prefer_const_border_radius

v0.4.0 Warning Fix Widget Replacement

Flags usages of BorderRadius.circular() which should be replaced with BorderRadius.all(Radius.circular(...)). The BorderRadius.circular() factory delegates to BorderRadius.all(Radius.circular()) internally, but it cannot be made const because it is a factory constructor.

Why use this rule

BorderRadius.circular(8) is a factory, and a factory can never be const. It forwards straight to BorderRadius.all(Radius.circular(8)), which can. Writing the second form lets the whole surrounding expression be constant, so it is allocated once at compile time instead of on every build.

Don’t

// A rounded card decoration rebuilt on every frame.
Container(
  decoration: BoxDecoration(
    color: const Color(0xFFEEEEEE),
    borderRadius: BorderRadius.circular(8), // LINT
  ),
  child: const Text('Hello'),
);

Do

Container(
  decoration: const BoxDecoration(
    color: Color(0xFFEEEEEE),
    borderRadius: BorderRadius.all(Radius.circular(8)),
  ),
  child: const Text('Hello'),
);

Examples

A shared radius constant

Once the expression is const-capable, it can be hoisted to a top-level constant and reused:

// Don't — a new object per call site, per build
ClipRRect(
  borderRadius: BorderRadius.circular(12),
  child: const Text('Clipped'),
);

// Do
const kCardRadius = BorderRadius.all(Radius.circular(12));

ClipRRect(borderRadius: kCardRadius, child: const Text('Clipped'));

Any argument shape is reported

The radius does not have to be a literal — the rule reports the factory call itself:

// Don't
final radius = BorderRadius.circular(spacing * 2);

// Do
final radius = BorderRadius.all(Radius.circular(spacing * 2));

Here the value is not constant, so const is not available — but the explicit form is still what the rule asks for, and it becomes const the moment spacing does.

Known limitations

The fix does not add const for you. It rewrites BorderRadius.circular(r) into BorderRadius.all(Radius.circular(r)) and stops there. Adding the keyword is the SDK’s prefer_const_constructors job — turn that on to get the second half of the win.

Only BorderRadius.circular is matched. BorderRadius.horizontal, BorderRadius.vertical and BorderRadius.only are factories too, but they take Radius arguments rather than doubles, so there is no mechanical rewrite to offer.

Configuration

This rule is in the opinionated preset, so it is on with preset: opinionated, or by name:

rules:
  prefer_const_border_radius: true

To turn it off again:

rules:
  prefer_const_border_radius: false

To keep the rule on but skip certain paths, use per-rule exclude.

avoid_border_all — Use Border.fromBorderSide instead of Border.all for const support.
avoid_expanded_as_spacer — Use Spacer instead of Expanded with an empty child.
avoid_incorrect_image_opacity — Use Image’s opacity parameter instead of wrapping in Opacity.
avoid_wrapping_in_padding — Avoid wrapping widgets that support padding in a Padding widget.