Introducing the Memento Pattern - Building the second piece of the memento pattern
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In consonance with the concepts that I expressed in the prior section, the schema imposed by the memento pattern must be completed by creating an additional class, called the caretaker. As its name implies, this class will take care of holding a specific state of the previously defined originator, completing the logic implemented by the pattern in question.
However, before I teach you how to put the two pertinent classes to work together, first let me show you the signature that corresponds to the caretaker. In this you will understand later how this new class interacts with the respective originator.
Here is the definition for a brand new class, called "ArrayElementSelector." It plays the role of the caretaker. Have a look at its signature, please:
// define ArrayElementSelector' class (in this case, this is the Caretaker class)
class ArrayElementSelector{
// the constructor accepts the originator as the unique parameter
public function __construct(ArrayProcessor $arrayProcessor){
$this->setInputArray($arrayProcessor);
$this->setArrayIndex($arrayProcessor);
}
// set input array by using originator object (ArrayProcessor)
public function setInputArray(ArrayProcessor $arrayProcessor){
$this->inputArray=$arrayProcessor->getInputArray();
}
// get input array by using originator object (ArrayProcessor)
public function getInputArray(ArrayProcessor $arrayProcessor){
$arrayProcessor->setInputArray($this->inputArray);
}
// set array index by using originator object (ArrayProcessor)
public function setArrayIndex(ArrayProcessor $arrayProcessor){
$this->arrayIndex=$arrayProcessor->getArrayIndex();
}
// get array index by suing originator object (ArrayProcessor)
public function getArrayIndex(ArrayProcessor $arrayProcessor){
$arrayProcessor->setArrayIndex($this->arrayIndex);
}
}
As you can see, the class shown above implements a classic structure for a caretaker class. As you'll recall from the concepts that I explained in the introduction of this article, the previous class is only capable of accessing the originator via its interface, and never directly. Pretty weird, right?
Of course, this condition is clearly reflected by each of the methods exposed by the respective caretaker. As you can see, these methods access and set the index of the given input array (and its elements too) only by using the originator object.
Besides, I'd like you to pay attention to the signature of the following method, which is extremely demonstrative:
// set array index by using originator object (ArrayProcessor)
public function setArrayIndex(ArrayProcessor $arrayProcessor){
$this->arrayIndex=$arrayProcessor->getArrayIndex();
}
Do you see how the value of the "arrayIndex" property is accessed by using an ArrayProcessor's method? I bet you do! As you'll see soon, this condition is crucial for holding the state of a specific property that belongs to the originator, which leads straight to the definition of the memento pattern.
All right, now that you hopefully learned how the originator class does its business, as well as how the corresponding caretaker interacts with it, it's time to jump forward and develop a hands-on example. This example will demonstrate in a friendly fashion how the previous "ArrayElementSelector" class is capable of maintaining the value of the "arrayIndex" property that corresponds to "ArrayProcessor."
Do you want to see how this example will be created? Please visit the following section and keep reading.
Next: Implementing the memento pattern >>
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