"Use key in widget constructors" warning each time I create a new widget

Question

I've started getting a warning every time I create a new class.

Use key in widget constructors.

Prefer const with constant constructor.

and so on. Where these errors are coming from and how do I get rid of it?

Assentor

In VS Code you can do Ctrl+. to 'quick fix' the error and add Key to the class constructor in many cases. That said, i disabled the lint because most of my components are not intended to be re-used and having a key argument is pointless. You can always add one later. – Disturbing

@Disturbing Thanks but it wasn't the question about. Even in Android Studio, you can do control + enter to add that code. – Assentor

Answer 1

Solution 1:

If you don't want to ignore the warning, just do a super.key

FooPage({super.key});

Solution 2:

Open analysis_options.yaml file which is located at the root of your project:

include: package:flutter_lints/flutter.yaml

# Add these lines
linter:
  rules:
    use_key_in_widget_constructors: false

Solution 3:

To disable the rule for a specific file, put this line anywhere in your code.

// ignore_for_file: prefer_const_constructors

Solution 4:

As @Sigiria suggested in the comments, you can run:

dart fix --apply

Solution 5:

Recently Flutter team created a new package flutter_lints and it's added to your analysis_options.yaml file by default for newly created project.

You can simply remove the following line from your analysis_options.yaml file:

include: package:flutter_lints/flutter.yaml

However, I would suggest you to at least have some rules in it, for example, you can add these rules (from pedantic) and remove the one you don't want.

linter:
  rules:
    - always_declare_return_types
    - always_require_non_null_named_parameters
    - annotate_overrides
    - avoid_init_to_null
    - avoid_null_checks_in_equality_operators
    - avoid_relative_lib_imports
    - avoid_return_types_on_setters
    - avoid_shadowing_type_parameters
    - avoid_single_cascade_in_expression_statements
    - avoid_types_as_parameter_names
    - await_only_futures
    - camel_case_extensions
    - curly_braces_in_flow_control_structures
    - empty_catches
    - empty_constructor_bodies
    - library_names
    - library_prefixes
    - no_duplicate_case_values
    - null_closures
    - omit_local_variable_types
    - prefer_adjacent_string_concatenation
    - prefer_collection_literals
    - prefer_conditional_assignment
    - prefer_contains
    - prefer_equal_for_default_values
    - prefer_final_fields
    - prefer_for_elements_to_map_fromIterable
    - prefer_generic_function_type_aliases
    - prefer_if_null_operators
    - prefer_inlined_adds
    - prefer_is_empty
    - prefer_is_not_empty
    - prefer_iterable_whereType
    - prefer_single_quotes
    - prefer_spread_collections
    - recursive_getters
    - slash_for_doc_comments
    - sort_child_properties_last
    - type_init_formals
    - unawaited_futures
    - unnecessary_brace_in_string_interps
    - unnecessary_const
    - unnecessary_getters_setters
    - unnecessary_new
    - unnecessary_null_in_if_null_operators
    - unnecessary_this
    - unrelated_type_equality_checks
    - unsafe_html
    - use_full_hex_values_for_flutter_colors
    - use_function_type_syntax_for_parameters
    - use_rethrow_when_possible
    - valid_regexps

Assentor

Is there any option to solve the warning automatically? – Tutu

There's no automatic way of doing it. I recommend you use solution 1. – Assentor

It is. You can use: "dart fix --apply". This worked for me – Tutu

you have to be careful with that dart command. That generated 20 issues for me – Underpinning

Also I don't understand many of the changes it did like changing forEach to for – Underpinning

Answer 2

add this in your code right before the @override line

const FooPage({Key? key}) : super(key: key);

always add const before the widget command like EdgeInsets, spacer, SizedBox, BorderRadius etc

Carlow

Answer 3

You can use prefer_const_constructors : false in analysis_options.yaml

Donner

What is the difference between analyzer: errors: and linter.rules – Unparliamentary