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Many Lints

prefer_moving_to_variable

v1.0.0WarningConfigurableCode Quality

This rule is in the pedantic preset.

This rule warns when the same property-access or invocation chain is repeated inside one block, and could be computed once into a variable. It reports at the first occurrence, which is where the variable belongs.

Why use this rule

Section titled “Why use this rule”

A chain like Theme.of(context) written three times is three lookups and three chances to edit one of them and miss the others. Naming it states once what the code is talking about, and every later line reads as an operation on that thing rather than a repeated derivation.

The win grows with the length of the chain: context.dependOnInheritedWidgetOfExactType<Foo>()!.bar.baz is noise at every call site, and a name is the only thing that makes the surrounding line legible.

See also: Effective Dart: usage

Don’t

Section titled “Don’t”
return Container(
  // LINT: 'Theme.of(context)' is repeated 2 times in this block.
  color: Theme.of(context).colorScheme.secondary,
  child: Text(
    'Text with a background color',
    style: Theme.of(context).textTheme.bodyMedium,
  ),
);

Do

Section titled “Do”
final theme = Theme.of(context);


return Container(
  color: theme.colorScheme.secondary,
  child: Text(
    'Text with a background color',
    style: theme.textTheme.bodyMedium,
  ),
);

What is not reported

Section titled “What is not reported”

A chain is only worth naming when re-evaluating it is redundant, so several shapes are left alone:

  • A call made for its effect. print(x) twice is two prints; hoisting it would change what the code does.
  • Anything that allocates or awaits — Thing(), await f(), a cascade. Re-evaluating those is usually the point, so reusing one result is observably different.
  • A chain inside a closure. The closure may run a different number of times, so lifting the chain out of it changes when the work happens.
  • An assignment target. a.b.c = 1 writes; naming the target does not apply.
  • A short property chain, by default a.b, which reads no worse than the variable would. See min_chain_length.

An invocation is always long enough to name, whatever min_chain_length says: repeating Theme.of(context) repeats the work, where repeating a field read only repeats the text.

When a chain and its own prefix both repeat the same number of times, only the longest is reported — naming it subsumes the shorter one, so two diagnostics would ask for the same edit twice.

Turning this rule off

Section titled “Turning this rule off”

This rule appears only in the pedantic preset because some codebases deliberately keep short expressions at their call sites.

many_lints.yaml
rules:
  prefer_moving_to_variable: true

To turn it off again:

many_lints.yaml
rules:
  prefer_moving_to_variable: false

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

Options

Section titled “Options”
analysis_options.yaml
many_lints:
  rules:
    prefer_moving_to_variable:
      max_extra_occurrences: 1
      min_chain_length: 3
      ignored_invocations: [of, watch]
      ignored_targets: [Theme, MediaQuery]
Option Type Default Description
max_extra_occurrences int 0 How many extra repetitions to tolerate. 0 reports the second occurrence; 1 waits for the third
allowed_duplicated_chains int 0 Deprecated compatibility alias for max_extra_occurrences
min_chain_length int 2 The shortest pure-property chain worth naming, in links. a.b is 1, a.b.c is 2. Chains containing an invocation ignore this
ignored_invocations list of strings [] Method names that exempt any chain containing them
ignored_targets list of strings [] Class, enum or variable names that exempt any chain reaching them
Section titled “Related rules”