Command pattern for undo in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Command pattern in software design?

The Command pattern is a design pattern that turns a request into a stand-alone object containing all information about the request. This allows for parameterizing methods with different requests, queuing or logging requests, and supporting undoable operations.

intermediate

How does the Command pattern help implement undo functionality?

Each command object stores the action and its state before execution. To undo, the command calls its undo method, reversing the action using the saved state.

beginner

What are the main components of the Command pattern?

1. Command Interface: declares execute and undo methods.<br>2. Concrete Command: implements the interface and defines actions.<br>3. Invoker: asks the command to carry out the request.<br>4. Receiver: knows how to perform the operations.<br>5. Client: creates command objects and sets receivers.

intermediate

Why is storing command history important in undo implementations?

Storing command history allows the system to track executed commands in order. This history enables undoing commands in reverse order, ensuring correct reversal of actions.

beginner

Give a simple real-life analogy for the Command pattern with undo.

Think of a remote control (Invoker) that sends commands to a TV (Receiver). Each button press is a command object. If you want to undo the last action, you press the undo button, which tells the remote to reverse the last command, like turning the volume back down.

Which component in the Command pattern is responsible for executing and undoing actions?

A Concrete Command

B Invoker

C Receiver

D Client

The Concrete Command implements the execute and undo methods to perform and reverse actions.

Why do we need to store the state before executing a command for undo?

A To speed up execution

B To log user activity

C To reverse the action correctly

D To notify the client

Storing the state before execution allows the undo method to restore the previous state, effectively reversing the action.

What role does the Invoker play in the Command pattern?

A Executes commands

B Creates commands

C Stores command history

D Performs the actual operation

The Invoker asks the command to execute or undo the action but does not know the details of the operation.

In the Command pattern, what is the Receiver responsible for?

A Undoing commands

B Executing the actual operation

C Storing commands

D Invoking commands

The Receiver knows how to perform the actual work when a command is executed.

Which of the following is NOT a benefit of using the Command pattern for undo?

A Enables undo and redo

B Supports queuing and logging

C Decouples sender and receiver

D Automatically optimizes performance

The Command pattern does not automatically optimize performance; it focuses on decoupling and flexibility.

Explain how the Command pattern enables undo functionality in a software system.

Think about how each action can be reversed by storing information and calling undo.

Describe the roles of the main components in the Command pattern and how they interact.

Consider who creates, who calls, and who does the work.