Command pattern là gì? Ứng dụng Undo/Redo?

Command encapsulate một request thành object, cho phép parameterize clients với different requests, queue/log requests, và support undoable operations.

Cấu trúc:

interface Command {
  execute(): void
  undo(): void
}

class CommandManager {
  private historyStack: Command[] = []
  private redoStack: Command[] = []

  run(cmd: Command) {
    cmd.execute()
    this.historyStack.push(cmd)
    this.redoStack = []
  }

  undo() {
    const cmd = this.historyStack.pop()
    if (cmd) { cmd.undo(); this.redoStack.push(cmd) }
  }

  redo() {
    const cmd = this.redoStack.pop()
    if (cmd) { cmd.execute(); this.historyStack.push(cmd) }
  }
}

Ứng dụng thực tế: text editor (Ctrl+Z), transaction trong database, task queue (Bull, BullMQ), HTTP request retry.

• Trong Redux, mỗi dispatch(action) là Command — redux-undo library implement Undo/Redo bằng Command pattern.
• Dùng khi: cần undo/redo, transaction, audit log, retry; khi muốn queue và schedule requests.
• Không dùng khi: simple method call đủ dùng — Command overhead không cần thiết.

Command encapsulates a request as an object, allowing clients to be parameterized with different requests, to queue or log requests, and to support undoable operations.

Structure:

interface Command {
  execute(): void
  undo(): void
}

class CommandManager {
  private historyStack: Command[] = []
  private redoStack: Command[] = []

  run(cmd: Command) {
    cmd.execute()
    this.historyStack.push(cmd)
    this.redoStack = []
  }

  undo() {
    const cmd = this.historyStack.pop()
    if (cmd) { cmd.undo(); this.redoStack.push(cmd) }
  }

  redo() {
    const cmd = this.redoStack.pop()
    if (cmd) { cmd.execute(); this.historyStack.push(cmd) }
  }
}

Real-world uses: text editors (Ctrl+Z), database transactions, task queues (Bull, BullMQ), HTTP request retry.

• In Redux, every dispatch(action) is a Command — the redux-undo library implements Undo/Redo using this pattern.
• Use it when you need undo/redo, transactions, audit logs, or retries, or when you want to queue and schedule requests.
• Skip it when a simple method call suffices — Command overhead isn't always justified.