Template Method

Template Method is a behavioral design pattern that defines the skeleton of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.

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We need to create a tax calculator. With this, we will have our budget and it will show the specific amount after calculating the taxes.

The problem

If we do it this way, we can notice a problem: Taxes have the same minimum and maximum rate pattern, so every time we create a tax it would be repeated a lot but only with small changes.

<?php
class Budget 
{
    public float $value;
    public int $quantityItems;
}

<?php
class TaxCalculator
{
    public function calc(Budget $budget, Tax $tax) : float
    {
        return $tax->calculateTax($budget);
    }
}

<?php
interface Tax
{
    public function calculateTax(Budget $budget) : float;
}

<?php
class Ikcv implements Tax
{

    public function calculateTax(Budget $budget): float
    {
        if ($budget->value > 300 && $budget->quantityItems > 3) {
            return $budget->value * 0.04;
        }

        return $budget->value * 0.025;
    }
}

<?php
class Icpp implements Tax
{

    public function calculateTax(Budget $budget): float
    {
        if ($budget->value > 500) {
            return $budget->value * 0.03;
        }

        return $budget->value * 0.02;
    }
}

The solution

Now, using the Template Method pattern, we created an abstract class to help with the rule, making a standardization, which only needs to fill in the data.

<?php
abstract class TaxWith2Rates implements Tax
{
   public function calculateTax(Budget $budget): float
   {
       if ($this->shouldMaximumRateApply($budget)) {
           return $this->calculateMaxTax($budget);
       }

       return $this->calculateMinTax($budget);
   }

   abstract public function shouldMaximumRateApply(Budget $budget) : bool;
   abstract public function calculateMaxTax(Budget $budget) : float;
   abstract public function calculateMinTax(Budget $budget) : float;
}

<?php
class Icpp extends TaxWith2Rates
{
    public function shouldMaximumRateApply(Budget $budget): bool
    {
       return $budget->value > 500;
    }

    public function calculateMaxTax(Budget $budget): float
    {
        return $budget->value * 0.03;
    }

    public function calculateMinTax(Budget $budget): float
    {
        return $budget->value * 0.02;
    }
}

<?php
class Ikcv extends TaxWith2Rates
{
    public function shouldMaximumRateApply(Budget $budget): bool
    {
        return $budget->value > 300 && $budget->quantityItems > 3;
    }

    public function calculateMaxTax(Budget $budget): float
    {
       return $budget->value * 0.04;
    }

    public function calculateMinTax(Budget $budget): float
    {
        return $budget->value * 0.025;
    }
}

---

Installation for test

composer install

php wrong/test.php
php right/test.php