apply-command-pattern

Apply Command Pattern

Encapsulate a request as an object, allowing requests to be parameterized, queued, logged, and undone.

Why This Is Best Practice

Adopted by: Java's javax.swing.Action (every button and menu item in Java Swing wraps a Command — in every Java GUI application), Redux (JavaScript's most-used state management library — every state change is a dispatched Action, i.e., a Command), Git's commit model (every commit is an immutable Command object with undo via revert), and database transaction logs (write-ahead logging records Commands for recovery). Impact: GoF documents that Command enables undo/redo, logging, and transactional rollback without changing invoker or receiver code. Redux's adoption of Command (85M weekly npm downloads) demonstrates the pattern's value at scale: time-travel debugging (replay/undo arbitrary state changes) is only possible because every change is a serializable Command object. Why best: Direct method calls — the alternative — cannot be queued, logged, or undone without invasive change to every caller. Command objects turn operations into first-class values that can be stored, transmitted, and replayed.

Sources: Gamma et al. (1994) pp. 233–242; Redux documentation; Git internals documentation

Steps

Step 1: Define the command interface

from abc import ABC, abstractmethod

class Command(ABC):
    @abstractmethod
    def execute(self) -> None: ...

    @abstractmethod
    def undo(self) -> None: ...

Step 2: Implement concrete commands that wrap a receiver

class TextEditor:
    def __init__(self):
        self.text = ""

class InsertTextCommand(Command):
    def __init__(self, editor: TextEditor, position: int, text: str):
        self._editor = editor
        self._position = position
        self._text = text

    def execute(self) -> None:
        self._editor.text = (
            self._editor.text[:self._position]
            + self._text
            + self._editor.text[self._position:]
        )

    def undo(self) -> None:
        self._editor.text = (
            self._editor.text[:self._position]
            + self._editor.text[self._position + len(self._text):]
        )

Step 3: Build an invoker that manages command history

class CommandHistory:
    def __init__(self):
        self._history: list[Command] = []

    def execute(self, command: Command) -> None:
        command.execute()
        self._history.append(command)

    def undo(self) -> None:
        if self._history:
            self._history.pop().undo()

Step 4: Use the invoker — sender doesn't know the receiver

editor = TextEditor()
history = CommandHistory()

history.execute(InsertTextCommand(editor, 0, "Hello"))
history.execute(InsertTextCommand(editor, 5, " World"))
print(editor.text)   # "Hello World"

history.undo()
print(editor.text)   # "Hello"

Step 5: For queuing or batch execution, collect commands before executing

batch: list[Command] = [
    InsertTextCommand(editor, 0, "Line 1\n"),
    InsertTextCommand(editor, 7, "Line 2\n"),
]

for cmd in batch:
    history.execute(cmd)

Macro recording, test scripts, and transaction replay all use this pattern.

When NOT to Use

• Simple, non-undoable operations — if undo/redo, queuing, and logging are not required, encapsulating the call in a Command object adds overhead without benefit.
• When operations have no receiver — if the command is just a function call with no state to capture, a first-class function (lambda, partial) is simpler.

Common Mistakes

Command that holds too much state. If a command stores a deep snapshot of receiver state to support undo, consider apply-memento-pattern for the snapshot instead.

Forgetting that undo must perfectly reverse execute. undo() must leave the system in exactly the state it was in before execute(). Test execute(); undo() as a no-op pair.

Mutable command state. A command stored in history must not change after execution. If parameters can change externally, copy them in the constructor.