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I'm trying to use the UI-Thread, so I've written a simple test activity. But I think I've misunderstood something, because on clicking the button - the app does not respond anymore
public class TestActivity extends Activity {
Button btn;
int i = 0;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
btn = (Button)findViewById(R.id.btn);
btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
runThread();
private void runThread(){
runOnUiThread (new Thread(new Runnable() {
public void run() {
while(i++ < 1000){
btn.setText("#"+i);
try {
Thread.sleep(300);
catch (InterruptedException e) {
e.printStackTrace();
–
–
–
Just wrap it as a function, then call this function from your background thread.
public void debugMsg(String msg) {
final String str = msg;
runOnUiThread(new Runnable() {
@Override
public void run() {
mInfo.setText(str);
You have it back-to-front. Your button click results in a call to runOnUiThread(), but this isn't needed, since the click handler is already running on the UI thread. Then, your code in runOnUiThread() is launching a new background thread, where you try to do UI operations, which then fail.
Instead, just launch the background thread directly from your click handler. Then, wrap the calls to btn.setText() inside a call to runOnUiThread().
–
There are several techniques using of runOnUiThread(), lets see all
This is my main thread (UI thread) called AndroidBasicThreadActivity and I'm going to update it from a worker thread in various ways -
public class AndroidBasicThreadActivity extends AppCompatActivity
public static TextView textView;
@Override
protected void onCreate(Bundle savedInstanceState)
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_android_basic_thread);
textView = (TextView) findViewById(R.id.textview);
MyAndroidThread myTask = new MyAndroidThread(AndroidBasicThreadActivity.this);
Thread t1 = new Thread(myTask, "Bajrang");
t1.start();
1.) By passing Activity's instance as an argument on worker thread
class MyAndroidThread implements Runnable
Activity activity;
public MyAndroidThread(Activity activity)
this.activity = activity;
@Override
public void run()
//perform heavy task here and finally update the UI with result this way -
activity.runOnUiThread(new Runnable()
@Override
public void run()
AndroidBasicThreadActivity.textView.setText("Hello!! Android Team :-) From child thread.");
2.) By using View's post(Runnable runnable) method in worker thread
class MyAndroidThread implements Runnable
Activity activity;
public MyAndroidThread(Activity activity)
this.activity = activity;
@Override
public void run()
//perform heavy task here and finally update the UI with result this way -
AndroidBasicThreadActivity.textView.post(new Runnable()
@Override
public void run()
AndroidBasicThreadActivity.textView.setText("Hello!! Android Team :-) From child thread.");
3.) By using Handler class from android.os package
If we don't have the context (this/ getApplicationContext()) or Activity's instance (AndroidBasicThreadActivity.this) then we have to use Handler class as below -
class MyAndroidThread implements Runnable
Activity activity;
public MyAndroidThread(Activity activity)
this.activity = activity;
@Override
public void run()
//perform heavy task here and finally update the UI with result this way -
new Handler(Looper.getMainLooper()).post(new Runnable() {
public void run() {
AndroidBasicThreadActivity.textView.setText("Hello!! Android Team :-) From child thread.");
–
–
–
We use Worker Thread to make Apps smoother and avoid ANR's. We may need to update UI after the heavy process in worker Tread.
The UI can only be updated from UI Thread. In such cases, we use Handler or runOnUiThread both have a Runnable run method that executes in UI Thread.
The onClick method runs in UI thread so don't need to use runOnUiThread here.
Using Kotlin
While in Activity,
this.runOnUiThread {
// Do stuff
From Fragment,
activity?.runOnUiThread {
// Do stuff
Using Java,
this.runOnUiThread(new Runnable() {
void run() {
// Do stuff
@UiThread
public void logMsg(final String msg) {
new Handler(Looper.getMainLooper()).post(new Runnable() {
@Override
public void run() {
Log.d("UI thread", "I am the UI thread");
You can use from this sample :
In the following example, we are going to use this facility to publish the result from a
synonym search that was processed by a background thread.
To accomplish the goal during the OnCreate activity callback, we will set up
onClickListener to run searchTask on a created thread.
When the user clicks on the Search button, we will create a Runnable anonymous
class that searches for the word typed in R.id.wordEt EditText and starts the
thread to execute Runnable.
When the search completes, we will create an instance of Runnable SetSynonymResult
to publish the result back on the synonym TextView over the UI thread.
This technique is sometime not the most convenient one, especially when we don't
have access to an Activity instance; therefore, in the following chapters, we are
going to discuss simpler and cleaner techniques to update the UI from a background
computing task.
public class MainActivity extends AppCompatActivity {
class SetSynonymResult implements Runnable {
String synonym;
SetSynonymResult(String synonym) {
this.synonym = synonym;
public void run() {
Log.d("AsyncAndroid", String.format("Sending synonym result %s on %d",
synonym, Thread.currentThread().getId()) + " !");
TextView tv = (TextView) findViewById(R.id.synonymTv);
tv.setText(this.synonym);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button search = (Button) findViewById(R.id.searchBut);
final EditText word = (EditText) findViewById(R.id.wordEt);
search.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Runnable searchTask = new Runnable() {
@Override
public void run() {
String result = searchSynomim(word.getText().toString());
Log.d("AsyncAndroid", String.format("Searching for synonym for %s on %s",
word.getText(), Thread.currentThread().getName()));
runOnUiThread(new SetSynonymResult(result));
Thread thread = new Thread(searchTask);
thread.start();
static int i = 0;
String searchSynomim(String word) {
return ++i % 2 == 0 ? "fake" : "mock";
Source :
asynchronous android programming Helder Vasconcelos
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
gifImageView = (GifImageView) findViewById(R.id.GifImageView);
gifImageView.setGifImageResource(R.drawable.success1);
new Thread(new Runnable() {
@Override
public void run() {
try {
//dummy delay for 2 second
Thread.sleep(8000);
} catch (InterruptedException e) {
e.printStackTrace();
//update ui on UI thread
runOnUiThread(new Runnable() {
@Override
public void run() {
gifImageView.setGifImageResource(R.drawable.success);
}).start();
It's because:
1) the implicit this in your call to runOnUiThread is referring to AsyncTask, not your fragment.
2) Fragment doesn't have runOnUiThread.
However, Activity does.
Note that Activity just executes the Runnable if you're already on the main thread, otherwise it uses a Handler. You can implement a Handler in your fragment if you don't want to worry about the context of this, it's actually very easy:
// A class instance
private Handler mHandler = new Handler(Looper.getMainLooper());
// anywhere else in your code
mHandler.post(<your runnable>);
// ^ this will always be run on the next run loop on the main thread.
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