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Abraham Williams
Abraham Williams

Posted on • Originally published at

A month of Flutter: post model and mock data

Originally posted on

Now that there is a basic user interface, I'm going to create some mock data to display in it. The mock data will be stored in a JSON file and will include just the basic fields needed right now: id, createdAt, username, imageUrl, and text. For imageUrl I grabbed some random images from Unsplash. text is actually empty as I haven't decided what text to use in the mocks yet.

This is a similar format to what you might expect from a JSON API. In the future there will probably be a User object and maybe an Image object that includes URLs to different sizes, etc.

    "id": "7d3d8bd1-b9a6-4e1f-8e4e-dca6f4861441",
    "imageUrl": "",
    "createdAt": "2018-12-09T15:35:54.006Z",
    "text": "",
    "username": "woodstock"
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There are a number of ways to consume JSON in Flutter, but for now I'm going with the simple dart:convert pattern with a Post class.

class Post {
  const Post({
    @required this.username,
    @required this.createdAt,
    @required this.text,
    @required this.imageUrl,

  Post.fromMap(Map<String, dynamic> data)
      : id = data['id'],
        username = data['username'],
        createdAt = DateTime.parse(data['createdAt']),
        text = data['text'],
        imageUrl = data['imageUrl'];

  final String id;
  final String username;
  final DateTime createdAt;
  final String text;
  final String imageUrl;
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I'm not a fan of optional values so I require everything. This way I don't have to check if a property exists; I just have to check that the property has a value I can work with.

Along with the default constructor I'm also defining a named constructor, fromMap, that uses an an initializer list. fromMap takes in a Map of the raw data and translates it to class variables. Note that the JSON datetime string is being converted to a Dart DateTime instance.

One downside of this code is there isn't a defined format for what the raw JSON data contains. For example, if I remove DateTime.parse the linter will not complain and there will be a failure at runtime when the createdAt string from the data tries to be set to the DateTime type property in Post. I have not yet found a way to define the key names and value types in a Map.

As the data model will change during development, it's important to make sure Post continues to work correctly. I will start with a pretty simple test. It loads the first mock data object, instantiates a Post instance, and asserts each value is available as expected. This is a plain Dart test (as opposed to a Flutter widget test) so I will have to add the Dart test package to dev_dependencies.

group('Post', () {
  test('fromMap', () async {
    final Post post = Post.fromMap(await postData());

    expect(, '7d3d8bd1-b9a6-4e1f-8e4e-dca6f4861441');
    expect(post.username, 'woodstock');
    expect(post.imageUrl, '');
    expect(post.createdAt, DateTime.parse('2018-12-09T15:35:54.006Z'));
    expect(post.text, '');
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There is a new helper function, postData, that loads the JSON file, parses it, and returns the first object.

dynamic postData() async {
  final dynamic data = await File('../assets/posts.json').readAsString();
  return json.decode(data).first;
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I've implemented the mock data in a JSON file instead of creating a List in Dart because that forces me to use the mock data in an asynchronous way. The real data will be async when pulled from Firestore so I am starting with that pattern now.

Code changes

Discussion (3)

jel111 profile image

I am really liking this series. I am going through the docs and Codelabs now hoping to build an app with this. Thanks!

abraham profile image
Abraham Williams Author

Fantastic! Glad to hear it.

vinceramces profile image
Vince Ramces Oliveros