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A private setter restricts the mutation of a property to the library in which it is defined. In Dart, privacy is enforced at the library level (typically a single file) rather than the class level, and is denoted syntactically by prefixing an identifier with an underscore (_). To implement a private setter, a developer typically employs a private backing field combined with a public getter. This exposes the value for reading externally while preventing external assignment.

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Syntax and Implementation

Dart does not use access modifier keywords like private or protected. Instead, the presence of the underscore determines visibility.

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1. The Backing Field Pattern

The most common implementation involves defining a private variable (the backing field) and a public getter. The setter is effectively “private” because the field itself cannot be accessed outside the library. 
// library_a.dart
class Session {
  // Private backing field.
  // Only accessible within library_a.dart.
  String _token = '';

  // Public getter.
  // Accessible from any library importing library_a.dart.
  String get token => _token;

  // Internal method mutating the private field.
  void generateNewToken(String seed) {
    _token = 'hashed_$seed'; 
  }
}

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2. Explicit Private Setter

While less common, you can explicitly define a setter method with a private identifier. This allows for validation or logic execution when the value is modified internally or by other classes within the same file. 
class Configuration {
  int _timeout = 30;

  // Public getter
  int get timeout => _timeout;

  // Explicit private setter
  // Syntax: set _identifier(Type value)
  set _internalTimeout(int value) {
    if (value > 0) {
      _timeout = value;
    }
  }

  void reset() {
    // Invoking the private setter
    _internalTimeout = 60; 
  }
}

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Visibility Rules

The Dart compiler enforces the following rules regarding private setters and fields:
  1. Library Scope: A property named _variable is visible to all classes and functions defined within the same .dart file (library).
  2. External Access: Attempting to assign a value to _variable from a different library (a separate file) results in a compilation error: The setter '_variable' isn't defined for the class.

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Access Comparison

ContextRead Access (via Public Getter)Write Access (via Private Field/Setter)
Same ClassAllowedAllowed
Same Library (File)AllowedAllowed
External LibraryAllowedCompilation Error
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