PHP 类继承中协变与逆变规则下的代码复用与类型安全实践

// Foo 类层次结构
class BaseFooClass {
    protected $keys = [];
    private $map = [];
    public function __construct($keyValuePairs) {
        foreach($this->keys as $key => $value) {
            $this->map[$key] = $keyValuePairs[$key] ?? null;
        }
    }
}

class ChildFooClass1 extends BaseFooClass {
    protected $keys = ['foo1_a', 'foo1_b'];
}

class ChildFooClass2 extends BaseFooClass {
    protected $keys = ['foo2_a', 'foo2_b', 'foo2_c'];
}

// ... (可能存在上百个 ChildFooClass)

// Bar 类层次结构
abstract class BaseBarClass {
    protected $classIndex;
    protected function getFooBase(int $dataIndex) : ?BaseFooClass 
    {
        // 假设 GetRemoteData 是一个全局函数，根据 $classIndex 和 $dataIndex 获取数据
        // 如果 $classIndex 为 1，则 $keyValuePairs 可能形如 ['foo1_a' => value1, 'foo1_b' => value2]
        $keyValuePairs = GetRemoteData($this->classIndex, $dataIndex);
        if (checkDataIntegrity($keyValuePairs)) {
            $class = "ChildFooClass" . $this->classIndex;
            return new $class($keyValuePairs); // 动态实例化具体的 ChildFooClass
        }
        return null;
    }
}

class ChildBarClass1 extends BaseBarClass {
    protected $classIndex=1;
    public function getFoo(int $dataIndex) : ?ChildFooClass1 
    {
        // 这一行在静态分析或某些情况下可能被视为违反协变/逆变规则
        return $this->getFooBase($dataIndex);
    }
}

class ChildBarClass2 extends BaseBarClass {
    protected $classIndex=2;
    // getFoo 的输入参数在不同的 BarClass 中可能不同
    public function getFoo($someInput) : ?ChildFooClass2
    {
        $dataIndex = $this->calculateDataIndex($someInput);
        // 这一行同样可能被视为违反协变/逆变规则
        return $this->getFooBase($dataIndex);
    }

    private function calculateDataIndex($input) { /* ... */ return 0; }
}

// Foo 类层次结构保持不变
class BaseFooClass {
    protected $keys = [];
    private $map = [];
    public function __construct($keyValuePairs) {
        foreach($this->keys as $key => $value) {
            $this->map[$key] = $keyValuePairs[$key] ?? null;
        }
    }
}

class ChildFooClass1 extends BaseFooClass {
    protected $keys = ['foo1_a', 'foo1_b'];
}

class ChildFooClass2 extends BaseFooClass {
    protected $keys = ['foo2_a', 'foo2_b', 'foo2_c'];
}

// Bar 类层次结构 - 关键修改在 BaseBarClass
abstract class BaseBarClass {
    protected $classIndex;

    // PHP 8.0+ 推荐使用 mixed
    protected function getFooBase(int $dataIndex) : mixed 
    // PHP 7.4- 可以移除返回类型声明:
    // protected function getFooBase(int $dataIndex) 
    {
        $keyValuePairs = GetRemoteData($this->classIndex, $dataIndex);
        if (checkDataIntegrity($keyValuePairs)) {
            $class = "ChildFooClass" . $this->classIndex;
            // 运行时这里会返回 ChildFooClass1, ChildFooClass2 等具体类型
            return new $class($keyValuePairs); 
        }
        return null;
    }
}

class ChildBarClass1 extends BaseBarClass {
    protected $classIndex=1;
    public function getFoo(int $dataIndex) : ?ChildFooClass1 
    {
        // 现在这一行是类型安全的，因为 getFooBase 实际返回的是 ChildFooClass1
        // 且其声明不再阻碍这种赋值
        return $this->getFooBase($dataIndex);
    }
}

class ChildBarClass2 extends BaseBarClass {
    protected $classIndex=2;
    public function getFoo($someInput) : ?ChildFooClass2
    {
        $dataIndex = $this->calculateDataIndex($someInput);
        // 同理，这里也是类型安全的
        return $this->getFooBase($dataIndex);
    }

    private function calculateDataIndex($input) { /* ... */ return 0; }
}

// 假设的辅助函数
function GetRemoteData(int $classIndex, int $dataIndex): array {
    // 模拟从远程获取数据
    if ($classIndex === 1) {
        return ['foo1_a' => "value1_a_{$dataIndex}", 'foo1_b' => "value1_b_{$dataIndex}"];
    } elseif ($classIndex === 2) {
        return ['foo2_a' => "value2_a_{$dataIndex}", 'foo2_b' => "value2_b_{$dataIndex}", 'foo2_c' => "value2_c_{$dataIndex}"];
    }
    return [];
}

function checkDataIntegrity(array $data): bool {
    // 模拟数据完整性检查
    return !empty($data);
}

// 示例用法
$bar1 = new ChildBarClass1();
$foo1 = $bar1->getFoo(10);
if ($foo1 instanceof ChildFooClass1) {
    echo "ChildBarClass1::getFoo 返回了 ChildFooClass1 实例。\n";
    // var_dump($foo1);
}

$bar2 = new ChildBarClass2();
$foo2 = $bar2->getFoo("some_input_string");
if ($foo2 instanceof ChildFooClass2) {
    echo "ChildBarClass2::getFoo 返回了 ChildFooClass2 实例。\n";
    // var_dump($foo2);
}