How to conditionally add widgets to a list?

Issue

In flutter, widgets such as Row/ListView/Stack don’t handle null children. So if we want to conditionally add widgets as children I usually do the following:

Row(
  children: <Widget>[
    foo == 42 ? Text("foo") : Container(),
  ],
);

But this feels weird to add an empty container.

Another solution is a where filter :

Row(
  children: <Widget>[
    foo == 42 ? Text("foo") : null,
  ].where((t) => t != null).toList(),
);

This solves the empty container problem but we still have an ugly ternary and it is tiresome to write.

Is there any better solution?

Solution

EDIT:

Since Dart 2.2, new syntaxes supports this natively:

Column(
  children: [
    if (foo != null) Text(foo),
    Bar(),
  ],
);

This problem is currently debated on github here.

But for now, you can use dart sync* functions:

Row(
  children: toList(() sync* {
    if (foo == 42) {
      yield Text("foo");
    }
  }),
);

where toList is:

typedef Iterable<T> IterableCallback<T>();

List<T> toList<T>(IterableCallback<T> cb) {
  return List.unmodifiable(cb());
}

Not only it solves the conditional addition problem; it also allows for a “spread operator” thanks to yield*. Example:

List<Widget> foo;

Row(
  children: toList(() sync* {
    yield Text("Hello World");
    yield* foo;
  }),
);

Answered By – Rémi Rousselet

Answer Checked By – Terry (FlutterFixes Volunteer)

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