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
<?php
namespace 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
<?php
namespace 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
<?php
namespace 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
<?php
namespace 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。
No Comments