82 lines
2.1 KiB
Markdown
82 lines
2.1 KiB
Markdown
---
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layout: pattern
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title: Callback
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folder: callback
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permalink: /patterns/callback/
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categories: Idiom
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tags:
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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
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(execute) the argument at some convenient time.
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## Explanation
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Real world example
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> We need to be notified after 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 the executor which will be called at 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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public final class SimpleTask extends Task {
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private static final Logger LOGGER = getLogger(SimpleTask.class);
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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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* when some arbitrary synchronous or asynchronous action must be performed after execution of some defined activity.
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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 callback that will be triggered every time when barrier is tripped.
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