简单来说，LSP是指同一超类的对象应该能够在不破坏任何东西的情况下相互交换。

例如，如果我们有一个从Animal类派生的Cat和Dog类，那么任何使用Animal类的函数都应该能够使用Cat或Dog，并且行为正常。

简单来说，LSP是指同一超类的对象应该能够在不破坏任何东西的情况下相互交换。

例如，如果我们有一个从Animal类派生的Cat和Dog类，那么任何使用Animal类的函数都应该能够使用Cat或Dog，并且行为正常。

我想每个人都了解LSP在技术上是什么:你基本上希望能够从子类型细节中抽象出来，并安全地使用超类型。

所以利斯科夫有3条基本规则:

Signature Rule : There should be a valid implementation of every operation of the supertype in the subtype syntactically. Something a compiler will be able to check for you. There is a little rule about throwing fewer exceptions and being at least as accessible as the supertype methods. Methods Rule: The implementation of those operations is semantically sound. Weaker Preconditions : The subtype functions should take at least what the supertype took as input, if not more. Stronger Postconditions: They should produce a subset of the output the supertype methods produced. Properties Rule : This goes beyond individual function calls. Invariants : Things that are always true must remain true. Eg. a Set's size is never negative. Evolutionary Properties : Usually something to do with immutability or the kind of states the object can be in. Or maybe the object only grows and never shrinks so the subtype methods shouldn't make it.

所有这些属性都需要保留，并且额外的子类型功能不应该违反超类型属性。

如果这三件事都处理好了，那么您就从底层的东西中抽象出来了，并且您正在编写松散耦合的代码。

来源:程序开发在Java -芭芭拉利斯科夫

长话短说，让我们留下矩形矩形和正方形，实际的例子，当扩展一个父类时，你必须要么保留确切的父API，要么扩展IT。

假设您有一个基本ItemsRepository。

class ItemsRepository
{
    /**
    * @return int Returns number of deleted rows
    */
    public function delete()
    {
        // perform a delete query
        $numberOfDeletedRows = 10;

        return $numberOfDeletedRows;
    }
}

以及扩展它的子类:

class BadlyExtendedItemsRepository extends ItemsRepository
{
    /**
     * @return void Was suppose to return an INT like parent, but did not, breaks LSP
     */
    public function delete()
    {
        // perform a delete query
        $numberOfDeletedRows = 10;

        // we broke the behaviour of the parent class
        return;
    }
}

然后，您可以让客户端使用Base ItemsRepository API并依赖它。

/**
 * Class ItemsService is a client for public ItemsRepository "API" (the public delete method).
 *
 * Technically, I am able to pass into a constructor a sub-class of the ItemsRepository
 * but if the sub-class won't abide the base class API, the client will get broken.
 */
class ItemsService
{
    /**
     * @var ItemsRepository
     */
    private $itemsRepository;

    /**
     * @param ItemsRepository $itemsRepository
     */
    public function __construct(ItemsRepository $itemsRepository)
    {
        $this->itemsRepository = $itemsRepository;
    }

    /**
     * !!! Notice how this is suppose to return an int. My clients expect it based on the
     * ItemsRepository API in the constructor !!!
     *
     * @return int
     */
    public function delete()
    {
        return $this->itemsRepository->delete();
    }
} 

当用子类替换父类破坏了API的契约时，LSP就被破坏了。

class ItemsController
{
    /**
     * Valid delete action when using the base class.
     */
    public function validDeleteAction()
    {
        $itemsService = new ItemsService(new ItemsRepository());
        $numberOfDeletedItems = $itemsService->delete();

        // $numberOfDeletedItems is an INT :)
    }

    /**
     * Invalid delete action when using a subclass.
     */
    public function brokenDeleteAction()
    {
        $itemsService = new ItemsService(new BadlyExtendedItemsRepository());
        $numberOfDeletedItems = $itemsService->delete();

        // $numberOfDeletedItems is a NULL :(
    }
}

你可以在我的课程中学习更多关于编写可维护软件的知识:https://www.udemy.com/enterprise-php/

A square is a rectangle where the width equals the height. If the square sets two different sizes for the width and height it violates the square invariant. This is worked around by introducing side effects. But if the rectangle had a setSize(height, width) with precondition 0 < height and 0 < width. The derived subtype method requires height == width; a stronger precondition (and that violates lsp). This shows that though square is a rectangle it is not a valid subtype because the precondition is strengthened. The work around (in general a bad thing) cause a side effect and this weakens the post condition (which violates lsp). setWidth on the base has post condition 0 < width. The derived weakens it with height == width.

因此，可调整大小的正方形不是可调整大小的矩形。

利科夫替换原则指出，如果程序模块使用基类，则基类的引用可以被派生类替换，而不会影响程序模块的功能。

派生类型必须能够完全替代它们的基类型。

示例- java中的协变返回类型。