Dart - How to Check if an optional stream isEmpty and return a widget based on that

Question

What's the best way to set up a ternary expression for an optional list stream (Stream<List<SubFoo>>?)?

If there are items in the list, I want to display them in a StreamBuilder, but if the list is empty, or the stream property is null, I want to return a different view.

It's optional because my app works as follows:

• Check if there's a Foo, if so, subscribe to the stream of SubFoo for that Foo
• If no Foo, wait until the user manually creates one, then subscribe to the stream of SubFoo

My widget tree should return one of three views, depending on the data there:

Has Foo?	Has subFoo?	Widget to be returned
No	No	NoFooView()
Yes	Yes, but the stream is empty	NoSubFooView()
Yes	Yes, with active elements	StreamBuilderView()

Here's how I currently have it set, but even when there are items in my subFooStream (the third case in the table), the app still shows NoSubFooView():

return viewModel.foo == null ? NoFooView() : viewModel.subFooStream?.isEmpty != true ? NoSubFooView() : StreamBuilder(...);

Answer 1

I didn't do it as a ternary expression. I wrapped the second if/else condition within my stream builder. If there is a snapshot with data, and the length is greater than 0, it will return the StreamBuilderView(), and if not, it will return the NoSubFooView().

viewmodel.Foo == null ? NoFooView() : 
StreamBuilder<List<SubFooViewModel>>(
                stream: subFooStream,
                builder: ((context, snapshot) {
                  if (snapshot.data == null ||
                      snapshot.hasData == false ||
                      snapshot.data?.length == 0) {
                    return NoSubFooView(vm: vm);
                  } else {
                    return StreamBuilderView(...);
                  }
                }),
              ),