How iOS Implements Undo/Redo with Command & Observer Patterns

Current keyboards—both system‑provided and third‑party—offer undo and redo features. Do you know which design patterns power them? They rely on the Command pattern together with the Observer pattern .

Simple Introduction

What is the Command (Command) pattern?

The Command pattern is a behavioral design pattern that encapsulates an operation, method call, or command into an object, which can be executed at an appropriate time.

Pattern type: Behavioral pattern

Definition: Encapsulate a request as an object.

Typical scenario: Like a guard ready to act on command—e.g., a menu button that triggers a specific action when clicked.

Class diagram:

The invoker holds a composite command class, and the command class references the command receiver.

What is the Observer (Observer) pattern?

Pattern type: Behavioral pattern

Definition: Defines a one‑to‑many relationship where a change in the subject (observable) automatically notifies observers, allowing them to update their state.

Typical scenario: Controllers observe model properties; when a property changes, the view updates. Keyboard input is observed and then displayed.

Class diagram:

The observable holds references to its observers.

Real‑World Use in the Cocoa Touch Framework

Apple provides NSUndoManager for undo/redo management, which consists of three main components:

Redo stack

Undo stack

Command object:

NSInvocation

When entering the undo stack, NSInvocation executes the undo command; when entering the redo stack, it executes the redo command.

To combine these patterns in your own implementation:

Add an NSInvocation execution object to the Command.

Include an observer list in the Command so that executing a command notifies its observers.

Create a manager class similar to NSUndoManager to handle the overall command list.