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https://github.com/tiennm99/java-design-patterns.git
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Ilkka Seppälä
119 lines
4.3 KiB
Markdown
119 lines
4.3 KiB
Markdown
---
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title: Callback
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category: Functional
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language: en
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tag:
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- Asynchronous
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- Decoupling
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- Idiom
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- Reactive
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---
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## Intent
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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.
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## Also known as
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* Event-Subscription
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* Listener
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## Explanation
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Real world example
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> 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.
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In plain words
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> Callback is a method passed to an executor which will be called at a defined moment.
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Wikipedia says
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> 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.
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**Programmatic Example**
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Callback is a simple interface with single method.
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```java
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public interface Callback {
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void call();
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}
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```
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Next we define a task that will execute the callback after the task execution has finished.
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```java
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public abstract class Task {
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final void executeWith(Callback callback) {
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execute();
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Optional.ofNullable(callback).ifPresent(Callback::call);
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}
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public abstract void execute();
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}
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@Slf4j
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public final class SimpleTask extends Task {
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@Override
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public void execute() {
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LOGGER.info("Perform some important activity and after call the callback method.");
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}
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}
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```
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Finally, here's how we execute a task and receive a callback when it's finished.
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```java
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var task=new SimpleTask();
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task.executeWith(()->LOGGER.info("I'm done now."));
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```
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## Class diagram
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## Applicability
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Use the Callback pattern when
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* Asynchronous event handling in GUI applications or event-driven systems
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* Implementing notification mechanisms where certain events need to trigger actions in other components.
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* Decoupling modules or components that need to interact without having a direct dependency on each other
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## Known uses
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* GUI frameworks often use callbacks for event handling, such as user interactions (clicks, key presses)
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* Node.js heavily relies on callbacks for non-blocking I/O operations
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* Frameworks that deal with asynchronous operations, like Promises in JavaScript, use callbacks to handle the resolution or rejection of asynchronous tasks
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## Consequences
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Benefits:
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* Decouples the execution logic of an operation from the signaling or notification logic, enhancing modularity and reusability
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* Facilitates asynchronous processing, improving the responsiveness and scalability of applications
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* Enables a reactive programming model where components can react to events as they occur
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Trade-offs:
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* Callback hell or pyramid of doom: Deeply nested callbacks can lead to code that is hard to read and maintain
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* Inversion of control can lead to harder-to-follow code flow, making debugging more challenging
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* Potential issues with error handling, especially in languages or environments where exceptions are used, as errors might need to be propagated through callbacks
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## Related patterns
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* [Observer](https://java-design-patterns.com/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
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* [Command](https://java-design-patterns.com/patterns/command/): Callbacks can be implemented as Command objects in scenarios where more flexibility or statefulness is required in the callback operation
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* [Promise](https://java-design-patterns.com/patterns/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
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## Real world examples
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* [CyclicBarrier](http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CyclicBarrier.html#CyclicBarrier%28int,%20java.lang.Runnable%29) constructor can accept a callback that will be triggered every time a barrier is tripped.
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* [JavaScript: The Good Parts](https://amzn.to/3TiQV61)
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* [Node.js Design Patterns - Third edition: Design and implement production-grade Node.js applications using proven patterns and techniques](https://amzn.to/3VssjKG)
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