# Kotlin delegates in Android - real life examples

As the Kotlin in Android application development has been officially supported for some time, many Android developers feel quite fluent and comfortable using this language. Nevertheless, there are always some new small things that we still discover in Kotlin, which is very exciting. Recently I’ve discovered such an exciting thing which was the usage of Kotlin delegate to solve some specific problem. So first of, let’s discuss the problem.

There is a very good media player library from Google which is called _ExoPlayer_. I bet that most of you have heard of it. Recently, they’ve added a very nice feature - support for Chromecast. Thanks to this feature, it is much easier to switch between a local media player and Chromecast. What they’ve actually done is creating a _CastPlayer_ class and made it implement _Player_ interface. The _ExoPlayer_ class, which is meant for local playback, also implements _Player_ interface. So now, we have two media players unified by _Player_ interface. Sounds good as we can have one _Player_ variable that holds reference to either a remote or a local player.


![Inheritance diagram](https://www.dropbox.com/s/epo88xf5wmshb2z/1.jpeg?raw=1)


As you’ve probably guessed, I had to create a video player that would support local playback and Chromecast. I wanted both _ExoPlayer_ and _CastPlayer_ to have some additional methods like _loadMedia(playerMedia: PlayerMedia)_. My first thought was “Let’s use inheritance”, as simple as that. So I’ve created two classes _VideoExoPlayer_ and _VideoCastPlayer_, where both implement _ExtendedPlayer_ interface.


![Inheritance diagram 2](https://www.dropbox.com/s/qd91pwvpgdvo2ba/Inheritence2.jpeg?raw=1)


Looks fine so far. Having a reference to _ExtendedPlayer_ instance, we are able to access all the methods from _Player_ interface and additional _loadMedia(_) method.

But…nope :(

Take a look at _VideoCastPlayer_ class


![VideoCastPlayer](https://www.dropbox.com/s/lqtazwp3rp77nl6/VideoCastPlayerclass.png?raw=1)


The _CastPlayer_ class is final. We cannot inherit from it.

So if not inheritance then maybe…COMPOSITION. Right! As someone smart has said **“Composition over inheritance”** (probably it was Einstein).

So we can simply pass a _CastPlayer_ instance as an argument and call methods we want on it. Here is how _VideoCastPlayer_ class looks using composition.


![VideoCastPlayer](https://www.dropbox.com/s/us7gapaklkg53dk/VideoCastPlayer2.jpeg?raw=1)


Unfortunately, as _ExtendedPlayer_ implements _Player_ interface, we have to override all its methods. We don’t really want to do this as we’d like to keep implementations from _CastPlayer_ class. Is there any solution to this?

**Here comes the magic of Kotlin delagates!** By using delegate we can implement all of _Player_ interface methods using _castPlayer_ instance given in constructor. Just check this out:


![ExtendedPlayer](https://www.dropbox.com/s/u4zn4ckh0lfxe4n/ExtendedPlayerclass.png?raw=1)


What we have done here is changing _ExtendedPlayer_ from interface to abstract class, passing the _player_ argument in constructor and **letting this argument implement all the methods from _Player_ interface using delegation.** Pure magic, isn’t it?

As you can see in the picture below, there is no more error message that we have to implement _Player_ interface methods.


![VideoCastPlayer final](https://www.dropbox.com/s/alm9vlahbb0zx0p/VideoCastPlayerfinal.png?raw=1)


I think it is really worth having this superpower in your pocket.


kotlin_delegates_in_android_cover-3a8d29fa1263a4f8001a62c9f190e1f7

https://strapi-appunite.s3.eu-central-1.amazonaws.com/covers-with-text/20372217-db1b-4f6e-b198-63e325f28a02.png

Real life example of using Kotlin delegate in Android. A tricky way to overcome final class inheritance.

Kotlin Delegates in Android - Real Life Examples

# The decalogue of a pro media app
In this short article, I’d like to reveal the 10 ancient commandments that will help you create a pro media application. I’ll present things that are important to implement and those that are nice to have. Let’s just point them out.

### 1. You shall support AudioFocus.
This is a thing that some media apps do not handle, and when not handled, it may spoil UX of other apps. Basically, audio focus helps to prevent two or more apps from playing audio in the same time. For instance, it gives a developer a possibility to pause playback when a call is coming or to lower volume when a text message arrives. It would be really undesirable for a user to hear the audio from both, your media app and Youtube app when switching from one to another or experience a situation when during a call, music from your app is still playing. Please remember to always handle it. Luckily for you, if you use ExoPlayer library, it handles audio focus out of the box starting from version 2.9.0.

More info here ->  
<https://developer.android.com/guide/topics/media-apps/audio-focus>  
<https://medium.com/google-exoplayer/easy-audio-focus-with-exoplayer-a2dcbbe4640e>

### 2. Don’t be noisy.
Have you ever made a joke on your friend unplugging his headphones jack from his smartphone while he was listening to music during a history class? Yeah, it made music play loud. It happened because the app didn’t handle “Becoming noisy” action broadcast by system in such situations. Basically, what you should do, is to register a broadcast receiver with the following IntentFilter:
`IntentFilter(AudioManager.ACTION_AUDIO_BECOMING_NOISY)`.  
Once the system notifies you about becoming noisy action, you should just pause the playback.

More info here ->
<https://developer.android.com/guide/topics/media-apps/volume-and-earphones>

### 3. Honour Android Media Session with all the information you’ve got.
Android system holds information about all media sessions. Each media session has its own session token. Everyone owning it, is able to interact with the session. For instance, your smartwatch using session token can control the music playback in your phone. Media session can hold information about playback state and media metadata. Thanks to that, it is possible to:
- control the playback through so called media buttons, like the ones from the headset
- display artwork on lock screen (more about it later)
- display things like music title, subtitle and duration in your car or smartwatch display
- make your media notification look awesome (more about it later)
- and more.

A class responsible for media session creation is `MediaSessionCompat`. The instance of it should be created when you start your media service. This class has such methods as `setMetadata(MediaMetadataCompat)` and `setPlaybackState(PlaybackStateCompat)` which you should be using to give Android system all information about your media.  
In order to receive media buttons events in your app, like pressing a play button from your headset, you have to register a `MediaButtonReceiver`. Then, you should pass an Intent from a receiver to your media session like this:   `MediaButtonReceiver.handleIntent(mediaSession, intent)`.   
That way, all the media buttons actions will be reflected in your media session callback (`MediaSessionCompat.Callback`). For instance, pressing a play button on your headset will call either `onPlay()` or `onPause()` method in the callback, depending on playback state you have passed to session using `setPlaybackState(PlaybackStateCompat)` method.

### 4. Don’t be lazy and display Media notification.
If your app is going to be playing media in background, it is super useful for a user to display a media notification. Having it, a user is able to control the playback, know what is currently being played, or get back to the app through notification. 
If you’ve ever seen Spotify media notification, you could think it looks so cool. Saying cool, I mean things like notification background color created from provided media image dominant color, and text colors adjusted to this color, so they look nice and clear on any background. Surprisingly or not, it is just a native Android notification with media style.
This is how to create a basic media style of notification.
```kotlin
NotificationCompat.MediaStyle()
 .setMediaSession(mediaSession.sessionToken)
 .setShowActionsInCompactView(0, 1, 2)
 ```
Just pass it to `NotificationCompat.Builder().setStyle(mediaStyle)` method. A very important thing to remember is to provide the media session token and, as mentioned before, ensure that media session has all information about our media and playback.

### 5. Remember about a lock screen.
As an addition to media notification, it is worth implementing lock screen controls. It will give a user an opportunity to control the playback without unlocking the phone. In order to display a media notification on the lock screen, you just need to set public visibility on it, just like this:
`NotificationCompat.Builder().setVisibility(NotificationCompar.VISIBILITY_PUBLIC` .
There is one more fancy thing about a lock screen. We can set a full screen background on it. You just have to pass a bitmap to media session metadata.
`metadata.putBitmap(MediaMetadataCompat.METADATA_KEY_ALBUM_ART, bitmap)`
Just keep in mind that maximum allowed size of the bitmap is **320dp**, thus, if the image is fetched from the network, it may be worth requesting a smaller size image. If each of your songs has a different image and you’d like images to load fast on the lock screen, it may be worth considering loading smaller size blurred image.

### 6. Don’t let your Service die. Use foreground service.
In order to play media in background, you have to start a Service and embed a player within. However, since Android Oreo, we have to remember to start a Service as a foreground service. Otherwise, the system will kill our Service very soon after we put the app in background. When starting a service as foreground, we have to pass a Notification to `startForeground(Notification)` method. That is a requirement, however, it is not a problem, as you’d probably want to display a media notification.  
More info here -> <https://developer.android.com/about/versions/oreo/background>

### 7. Remember about the WakeLock and WiFiLock.
If your app plays media in background and a user turns off the screen, a device may go to sleep while your service is running. Android system tries to save battery and may shut down CPU, WiFi and other resources. You may want to prevent the system from doing this. To keep CPU awake, you should acquire a `PowerManager.WakeLock`. However, if you’d like WiFi to keep running, use the `WifiManager.WifiLock`.

### 8. You shall shine and keep the screen on.
If your app is going to play videos, you don’t want the screen to turn off while the video is playing. It may be very annoying to a user. To keep screen on, you have two options, either add a flag to activity window 
`window.addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON)`
or add an attribute to your root layout
`android:keepScreenOn=”true”`

### 9. Remember about Audio Tracks limitation.
The `AudioTrack` class manages and plays a single audio resource for Java applications. There is a strict system limitation that allows for only **32** instances of this class in the whole system. A single player instance (`ExoPlayer` or `MediaPlayer`) usually holds one instance of active AudioTrack so it shouldn’t be a big deal. However, if you use many players or, just forget to release player resources, you, or any other app in the system, might get into trouble. So please always release your player resources.

### 10. You shall display your media on a big screen with Chromecast.
`Chromecast` is a device from Google that enables users with a mobile device or personal computer to play Internet-streamed multimedia content on a television or home audio system through mobile and web apps that support the Google Cast technology. It is quite a popular device so it may be worth considering support Chromecast in your multimedia app. To implement Chromecast, you can use Cast SDK from Google <https://developers.google.com/cast/>. If you use ExoPlayer, it has a CastExtension that makes things easier. However, this extension is a little bit buggy so I don’t fully recommend it, at least in the version 2.9.2 of ExoPlayer. As a side note, I’ve heard from a dev working on ExoPlayer, that they are building something completely new that will resolve all known issues.

## Summary

That’s it. I hope you’re gonna be a good man and you will obey all the commandments. I’ve tried to be very concise and didn’t want to put a lot of code, however, most of the points mentioned, could be detailed in a separate article. I hope this post will help you create a pro media application or, at least, has left some useful knowledge in the back of your head :)

If you don’t know yet what superpower is hidden behind Kotlin delegates, check out my previous article <https://medium.com/@madamczewski/kotlin-delegates-in-android-real-life-examples-part-1-17c0e8f0869>


The-decaloque-of-a-pro-media-app-1

https://strapi-appunite.s3.eu-central-1.amazonaws.com/covers-with-text/d33c83e6-f842-4f75-b376-b89b2157b1a9.png

Marcin, our Android dev, reveals 10 ancient commandments that will help you create a pro media application.

The Decalogue of a Pro Media App

# Android Quiz #1

### Welcome to the first part of the Android Quiz! 

1. Are you bored with all the articles about MVP, MVVM, Kotlin coroutines vs RxJava, etc? 
2. Do you want to have a little fun?
3. Do you want to test your knowledge and maybe learn something new?

`if (yesCount < 2) return`

In this first part of the Android Quiz, I've prepared a form with a couple of questions about Android, Kotlin and Java.
So make yourself coffee or tea, take a 5-minute break and let's have fun. You can do it anonymously or type in your name. Later on, after submitting a form, you will be able to check whether your answers are correct or not. If an answer was wrong, don't worry, I've prepared a link with explanation so you can learn more about it. At the end of the test, you will be able to see everyone's results in the summary chart. This will let curious people know what was the most difficult for other people and what was a peace of cake. 

* I don't collect any data from you. Google account is not required.
* If some question or answer is either not clear or correct to you, please choose the most probable answer and leave a comment.
* If you really want to know how good you are, skip the questions you don't know the answer to. Don't try to guess as I don't provide explanations for correct answers :)

**Here we go !** 
<iframe src="https://docs.google.com/forms/d/e/1FAIpQLSfoR1mbg8AT5z5hAmNbDX92zGesfkuTmCPuAHqeW4Qq8OFFPg/viewform?embedded=true" width="640" height="500" frameborder="0" marginheight="0" marginwidth="0">Ładuję...</iframe>

This form is a part of this article and cannot be shared separately.

If you still don't know the 10 most important rules of a media app, then checkout my previous article [LINK](https://proandroiddev.com/the-decalogue-of-a-pro-media-app-6b6eac0156fc)


quize-android-1

https://strapi-appunite.s3.eu-central-1.amazonaws.com/covers-with-text/c42f990a-466e-4a38-bcad-8d6da65fb273.png

Marcin from our Android team prepared a form with a couple of questions about Android, Kotlin and Java to test your knowledge!

Android Quiz #1

# Android Quiz #2
### Welcome to the second part of the Android Quiz - the quiz for those who are interested in android software development 🎉

I’m glad that so many of you liked the first part of the Android Quiz. If you haven’t seen it yet, you can check it 
[here](https://proandroiddev.com/android-quiz-1-1f475c7170fd). Before we start, let’s quickly summarize the results of Part 1 and remind the rules.

**Part 1 summary**

![](https://dl.dropboxusercontent.com/s/eejk9v216w3t8pi/Screenshot%202019-02-03%2020.38.14.png?dl=0)

As we can see in the chart, the quiz wasn’t that easy. Only 7 out of above 500 people hit the highest score. Big congrats to them. The interesting thing is that the Gauss distribution is everywhere, also in the quiz results :)
The hardest questions for you were about Handler class and no-dpi directory. 

**Rules reminder**

* I don’t collect any data from you. Google account is not required.
* If any question or answer is either not clear or correct to you, please choose the most probable answer and leave a comment.
* If you really want to know how good you are, skip the questions you don’t know the answer to. Don’t try to guess as I don’t provide explanations for correct answers :)

Here we go ! 
Psssst…Just scroll down the form to proceed ;)

<iframe src="https://docs.google.com/forms/d/e/1FAIpQLSdv5u-e28Ol177RuqmHcdZwl_2Ayu-HxJ_AAkbBzyH7P4dI-A/viewform?embedded=true" width="640" height="500" frameborder="0" marginheight="0" marginwidth="0">Loading...</iframe>

If for any reason you can’t see the quiz, here is the direct [LINK ](https://goo.gl/forms/F9cjgspMZMeMhccO2)

---

If you liked the Quiz, remember to clap 👏. It gives me motivation to create more. If you have any good questions for part 3 of Android Quiz, let me know in a private message or email me at marcin.adamczewski@appunite.com. I will include your name next to the question :)

Any feedback is very very welcome. 

This form is a part of this article and cannot be shared separately.


quiz-part2-1

https://strapi-appunite.s3.eu-central-1.amazonaws.com/covers-with-text/01c4ed40-e568-4e31-b36e-61064eecd8ee.png

Welcome to the second part of the Android Quiz - the quiz for those who are interested in android software development 🎉

Articles

Let’s Work Together!