Handler in Android
In Android, some times we want to perform long-running tasks using the background thread with the help of a thread class.
In Android application we have two types of threads:
Example We want to download a file from a remote server and update the UI progress bar for showing the file downloading status. For that we use a background thread to download the file from a remote server, otherwise, if we initialize a long-running task in Main Thread (UI Thread), the UI will slow
We can do this with the help of background thread. However, the problem is that we want to update the UI from a background thread. The background thread will not allow us to communicate directly to the main thread.
Handler
To communicate main thread and a background thread with the help of Handler.
A handler works as a mediator between the background thread and the main thread.
Uses of Handler
To schedule messages and taks to be executed at some point in the future.
To queue an action to be performed on a different thread than your own
Let see Example
XML file
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="https://schemas.android.com/apk/res/android"
xmlns:app="https://schemas.android.com/apk/res-auto"
xmlns:tools="https://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="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="160dp"
android:layout_marginLeft="160dp"
android:layout_marginTop="139dp"
android:layout_marginBottom="168dp"
android:background="@color/colorPrimary"
android:textColor="#FFF"
android:text="Start"
android:textSize="18sp"
app:layout_constraintBottom_toTopOf="@+id/progressBar"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<ProgressBar
android:id="@+id/progressBar"
style="?android:attr/progressBarStyleHorizontal"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_marginStart="27dp"
android:layout_marginLeft="27dp"
android:layout_marginEnd="27dp"
android:layout_marginRight="27dp"
android:layout_marginBottom="344dp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/btnStart" />
<TextView
android:id="@+id/txtCount"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:textAlignment="center"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintTop_toBottomOf="@+id/progressBar"
tools:layout_editor_absoluteX="132dp" />
</androidx.constraintlayout.widget.ConstraintLayout>
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Code in Kotlin
MainActivity.kt
package com.rrtutors.handler
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.os.Handler
import android.os.Message
import android.view.View
import kotlinx.android.synthetic.main.activity_main.*
import android.R.attr.start
class MainActivity : AppCompatActivity() {
var MAX_COUNT = 100
var mHandlerThread: Handler? = null
private val START_PROGRESS = 100
private val UPDATE_COUNT = 101
var thread:Thread?=null;
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
btnStart.setOnClickListener(View.OnClickListener {
mHandlerThread?.sendEmptyMessage(START_PROGRESS);
})
thread= Thread(Runnable {
for (count in 0..100) {
try {
Thread.sleep(100)
} catch (ex: InterruptedException) {
ex.printStackTrace()
}
val message = Message()
message.what = UPDATE_COUNT
message.arg1 = count
mHandlerThread?.sendMessage(message)
}
})
mHandlerThread = object : Handler() {
override fun handleMessage(msg: Message) {
super.handleMessage(msg)
if (msg.what == START_PROGRESS) {
thread!!.start()
} else if (msg.what == UPDATE_COUNT) {
txtCount.text = "Progress Count : " + msg.arg1
progressBar.progress = msg.arg1
}
}
}
}
}
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Java Code
MainActivity.java
package com.rrtutors.handler;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
public class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity";
Button btnStar;
ProgressBar progressBar;
TextView txtCount;
int MAX_COUNT = 100;
Handler mHandlerThread;
private static final int START_PROGRESS = 100;
private static final int UPDATE_COUNT = 101;
Thread thread;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btnStar = findViewById(R.id.btnStart);
progressBar = findViewById(R.id.progressBar);
txtCount = findViewById(R.id.txtCount);
progressBar.setMax(MAX_COUNT);
btnStar.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
mHandlerThread.sendEmptyMessage(START_PROGRESS);
}
});
thread = new Thread(new Runnable() {
@Override
public void run() {
for (int count = 0; count <= 100; count++) {
try {
Thread.sleep(100);
} catch (InterruptedException ex) {
ex.printStackTrace();
}
Message message = new Message();
message.what = UPDATE_COUNT;
message.arg1 = count;
mHandlerThread.sendMessage(message);
}
}
});
mHandlerThread = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
if (msg.what == START_PROGRESS) {
thread.start();
} else if (msg.what == UPDATE_COUNT) {
txtCount.setText("Progress Count : " + msg.arg1);
progressBar.setProgress(msg.arg1);
}
}
};
}
}
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