Is it possible to use polymorphism with an abstraction layer for different widgets in Flutter?

Make your AbstractWidget extend or implement Widget. However, I must agree with Rémi Rousselet. An abstract class should not know anything about its children (that's why it is an abstraction, anyway). I would do, instead:

class Foo extends StatelessWidget {  @override  Widget build(BuildContext context) {    var widgetList = new List<Widget>();    for (var item in items) {      X content = fetchContentFromAPI();      widgetList.add(abstractWidgetWith(content: content, id: item.id));    }    return Column(children: widgetList);  }  Widget abstractWidgetWith({@required int id, @required X content}) {    switch (id) {      case 1:        return Implementation1(content);      default:        return Implementation2(content);    }  }}abstract class AbstractWidget {  final X content;  AbstractWidget(this.content);}class Implementation1 extends StatelessWidget implements AbstractWidget {  final X content;  Implementation1(this.content);  @override  Widget build(BuildContext context) {    // Display content in some type of way  }}class Implementation2 extends StatelessWidget implements AbstractWidget {  final X content;  Implementation2(this.content);  @override  Widget build(BuildContext context) {    // Display content in some type of way  }}

I just want to make an addition about something you said:

It's just that the extensive switch case goes against the design patterns and principles I've been taught.

The idea here is to always look for an abstraction instead of repeating condition structures. Notice repeating emphasizing. If the condition structure is used more than once, abstraction is mostly a better option. If this is not the case, you are probably overkilling the problem by creating abstraction.

Again, notice repeating emphasizing. You tend to need an abstraction when you have lots of condition structures, but I'll end up using ONE condition in the end. In other words, you can't get rid of the condition structures, you are just going to use it less.

In the context of this question, seems like you followed all the rules. This looks like a clean code to me.