Android Handler – Background Thread Communicate with UI thread

Every Android app has its own thread that runs UI objects, this thread is called Main Thread or UI Thread. If your app is running a background thread long operation and you want to publish the result in-app screen. But here is a problem, your app task is running in the background and Where UI Thread is different and only objects running on the UI Thread have to access other objects in UI Thread. So your background app task can’t communicate or send data directly to Main Thread (UI Thread). That time you have to use an Android Handler, that’s running on UI Thread.

In this tutorial, you will learn the following:

1. Overview Android Handler
2. Complete example Background thread communicate to the Main thread using Handler.

Android Handler allows you to communicate with the UI thread from other background threads. It’s useful in android as android doesn’t allow other threads to communicate directly with the UI thread.

Let’s Build an Android Handler example in kotlin:

In Android app development using kotlin language. In the example on Click button thread will run and update the UI counter and progress bar using Handler. You can see the counting in TextView label . Let’s see the steps to how to do it.

Step 1. Create a new project “Build Your First Android App in Kotlin“

Step 2. Add following code in “activity_main.xml” layout file

Where Button will start the thread and Textview show count and ProgressBar will show update progress.

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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">

    <ProgressBar
        android:id="@+id/progressBar"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_alignParentTop="true"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_margin="8dp"
        android:layout_marginTop="120dp"
        android:text="Count"
        android:textSize="36sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/progressBar" />

    <Button
        android:id="@+id/start_progress"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerVertical="true"
        android:layout_margin="16dp"
        android:layout_marginTop="156dp"
        android:text="Start"
        android:textSize="18sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView" />

</android.support.constraint.ConstraintLayout>

Step 3. Open the “MainActivity.kt” and add following code

On clicking button – where we are updating ProgressBar and while updating ProgressBar same time updating TextView by using the same Handler.

package `in`.eyehunt.androidhandler

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.os.Handler
import android.os.Message
import android.util.Log
import kotlinx.android.synthetic.main.activity_main.*


class MainActivity : AppCompatActivity() {

    lateinit var mHandlerThread: Handler
    lateinit var mThread: Thread

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        progressBar.max = 100

        mThread = Thread(Runnable {
            for (i in 0..100) {
                Log.d("I", ":$i")
                progressBar.progress = i
                try {
                    Thread.sleep(100)
                } catch (ex: InterruptedException) {
                    ex.printStackTrace()
                }

                val message = Message()
                message.what = COUNT
                message.arg1 = i
                mHandlerThread.sendMessage(message)
            }
        })

        // start counting
        start_progress.setOnClickListener {
            if (!mThread.isAlive) {
                val currentProgess = progressBar.progress
                mHandlerThread.sendEmptyMessage(START)
            }
        }

    }

    override fun onResume() {
        super.onResume()
        mHandlerThread = object : Handler() {
            override fun handleMessage(msg: Message) {
                super.handleMessage(msg)
                if (msg.what === START) {
                    mThread.start()
                } else if (msg.what === COUNT) {
                    textView.text = "Count " + msg.arg1
                }
            }
        }
    }

    companion object {
        private const val START = 100
        private const val COUNT = 101
    }
}

Step 4. Now Run the application, in an emulator or On you android device

Output screenshot Android Handler Dialog :

Download source code Android Handler in kotlin

https://github.com/EyeHunts/AndroidHandler

Note : This example (Project) is developed in Android Studio 3.1.3 . Tested on Android 9 ( Android-P), compile SDK version API 27: Android 8.0 (Oreo)

MinSdkVersion=”15″

TargetSdkVersion=”27″

Coding in Kotlin

Any suggestions always welcome, please do comment.