4.1 Added unsubscribeOn

Author: Froussios

Part 4 - Concurrency/1. Scheduling and threading.md

@@ -160,6 +160,41 @@ We can see here that events start on thread that calls `onNext` and stay on that
 
 This is very useful if you, as the consumer of an observable, know that processing is time-consuming and you don't want to block the producing thread. A typical case for this is applications with a GUI. Handlers to GUI events are invoked on the GUI thread by default. In order not to have your application freeze for the duration of the processing, you can take the processing to another thread. You can use `observeOn` again to return to the GUI thread, when the data is ready to be displayed.
 
+### unsubscribeOn
+
+As we have seen, some observables depend on resources which are leased on subscription and released when subscriptions end. Typically, releasing resources is cheap, but there can be cases where releasing a resource may require ceremony, e.g. coordinating over a network. If you need control over the thread that will execute the unsubscription code, you can define that with the `unsubscribeOn` method. 
+
+```java
+Observable<Object> source = Observable.using(
+	() -> {
+		System.out.println("Subscribed on " + Thread.currentThread().getId());
+		return Arrays.asList(1,2);
+	},
+	(ints) -> {
+		System.out.println("Producing on " + Thread.currentThread().getId());
+		return Observable.from(ints);
+	},
+	(ints) -> {
+		System.out.println("Unubscribed on " + Thread.currentThread().getId());
+	}
+);
+
+source
+	.unsubscribeOn(Schedulers.newThread())
+	.subscribe(System.out::println);
+```
+Output
+```
+Subscribed on 1
+Producing on 1
+1
+2
+Unubscribed on 11
+```
+
+The `using` method executes 3 functions, one that leases a resource, one that uses it and one the releases it. With `unsubscribeOn` we only affected the function that releases the resource.
+
+
 ## Schedulers
 
 The `observeOn` and `subscribeOn` methods take as an argument a [Scheduler](http://reactivex.io/RxJava/javadoc/rx/Scheduler.html). A `Scheduler`, as the name suggests, is a tool that can schedule individual actions to be performed. The specifics of how the action will be invoked depends on the implementation of the scheduler used. You can create your own implementation of scheduler, but Rx already has you covered with a set of Schedulers for the common cases. You can get the existing implementations from the factory methods on [Schedulers](http://reactivex.io/RxJava/javadoc/rx/schedulers/Schedulers.html).