UseCase specification comparison (class vs function, explicit vs implicit deps)

main.dart

void main() {
  final di = DIContainer();
  
  final quarterExpenses1 = di.getComputeQuarterExpensesUseCase().perform(100000);
  print(quarterExpenses1);
  final quarterExpenses2 = di.getComputeQuarterExpensesUseCaseFunction()(100000);
  print(quarterExpenses2);
  final quarterExpenses3 = di.getExplicitComputeQuarterExpensesUseCaseFunction()(100000);
  print(quarterExpenses3);
}

// A very simplified version of DI container
class DIContainer {
  TaxCalculator getTaxCalculator() {
    return TaxCalculator(
      fopTaxPercent: 0.05,
      socialTaxPerQuarter: 3960,
    );
  }
  
  ComputeQuarterExpensesUseCase getComputeQuarterExpensesUseCase() {
    return ComputeQuarterExpensesUseCase(getTaxCalculator());
  }
  
  ComputeQuarterExpensesUseCaseFunction getComputeQuarterExpensesUseCaseFunction() {
    return (income) => computeQuarterExpensesUseCase(getTaxCalculator(), income);
  }
  
  ComputeQuarterExpensesUseCaseFunction getExplicitComputeQuarterExpensesUseCaseFunction() {
    return (income) => computeQuarterExpensesUseCaseExplicit(getTaxCalculator().quarterTaxes, income);
  }
}

// A "dependency"
class TaxCalculator {
  const TaxCalculator({
    required this.fopTaxPercent,
    required this.socialTaxPerQuarter,
  });
  
  final double fopTaxPercent;
  final double socialTaxPerQuarter;
  
  double quarterTaxes(double income) {
    return income * fopTaxPercent + socialTaxPerQuarter;
  }

  double yearTaxes(double income) {
    return income * fopTaxPercent + socialTaxPerQuarter * 4;
  }
}

const moviesSpendingPerQuarter = 200.0;
const gamesSpendingPerQuarter = 1000.0;

// Use-case as class
class ComputeQuarterExpensesUseCase {
  ComputeQuarterExpensesUseCase(this.taxCalc);
  
  final TaxCalculator taxCalc;
  
  double perform(double income) {
    return income - taxCalc.quarterTaxes(income) - moviesSpendingPerQuarter - gamesSpendingPerQuarter;
  }
}

// Use-case as function
typedef ComputeQuarterExpensesUseCaseFunction = double Function(double income);

// same explicitness as [ComputeQuarterExpensesUseCase]
double computeQuarterExpensesUseCase(TaxCalculator taxCalc, double income) {
  return income - taxCalc.quarterTaxes(income) - moviesSpendingPerQuarter - gamesSpendingPerQuarter;
}

// more explicit
double computeQuarterExpensesUseCaseExplicit(double Function(double income) getQuarterTaxes, double income) {
  return income - getQuarterTaxes(income) - moviesSpendingPerQuarter - gamesSpendingPerQuarter;
}