Challenge 3 — Solution

Task: Using the Strategy pattern's ShoppingCart from Chapter 2, write a
test that uses createMock() to create a fake DiscountStrategy whose
apply() method is programmed (via willReturn) to always return 50,
then asserts that ShoppingCart's checkout() correctly returns 50
regardless of the price passed in. Explain in a comment why this test
never touches PercentageDiscount or FixedAmountDiscount at all.

<?php
use PHPUnit\Framework\TestCase;

class ShoppingCartTest extends TestCase {
    public function testCheckoutUsesDiscountStrategyResult() {
        $mockDiscount = $this->createMock(DiscountStrategy::class);
        $mockDiscount->method('apply')->willReturn(50.0);

        $cart = new ShoppingCart($mockDiscount);

        $this->assertEquals(50.0, $cart->checkout(999.0));
        $this->assertEquals(50.0, $cart->checkout(1.0));
    }
}

// Why this test never touches PercentageDiscount or FixedAmountDiscount
// at all:
// ShoppingCart's own constructor only requires an object implementing
// the DiscountStrategy interface - it has no idea, and no way to know,
// whether it received a real PercentageDiscount, a real
// FixedAmountDiscount, or this test's own mock object. createMock()
// generates a fake object that satisfies the DiscountStrategy
// interface's type requirement without needing either real
// implementation to exist or be instantiated at all. This test is
// therefore verifying ONE specific thing in complete isolation:
// "does ShoppingCart->checkout() correctly return whatever its
// injected DiscountStrategy's apply() method returns" - a question
// that has nothing to do with how any particular discount is actually
// calculated. Testing PercentageDiscount's own math (10% off, etc.)
// would be a separate, dedicated test entirely.
?>


Expected test run output:
PHPUnit 10.x

.                                                                    1 / 1 (100%)

OK (1 test, 2 assertions)

Notes:
- willReturn(50.0) programs the mock to always return exactly 50.0 from
  apply(), completely regardless of what value is actually passed into
  it - which is precisely why calling checkout() with two wildly
  different prices (999.0 and 1.0) both still return 50.0 in this test.
- This directly demonstrates the chapter's own central point: code
  written against an interface (DiscountStrategy) is far easier to
  mock than code tied directly to a concrete class, since the mock
  only needs to satisfy the interface's method signature, not
  replicate any real discount calculation logic.
- A separate, dedicated test for PercentageDiscount's own apply()
  method (verifying the actual percentage math) would still be needed
  elsewhere - this test's own job is narrowly scoped to ShoppingCart's
  own behavior alone.