Menguasai konteks Android

Konteks dalam Android adalah salah satu objek yang paling sering digunakan dan disalahgunakan. Tetapi kebanyakan artikel di web memberi tumpuan kepada definisi apa itu. Saya tidak dapat mencari sumber yang baik yang memberi saya pandangan dan membantu saya memahami gambaran yang lebih besar. Oleh itu, saya cuba mempermudahkan perkara dengan artikel ini.

Kata Pengantar

Misi saya untuk artikel ini adalah untuk membantu anda menguasai Konteks Android. Ini adalah salah satu topik utama pembangunan Android, dan hampir tidak ada pembangun yang menggunakan konteks sepenuhnya dan dengan cara ia dirancang.

Saya pada awalnya menerbitkan artikel ini sebagai rangkaian empat catatan di laman web saya. Sekiranya anda berminat membaca bab demi bab, sila baca di sana.

Bermula

Adakah anda pernah temui soalan ini: Apakah perbezaan antara getContext(), this, getBaseContext(), dan getApplicationContext()? Sekiranya ya, artikel ini akan membantu menjelaskan kebingungan anda.

Catatan: anda harus mengetahui asas-asas pengembangan Android, seperti Activity, Fragments, Broadcast Receiver, dan blok bangunan lain. Sekiranya anda seorang pembangun baru yang baru memulakan perjalanan ke dunia Android, ini mungkin bukan tempat terbaik untuk memulakan.

Apa itu konteksnya?

Mari kita hadapi, Konteks adalah salah satu ciri API Android yang paling kurang dirancang. Anda boleh menyebutnya sebagai objek "Tuhan".

Kit aplikasi atau aplikasi Android (APK) adalah sekumpulan komponen. Komponen-komponen ini didefinisikan dalam Manifest, dan terdiri terutamanya dari Activity (UI), Service (Background), BroadcastReceiver (Action), ContentProvider (Data), dan Resources (gambar, string dll).

Pembangun boleh memilih untuk mendedahkan komponen tersebut ke sistem menggunakan penapis maksud. Contohnya: hantar e-mel atau kongsi gambar. Mereka juga boleh memilih untuk mendedahkan komponen hanya kepada komponen aplikasi mereka yang lain.

Begitu juga, sistem operasi Android juga dirancang untuk memaparkan komponen. Beberapa yang terkenal ialah WifiManager, Vibrator, dan PackageManager.

Konteks adalah jambatan antara komponen. Anda menggunakannya untuk berkomunikasi antara komponen, membuat komponen, dan mengakses komponen.

Komponen anda sendiri

Kami menggunakan konteks untuk memberi contoh komponen kami dengan Activity, Content Provider, BroadcastReceiver, dan sebagainya. Kami menggunakannya untuk mengakses sumber dan sistem fail juga.

Komponen anda dan komponen sistem

Konteks berfungsi sebagai titik masuk ke sistem Android. Beberapa komponen Sistem yang digunakan dengan baik adalah WifiManager, Vibrator, dan PackageManager. Anda boleh mengakses WifiManager menggunakan context.getSystemService(Context.WIFI_SERVICE).

Dengan cara yang sama, anda dapat menggunakan konteks untuk mengakses sistem fail yang didedikasikan untuk aplikasi anda sebagai pengguna dalam OS.

Komponen anda sendiri dan komponen aplikasi lain

Berkomunikasi antara komponen anda sendiri dan komponen aplikasi lain hampir sama jika anda menggunakan pendekatan penapis maksud. Bagaimanapun, setiap komponen adalah warganegara yang setara dalam Android.

Contoh niat yang digunakan untuk menghantar e-mel adalah di bawah. Semua komponen yang menawarkan tindakan niat ini akan diserahkan kepada pengguna yang dapat memilih apa yang akan digunakan.

Intent emailIntent = new Intent(android.content.Intent.ACTION_SEND);

Ringkasan

Mari kita bersetuju bahawa semua yang ada di Android adalah komponen. Konteks adalah jambatan antara komponen. Anda menggunakannya untuk berkomunikasi antara komponen, membuat komponen, dan mengakses komponen. Saya harap definisinya sekarang jelas.

Berbagai jenis Konteks

Terdapat banyak cara untuk mendapatkan penekanan pada konteks ( reka bentuk buruk dilihat ).

Selalunya kita menggunakan salah satu perkara berikut apabila kita memerlukan konteks:

- Application instance as context- Activity - Instance of your activity (this) - getApplicationContext() in Activity - getBaseContext() in Activity- Fragment - getContext() in Fragment- View - getContext() in View- Broadcast Receiver - Context received in broadcast receiver- Service - Instance of your service (this) - getApplicationContext() in Service- Context - getApplicationContext() in Context instance

Saya membahagikan jenis konteks kepada dua kategori: Konteks UI dan Konteks Bukan UI . Perbezaan ini akan membantu anda memahami n-waysdengan lebih baik.

Konteks UI

Pada hakikatnya, hanya ContextThemeWrapper adalah UI Konteks - yang bermaksud Konteks + Tema anda .

Aktiviti dilanjutkan ContextThemeWrapper. Inilah sebabnya, apabila anda mengembang XML apa pun, pandangan anda bertema. Sekiranya anda menaikkan susun atur anda dengan konteks Bukan UI, susun atur anda tidak akan bertema. Teruskan, cubalah.

Apabila anda menggunakan Activity sebagai placeholder untuk Konteks, anda dijamin akan menggunakan UI Context. Sekiranya anda menggunakan kaedah getContext dari Fragment, anda secara tidak langsung menggunakan Activity (jika anda melampirkan Fragment via fragmentManager dalam aktiviti).

Tetapi view.getContext()tidak dijamin menjadi Konteks UI.

Sekiranya Paparan dibuat menggunakan Layout Inflater dan lulus UI Context, anda akan mendapat UI Context kembali. Tetapi jika ia ditunjukkan dengan tidak meneruskan UI Konteks, anda akan mendapatkan kembali konteks yang lain.

UI Context- Activity - Instance of your activity (this)- Fragment - getContext() in Fragment- View - getContext() in View (if View was constructed using UI-Context)

Konteks Bukan UI

Anything except UI Context is Non-UI Context. Technically, anything which is not ContextThemeWrapper is Non-UI Context.

Non-UI Context is allowed do almost everything UI-Context can do (bad design spotted). But as we pointed out above, you lose theming.

Non-UI Context- Application instance as context- Activity - getApplicationContext() in Activity- Broadcast Receiver - Context received in broadcast receiver- Service - Instance of your service (this) - getApplicationContext() in Service- Context - getApplicationContext() in Context instance

Tip: All context types are supposed to be short lived except Application context. This is the one you get from your application class or from using the getApplicationContext() method when you have context access.

Summary

We have simplified it a little bit by putting Context in two buckets. UI Context is Context + Theming, and technically any class which is a subclass of ContextThemeWrapper comes in this bucket. Non-UI Context is all other types of Context.

Where to use what

The question arises: what will go wrong if you use context in the wrong place? Following are a few scenarios:

Scenario 1

Lets say you are inflating a layout and you use Non-UI Context. What may go wrong? You can guess in this case: you will not get a themed layout. Not so bad, hmm? It’s bearable.

Scenario 2

You pass UI-Context to someplace where all it needs is resource access or file system access. What can no wrong? Short Answer: Nothing. Remember, UI-Context = Context + Theme. It will gladly serve as context for you.

Scenario 3

You pass UI-Context to someplace where all it needs is resource access or file system access but it is a long operation in the background. Say downloading a file. Now what can go wrong? Short Answer: Memory leak.

If you are lucky and download completes quickly, the object is released and everything is fine. Sun is shining and birds are chirping. This is one of the most common mistakes developers make. They pass the reference of UI-Context to long living objects, and sometimes it has zero side effect.

However, sometimes Android wants to claim memory for either one of your next component’s requirements or another component’s requirements, and woooshhhh!!! You run out of memory in your app. Don’t worry, I will explain.

Memory Leak or Crash! That’s it.

Yes this is the worst case scenario when you use context in the wrong place. If you are new to the app development world, let me share some wisdom. Memory leaks are inversely proportional to your experience. Every Android developer has leaked memory. There is no shame in doing so.

Shame is when you repeat the mistake again and leak it the same way. If you leak memory a different way every time, congrats you are growing. I have explained what a Memory leak is with a short story here.

Okay I get it, but what is the relation of Context here?

Say it aloud, “Bad Design Spotted".

Almost everything in Android needs access to Context. Naive developers pass UI Context, because that’s what they have access to very easily. They pass short-living context (usually Activity context) to long living objects and before the memory/money is returned back to system, they hit a crisis. Woooshhh!!!

The simplest way to deal with this is with Async Task or Broadcast Receiver. But discussing them isn’t in the scope of this article.

Summary

  • Do you need to access UI related stuff? Use UI-Context. Inflating Views and showing dialogue are the two use cases I can think of.
  • Otherwise, Use Non UI Context.
  • Make sure you do not pass short-living context to long-living objects.
  • Pass knowledge, help people, plant trees and invite me for a coffee.

Tips and Tricks

What is the difference between this, getApplicationContext() and getBaseContext()?

This is one question every Android developer have encountered in their lives. I will try to simplify it as much as possible. Let’s take a step back and revisit the basics.

We know there are many factors in mobile devices. For instance, configuration changes all the time, and locale can change explicitly or implicitly.

All of these changes trigger apps to re-create so they can pick the right resources that are the best match to their current configuration. Portrait, Landscape, Tablet, Chinese, German, and so on. Your app needs the best possible resources to deliver the best user experience. It is the Context which is responsible for delivering those best match resources.

Try answering this question:

The user’s configuration is currently in portrait and you want to access landscape resources. Or the user locale is en and you want to access uk resources. How will you do it?

Below are some magical methods from Context:

There are many createX methods, but we are mainly interested in createConfigurationContext. Here is how you can use it:

Configuration configuration = getResources().getConfiguration();configuration.setLocale(your_custom_locale);context = createConfigurationContext(configuration);

You can get a hold of any type of Context you desire. When you call any method on the new Context you just got, you will get access to resources based on the configuration you had set.

I know it is amazing. You can send me thank you card.

Similarly, you can create a Themed Context and use it to inflate views with the theme you want.

ContextThemeWrapper ctw = new ContextThemeWrapper(this, R.style.YOUR_THEME);

Let’s come back to the tricky question we asked above and discuss Activity Context.

What is the difference between this, getApplicationContext()and getBaseContext()?

These are the possible ways you can get a hold on Context when you are in the Activity scope.

thispoints to Activity itself, our UI Context and short life context. getApplicationContext() points to your application instance which is Non-UI and long living context.

baseContext is the base of your Activity Context which you can set using a delegate pattern. You already know you can create Context with any xyz configuration you want. You can combine your xyz configuration knowledge with Base Context and your Activity will load resources as you desire.

Here is the method you can use:

@Overideprotected void attachBaseContext (Context base) {super.attachBaseContext(useYourCustomContext);}

Once BaseContext is attached, your Activity will delegate calls to this object. If you do not attach to Activity, it remains baseContext and you get Activity when you call getBaseContext.

Conclusion

We can say Context is the life of your android app. From Android’s point of view, it is your app. You can do almost nothing without Context. Without it, your app is plain Java code.

Context + Java code => Android

Good or bad, it is the design we have and we have to make the best of it. From the first part of this article, we learned that we use it to communicate between components, instantiate components, and access components.

In the next part, we learned that Context can be UI or NonUI, Short Lived or Long lived.

Following that, we learned that you need to choose context carefully otherwise you have to deal with memory leaks and other UI issues.

Finally, you saw that Context is responsible for loading best match resources for your app and you can configure it as you want. We also learned the difference between this, applicationContext and baseContext.

Many developers will advise you to use only application context. Do not use Application Context everywhere from the fear of a memory leak. Understand the root cause and always use the right Context in the right place.

You, my dear friend, are a master of Android Context now. You can suggest the next topic you want to understand. Click here to suggest.

Below are links from the original Series Mastering Android Contexton my blog.

Chapter 1

What the heck is Context? Why do we need it and what are various use cases in day to day development?

Chapter 2

Simplifying Context. We will discuss how many types of context are there and which ones are you suppose to use.

Chapter 3

Where to use UI Context and where to use Non UI-Context. How using context at wrong place may lead to memory leaks.

Chapter 4

My UI Context also offers me multiple types of context. Let’s answer this question and see how to avoid common pitfalls.

Training

Do you know that many times your app is crashing because your developers are not using Context properly? Let’s learn together. I offer training in Android, Java, and Git.

Want to master Android themes? Check out our series with more than 3k upvotes.

Feel free to share your feedback and questions. Happy Coding.

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