Running User Interface Thread in Android using Kotlin - GeeksforGeeks

23 Sep, 2021
User Interface Thread or UI-Thread in Android is a Thread element responsible for updating the layout elements of the application implicitly or explicitly. This means, to update an element or change its attributes in the application layout ie the front-end of the application, one can make use of the UI-Thread. 
Realizing UI Thread: For example, a thread action is started, and the developer wants to update the front-end element with respect to the thread elements, runOnUIThread{...} function can be used.Below is an example of the application where UI thread is used. 
Sample App in which the text in the TextView is changed every second  
Initially, the application will show a Welcome message and as soon as the start button is clicked, it will show 2 messages, “love gfg” and “not gfg” alternatively at each second. 
Approach: Step 1: Add the below code in activity_main.xml. Here, add a TextView and a button to our MainActivity layout.
HTML
<?xml version="1.0" encoding="utf-8"?><RelativeLayout 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">     <TextView        android:id="@+id/tv1"        android:layout_width="210sp"        android:layout_height="100sp"        android:layout_centerInParent="true"        android:gravity="center"        android:textSize="50sp"        android:text="Welcome"        />     <Button        android:id="@+id/btnStart"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="Start"        android:layout_below="@id/tv1"        android:layout_centerHorizontal="true"        /> </RelativeLayout>
Step 2: Add the below code in MainActivity. Here OnClickListener is added with the button. So it is invoked whenever the user clicks the button. In the listener, an infinite loop is created in the main thread and using the UI thread the text is changed after every second. 
Java
class MainActivity : AppCompatActivity() {    override fun onCreate(savedInstanceState: Bundle?)    {        super.onCreate(savedInstanceState)        setContentView(R.layout.activity_main)         // Assigning Layout elements        val tv = findViewById<TextView>(R.id.tv1)        val btn = findViewById<Button>(R.id.btnStart)        val msg1 = "love gfg" val msg2 = "not gfg"         // Button onClick action        btn.setOnClickListener        {            // Declaring Main Thread            Thread(Runnable {                while (true) {                    // Updating Text View at current                    // iteration                    runOnUiThread{ tv.text = msg1 }                     // Thread sleep for 1 sec                    Thread.sleep(1000)                    // Updating Text View at current                    // iteration                    runOnUiThread{ tv.text = msg2 }                     // Thread sleep for 1 sec                    Thread.sleep(1000)                }            }).start()        }    }}
Note: The While loop must be declared only inside the Thread. If a Thread is declared inside a while loop, the program doesn’t work and crashes.
Sample Timer App From the basic concept of the above code, a timer app can be designed. Below is the code for the same: 
Approach: Step 1: Add the below code in MainActivity layout. Here a button, edittext, and textview are added. The button is used to start the timer, edittext is used to take the input from the user, and textview is used to display the time left. 
XML
<?xml version="1.0" encoding="utf-8"?><RelativeLayout 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/btnStart"        android:layout_width="100sp"        android:layout_height="50sp"        android:layout_centerInParent="true"        android:text="Start"        />     <EditText        android:id="@+id/et1"        android:layout_width="100sp"        android:layout_height="100sp"        android:layout_centerHorizontal="true"        android:layout_above="@id/btnStart"        android:textSize="50sp"        android:inputType="number"        android:gravity="center"        android:background="@color/colorPrimary"        android:textColor="@color/colorAccent"        />     <TextView        android:id="@+id/tv1"        android:layout_width="100sp"        android:layout_height="100sp"        android:layout_centerHorizontal="true"        android:layout_below="@id/btnStart"        android:gravity="center"        android:textSize="50sp"        android:background="@color/colorPrimaryDark"        android:textColor="@color/colorAccent"        /> </RelativeLayout>
Step 2: Add the below code in MainActivity class. Here we add the onClickListener with the button. As the button is clicked, runOnUiThread function is used to display the time left.
Java
class MainActivity : AppCompatActivity() {    override fun onCreate(savedInstanceState: Bundle?)    {        super.onCreate(savedInstanceState)        setContentView(R.layout.activity_main)         // Assigning Layout elements        val et = findViewById<EditText>(R.id.et1)        val btn = findViewById<Button>(R.id.btnStart)        val tv = findViewById<TextView>(R.id.tv1)               // Button onClick action        btn.setOnClickListener{            // Converting Edit Text input to String            val stringTime= (et.text).toString()            // Converting stringTime to Integer            val intTime= Integer.parseInt(stringTime)            // Declaring Main Thread             Thread(Runnable {                  // For loop Decrement                  for (i in intTime downTo 0) {                       // Updating Text View at                       // current iteration                        runOnUiThread{                            tv.text = i.toString()                         }                    // Thread sleep for 1 sec                        Thread.sleep(1000)                          }                      }).start()        }    }}
 
simmytarika5
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