prefer_typedefs_for_callbacks

v1.0.0WarningConfigurableType Annotations

This rule flags an inline function type with two or more parameters, written where a typedef would name it.

void Function(String, int, bool) tells the reader the shape and nothing else — what the String is, what the bool means, and whether two such parameters are the same concept. A typedef gives the signature a name that can be reused, documented and searched for.

This rule is in the pedantic preset. min_parameters is the threshold at which reporting starts: at the default 2, a two-parameter inline type is already reported, while void Function() and void Function(String) are not. That is why Flutter ships VoidCallback and ValueChanged<T> for those shapes and stops there.

Don’t

An event callback carried through a signature, spelled out inline at every hop:

void listen(void Function(String, int) onEvent) {}

class EventBus {
  EventBus(this.onEvent);

  final void Function(String, int) onEvent;
}

Do

Name it once, then use the name:

typedef EventHandler = void Function(String name, int code);

void listen(EventHandler onEvent) {}

class EventBus {
  EventBus(this.onEvent);

  final EventHandler onEvent;
}

One parameter or none is never reported

At the default min_parameters: 2, these are below the threshold and pass as written:

void onTap(void Function() callback) {}

void onChanged(void Function(String value) callback) {}

The typedef’s own body is exempt

Declaring the typedef is the fix, so the function type inside it is never reported however many parameters it has:

typedef ProgressReport = void Function(int sent, int total, bool done);

Reporting single-parameter types too

With min_parameters: 1, a one-parameter inline type is reported as well:

rules:
  prefer_typedefs_for_callbacks:
    min_parameters: 1

// Don't
void onChanged(void Function(String value) callback) {}

// Do
typedef TextChanged = void Function(String value);

void onChanged(TextChanged callback) {}

Enabling this rule

This rule is in the pedantic preset, so it is enabled by preset: pedantic or by name:

rules:
  prefer_typedefs_for_callbacks:
    enabled: true

many_lints:
  rules:
    prefer_typedefs_for_callbacks:
      min_parameters: 2

rules:
  prefer_typedefs_for_callbacks:
    min_parameters: 2

Option	Type	Default	Description
`min_parameters`	int	`2`	Fewest parameters an inline function type must have before it is reported. At `2`, a two-parameter type is reported

Turning this rule off

To disable this rule:

rules:
  prefer_typedefs_for_callbacks: false

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

prefer_explicit_function_type — Prefer explicit function type annotations over the bare ‘Function’ type.
prefer_explicit_parameter_names — Name the parameters of a function type.
prefer_void_callback — Use ‘VoidCallback’ instead of ‘void Function()’.
prefer_async_callback — Use ‘AsyncCallback’ instead of ‘Future<void> Function()’.