Hello,
Today I will show you how to benefit from Handlers in Android 🙂
First of all, Handlers aren’t some new concept, they were there long ago. How long? Well, from API level 1. Still, I always felt Android developers don’t use them enough, me included.
When I’ve discover the brilliance of Handlers, I was surprised how handy they are and how easy is to use them.
So, what does a Handler do? A few things, really.
- Schedule and process messages
- Schedule and execute Runnables
- Perform its actions on a different Thread that a Handler was created in
- Being reused as many times as needed
Pretty nice stuff to begin with. One drawback, is that it creates an ambiguity, because Handler is not a Runnable (which is a type of class with a run code inside) and neither a Thread (independent flow of execution). You can think of a Handler as an advanced mechanism to handle a queue. Doesn’t matter if this queue contains Messages or Runnables, and whether they should be processed on a main or some background thread. Handler was created to handle messages, literally, and this is the main bit to remember.
A good example of using a Handler is when you have a Runnable, you do something in the background thread, and then – you want to update UI at some point. In this case, you initialize a Handler as new Handler(Looper.getMainLooper), call handler.post() and do the UI job inside the post(). Useful, isn’t it?
We all know, that a single AsyncTask can be executed only once. This is not a case with Handlers. There is even a special class to take care of a few Handlers – HandlerThread, which in turn takes care of a Looper, internally, so you don’t have to worry about it all the time. Here you can find a good explanation how it works, as well as see some code.
To show you different kind of Handler, Thread, and Runnable examples, I have created an Android test project – HandlersExample. I’ve pasted the main code part below. It has lots of comments, so everything should be self-explanatory. Run this code and experiment with Handlers, – they’re worth it!
package com.lomza.examples.handlers;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.support.annotation.UiThread;
import android.support.v7.app.AppCompatActivity;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import java.lang.ref.WeakReference;
import java.util.concurrent.TimeUnit;
/**
* Main activity with methods to demonstrates the usage of Handlers, Runnables, and Messages :)
*
* @author Antonina Tkachuk
*/
public class MainActivity extends AppCompatActivity {
private TextView tv01;
private TextView tv02;
private TextView tv03;
private TextView tv04;
private TextView tv05;
private TextView tv06;
private TextView tv07;
private Button button07;
private final Handler mainThreadHandler = new Handler(Looper.getMainLooper());
private Handler currentThreadHandler;
private Handler decorViewHandler;
private final ChangeTextHandler customHandler = new ChangeTextHandler(this);
private final Handler notLeakyHandler = new Handler();
private final Runnable notLeakyRunnable = new ChangeTextRunnable(this, "Hi from leak-safe Runnable!");
private static final String TAG = "[Handlers]";
private static final String BUNDLE_KEY = "greeting";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initView();
}
@Override
protected void onStart() {
super.onStart();
postTaskWithOrdinaryThread();
postTaskWithHandlerOnMainThread();
postTaskWithHandlerOnCurrentThread();
postTaskInsideBackgroundTask();
//postTaskWithHandlerOnBackgroundThread();
postTaskWithNotLeakyHandlerAndRunnable();
sendMessageToChangeTextHandler();
}
@Override
protected void onStop() {
super.onStop();
// when posting Runnables or Messages, always remember to call removeCallbacks() or removeMessages()
// or removeCallbacksAndMessages() for both.
// this ensures that all pending tasks don't execute in vain; for instance, the user has left our activity
// and he doesn't really care if some job is finished or not, so it's our responsibility to cancel it
// pass null to remove ALL callbacks and messages
mainThreadHandler.removeCallbacks(null);
currentThreadHandler.removeCallbacks(null);
if (decorViewHandler != null)
decorViewHandler.removeCallbacks(null);
customHandler.removeCallbacksAndMessages(null);
notLeakyHandler.removeCallbacks(notLeakyRunnable);
}
private void initView() {
tv01 = (TextView) findViewById(R.id.tv_01);
tv02 = (TextView) findViewById(R.id.tv_02);
tv03 = (TextView) findViewById(R.id.tv_03);
tv04 = (TextView) findViewById(R.id.tv_04);
tv05 = (TextView) findViewById(R.id.tv_05);
tv06 = (TextView) findViewById(R.id.tv_06);
tv07 = (TextView) findViewById(R.id.tv_07);
button07 = (Button) findViewById(R.id.button_07);
button07.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
postTaskWithThisWindowAndTextViewHandlers();
}
});
}
private void postTaskWithOrdinaryThread() {
Runnable notForHandlerTask = new Runnable() {
@Override
public void run() {
// as written here - https://developer.android.com/guide/components/processes-and-threads.html#Threads,
// do NOT access the Android UI toolkit from outside the UI thread, as sth unexpected may happen
// for instance, you might get android.view.ViewRootImpl$CalledFromWrongThreadException
tv01.setText("Hi from Thread(runnable)!");
// if you call thread.run(), this would be TRUE, as no new Thread would be created
// read the explanation here - http://stackoverflow.com/a/35264580/655275
Log.d(TAG, "[postTaskWithOrdinaryThread] Current looper is a main thread (UI) looper: "
+ (Looper.myLooper() == Looper.getMainLooper()));
}
};
Thread thread = new Thread(notForHandlerTask);
thread.start();
}
@UiThread
private void postTaskWithHandlerOnMainThread() {
Runnable mainThreadTask = new Runnable() {
@Override
public void run() {
// since we use Looper.getMainLooper(), we can safely update the UI from here
tv02.setText("Hi from Handler(Looper.getMainLooper()) post!");
Log.d(TAG, "[postTaskWithHandlerOnMainThread] Current looper is a main thread (UI) looper: "
+ (Looper.myLooper() == Looper.getMainLooper()));
}
};
mainThreadHandler.post(mainThreadTask);
}
private void postTaskWithHandlerOnCurrentThread() {
currentThreadHandler = new Handler();
Runnable currentThreadTask = new Runnable() {
@Override
public void run() {
// since we use current thread (and from onCreate(), it's the UI thread), we can safely update the UI from here
tv03.setText("Hi from Handler() post!");
Log.d(TAG, "[postTaskWithHandlerOnCurrentThread] Current looper is a main thread (UI) looper: "
+ (Looper.myLooper() == Looper.getMainLooper()));
}
};
currentThreadHandler.post(currentThreadTask);
}
private void postTaskInsideBackgroundTask() {
Thread backgroundThread = new Thread(new Runnable() {
@Override
public void run() {
// pretend to do something "background-y"
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
mainThreadHandler.post(new Runnable() {
@Override
public void run() {
tv04.setText("Hi from a Handler inside of a background Thread!");
}
});
}
});
backgroundThread.start();
}
@UiThread
private void postTaskWithThisWindowAndTextViewHandlers() {
// this line will return null from onCreate() (and even if called from onResume()) and cause NPE when trying to post();
// this is because the handler isn't attached to the view if it's not fully visible
decorViewHandler = getWindow().getDecorView().getHandler();
Runnable decorViewTask = new Runnable() {
@Override
public void run() {
// View's post() uses UI handler internally
tv07.post(new Runnable() {
@Override
public void run() {
tv07.setText("Hi from getWindow().getDecorView().getHandler() > TextView.post()!");
Log.d(TAG, "[postTaskWithThisWindowAndTextViewHandlers] Current looper is a main thread (UI) looper: "
+ (Looper.myLooper() == Looper.getMainLooper()));
}
});
}
};
decorViewHandler.post(decorViewTask);
}
private void postTaskWithHandlerOnBackgroundThread() {
final Runnable pretendsToBeSomeOtherTask = new Runnable() {
@Override
public void run() {
Log.d(TAG, "[postTaskWithHandlerOnBackgroundThread] Is there a looper? " + (Looper.myLooper() != null));
// you'll get java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
// this is because Handlers need to have a Looper associated with them; when the task was run on the main thread,
// main thread looper was used, but if we call this from the background thread, there is NO looper to use
// read more - https://developer.android.com/reference/android/os/Looper.html
postTaskWithHandlerOnCurrentThread();
}
};
final Thread thread = new Thread(pretendsToBeSomeOtherTask);
thread.start();
}
private void postTaskWithNotLeakyHandlerAndRunnable() {
// in order to eliminate leaks, both Handler and Runnable should be static
// static inner classes do not hold an implicit reference to the outer class
// it seems like a lot of useless work, but it's the most accurate and bug-free way
// read more - http://www.androiddesignpatterns.com/2013/01/inner-class-handler-memory-leak.html
notLeakyHandler.postDelayed(notLeakyRunnable, 500);
}
private static class ChangeTextRunnable implements Runnable {
private final WeakReference activity;
private final String greetingMessage;
public ChangeTextRunnable(MainActivity activity, String greetingMessage) {
this.activity = new WeakReference<>(activity);
this.greetingMessage = greetingMessage;
}
public void run() {
if (greetingMessage == null) {
Log.e(TAG, "The message is null ChangeTextRunnable.run()!");
return;
}
MainActivity activity = this.activity.get();
if (activity == null) {
Log.e(TAG, "Activity is null ChangeTextRunnable.run()!");
return;
}
activity.tv05.setText(greetingMessage);
}
}
// === OBTAIN AND HANDLE A MESSAGE ===
private void sendMessageToChangeTextHandler() {
Message messageToSend = customHandler.obtainMessage();
Bundle bundle = new Bundle();
bundle.putString(BUNDLE_KEY, "Hi from custom inner Handler!");
messageToSend.setData(bundle);
messageToSend.what = 6;
customHandler.sendMessage(messageToSend);
}
private static class ChangeTextHandler extends Handler {
private final WeakReference activity;
public ChangeTextHandler(MainActivity activity) {
this.activity = new WeakReference<>(activity);
}
@Override
public void handleMessage(Message msg) {
MainActivity activity = this.activity.get();
if (activity == null) {
Log.e(TAG, "Activity is null ChangeTextHandler.handleMessage()!");
return;
}
final String text = (String) msg.getData().get(BUNDLE_KEY);
if (!TextUtils.isEmpty(text)) {
switch (msg.what) {
case 6:
activity.tv06.setText(text);
break;
default:
activity.tv01.setText(text);
break;
}
}
}
}
// === END - OBTAIN AND HANDLE A MESSAGE ===
}
Thanks for reading and, as always, leave your comments below 😉
Useful links
Communicating with the UI Thread
How to Leak a Context: Handlers & Inner Classes
Activitys, Threads, & Memory Leaks
Android background processing with Handlers, AsyncTask and Loaders – Tutorial
I would like to thank Oliver Mueller, as I have used the image from his wonderful blog post in the header of this post.
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