Flutter: SizedBox Vs Container, why use one instead of the other

flutter

When I start to think about those two componentes I find myself arguing about why should I one instead of the other. Some questions that come to my mind:

  1. What are the differences between a Container and SizedBox?

  2. I understand that Container can have other parameters like padding or decoration, but if I will not use those, why should I use a SizedBox instead of a Container?

  3. There are performance differences between them?

Best Answer

Small Update: When used for whitespace, there is now even a linter warning to prefer SizedBox instead of Container. The main advantage seems to be that SizedBox can be const and won't even create a new instance during runtime.

Thanks to the magic of open source, you don't have to guess too much.

Container is basically just a convenience widget which sometimes saves you to nest 4 other widgets. If you pass width/height into the Container:

       constraints =
        (width != null || height != null)
          ? constraints?.tighten(width: width, height: height)
            ?? BoxConstraints.tightFor(width: width, height: height)
          : constraints,

Which will result in:

    if (constraints != null)
      current = ConstrainedBox(constraints: constraints, child: current);

And the ConstrainedBox in effect is pretty much the same as a SizedBox, just more flexible.

A SizedBox will do:

  @override
  RenderConstrainedBox createRenderObject(BuildContext context) {
    return RenderConstrainedBox(
      additionalConstraints: _additionalConstraints,
    );
  }

  BoxConstraints get _additionalConstraints {
    return BoxConstraints.tightFor(width: width, height: height);
  }

ie. It is effectively the same. If you only use Container for width/height there might be a very minor minor negligible performance overhead. but you most certainly will not be able to measure it.. But I would still recommend SizedBox because it's way clearer. imho.

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