dart-use-primary-constructors

Dart Primary Constructors & New Constructor Syntax Skill

Use this skill when helping users write, refactor, or debug code using Dart's Primary Constructors feature.

Dart Version Requirements

• Dart 3.13 and above: Primary constructors are enabled by default.
• Dart 3.12: The feature is available but experimental. Users must explicitly enable the experiment flag primary-constructors via --enable-experiment=primary-constructors or in analysis_options.yaml:

analyzer:
  enable-experiment:
    - primary-constructors

• Dart 3.11 and earlier: Primary constructors are not supported.

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1. Overview

Primary Constructors allow developers to declare a non-redirecting generative constructor as well as a set of instance variables directly in the class header. This significantly reduces boilerplate and improves code readability.

Key Benefits

• Combines field declaration, parameter declaration, and initialization into a single declaration known as a declaring parameter declaration.
• Enables safe reference to constructor parameters in non-late field initializers (Primary Initializer Scope).
• Allows empty declaration bodies to be represented concisely with a semicolon (;).
• Introduces abbreviated concise syntax for in-body constructors.

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2. Syntax Reference

2.1 Basic Class Header Syntax

To declare a primary constructor, place a parameter list immediately after the type name (and optional type parameters):

// Declares fields x and y, and a generative constructor Point(this.x, this.y)
class Point(var int x, var int y);

// Declares final fields
class PointFinal(final int x, final int y);

2.2 Declaring, Initializing, and Plain Parameters

A primary constructor parameter list distinguishes between three types of parameters:

1. Declaring Parameters: Indicated by the var or final modifier (e.g., final int x). They implicitly create a corresponding instance field in the class.
2. Initializing Parameters: Indicated by the this. or super. prefix (e.g., this.x or super.x). They initialize an existing field or a super constructor parameter, respectively.
3. Regular Parameters: Declared without modifiers (e.g., int y). They do not become fields and are only available during initialization (e.g., in field initializers or the this : initializer list in the class body).