description: This query explores how to populate a DropdownButton with JSON data fetched asynchronously, like from an API.
    • Code:
      import 'package:flutter/material.dart'; import 'package:http/http.dart' as http; import 'dart:convert'; Future<List<dynamic>> fetchItems() async { final response = await http.get(Uri.parse('https://example.com/api/items')); if (response.statusCode == 200) { return json.decode(response.body); } else { throw Exception('Failed to load items'); } } class MyDropdown extends StatefulWidget { @override _MyDropdownState createState() => _MyDropdownState(); } class _MyDropdownState extends State<MyDropdown> { List<dynamic>? items; String? selectedItem; @override void initState() { super.initState(); fetchItems().then((data) { setState(() { items = data; }); }); } @override Widget build(BuildContext context) { return DropdownButton<String>( value: selectedItem, onChanged: (newValue) { setState(() { selectedItem = newValue; }); }, items: items?.map((item) { return DropdownMenuItem<String>( value: item['name'], child: Text(item['name']), ); }).toList(), ); } }

  3. Query: "Flutter DropdownButton with key-value pairs from JSON"

    • Description: This query discusses creating a DropdownButton from a list of key-value pairs in JSON data and setting the value and display text.
    • Code:
      import 'dart:convert'; import 'package:flutter/material.dart'; String jsonData = ''' [ {"id": 1, "name": "Red"}, {"id": 2, "name": "Green"}, {"id": 3, "name": "Blue"} ] '''; List<dynamic> items = json.decode(jsonData); String? selectedItem; @override Widget build(BuildContext context) { return DropdownButton<String>( value: selectedItem, onChanged: (newValue) { setState(() { selectedItem = newValue; }); }, items: items.map((item) { return DropdownMenuItem<String>( value: item['id'].toString(), // Key child: Text(item['name']), // Display text ); }).toList(), ); }

  4. Query: "Flutter DropdownButton with custom data types from JSON"

    • Description: This query explores how to create a DropdownButton with custom data types from JSON, such as populating the dropdown with complex objects.
    • Code:
      class Item { final int id; final String name; Item(this.id, this.name); @override String toString() => name; // To display in dropdown } import 'dart:convert'; import 'package:flutter/material.dart'; String jsonData = ''' [ {"id": 1, "name": "Item A"}, {"id": 2, "name": "Item B"}, {"id": 3, "name": "Item C"} ] '''; List<Item> items = (json.decode(jsonData) as List) .map((data) => Item(data['id'], data['name'])) .toList(); Item? selectedItem; @override Widget build(BuildContext context) { return DropdownButton<Item>( value: selectedItem, onChanged: (Item? newValue) { setState(() { selectedItem = newValue; }); }, items: items.map((item) { return DropdownMenuItem<Item>( value: item, child: Text(item.name), ); }).toList(), ); }

  5. Query: "Flutter DropdownButton with JSON and default selection"

    • Description: This query explores how to create a DropdownButton from JSON with a default selection.
    • Code:
      import 'dart:convert'; import 'package:flutter/material.dart'; String jsonData = ''' [ {"id": 1, "name": "Option 1"}, {"id": 2, "name": "Option 2"}, {"id": 3, "name": "Option 3"} ] '''; List<dynamic> items = json.decode(jsonData); String selectedItem = items[1]['name']; // Default selection @override Widget build(BuildContext context) { return DropdownButton<String>( value: selectedItem, onChanged: (newValue) { setState(() { selectedItem = newValue!; }); }, items: items.map((item) { return DropdownMenuItem<String>( value: item['name'], child: Text(item['name']), ); }).toList(), ); }

  6. Query: "Flutter DropdownButton with JSON and custom callback"

    • Description: This query discusses how to implement a custom callback to execute additional code when the DropdownButton value changes.
    • Code:
      import 'dart:convert'; import 'package:flutter/material.dart'; String jsonData = ''' [ {"id": 1, "name": "First"}, {"id": 2, "name": "Second"}, {"id": 3, "name": "Third"} ] '''; List<dynamic> items = json.decode(jsonData); String? selectedItem; void onDropdownChanged(String? newValue) { print("Dropdown changed to: $newValue"); } @override Widget build(BuildContext context) { return DropdownButton<String>( value: selectedItem, onChanged: (newValue) { setState(() { selectedItem = newValue!; }); onDropdownChanged(newValue); // Custom callback }, items: items.map((item) { return DropdownMenuItem<String>( value: item['name'], child: Text(item['name']), ); }).toList(), ); }

  7. Query: "Flutter DropdownButton with JSON and default select text"

    • Description: This query discusses how to set a default text when nothing is selected in the DropdownButton.
    • Code:
      import 'dart:convert'; import 'package:flutter/material.dart'; String jsonData = ''' [ {"id": 1, "name": "Choice 1"}, {"id": 2, "name": "Choice 2"}, {"id": 3, "name": "Choice 3"} ] '''; List<dynamic> items = json.decode(jsonData); String? selectedItem; @override Widget build(BuildContext context) { return DropdownButton<String>( hint: Text("Select a choice"), // Default text value: selectedItem, onChanged: (newValue) { setState(() { selectedItem = newValue!; }); }, items: items.map((item) { return DropdownMenuItem<String>( value: item['name'], child: Text(item['name']), ); }).toList(), ); }

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