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
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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：
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- 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
<?phpnamespace DesignPatterns\Behavioral\Memento;class Memento{ /* @var mixed */ private $state; /** * @param mixed $stateToSave */ public function __construct($stateToSave) { $this->state = $stateToSave; } /** * @return mixed */ public function getState() { return $this->state; }}Originator.php
<?phpnamespace DesignPatterns\Behavioral\Memento;class Originator{ /* @var mixed */ private $state; // 这个类还可以包含不属于备忘录状态的额外数据 /** * @param mixed $state */ public function setState($state) { // 必须检查该类子类内部的状态类型或者使用依赖注入 $this->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
<?phpnamespace DesignPatterns\Behavioral\Memento;class Caretaker{ protected $history = array(); /** * @return Memento */ public function getFromHistory($id) { return $this->history[$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
<?phpnamespace DesignPatterns\Behavioral\Memento\Tests;use DesignPatterns\Behavioral\Memento\Caretaker;use DesignPatterns\Behavioral\Memento\Memento;use DesignPatterns\Behavioral\Memento\Originator;/** * MementoTest用于测试备忘录模式 */class MementoTest extends \PHPUnit_Framework_TestCase{ public function testUsageExample() { $originator = new Originator(); $caretaker = new Caretaker(); $character = new \stdClass(); // new object $character->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。
