Develop Reference

Is there an event that triggers when the app is closed?

As the title says.
Specifically, I am writing an app that prints data to files over the course of runtime. I want to know when I can tell my PrintWriters to save the files. I understand that I can probably do autosave every X minutes, but I am wondering if Android Studio will let me save on close instead. I tried using onDestroy but the code block never executed. (To be precise, I started the app, did a few things, closed the app, clicked Recents, and swiped the app away. The debugger showed that the app never got to that code.)
My current solution attempts to catch the surrounding circumstances by checking for key presses but this only works for the back and volume buttons and not the home, recent, or power buttons.
#Override public boolean onKeyDown(int key, KeyEvent event) {
close();
return false;
}

There's a built in hook to the Activity lifecycle to save your state- onSaveInstanceState. There's even a bundle passed into you to save your state into for it to be restored (the matching function is onResumeInstanceState). And as a free bonus, if you call super.onSaveInstanceState and super.onRestoreInstanceState, it will automatically save the UI state of your app for all views with an id.

Please check the activity lifecycle:
https://developer.android.com/guide/components/activities/activity-lifecycle
Or if you're using a fragment:
https://developer.android.com/guide/fragments/lifecycle
Consider using one of these two:
#Override public void onStart() {
super.onStart();
close();
}
#Override public void onPause() {
super.onPause();
close();
}

How to show Online of a user?

I'm trying to get an app to show all the users online... I'm using the following way to achieve it
#Override
public void onStart(){
super.onStart();
mDatabaseReference.child("Online").setValue(true);
}
#Override
public void onStop(){
super.onStop();
mDatabaseReference.child("Online").setValue(false);
}
I'm not using ondisconnect because it shows offline only if the app is completely closed(not running in the background). SO i used this method on each activity... But the problem is that whenever i open an activity it shows online and the next second turns offline... I'm guessing that its because the prev activity closes after opening the new activity so the presents activity's on start is executed before the next activity's on stop. So since the activity's onstop is executed last it shows offline. How do i solve this problem

Your guess is completely correct.
Starting a new Activity will cause the onPause() and onStop() methods to be called for the first Activity.
Based on your description, I'm assuming that you want the Online status to remain true for as long as the app remains in the foreground and you are putting it in every Activity because your app doesn't only open from a single main Activity.
Your current code will work without a problem if you switched to using a Single-Activity Architecture, which is simply to use a single Activity and have it display different Fragments instead of new Activities. This solution will work with your existing code because onStop() and onStart() will only be called when your app enters the background.
If you look at the Navigation section in the official Android Developers Blog, you'll see that Google wants to encourage developers to switch to using the Single-Activity Architecture.
However, if you still wish to use multiple Activities, then you might want to consider an alternative way to keep track of Online status to take into account of multiple Activities.
For example, rather than using a simple boolean value, you can use an int value.
#Override
public void onStart(){
super.onStart();
mDatabaseReference.child("Online").addValue();
}
#Override
public void onStop(){
super.onStop();
mDatabaseReference.child("Online").removeValue();
}
With the addValue() and removeValue() being:
private void addValue(){
activityCount++;
onlineStatus = true;
}
private void removeValue(){
activityCount--;
if(activityCount <= 0)
onlineStatus = false;
}
Please keep in mind that this is only an example and doesn't take into account of how your app is designed. Bottom line is, you'll have to think of a solution that takes into account of multiple Activities that are displayed.
I do heavily suggest the Single-Activity approach.

Android QR Scanner: How to quit ZXingScannerView.ResultHandler to get back to where I came from

In my first steps in exploring Android I now start with QR scanning.
Works all pretty well. But I am not able to come back from the ResultHandler after read the QR successfully to my MainActivity.
public class MainActivity extends AppCompatActivity implements
ZXingScannerView.ResultHandler
{
private ZXingScannerView mScannerView
....
#Override
public void handleResult(Result rawResult)
{
// my results are ok in rawResult
// the scanner does not scan anymore but it is still there
// how to go back to my main activity???
}
public void ClickButton (View view)
{
mScannerView = new ZXingScannerView(this);
setContentView(mScannerView);
mScannerView.setResultHandler(this);
mScannerView.startCamera();
}
}
}
I tried
mScannerview.stopCameraPreview
mScannerView.stopCamera
this.finish
setContentView(R.layout.activity_main); // shows my activity_main
// but I can not click anything
Thanks!!
EDIT
I added some code to describe it a bit better. The idea is from
https://www.numetriclabz.com/android-qr-code-scanner-using-zxingscanner-library-tutorial/

Your question isn't clear but I'm assuming you want to restart the scan process. Normally, you'd have to restart the SurfaceHolder to be in preview mode. Luckily for you the ZXingScannerView already has a method to do that. Call mScannerView.resumeCameraPreview(this) to restart the scan process.
Otherwise can you clarify? You say you want to go back but you're already in MainActivity

If you want to go back into activities/fragments stack you can try Activity.onBackPressed()
if you are in a fragment you must call this method against attached Activity
What do you want is not going back to your activity. You want to restore activity's layout.
I think the better choice is to add ScannerView to your activity's layout file with android:visibility="gone". Then in on click you can get this view and change it's visibility to VISIBILE.
Then when you have handled scanning result, you can reset yuoir ScannerView to visibility = GONE

I too was stuck with this problem for an hour, just like you. And later realised..
To solve this problem DON NOT implementing the ZXingScannerView in the same activity or fragment. Instead start a new activity when you click the button and this activity is just for the ZXingScannerView
Once the Scan is done finish and pass the data back to your activity or fragment

Just restart your MainActivity before this.finish()
the code below will start your main activity through intent...
worked fine for me
startActivity(new Intent(this,MainActivity.class));
this.finish();

remove from onCreate method this line setContentView(your layout) and when you finish scan write it after you stoped the camera then you can use your layout after scan

I had a bit of a look into android concepts and activities.
I put the QR handling in a 2nd activity and it worked well with the finish ().
Thanks for help anyway!!

I think it's too late, but I came with the same problem and I had so find a solution by myself.
You were on the right way, you need two steps more.
I called the methods where I link and set the listener of any buttons
There are the methods
Basically you were right where you set the content view, but you need to give the buttons their functionality back.
(I know its late, but better late than never). Good luck!

When bringing Android activities from the stack to the front, how do I refresh (reinitialize) them? So to run onCreate again etc

If I'm bringing Android activities from the stack to the front, how do I refresh them? So to run onCreate again etc.
My code below, in conjunction with setting activities in the Android manifest to android:launchMode="singleTask" allows me to initiate an activity if that activity is not already active within the stack, if it is active within the stack it is then brought to the front.
How do I then, if the activity is brought to the front refresh it so that onCreate is ran again etc.
Intent intent = new Intent(myActivity.this,
myActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_REORDER_TO_FRONT);
startActivity(intent);

I think FLAG_ACTIVITY_CLEAR_TOP will resolved your problem:
Intent intent = new Intent(myActivity.this, myActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(intent);

I don't think there is an explicit way to refresh onCreate, perhaps you may want to add the code you want reloaded into onResume.
This workaround may work if you want to keep your code in onCreate.
finish();
startActivity(getIntent());

If all you want to do is refresh the content, you should override the onResume method, and add in the code to perform the refresh in this method. To do this, use the following code within the activity that you want to perform the refresh, (ie, not the same activity that you are calling startActivity() from):
#Override
protected void onResume(){
super.onResume();
//add your code to refresh the content
}
Tip: If you are using Android Studio, press Alt+Insert (while you have the Java file open), then click Override Methods, find onResume, and it should provide you with a basic template for the method.
The diagram I added shows the order that the methods are run (this is known as the Activity Lifecycle). onCreate() is run whenever an Activity is first created, followed by onStart(), followed by onResume(). As you can see, when a user returns to an Activity, onCreate() is not run again. Instead, onResume() is the first method that is called. Therefore, by putting your code into the onResume() method, it will be run when the user returns to the activity. (Unlike onCreate(), which will not be run again).
Extra info: Since you will be initially setting the data in onCreate() and then refreshing it within onResume(), you might want to consider moving all of your code used to initially set the data to onResume() as well. This will prevent redundancy.
Edit: Based on your following comment, I can give the following solution:
I'm wanting to properly refresh the page, e.g. if there is a variable count initialised at 0. And though running the activity it's has became equal to 300. When the activity is called (intent) then refreshed, count will once again be equal to it's initial value. Do you know how to do this?
Without your current activity's code, there is not much to work with, but here is some pseudo-code as to how I would accomplish your problem:
public class MyActivity extends Activity {
TextView numberTextView;
int numberToDisplay;
#Override
protected void onCreate(Bundle bundle){
super.onCreate(bundle);
setContentView(myContent);
numberTextView = (TextView) findViewById(R.id.numberTextView);
numberTextView.setText(numberToDisplay+"")//converts the integer to a string
}
#Override
protected void onResume(){
super.onResume();
numberToDisplay = 0;
numberTextView.setText(numberToDisplay+"");
}
}

Android activity restart

Hey guys, i am making an android application where i want to show a dialog box about legal agreement everytime the application starts, i have a public method showalert(<>); which shows an alertdialog by building a dialog with alertbuilder. I added a call to showalert() method on the onCreate() method of the main activity to show it, but whenever the user rotates the screen, he gets the dialog everytime. The activity restarts itself when the phone is rotated. I tried adding android:configChanges="keyboardHidden|orientation" to my manifest but that doesnt help on this case. Also can i know how to register a new application class on manifest file. I am trying to create an application class and put the code to show dialog on the new class's oncreate method. But i am not being able to load the class when the app starts.
I also checked Activity restart on rotation Android but i dont seem to get a thing. I am pretty much a newbie to android programming, could someone simplify that for me?
Any help would be appreciated. :)

you could maybe look at the onRetainNonConfigurationInstance() activity method, which is called just before destroying and re-creating the activity on screen orientation change.
it allows you to retain an object that could for instance contain a test variable to know if your legal thing was already shown or not.. example :
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
final String test = (String) getLastNonConfigurationInstance();
if (!("textAlreadyShown").equals(test)) {
//here : show your dialog
}
}
#Override
public String onRetainNonConfigurationInstance() {
return "textAlreadyShown";
}

Set the main activity to an activity that just shows the legal notice, when it is accepted/cleared, show a second activity ( which is currently the main activity )?