Dart Mixin Class

mixin class Diagnostic {
  int errorCount = 0;
  
  void recordError() {
    errorCount++;
  }
}

// 1. Instantiation (acting as a class)
Diagnostic diagnosticTool = Diagnostic();

// 2. Extension (acting as a superclass)
class AdvancedDiagnostic extends Diagnostic {
  void reset() => errorCount = 0;
}

// 3. Mixing in (acting as a mixin)
class NetworkService with Diagnostic {
  void fetch() {
    // Can access mixin class members directly
    recordError(); 
  }
}

// INVALID: Cannot declare a constructor in a mixin class
mixin class Diagnostic {
  Diagnostic(this.errorCount); 
}

class BaseSystem {}

// INVALID: A mixin class cannot have an extends clause
mixin class Diagnostic extends BaseSystem {} 

// INVALID: The 'on' clause is exclusive to pure mixins
mixin class Diagnostic on BaseSystem {}