Scopes in Kotlin Coroutines - GeeksforGeeks

15 Oct, 2021
Prerequisites:
Kotlin Coroutines on Android
Suspend Function In Kotlin Coroutines
Scope in Kotlin’s coroutines can be defined as the restrictions within which the Kotlin coroutines are being executed. Scopes help to predict the lifecycle of the coroutines. There are basically 3 scopes in Kotlin coroutines:
Global ScopeLifeCycle ScopeViewModel Scope
Global Scope
LifeCycle Scope
ViewModel Scope
Import following dependencies to build.gradle (app) level file.
implementation ‘org.jetbrains.kotlinx:kotlinx-coroutines-core:1.3.5’
implementation ‘org.jetbrains.kotlinx:kotlinx-coroutines-android:1.3.5’
def arch_version = ‘2.2.0-alpha01’
implementation “androidx.lifecycle:lifecycle-viewmodel-ktx:$arch_version”
implementation “androidx.lifecycle:lifecycle-runtime-ktx:$arch_version”  
Global Scope is one of the ways by which coroutines are launched. When Coroutines are launched within the global scope, they live long as the application does. If the coroutines finish it’s a job, it will be destroyed and will not keep alive until the application dies, but let’s imagine a situation when the coroutines has some work or instruction left to do, and suddenly we end the application, then the coroutines will also die, as the maximum lifetime of the coroutine is equal to the lifetime of the application. Coroutines launched in the global scope will be launched in a separate thread. Below is the example which shows that the in global scope coroutines are launched in a separate thread.
Kotlin
import android.os.Bundleimport android.util.Logimport androidx.appcompat.app.AppCompatActivityimport kotlinx.coroutines.GlobalScopeimport kotlinx.coroutines.launch class MainActivity : AppCompatActivity() {    val TAG = "Main Activity"    override fun onCreate(savedInstanceState: Bundle?) {        super.onCreate(savedInstanceState)        setContentView(R.layout.activity_main)        GlobalScope.launch {            Log.d(TAG, Thread.currentThread().name.toString())        }        Log.d("Outside Global Scope", Thread.currentThread().name.toString())    }}
 
 
Below is the Log-Output for the above program:
 
 
As it is known that coroutines launched in global scope live as long as the application does, but there are very rare chances when the developer needs the coroutines to be live as long as the application does. The main problem with the coroutines launched in the global scope is that when the activity in which coroutines is launched dies, the coroutines will not die with the activity, since the lifetime of coroutines is decided on the basis of application lifetime, not the activity lifetime. Since the coroutine is using the resources of the activity in which it is launched, and now since that activity has died, the resources will not be garbage collected as a coroutine is still referring to that resources. This problem can lead to a memory leak. So using global scope all the time is not always a good idea. Let’s try to launch a coroutine and run an infinite loop with a delay of 1 sec and launch another coroutine within the global scope after the delay of 5sec from the starting by terminating the first activity and intent to another activity. we can see in the output that even after the first activity is being terminated programmatically, the coroutine associated with the first activity does not die. Let’s try to understand what written in the above paragraph programmatically. Below is the code for both the activity_main.xml and the MainActivity.kt file.
 
XML
Kotlin
<?xml version="1.0" encoding="utf-8"?><androidx.constraintlayout.widget.ConstraintLayout    xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:app="http://schemas.android.com/apk/res-auto"    xmlns:tools="http://schemas.android.com/tools"    android:layout_width="match_parent"    android:layout_height="match_parent"    tools:context=".MainActivity">     <Button        android:id="@+id/btnStartActivity"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:layout_marginStart="167dp"        android:layout_marginTop="320dp"        android:layout_marginEnd="156dp"        android:layout_marginBottom="363dp"        android:text="Start Activity"        android:textSize="22sp"        app:layout_constraintBottom_toBottomOf="parent"        app:layout_constraintEnd_toEndOf="parent"        app:layout_constraintStart_toStartOf="parent"        app:layout_constraintTop_toTopOf="parent" /> </androidx.constraintlayout.widget.ConstraintLayout>
// program for main activity which intent to another activity// it uses global scope to launch the coroutine import android.content.Intentimport android.os.Bundleimport android.util.Logimport androidx.appcompat.app.AppCompatActivityimport kotlinx.android.synthetic.main.activity_main.*import kotlinx.coroutines.GlobalScopeimport kotlinx.coroutines.delayimport kotlinx.coroutines.launch const val TAG = "Main Activity" class MainActivity : AppCompatActivity() {    override fun onCreate(savedInstanceState: Bundle?) {        super.onCreate(savedInstanceState)        setContentView(R.layout.activity_main)         btnStartActivity.setOnClickListener {            // coroutine will launch when button got pressed            GlobalScope.launch {                // infinite loop                while (true) {                    delay(1000L)                    Log.d(TAG, "Still Running..")                }            }             GlobalScope.launch {                delay(5000L)                // new activity will get intended after 5 sec                val intent = Intent(this@MainActivity, SecondActivity::class.java)                startActivity(intent)                // calling finish() method,so that first activity will not be alive                // after being intended to second activity                finish()            }        }    }}
 
 
Go to app > java > 1st package name > right-click > New > Activity > Empty Activity to create a new activity and named it as SecondActivity. Below is the code for both the activity_second.xml and SecondActivity.kt file.
 
XML
Kotlin
<?xml version="1.0" encoding="utf-8"?><androidx.constraintlayout.widget.ConstraintLayout    xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:app="http://schemas.android.com/apk/res-auto"    xmlns:tools="http://schemas.android.com/tools"    android:layout_width="match_parent"    android:layout_height="match_parent"    tools:context=".SecondActivity">     <!--Various attributes for button-->    <Button        android:id="@+id/btnSecondActivity"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:layout_marginStart="166dp"        android:layout_marginTop="331dp"        android:layout_marginEnd="130dp"        android:layout_marginBottom="352dp"        android:gravity="center"        android:text="Second Activity"        android:textSize="22sp"        app:layout_constraintBottom_toBottomOf="parent"        app:layout_constraintEnd_toEndOf="parent"        app:layout_constraintStart_toStartOf="parent"        app:layout_constraintTop_toTopOf="parent" /> </androidx.constraintlayout.widget.ConstraintLayout>
// program for second activity import android.os.Bundleimport android.util.Logimport android.widget.Toastimport androidx.appcompat.app.AppCompatActivity class SecondActivity : AppCompatActivity() {    override fun onCreate(savedInstanceState: Bundle?) {        super.onCreate(savedInstanceState)        setContentView(R.layout.activity_second)        Log.i("Second Activity", "Second Activity Running ....")        Toast.makeText(this, "Second Activity", Toast.LENGTH_SHORT).show()    }}
 
 
Output: 
 
 
It can be seen in the log out below that even after the second activity is being launched, the coroutine of the main activity is still running. The oval circle is used to show the timestamps.
 
 
The lifecycle scope is the same as the global scope, but the only difference is that when we use the lifecycle scope, all the coroutines launched within the activity also dies when the activity dies. It is beneficial as our coroutines will not keep running even after our activity dies. In order to implement the lifecycle scope within our project just launch the coroutine in lifecycle scope instead of global scope, ie just change the global scope to lifecycle scope in the main activity within which the infinite loop is running. All the code will remain the same except for some changes in the code of the main activity as mentioned above.
 
Kotlin
// program to show how lifecycle scope works import android.content.Intentimport android.os.Bundleimport android.util.Logimport androidx.appcompat.app.AppCompatActivityimport androidx.lifecycle.lifecycleScopeimport kotlinx.android.synthetic.main.activity_main.*import kotlinx.coroutines.GlobalScopeimport kotlinx.coroutines.delayimport kotlinx.coroutines.launch const val TAG = "Main Activity" class MainActivity : AppCompatActivity() {    override fun onCreate(savedInstanceState: Bundle?) {        super.onCreate(savedInstanceState)        setContentView(R.layout.activity_main)         btnStartActivity.setOnClickListener {            // launching the coroutine in the lifecycle scope            lifecycleScope.launch {                while (true) {                    delay(1000L)                    Log.d(TAG, "Still Running..")                }            }             GlobalScope.launch {                delay(5000L)                val intent = Intent(this@MainActivity, SecondActivity::class.java)                startActivity(intent)                finish()            }        }    }}
 
 
Log Output: 
 
The oval circle is used to show the timestamps.
 
 
It can be seen in the above log output that the main activity stops get printing after the launch of the second activity.
 
 
It is also the same as the lifecycle scope, only difference is that the coroutine in this scope will live as long the view model is alive. ViewModel is a class that manages and stores the UI-related data by following the principles of the lifecycle system in android. If one wants to dig deeper into what basically view model class is can refer to this link of android official docs. 
 
kashishsoda
android
Android
Kotlin
Android
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