PHP 设计模式系列 -- 备忘录模式（Memento）

1、模式定义

备忘录模式又叫做快照模式（Snapshot）或 Token 模式，备忘录模式的用意是在不破坏封装性的前提下，捕获一个对象的内部状态，并在该对象之外保存这个状态，这样就可以在合适的时候将该对象恢复到原先保存的状态。

我们在编程的时候，经常需要保存对象的中间状态，当需要的时候，可以恢复到这个状态。比如，我们使用Eclipse进行编程时，假如编写失误（例如不小心误删除了几行代码），我们希望返回删除前的状态，便可以使用Ctrl+Z来进行返回。这时我们便可以使用备忘录模式来实现。

2、UML类图

备忘录模式所涉及的角色有三个：备忘录(Memento)角色、发起人(Originator)角色、负责人(Caretaker)角色。

这三个角色的职责分别是：

• 发起人：记录当前时刻的内部状态，负责定义哪些属于备份范围的状态，负责创建和恢复备忘录数据。
• 备忘录：负责存储发起人对象的内部状态，在需要的时候提供发起人需要的内部状态。
• 管理角色：对备忘录进行管理，保存和提供备忘录。

3、示例代码

Memento.php

state = $stateToSave;    }    /**     * @return mixed     */    public function getState()    {        return $this->state;    }}

Originator.php

state = $state;    }    /**     * @return Memento     */    public function getStateAsMemento()    {        // 在Memento中必须保存一份隔离的备份        $state = is_object($this->state) ? clone $this->state : $this->state;        return new Memento($state);    }    public function restoreFromMemento(Memento $memento)    {        $this->state = $memento->getState();    }}

Caretaker.php

history[$id];    }    /**     * @param Memento $state     */    public function saveToHistory(Memento $state)    {        $this->history[] = $state;    }    public function runCustomLogic()    {        $originator = new Originator();        //设置状态为State1        $originator->setState("State1");        //设置状态为State2        $originator->setState("State2");        //将State2保存到Memento        $this->saveToHistory($originator->getStateAsMemento());        //设置状态为State3        $originator->setState("State3");        //我们可以请求多个备忘录, 然后选择其中一个进行回滚                //保存State3到Memento        $this->saveToHistory($originator->getStateAsMemento());        //设置状态为State4        $originator->setState("State4");        $originator->restoreFromMemento($this->getFromHistory(1));        //从备忘录恢复后的状态: State3        return $originator->getStateAsMemento()->getState();    }}

4、测试代码

Tests/MementoTest.php

name = "Gandalf";        // connect Originator to character object        $originator->setState($character);        // work on the object        $character->name = "Gandalf the Grey";        // still change something        $character->race = "Maia";        // time to save state        $snapshot = $originator->getStateAsMemento();        // put state to log        $caretaker->saveToHistory($snapshot);        // change something        $character->name = "Sauron";        // and again        $character->race = "Ainur";        // state inside the Originator was equally changed        $this->assertAttributeEquals($character, "state", $originator);        // time to save another state        $snapshot = $originator->getStateAsMemento();        // put state to log        $caretaker->saveToHistory($snapshot);        $rollback = $caretaker->getFromHistory(0);        // return to first state        $originator->restoreFromMemento($rollback);        // use character from old state        $character = $rollback->getState();        // yes, that what we need        $this->assertEquals("Gandalf the Grey", $character->name);        // make new changes        $character->name = "Gandalf the White";        // and Originator linked to actual object again        $this->assertAttributeEquals($character, "state", $originator);    }    public function testStringState()    {        $originator = new Originator();        $originator->setState("State1");        $this->assertAttributeEquals("State1", "state", $originator);        $originator->setState("State2");        $this->assertAttributeEquals("State2", "state", $originator);        $snapshot = $originator->getStateAsMemento();        $this->assertAttributeEquals("State2", "state", $snapshot);        $originator->setState("State3");        $this->assertAttributeEquals("State3", "state", $originator);        $originator->restoreFromMemento($snapshot);        $this->assertAttributeEquals("State2", "state", $originator);    }    public function testSnapshotIsClone()    {        $originator = new Originator();        $object = new \stdClass();        $originator->setState($object);        $snapshot = $originator->getStateAsMemento();        $object->new_property = 1;        $this->assertAttributeEquals($object, "state", $originator);        $this->assertAttributeNotEquals($object, "state", $snapshot);        $originator->restoreFromMemento($snapshot);        $this->assertAttributeNotEquals($object, "state", $originator);    }    public function testCanChangeActualState()    {        $originator = new Originator();        $first_state = new \stdClass();        $originator->setState($first_state);        $snapshot = $originator->getStateAsMemento();        $second_state = $snapshot->getState();        // still actual        $first_state->first_property = 1;        // just history        $second_state->second_property = 2;        $this->assertAttributeEquals($first_state, "state", $originator);        $this->assertAttributeNotEquals($second_state, "state", $originator);        $originator->restoreFromMemento($snapshot);        // now it lost state        $first_state->first_property = 11;        // must be actual        $second_state->second_property = 22;        $this->assertAttributeEquals($second_state, "state", $originator);        $this->assertAttributeNotEquals($first_state, "state", $originator);    }    public function testStateWithDifferentObjects()    {        $originator = new Originator();        $first = new \stdClass();        $first->data = "foo";        $originator->setState($first);        $this->assertAttributeEquals($first, "state", $originator);        $first_snapshot = $originator->getStateAsMemento();        $this->assertAttributeEquals($first, "state", $first_snapshot);        $second       = new \stdClass();        $second->data = "bar";        $originator->setState($second);        $this->assertAttributeEquals($second, "state", $originator);        $originator->restoreFromMemento($first_snapshot);        $this->assertAttributeEquals($first, "state", $originator);    }    public function testCaretaker()    {        $caretaker = new Caretaker();        $memento1 = new Memento("foo");        $memento2 = new Memento("bar");        $caretaker->saveToHistory($memento1);        $caretaker->saveToHistory($memento2);        $this->assertAttributeEquals(array($memento1, $memento2), "history", $caretaker);        $this->assertEquals($memento1, $caretaker->getFromHistory(0));        $this->assertEquals($memento2, $caretaker->getFromHistory(1));    }    public function testCaretakerCustomLogic()    {        $caretaker = new Caretaker();        $result = $caretaker->runCustomLogic();        $this->assertEquals("State3", $result);    }}

5、总结

如果有需要提供回滚操作的需求，使用备忘录模式非常适合，比如数据库的事务操作，文本编辑器的 Ctrl+Z 恢复等。