Use `SizedBox` instead of `Container`, when need to add a whitespace to layout.

== Why is this an issue?

In Flutter https://api.flutter.dev/flutter/widgets/SizedBox-class.html[`SizedBox`] represents a widget of a specified size. It is used to control the dimensions of the child widget. https://api.flutter.dev/flutter/widgets/Container-class.html[`Container`] is another Flutter widget, that doesn't only provide width and height, but also applies additional constraints, decorations, transformations, etc. Thus `Container` is a more advanced and at the same time a much heavier widget. It is recommended to not use it, if the only need is to add a whitespace. `SizedBox` is a better option for such simple cases. You can even benefit from a const constructor of a `SizedBox`, if applicable in your case.

The rule reports issue on the `Container` instance creation, if:

* both `width` and `heigh` are specified and there are no other parameters except optional `key`
* `child` is specified, at least one of `width` or `heigh` is specified  and there are no other parameters except optional `key`

=== What is the potential impact?

A misuse of a `Container` widget may affect you performance and make the layout overly complicated.

== How to fix it

Replace `Container` with `SizedBox`

=== Code examples

==== Noncompliant code example

[source,dart,diff-id=1,diff-type=noncompliant]
----
List<Widget> widgets(){
  return [
    Container(width: 4, height: 5),
    Container(key: Key("MyWidget"), child: MyWidget(), height: 5)
  ];
}
----

==== Compliant solution

[source,dart,diff-id=1,diff-type=compliant]
----
List<Widget> widgets(){
  return [
    SizedBox(width: 4, height: 5),
    SizedBox(key: Key("MyWidget"), child: MyWidget(), height: 5)
  ];
}
----

== Resources

=== Documentation

* Dart Docs - https://dart.dev/tools/linter-rules/sized_box_for_whitespace[Dart Linter rule - sized_box_for_whitespace]
* Flutter API Reference - https://api.flutter.dev/flutter/widgets/Container-class.html[widgets - Container class]
* Flutter API Reference - https://api.flutter.dev/flutter/widgets/SizedBox-class.html[widgets - SizedBox class]

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Use a 'SizedBox' to add whitespace to a layout.

=== Highlighting

`Container` constructor invocation without parameters.


endif::env-github,rspecator-view[]