Callback

Intent

Callback is a piece of executable code that is passed as an argument to other code, which is expected to call back (execute) the argument at some convenient time.

Also known as

• Event-Subscription
• Listener

Explanation

Real world example

We need to be notified after the executing task has finished. We pass a callback method for the executor and wait for it to call back on us.

In plain words

Callback is a method passed to an executor which will be called at a defined moment.

Wikipedia says

In computer programming, a callback, also known as a "call-after" function, is any executable code that is passed as an argument to other code; that other code is expected to call back (execute) the argument at a given time.

Programmatic Example

Callback is a simple interface with single method.

public interface Callback {

  void call();
}

Next we define a task that will execute the callback after the task execution has finished.

public abstract class Task {

  final void executeWith(Callback callback) {
    execute();
    Optional.ofNullable(callback).ifPresent(Callback::call);
  }

  public abstract void execute();
}

@Slf4j
public final class SimpleTask extends Task {

  @Override
  public void execute() {
    LOGGER.info("Perform some important activity and after call the callback method.");
  }
}

Finally, here's how we execute a task and receive a callback when it's finished.

    var task = new SimpleTask();
    task.executeWith(() -> LOGGER.info("I'm done now."));

Class diagram

Callback

Applicability

Use the Callback pattern when

• Asynchronous event handling in GUI applications or event-driven systems
• Implementing notification mechanisms where certain events need to trigger actions in other components.
• Decoupling modules or components that need to interact without having a direct dependency on each other

Known uses

• GUI frameworks often use callbacks for event handling, such as user interactions (clicks, key presses)
• Node.js heavily relies on callbacks for non-blocking I/O operations
• Frameworks that deal with asynchronous operations, like Promises in JavaScript, use callbacks to handle the resolution or rejection of asynchronous tasks

Consequences

Benefits:

• Decouples the execution logic of an operation from the signaling or notification logic, enhancing modularity and reusability
• Facilitates asynchronous processing, improving the responsiveness and scalability of applications
• Enables a reactive programming model where components can react to events as they occur

Trade-offs:

• Callback hell or pyramid of doom: Deeply nested callbacks can lead to code that is hard to read and maintain
• Inversion of control can lead to harder-to-follow code flow, making debugging more challenging
• Potential issues with error handling, especially in languages or environments where exceptions are used, as errors might need to be propagated through callbacks

Related patterns

Observer: Callbacks can be seen as a more dynamic and lightweight form of the Observer pattern, with the ability to subscribe and unsubscribe callback functions dynamically
Command: Callbacks can be implemented as Command objects in scenarios where more flexibility or statefulness is required in the callback operation
Promise: In some languages or frameworks, Promises or Futures can be used to handle asynchronous operations more cleanly, often using callbacks for success or failure cases

Real world examples

• CyclicBarrier constructor can accept a callback that will be triggered every time a barrier is tripped.
• JavaScript: The Good Parts
• Node.js Design Patterns - Third edition: Design and implement production-grade Node.js applications using proven patterns and techniques