Pamiątka w języku TypeScript
Pamiątka to behawioralny wzorzec projektowy umożliwiający zapisywanie “migawek” stanu obiektu i późniejsze jego przywracanie.
Wzorzec Pamiątka nie wpływa na wewnętrzną strukturę obiektu z którym współpracuje, ani na dane przechowywane w migawkach.
Złożoność:
Popularność:
Przykłady użycia: Zasada działania Pamiątki opiera się na serializacji, która jest dość powszechnie stosowana w TypeScript. Nie jest to jedyny, czy najefektywniejszy sposób zapisywania migawki stanu obiektu, ale pozwala na przechowywanie kopii zapasowych, chroniąc jednocześnie strukturę obiektu źródłowego przed innymi obiektami.
Przykład koncepcyjny
Poniższy przykład ilustruje strukturę wzorca Pamiątka ze szczególnym naciskiem na następujące kwestie:
- Z jakich składa się klas?
- Jakie role pełnią te klasy?
- W jaki sposób elementy wzorca są ze sobą powiązane?
index.ts: Przykład koncepcyjny
/**
* The Originator holds some important state that may change over time. It also
* defines a method for saving the state inside a memento and another method for
* restoring the state from it.
*/
class Originator {
/**
* For the sake of simplicity, the originator's state is stored inside a
* single variable.
*/
private state: string;
constructor(state: string) {
this.state = state;
console.log(`Originator: My initial state is: ${state}`);
}
/**
* The Originator's business logic may affect its internal state. Therefore,
* the client should backup the state before launching methods of the
* business logic via the save() method.
*/
public doSomething(): void {
console.log('Originator: I\'m doing something important.');
this.state = this.generateRandomString(30);
console.log(`Originator: and my state has changed to: ${this.state}`);
}
private generateRandomString(length: number = 10): string {
const charSet = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
return Array
.apply(null, { length })
.map(() => charSet.charAt(Math.floor(Math.random() * charSet.length)))
.join('');
}
/**
* Saves the current state inside a memento.
*/
public save(): Memento {
return new ConcreteMemento(this.state);
}
/**
* Restores the Originator's state from a memento object.
*/
public restore(memento: Memento): void {
this.state = memento.getState();
console.log(`Originator: My state has changed to: ${this.state}`);
}
}
/**
* The Memento interface provides a way to retrieve the memento's metadata, such
* as creation date or name. However, it doesn't expose the Originator's state.
*/
interface Memento {
getState(): string;
getName(): string;
getDate(): string;
}
/**
* The Concrete Memento contains the infrastructure for storing the Originator's
* state.
*/
class ConcreteMemento implements Memento {
private state: string;
private date: string;
constructor(state: string) {
this.state = state;
this.date = new Date().toISOString().slice(0, 19).replace('T', ' ');
}
/**
* The Originator uses this method when restoring its state.
*/
public getState(): string {
return this.state;
}
/**
* The rest of the methods are used by the Caretaker to display metadata.
*/
public getName(): string {
return `${this.date} / (${this.state.substr(0, 9)}...)`;
}
public getDate(): string {
return this.date;
}
}
/**
* The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
* doesn't have access to the originator's state, stored inside the memento. It
* works with all mementos via the base Memento interface.
*/
class Caretaker {
private mementos: Memento[] = [];
private originator: Originator;
constructor(originator: Originator) {
this.originator = originator;
}
public backup(): void {
console.log('\nCaretaker: Saving Originator\'s state...');
this.mementos.push(this.originator.save());
}
public undo(): void {
if (!this.mementos.length) {
return;
}
const memento = this.mementos.pop();
console.log(`Caretaker: Restoring state to: ${memento.getName()}`);
this.originator.restore(memento);
}
public showHistory(): void {
console.log('Caretaker: Here\'s the list of mementos:');
for (const memento of this.mementos) {
console.log(memento.getName());
}
}
}
/**
* Client code.
*/
const originator = new Originator('Super-duper-super-puper-super.');
const caretaker = new Caretaker(originator);
caretaker.backup();
originator.doSomething();
caretaker.backup();
originator.doSomething();
caretaker.backup();
originator.doSomething();
console.log('');
caretaker.showHistory();
console.log('\nClient: Now, let\'s rollback!\n');
caretaker.undo();
console.log('\nClient: Once more!\n');
caretaker.undo();
Output.txt: Wynik działania
Originator: My initial state is: Super-duper-super-puper-super.
Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: qXqxgTcLSCeLYdcgElOghOFhPGfMxo
Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: iaVCJVryJwWwbipieensfodeMSWvUY
Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: oSUxsOCiZEnohBMQEjwnPWJLGnwGmy
Caretaker: Here's the list of mementos:
2019-02-17 15:14:05 / (Super-dup...)
2019-02-17 15:14:05 / (qXqxgTcLS...)
2019-02-17 15:14:05 / (iaVCJVryJ...)
Client: Now, let's rollback!
Caretaker: Restoring state to: 2019-02-17 15:14:05 / (iaVCJVryJ...)
Originator: My state has changed to: iaVCJVryJwWwbipieensfodeMSWvUY
Client: Once more!
Caretaker: Restoring state to: 2019-02-17 15:14:05 / (qXqxgTcLS...)
Originator: My state has changed to: qXqxgTcLSCeLYdcgElOghOFhPGfMxo
