I'm writing an application that has several activities. The main activity can receive data via Intent, then start another activity that process that data.
I have a problem when finished the second activity, since the main activity runs the onResume method (where check and intentions of the process), and the original intention is still alive. So App calls the second activity again.
My question is to eliminate the Intent data after calling second activity, and thus the return of second intention not to repeat the cycle.
Thank you very much
set a class variable to 0 and then make it 1 instead.
So you don't do the looping.
Can I eliminate the Intent data after calling second activity
yes.
As soon as you call second activity(Or the second activity is started), first activity onPause method will be called. You can override onPause method in Activity1 and do what ever you like to do.
Ex:
#Override
protected void onPause() {
super.onPause();
getIntent().removeExtra("YourMainActivityData");
}
Try add this code after u start second activity in onResume:
setIntent(new Intent());
this will replace the return intent when u call getIntent();
Hope it helps.
I can't remove Intent data, so I wrote an alternative method. I did set the intent-filter at the second activity, and this activity does all the work. When user wants to go back, I start first activity and finih second one.
Perhaps it's a good method, perhaps not is. But it runs OK for me.
Thank you very much!
As I understand it, an activity being destroyed is not equivalently to an activity being finished.
Finished
The activity is removed from the back stack.
It can be triggered by the program (e.g. by calling finish()), or by the user pressing the back key (which implicitly calls finish()).
Finishing an activity will destroy it.
Destroyed
The Android OS may destroy an invisible activity to recover memory. The activity will be recreated when the user navigates back to it.
The activity is destroyed and recreated when the user rotates the screen.
Reference: Recreating an Activity
So how do I finish a destroyed activity? The finish() method requires an Activity object, but if the activity is destroyed, I have no Activity object - I am not supposed to be holding a reference to a destroyed activity, am I?
Case study:
I have an activity a, which starts b, which in turn starts c (using Activity.startActivity()), so now the back stack is:
a → b → c
In c, the user fills out a form and tap the Submit button. A network request is made to a remote server using AsyncTask. After the task is completed, I show a toast and finish the activity by calling c.finish(). Perfect.
Now consider this scenario:
While the async task is in progress, the user switches to another app. Then, the Android OS decided to destroy all 3 activities (a, b, c) due to memory constraints. Later, the async task is completed. Now how do I finish c?
What I have tried:
Call c.finish():
Can't, because c is destroyed.
Call b.finishActivity():
Can't, because b is destroyed.
Use Context.startActivity() with FLAG_ACTIVITY_CLEAR_TOP so as to raise b to the top, thus finishing c:
// appContext is an application context, not an activity context (which I don't have)
Intent intent = new Intent(appContext, B.class); // B is b's class.
intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
appContext.startActivity(intent);
Failed, appContext.startActivity() throws an exception:
android.util.AndroidRuntimeException: Calling startActivity() from outside of an Activity context requires the FLAG_ACTIVITY_NEW_TASK flag. Is this really what you want?
Edit: Clarification: I need to wait until the async task finishes and decide whether to finish c based on server's response.
android.util.AndroidRuntimeException: Calling startActivity() from
outside of an Activity context requires the FLAG_ACTIVITY_NEW_TASK
flag. Is this really what you want?
This exception used to occur when you are starting an activity from
the background thread or service. You need to pass
FLAG_ACTIVITY_NEW_TASK flag whenever you need the "launcher"
type of behavior.
Just add mIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK); to avoid this exception.
The way you are trying to kill the activity is not recommended, let the
android handle itself. There isn't any meaning to finish an activity
which is already destroyed.
Now, what you can do?
If you are facing problem in finishing activity when app is not in foreground, what you can do is to implement a security check which will finish the activity only when app is in foreground to go to back-stack activity or else just skip that step.
I think you are trying to kill the activity when app is in background. It seems a little bit difficult to do so, but you can make use of onUserLeaveHint to decide when app is going in the background in-order to finish the activity or you can finish the activity by adding finish(); in onStop(). Just make sure that asynctask's onPost() don't finish it again in-order to avoid the exception.
Have a look at android:clearTaskOnLaunch attribute and set it to true.
Google Doc says about this attribute is:
for example, that someone launches activity P from the home screen,
and from there goes to activity Q. The user next presses Home, and
then returns to activity P. Normally, the user would see activity Q,
since that is what they were last doing in P's task. However, if P set
this flag to "true", all of the activities on top of it (Q in this
case) were removed when the user pressed Home and the task went to the
background. So the user sees only P when returning to the task.
and i think this is the exact case which you want.
Hope this will give you some hint to achieve your desired task.
you can broadcast your action from the onPostExecute method in c and register a broadcast receiver to receive for that action in a and b. Then do finish in that receiver onRevice method
In c , AsyncTask,
void onPostExecute(Long result) {
----
Intent intent1 = new Intent("you custom action");
context.sendBroadcast(intent1);
}
In a and b
registerReceiver(new BroadcastReceiver() {
#Override
public void onReceive(Context context, Intent intent) {
finish();
}
},new IntentFilter("you custom action"));
Personally, I'd use the notification bar to notify the user of the status of his query.
This way, I'd avoid the entire issue of having an unfinished activity. And I'd only keep the activity unfinished only if the user had not clicked on the submit button yet.
Regarding android manual onDestroy() called exactly before activity is destroyed, so you can call finish in it (even you can stop your bg thread before killing the activity completly).
We can assume that if activity was killed we don't interested in bg thread either, and for example if bg thread is to download image or etc that needs to be completed - so you have to use service instead of asynctask.
Can't finish a destroyed activity directly, so just finish() it in its onCreate() (suggested by #Labeeb P). Here's how:
If the activity is already destroyed when trying to finish it, save a boolean flag somewhere instead.
if(activity != null)
{
// Activity object still valid, so finish() now.
activity.finish();
}
else
{
// Activity is destroyed, so save a flag.
is_activity_pending_finish = true;
}
If the flag needs to stay even if the app is destroyed, use persistent storage, e.g. SharedPreferences (suggested by #Labeeb P).
In the activity's onCreate(), check the flag and call finish().
#Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
if(is_activity_pending_finish)
{
is_activity_pending_finish = false; // Clear the flag.
// This activity should have been finished, so finish it now.
finish();
return;
}
...
}
If there're multiple instances of the same Activity class, you may need something more than a boolean flag to identify the specific instance of activity to finish.
Calling finish() in onCreate() is actually a legimate operation, as it is mentioned in the doc:
... you might call finish() from within onCreate() to destroy the activity. In this case, the system immediately calls onDestroy() without calling any of the other lifecycle methods.
Other considerations:
It may not be a good idea to finish an activity while the app is in background, especially if it is the only activity. Make sure that you don't confuse the user.
For better user experience, if you finish an activity while the app is in background, you may want to inform the user. Consider using toasts (good for short notices) or notifications (good for long operations that the user may have forgotten)(suggested by #Stephan Branczyk and #dilix).
Of course, an activity being destroyed doesn't necessary mean that the app is in background (there might be another foreground activity). Still, the above solution (calling finish() in onCreate()) works.
When the system tries to destroy your Activity, it calls onSaveInstanceState. In here you can call finish(). That's it.
Warning: I've never tried this, so I don't know for sure if there are any issues with calling finish() from onSaveInstanceState. If you try this, please comment and let me know how it works out.
Sorry for answering this almost 10 years later.
In my understanding the premise of the question is wrong, mainly this part:
"While the async task is in progress, the user switches to another app. Then, the Android OS decided to destroy all 3 activities (a, b, c) due to memory constraints. Later, the async task is completed. Now how do I finish c?"
In my understanding if the operating system decides to destroy all three activities due to memory constraints, it won't destroy only them, but the whole process, and this should be including the AsyncTask in question. So, the async task won't be able to complete as well.
Resource: https://medium.com/androiddevelopers/who-lives-and-who-dies-process-priorities-on-android-cb151f39044f
mainly this line from the article: "Note that while we’ll talk about specific components (services, activities), Android only kills processes, not components."
In todays world, I guess that a WorkManager would be used for running work that needs to be guaranteed to run.
Can anyone help me to understand synchronous and asynchronous activities in Android?
What is exactly meant by synchronous and asynchronous activity in Android?
StartActivity, StartSubActivity and StartAcivityForResult start an activity synchronously or asynchronously, or can they behave in both ways?
Please explain as I have gone through many articles but could not find any proper explaination over this.
First of all, only one activity can be running at a time on Android, so you'll never have two activities running at the same time. Use startActivity() when you want to "fire and forget", that is, you want to launch an activity but are not expecting it to return a value to your activity. In that case, the new activity will start and your activity will be paused; you might eventually regain control once the user returns to your activity.
Use startActivityForResult() when you are expecing a result from the activity you are launching. In this case, the calling activity should override onActivityResult(), which will be called when the launched activity exits and has a result to return to you (which it sets with setResult()).
In both cases, since the calling activity and the called activity are in the same task, it's "synchronous" in a certain sense (although I think using the terms "synchronous" and "asynchronous" can be confusing in this context). The calling activity won't appear on the screen until the called activity finishes.
A useful read to know more is:
* http://developer.android.com/guide/topics/fundamentals/tasks-and-back-stack.html
-
Bruno Oliveira (Android Developer Relations, Google)
Please tell me the Difference Between Synchronous Activities ans ASynchronous Activities..
and also about that
-startActivity() is Synchronous or Asynchronous
-startActivityForResult() is Synchronous or Asynchronous
and why..
From first principles, synchronous activities means that Activity A operation will depend on activity B eg(coordinating to work with a shared param) Asynchronous activities means that the operation of two activities are totally disconnected.
According to my general concept both startActvity() and startActivityForResult() are asynchronus as in the synchronus Activity the current activity will depend on the new activity.
For example :
suppose in current actvity:
Intent i=new Intent(First.this,Second.class);
startAcivity(i); //or startActivityForResult(i,0);
Log.v("First","Activity");
Now for any activity to be synchronus the later part of the current acivity (once the new activity is started) must be executed after the onCreate method of the new actvity(here Second class).
But generally this never happens. I mean the later part of the current Activity( once a new acivity is started by calling startActivity() or StartResultForActivity()) is never depended on the onCreate method of the New activity.
for more you can go through these links :
What is the mechanism behind startActivityForResult() in Android?
http://osdir.com/ml/Android-Developers/2009-12/msg04249.html
Hope from the above example you understood exactly what startActivity() & StartResultForActivity() are and what is synchronus & asynchronus Actvity is....:)
I have an activity that launches another activity with startActivityForResult method. I would like to terminate the called one programmatically but I don't know how to do this since in onActivityResult() method I have no information about the called activity and I cannot call finish() on it. How can I achieve this?
Thanks
The launched Activity can finish self:
setResult(RESULT_OK);
finish();
Try finishActivity(requestCode). According to the documentation it lets you finish an activity previously started with startActivityForResult. And if there are multiple such activities with the same request code, all will be finished.
Note: I haven't actually tried this myself, but that's what the documentation says! Experiment with that, see if it does what you want.
At the moment you call startActivityForResult your Activity will be closed or paused and the new activity is started. The only one that can finish the new activity is the new activity.
You could start a background task and let this background task somehow notify your activity the activity could now finish itself.
I don't know if a Handler created in Activity A and passed to a thread will remain valid if Activity A is paused and Activity B is active. But I would assume this works because both of the Activities are running in the same thread therefore they should share the same message queue.
This thing just bit me, so I thought I'll add a comment here:
if(readyToFinish()){
finish()
}
thisCodeWillBeExecuted()
My experience is that all your code in the stacktrace gets executed, before
the activity is finished. The documentation is not ideal at this point.