[New rule] Avoid redundant .of(context) calls inside build method

[enseitankad0]

Calling Theme.of(context) is an expensive operation. If possible this value should be cached.

I've heard about this optimization in Geekle 2022 Junior Track
"The things you can do with .of(context)"

• Warns about a potential error (problem)
• Suggests an alternate way of doing something (suggestion)
• Other (please specify:)

✅ GOOD

 @override
  Widget build(BuildContext context) {
    final theme = Theme.of(context);
    
    return Container(
      color: theme.disabledColor,
      child: Card(
        color: theme.primaryColor,
      ),
    );
  }

❌ BAD

 @override
  Widget build(BuildContext context) {
    return Container(
      color: Theme.of(context).disabledColor,
      child: Card(
        color: Theme.of(context).primaryColor,
      ),
    );
  }

[ADDITIONAL] I think this rule can be also used not only for .of call but should also suggest a developer to cache property instead of getting it every time when needed.

✅ GOOD

@override
  Widget build(BuildContext context) {
    final nameLength = user.userName.length;
    return Column(
      color: nameLength > 10 : Color.red : Color.blue,
      child: SizedBox(
        width : nameLength > 10 : 20.0 : 30.0
        child: Text('User')
      )
    );
  }

❌ BAD

 @override
  Widget build(BuildContext context) {
    return Column(
      color: user.userName.length > 10 : Color.red : Color.blue,
      child: SizedBox(
        width : user.userName.length > 10 : 20.0 : 30.0
        child: Text('User')
      )
    );
  }

[incendial]

@enseitankad0 have you tried https://dartcodemetrics.dev/docs/rules/common/prefer-moving-to-variable? Looks like it does exactly what you're asking for.

[enseitankad0]

Yes. Sorry but I wasn't aware about this rule existence. Thank you so much!

[incendial]

You're welcome!