Add custom task docs

docs/custom-tasks.md

@@ -0,0 +1,80 @@
+---
+title: Custom tasks
+permalink: /custom-tasks
+---
+Instead of passing simple callables to workers, this package also allows custom implementations of the `Task` interface to dispatch work in child processes or threads.
+
+## `Task`
+
+The `Task` interface has a single `run()` method that gets invoked in the worker to dispatch the work that needs to be done.
+
+```php
+<?php
+
+namespace Amp\Parallel\Worker;
+
+/**
+ * A runnable unit of execution.
+ */
+interface Task
+{
+    /**
+     * Runs the task inside the caller's context.
+     *
+     * Does not have to be a coroutine, can also be a regular function returning a value.
+     *
+     * @param Environment
+     *
+     * @return mixed|\Amp\Promise|\Generator
+     */
+    public function run(Environment $environment);
+}
+```
+
+Task instances are `serialize`'d in the main process and `unserialize`'d in the worker.
+That means that all data that is passed between the main process and a worker needs to be serializable.
+
+## `Environment`
+
+The passed `Environment` allows to persist data between multiple tasks executed by the same worker, e.g. database connections or file handles, without resorting to globals for that.
+Additionally `Environment` allows setting a TTL for entries, so can be used as a cache.
+
+```php
+<?php
+
+namespace Amp\Parallel\Worker;
+
+interface Environment extends \ArrayAccess
+{
+    /**
+     * @param string $key
+     *
+     * @return bool
+     */
+    public function exists(string $key): bool;
+
+    /**
+     * @param string $key
+     *
+     * @return mixed|null Returns null if the key does not exist.
+     */
+    public function get(string $key);
+
+    /**
+     * @param string $key
+     * @param mixed $value Using null for the value deletes the key.
+     * @param int $ttl Number of seconds until data is automatically deleted. Use 0 for unlimited TTL.
+     */
+    public function set(string $key, $value, int $ttl = null);
+
+    /**
+     * @param string $key
+     */
+    public function delete(string $key);
+
+    /**
+     * Removes all values.
+     */
+    public function clear();
+}
+```

docs/index.md

@@ -46,4 +46,4 @@ foreach ($responses as $url => $response) {
 If you just want to fetch multiple HTTP resources concurrently, it's better to use [Artax](https://amphp.org/artax/), our non-blocking HTTP client.
 
 The functions you call must be predefined or autoloadable by Composer so they also exist in the worker processes.
-Instead of simple callables, you can also enqueue `Task` instances with `Amp\Parallel\Worker\enqueue()`.
+Instead of simple callables, you can also [enqueue `Task` instances](./custom-tasks.md) with `Amp\Parallel\Worker\enqueue()`.