Getting Started With ReactiveX

What is Reactive?

ReactiveX is a library composing asynchronous and event-based programs by using observable pattern, Iterator pattern and functional programming.  It is developed by NetFlix.


What languages it supports?

You can find implementation of ReactiveX in many different programming languages like Java, JavaScript, C# etc. In every programming language it has different name as mentioned below-

Java - RxJava
JavaScript - RxJS
C++ - RxCpp
C# - Rx.NET
Scala - RxScala

As described above, its Android implementation is known as RxAndroid and below explanation is based with the reference of RxJava/RxAndroid. It is one of the most discussed libraries for enabling Reactive Programming in Android development.


What problem it solves/ how it simplifies things?

When you develop a complex Android application it contains lots of database operation, network calls, bitmap processing which introduce nested callbacks. Because of this application becomes complex and error prone. As a developer this also leads application difficult to understand as well. So ReactiveX offers an alternative approach that is both clear and concise, to manage asynchronous tasks and events.


Basics of RxJava

There are two main actors in RX world as mentioned:

Observables – An object that emits a stream of data or events
Observers/Subscribers – An object that acts upon the emitted data.

In RxJava and RxAndroid, observers are instances of the Observer interface, and observables are instances of the Observable class.

The standard flow of an Observable is to emit one or more data items with either complete successfully or with an error. An Observable can have multiple Observers so each data item emitted by the Observable will be received by each Observer.


Setting up RxAndroid

To use RxAndroid, add its compile dependency in the Android Studio project build.gradle.

compile 'io.reactivex:rxandroid:1.1.0'


Basics of Observable & Observer

Create Observable-

The Observable class has many static methods known as operators which is use to create Observable objects. There are different operators groups provided by RxAndroid. Some of them are:

Creating Observables
Create, Defer, Empty/Never/Throw, From, Interval, Just, Range
·        
       Transforming Observable Items
Buffer, FlatMap, GroupBy, Map, Scan, and Window
·         
       Filtering Observables
Debounce, Distinct, ElementAt, Filter, First, IgnoreElements, Last, Sample, Skip, SkipLast, Take, and TakeLast
·         
       Combining Observables
And/Then/When, CombineLatest, Join, Merge, StartWith, Switch, and Zip
·         
       Utility Operators
Delay, Do, Materialize/Dematerialize, ObserveOn, Serialize, Subscribe, SubscribeOn, TimeInterval, Timeout, Timestamp, and Using

To know all the operators groups refer Reactive doc.

The following code shows you how to use the just operator to create a very simple Observable that emits a String value.

Observable<String> stringObservable = Observable.just("Hello Android");
Above line of code emits "Hello Android"

Below code emits multiple Integer values:
Observable<Integer> integerObservable = Observable.just(2, 4, 8, 13, 23);

Create Observer-

To create an observer, create a class that implements the Observer interface. The Observer interface has overridden methods which are used to handle different types of notifications received from Observable.

Here is an observer that receives String value emitted by the observable:

Observer<String> stringObserver = new Observer<String>() {
    @Override
    public void onCompleted() {
        // this method gets called when the observable has no more data to emit
    }

    @Override
    public void onError(Throwable e) {
        // this method gets called when the observable encounters an error
    }

    @Override
    public void onNext(String s) {
        // this method gets called each time the observable emits data
    }
};

As explained onNext() method gets called to consume data events emitted by Observable.
onCompleted() gets called when no additional events coming and Observable finished its work
onError() gets called in case of unrecoverable error.


Subscribe Observer to Observable

To assign Observer to Observable use Subscribe method which returns Subscription object? The following code makes stringObserver observe stringObservable:

Subscription stringSubscription = stringObservable.subscribe(stringObserver);


UnSubscribe Observer to Observable

To stop observing items simply need to call unsubscribe() method on the returned Subscription object.

stringSubscription.unsubscribe();


Handling Asynchronous operations

Till now whatever we discussed on Observer and Observable will work perfectly with single thread (Main Thread/UI Thread). So to manage multiple Thread, ReativeX provides freedom from nested callbacks.

If you want to introduce multithreading in Observable operators, you can instruct particular Observables to operate on particular Schedulers. By default, an Observable notifies its observers on the same thread on which its Subscribe method is called.

The SubscribeOn operator changes this behavior by specifying a different Scheduler on which the Observable should operate. The ObserveOn operator specifies a different Scheduler that the Observable will use to send notifications to its observers. Using subscribeOn() and observeOn() operators you can explicitly specify which thread should run the background job and which thread should handle the user interface updates.

Example-
stringObservable
                                 .subscribeOn(Schedulers.io())
                                 .observeOn(AndroidSchedulers.mainThread())
                                .subscribe(stringObserver);    

Here stringObservable (Observable) will use the Schedulers.io() to subscribe, and will observe the results on the UI Thread using AndroidSchedulers.mainThread().

More on Scheduler refer here.


Conclusion


I hope this serves as a useful introduction to ReactiveX and an overview of its basic capabilities. It also has much more powerful concepts. To learn more about reactive extensions I encourage you to browse the resources available at ReactiveX.

To find more interesting topics on Software development follow me at https://medium.com/@ankit.sinhal

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