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Android - Start a new activity while the application is in the background

The first activity of my application is a splash screen, where some animation is displayed while some loading occurs in a background thread using AsyncTask.
Once the loading in the background is done, I want to start a new activity. What is the correct way to do that ?
Start a new activity directly from the onPostExecute method of the AsyncTask class.
Check if the current activity is displayed before starting the new activity :
If the current activity is display, start the new activity.
If the current activity is NOT display, use a flag (boolean) and check the flag during the onResume method in order to start the new activity there.
My main concern is :
If my application went to the background (due to an incoming phone call, a home key press, ...) and the background thread (AsyncTask) finished executing, and a new activity is started from the onPostExecute method while my application is still in the background : What happens ?
Will the new activity start directly as soon as my application is visible again ?

I faced a similar situation: my application went to background and after some time the app started an intent displaying another activity. However, the app's behavior now depends on the os that it's running:
on pre 4.4 devices the app silently opens the new activity and remains in background. When the user resumes the application, he is prompted with the second activity already.
on 4.4 devices (tested on 4.4.2 and 4.4.4) the app opens the second activity, but after 3-4 seconds, the app pops to foreground, interrumping the user.
Hope it helps anybody. I'm still looking solutions for the second case in order to prevent the app from popping to foreground.

From my experience i am answering your question
Question1
If you using AsyncTask you have to start new activity in OnPostExecute(). In my experience this is the correct way of doing it.
Question2
When ever your press the home key or receiving phone call. Your activity will go in to background until you exit the app by pressing back button(at that time your app is exited). So when you come back to your app your new activity will be visible if background process get finished. Otherwise you will see the start splash screen.

I think that it is dependent upon the OS of android. It has defined some built in priority model for each of the components.
Look at the answer given by commonsware.
change process priority in android
this gives brief idea about components priority.

Delay after pressing a button

After I press a button, Android parse a JSON file and pick the info it needs. Until yesterday, I was using an external library created by a user and it worked perfectly. But now, I don't want to depend on him, so I've searching info about Google's GSON. I've implemented this library with no problem, but now, after pressing the button that opens a new activity there's a delay.
This delay is due to the connection and parsing that are done before the activity shows.
How can I force the app to wait the Internet connection until de Activity is shown? It's a lil bit uncomfortable because after pressing the button, it seems that the app has frozen, but after all data is loaded, the new activity appears.
Thank you in advance!

Use AsyncTask or Handler for network operations. Do never put "long time" operations into the UI thread

Use asyncTask.. and if server communication is a success then show the new Activity or else exit..

it is possibl;e via Handler and AsyncTask see this How to set delay in Android onClick function you will get how to use Handler and see this for Asynctask https://stackoverflow.com/questions/7644567/need-a-simple-example-for-android-asynctask

Running a thread while Android app is in foreground

I have an Android app. I have a main activity, that has a button. When the button is clicked, another activity comes to the foreground. The thing is, I want to run a background thread that polls updates from the server. However, I want it to run only when the app is in foreground (either the main activity or the second one), and to stop polling when the user clicks the Home button or clicks the Back button till it's going back from the main activity.
But how do I know if the app is still in the foreground? I can catch the onPause of the main activity, but it's called also when I'm launching the second activity.
So how do I know when the app is in background?
Thanks

You should make a Service for the work you are doing in the background.
For stopping it when you click the Home or Back button, just make a listener for them and stop the Service when either one is pressed.

Seems easiest to me that each activity polls. Is it super important that it can poll when it is between the two activities? Otherwise you will have problems about knowing who is in front or not.

You can have a singleton with reference counting.
You main activity should add the first reference on it's onResume and from now, upon calling for every new activity (startActivity for example) you should add a reference.
Each activity should decrease the reference counting on its onPause.
Another option is to use services: Services

How to tell when an activity is shown

I have an app that contains a sequence of listviews, and those lists are populated with data retrieve from a webservice. I want to refresh this data when the user presses back to go to the previous list.
I have currently tried overriding the onWindowFocusChanged method, but this doesn't work, since when I start a webservice download, I bring up a progress dialog and close it upon completion. This causes a recursive effect (the dialog closes and gives focus back to the list activity).
Is there any way I can get when the activity is first shown? Similar to viewWillAppear for iOS?

Just override onResume() in the Activity. Just make sure to do whatever internet communication you do in a separate thread.

Loading an Alert Box from a Thread when the activity has gone away

I have a comment activity that loads a Thread and sends some data to a server; the activity is immediately finished once the submit button is pressed.
The user is then free to do other things in my application.
When the server responds an AlertDialog is shown.
The problem is that since the initial context has been destroyed, my application crashes.
I tried getApplicationContext() but still get an exception.

Put your network stuff in a Service, then show a status bar notification instead of a dialog.

Take a look at AsyncTask
From JavaDocs:
AsyncTask enables proper and easy use of the UI thread. This class allows to perform background operations and publish results on the UI thread without having to manipulate threads and/or handlers.