A month of Flutter: Stream transforms and failing tests

Abraham Williams Dec 13 '18 ・2 min read

Originally published on bendyworks.com.

Now that I've got a Post model with mock data and a StreamBuilder for rendering, it all needs to be wired together.

The main changes I'm making are in _MyHomePageState where I'm replacing the stream of ints with a call to the new _loadPosts method.

Stream<List<Post>> _loadPosts(BuildContext context) {
  return DefaultAssetBundle.of(context)
      .loadString('assets/posts.json')
      .then<List<dynamic>>((String value) => json.decode(value))
      .asStream()
      .map(_convertToPosts);
}

_loadPosts uses DefaultAssetBundle.of to get the most appropriate AssetBundle for loading files from the assets directory.

Reading the String contents of a file is asynchronous, after which the JSON contents can be decoded. The loading and decoding will happen as a Future but I want to be working on a stream so I'll use asStream to convert the Future to a Stream.

Finally the Stream will then get transformed using map and another new method _convertToPosts.

List<Post> _convertToPosts(List<dynamic> data) {
  return data.map((dynamic item) => Post.fromMap(item)).toList();
}

Every time an event comes down the stream, map will call the convert method on the value. In this case the value is a List of items and List's map will pass each item to Post.fromMap. Note that map on a List is different from map on a Stream. map on a List is lazy so we'll force it to execute with a final toList.

The rest of the changes are going thought the code base and updating List<int> types to be List<Post> types and passing Post instances into the rendering PostItems. I'll also replace the hard coded 'Prim Birb' text with the dynamic username from the Post instances.

As I was updating the tests to use the new mock JSON data, I kept running into failures. I was seeing tests timing out or the PostsList error handling showing errors instead of content. After some digging it turns out that the code in testWidgets can't access files or assets. With CI configured I can't merge code without passing tests so come back tomorrow to see how I fix them.