Command

Encapsulates a request as an object, enabling undo/redo, queuing, and logging of operations.

The Problem

You need to issue requests without knowing what operation is performed, who performs it, or when. You also want to support undo/redo.

How It Works

›Request is wrapped as an object with execute() and undo() methods
›Decouples the invoker (who sends the command) from the receiver (who handles it)
›Commands can be queued, logged, or replayed
›undo() reverses the effect of execute() — enables full history management
›Macro commands compose multiple commands into one

When to Use

✓Undo/redo functionality (text editors, drawing apps)

✓Transaction-based systems where rollback is needed

✓Queuing or scheduling actions for deferred execution

✓Logging and auditing all operations performed

Real-World Examples

🍎

iOS

NSUndoManager, UIAction, SwiftUI .onTapGesture closures

🤖

Android

Room database transactions, WorkManager Tasks, MenuItem.setOnMenuItemClickListener

🌐

Web

Redux actions & reducers, CQRS command objects, job queues (Bull, Celery)

Code Examples

Swift

protocol Command {
    func execute()
    func undo()
}

class TextEditor {
    private(set) var content = ""
    private var history = [Command]()

    func run(_ cmd: Command) { cmd.execute(); history.append(cmd) }
    func undoLast() { history.popLast()?.undo() }

    func appendCommand(_ text: String) -> Command {
        AppendCommand(editor: self, text: text)
    }

    fileprivate func append(_ t: String)    { content += t }
    fileprivate func removeLast(_ n: Int)   { content = String(content.dropLast(n)) }
}

class AppendCommand: Command {
    private let editor: TextEditor
    private let text: String
    init(editor: TextEditor, text: String) { self.editor = editor; self.text = text }
    func execute() { editor.append(text) }
    func undo()    { editor.removeLast(text.count) }
}

let ed = TextEditor()
ed.run(ed.appendCommand("Hello"))
ed.run(ed.appendCommand(" World"))
print(ed.content)   // "Hello World"
ed.undoLast()
print(ed.content)   // "Hello"