Develop Reference

startActivity should performed on main thread?

Need to do a transition from one activity to an other. Need startActivity(intent); executed on main thread on it can be a worker thread? I would choose main thread concept, because there are GUI related operations. What do you think?

Yes, it should, but as others stated is not required to do so.
Basically, "where", in the sense of Context, your new Activity should be placed in its stack or under/above others.
From documentation:
Launch a new activity. You will not receive any information about when the activity exits.
Note that if this method is being called from outside of an Activity Context, then the Intent must include the FLAG_ACTIVITY_NEW_TASK launch flag. This is because, without being started from an existing Activity, there is no existing task in which to place the new activity and thus it needs to be placed in its own separate task.

Previous answers have revealed that the startActivity method itself ensures the relevant operations are run on the main thread, so it doesn't actually matter where you call it: Is it safe to launch and activity from a non-ui thread?

If i'm not wrong... startActivity() internally executes in the UI thread so there's no discussion :)

Refresh current view after asynctask started in another activity

I have a situation where I have multiple activities that deal with a single global list of objects.
From activity A, I can start an asynctask which sends an HTTP Get request to retrieve an XML message then parses the XML file into a series of objects.
My aim is to be able to refresh all views, only if the underlying data structure changes.
This should ideally mean that I can launch activity A, call an asynctask, move onto activity B, C D etc, and when the asynctask completes (no matter what activity I am on) the current view should be updated.
I've looked into broadcast listeners but I am not sure I am on the right direction with this, would it be possible for somebody to explain if this is possible?
Thanks

Since AsyncTask instances are dependent of the caller Activity, I think this is not the correct approach. In your case, as you need to recover the result within any Activity, I'd use an unbound Service so it might be accessed whenever you want, wherever you want.
You might also use a local BroadcastReceiver which would send a signal to all of your Activitys and, those registering the action that sent your AsyncTask/Service, would perform the necessary actions you may need. Be careful as if your Activity has not been started, obviously that Activity won't be able to process what you've sent since that receiver should be unregistered.

Just post a runnable to the handler of the current Activity.
Your activities can implement a common interface, where a function "updateUI" or whatever exists, and that way your runnable can be one line, and be agnostic as to the current active instance.
As you probably know the postexecute part of the asynctask runs on the UI thread of the activity that launches it. Since that activity may be dead, just post to the handler in the "doinbackground" part of the async task and keep postexecute blank.
interface Updateable{
public Handler getHandler();
public void updateUI();
}
From Async Thread
final Updateable curr_activity = ...
Handler handle = curr_activity.getHandler();
handle.post(new Runnable() {
#Override
public void run() {
curr_activity.updateUI();
}
});

How to finish destroyed Activity

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.

Difference Between Synchronous Activities ans ASynchronous Activities

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....:)

Sending Activity to background without finishing

How can I make an activity go to background without calling its finish() method and return to the Parent activity that started this? I tried so much but I could not find a solution. So if you guys could help I would be very thankful.

The following will work if the Activity is running in a different task to its parent. To achieve this, see http://developer.android.com/guide/topics/manifest/activity-element.html#lmode.
public void onBackPressed () {
moveTaskToBack (true);
}
The current task will be hidden, but its state will remain untouched, and when it is reactivated it will appear just as it was when you left it.

If you have data you want to cache / store / process in the background, you can use an AsyncTask or a Thread.
When you are ready to cache / transition to parent, you would do something like the following in one of your child Activity methods (assuming you started child with startActivityForResult() )
Thread Approach:
Thread t1 = new Thread(new Runnable() {
#Override
public void run() {
// PUT YOUR CACHING CODE HERE
}
});
t1.start();
setResult( whatever );
finish();
You can also use a Handler if you need to communicate anything back from your new thread.
AsyncTask Approach:
new CacheDataTask().execute( data params );
set_result( whatever );
finish();
The AsyncTask is for situations in which you need to process something in a new thread, but be able to communicate back with the UI process.

Simply
protected void minimizeApp() {
Intent startMain = new Intent(Intent.ACTION_MAIN);
startMain.addCategory(Intent.CATEGORY_HOME);
startMain.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
startActivity(startMain);
}

Moving the task backward is NOT going to insure that you will maintain any state, unless you do that manually by responding to the Activity lifecycle events.
You should just start the second Activity via Intent using startActivity(), or, if you need to receive a result back from the second Activity, use startActivityForResult(). This allows you to receive a callback when the user finishes the Activity and returns to your first Activity.
Starting an Activity and getting results: http://developer.android.com/reference/android/app/Activity.html#StartingActivities

Try:
Intent toNextActivity = new Intent(CurrentActivity.this,
NextActivity.class);
CurrentActivity.startActivity(toNextActivity);
If you use this way, the method onPause() from CurrentActivity will be called and if you have a static variable (like a MediaPlayer object) in CurrentActivity it will continue to exist (or play if it is playing)..
I'm using that in my application but I found a better way to do that with services.
Hope this will help you!