Develop Reference

Cancel Action in onPause

In my login Activity, just after the use log in, I sync data and create several tables in sql.
If the user open another app before this process finish, I want to cancel it and delete tables.
I thought I could put it in onPause or onStop, but then, when the process finishes and go to the dashboard activity, onPause or onStop should be called, and I don't want this.
I assume the solution is simple, but I can't figure out how to solve it.
Any idea?

The logic behind your solution is straightforward - create a flag (say for example shouldRollback) to true when you start your transactions. Once your transaction are successfully completed, you set this flag to false. In your onPause() method, simply check for this flag. If it is true, you know you need to roll back the changes.
I would only suggest this if this is your requirement. Personally, I would process all database operations in a background thread. The thread can run in the background even if other applications are opened. There will be too much overhead (if you have a large number of transactions) to keep rolling back inserts. You could use the transaction methods from the SQLiteDatabase object to automatically roll back changes if they were not completed successfully.

How to deal with retained data after Activity comes to foreground when using more than one Activity?

At the moment I'm a little bit confused about the lifecycle management in Android. There are at least 4 possibilities to resume retained data after an Activity comes back to the foreground:
Android handling: If there is enough memory, Android stores and resumes the important data (checked radio buttons, text of EditText,-... and so on) after Activity restart, the user has the same state as before as the Activity went into background.
onPause, onResume: Overriding onPause and save the important data to a database or text file and resume it when onResume is executed next time.
onSavedInstance(Bundle), onRestoreInstance(Bundle): I can save the data as key-value-pair into bundles and restore them after onRestoreInstance is executed.
onRetainNonConfigurationInstance(), getLastNonConfigurationInstance(): I handle all my storage issues in one big object and read getLastNonConfigurationInstance() out when onCreate is executed.
Although it is confusing which approach is best, I guess it relies on development experience to know when to use which possibility. If you have some good examples for each I would be glad, but this is not my question. I wonder how to deal with all that when I have different Activities and one Activity will be killed by Android when it pauses in background:
In my case I have a MainActivity and a MessageActivity. The MessageActivity consists of a ViewSwitcher which consists of two states. State one is a radio button choice list. State two is an EditText with two buttons (send and abort). When I monkey test each state, hit the Android home button, and restart the application, the right Activity with the right state and the old data comes into foreground, when I leave the handling to Android. So that works.
But what happens when Android destroys the MessageActivity in background:
If I use the Android way, the data is lost and I guess MainActivity (instead of MessageActivity->state(1 or 2)) will start next time after I relaunch the application (is that correct?). So when I'd like to keep the data of MessageActivity, I have to use one of the other three possibilities.
How to do that neatly, when the application entry point (so the MainActivity) differs from the last active Activity. The problem is that I have to resume a special Activity with a special state of ViewSwitcher. I could start MessageActivity out of MainActivity with startActivity(Intent) in onStart() or onResume() method (because MainActivity is probably the entry point) but then I run into a lot of logical problems in Lifecycle management. Due to this fact I don't think that this is the right way to do that.
But, what's the right and best way to do that?

I guess MainActivity (instead of MessageActivity->state(1 or 2)) will start next time after I relaunch the application (is that correct?)
No, I don't believe this is correct, depending on what your code does in onCreate(). It certainly doesn't need to be correct if you go about things the right way. A simple way to test this is to rotate your screen, which recreates the running activities, unless you have overridden the default configuration change behaviour.
I recommend reading this section in the android docs carefully:
http://developer.android.com/guide/topics/fundamentals/activities.html#SavingActivityState
In particular:
even if you do nothing and do not implement onSaveInstanceState(), some of the activity state is restored by the Activity class's default implementation of onSaveInstanceState(). Specifically, the default implementation calls onSaveInstanceState() for every View in the layout, which allows each view to provide information about itself that should be saved. Almost every widget in the Android framework implements this method as appropriate, such that any visible changes to the UI are automatically saved and restored when your activity is recreated. For example, the EditText widget saves any text entered by the user and the CheckBox widget saves whether it's checked or not. The only work required by you is to provide a unique ID (with the android:id attribute) for each widget you want to save its state. If a widget does not have an ID, then it cannot save its state.
What this means is, that so long as you don't force any UI state in any onCreate() calls, your activity stack and UI state will be restored.
Personally, my preferred approach is to keep as little state as possible in member variables of my activities, saving and restoring it with onSave/RestoreInstanceState(), and relying on the default implementations to save the rest of the UI state (text box contents, etc). Data that should persist between sessions I commit straight to my DB or preferences as soon as it's changed (e.g. in the on-click handler). This means I don't need to worry about the activity lifecycle for that. As much as possible, my UI just presents a view of the data in my DB (using CursorAdapter etc.).
Edit:
Regarding restoration of the whole activity stack:
When the user leaves a task by pressing the HOME key, ... The system retains the state of every activity in the task. If the user later resumes the task by selecting the launcher icon that began the task, the task comes to the foreground and resumes the activity at the top of the stack.
(See http://developer.android.com/guide/topics/fundamentals/tasks-and-back-stack.html)

It's not my attempt for a best answer, but it's too long to get in the comments section.
First I will suggest not to rely on the "Android way" - this will result in inconsistent application behavior depending on the free memory of the device - bad practice.
My suggestion is to save your state-dependent data in key-value pairs in SharedPreferences, every time you go into onPause() in your MessageActivity. Store a flag in SharedPreferences, which indicates which was the Activity that was last opened (if you only have two Activities you can easily go 0/1 or true/false flags).
When you re-launch your application, it's normal to start the Activity marked in your AndroidManifest.xml as "entry point". So naturally you'll check the flag in onResume() in your MainActivity and start the other Activity if needed. In MessageActivity's onResume() check the values in SharedPreferences and fill in what's necessary...
If your application is "resumed" to the last Activity in the ActivityStack this will call onResume() in the last Activity in the ActivityStack.

The way I have handled an issue like this in the past, is to have a service running in the background, which handles the flow of information from different activities via either Intents and listeners (preferable, since they are the most easily decoupled solution), or if you are extremely careful, and the only viable solution for some reason is to store the data through direct property access or method calls, you can use static properties/methods on the service class as well. However, I would strongly recommend using the Intent/listener method as it is generally more flexible, thread safe, and decoupled. Additionally, it is wise to make sure that not much is happening at any point in time (in other words, only use this service for Intent handling) when it's not needed, otherwise the Service will tend to hog CPU time as well as RAM, when it's not really needed.
Some resources to look at when it comes to this approach would be IntentService and its related classes, including the superclass, Service. IntentService, however, it is worth noting handles a few more things about async Intent processing, etc that Service does not automatically come with.Hope this helps you!

login.setOnClickListener(new View.OnClickListener() {
public void onClick(View view) {
String name=username.getText().toString();
SharedPreferences settings = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());
SharedPreferences.Editor editor = settings.edit();
editor.putString("username", name);
if(name.equals("xxx")) {
Intent intent=new Intent(currentactivity.this,nextactivity.class);
intent.putExtras(bundle);
startActivityForResult(intent,0);
}
}
});

Application pause/resume state

My question is can i get to know when the entire application gets paused/resumed start/stop etc.
For example if i have 5 activities in my application. Whenever any activity gets paused/resumed android notify the activity by calling the onPause/onResume methods.
So there are two possible scenarios when my activity gets paused.
1. My activity-2 gets paused because my activity-3 gets invoked.
2. My activity-2 gets paused because of some outside activity like incoming call.
Here I am interested only tracking when my activity gets paused by outside activities not my own application activities.
So is there any android provided solution for this or do I have to write my customized solution.
Thanks
Dalvin

There is no solution provided by the API because it is not needed in most cases.
What you can do is to create an abstract activity and make all your activities inheriting from this abstract one.
In this abstract activity, by overriding onCreate, onResume, onPause, onDestroy, you can manage to count how many of your own activities are "alive", and then determine the state of your application.
This could work but it's not really the Android phylosophy

You can know the starting of the whole application on application.oncreate() but there is no indicator for the whole application pause. Most of the cases never needs it anyway.
So further read in the activity lifecycle and the application class.
Still you can do this option in your program by overriding the onPause in each class and save a value to the sharedPrefrences then check on this value all over the application

If I understand your question, you want your app to be able to distinguish between exiting the current activity within the context of your program or by an external event like a phone call. I have used the following method in the past to do this (although it may not be the best, it definitely works):
(1) Set up a value in SharedPreferences (the built in file for storing a program's data). Call it something like "exitStatus" which is set to 1 for an exit within the program code and 0 for an exit based on external events.
(2) Now, within each of your activities, set the value of exitStatus to 0 in onResume (which is called no matter how you enter). If your program exits due to an external event within that activity, this value will persist when the program is reloaded.
(3) At the end of your activity, at all points where you are going to transfer to another activity, first set exitStatus to 1. Then, when you arrive at the other activity, it will know that you arrived there from within your program.
(4) Thus, just to be clear, each of your activities can check exitStatus at the outset to see whether you are entering from within your program context (= 1) or after a non-local exit of some kind (= 0).
That's all there is to it. I use this method to be sure that load data for my app is present as it may be lost if a user turns off their device so that the app tries to pick up in the middle of things when they later reboot, etc.

Instead of making base activity and override onPause/onResume you can use
registerActivityLifecycleCallbacks(Application.ActivityLifecycleCallbacks callback)
where you can handle these states for application activities in one place.

Updating Activity UI from Different Activity Method?

I have a tab widget where one of the tabs is a chat-type feature. I want to update the chat data at an interval (variable depending on whether the chat tab is active or not).
The best approach seemed to be using an AsyncTask in my main TabActivity class, as that would avoid any issues of the chat activity being destroyed while in the background, while an AsyncTask was running. I wanted to ensure that the Activity isn't destroyed and recreated, thus causing my AsyncTask to be unable to modify the actual active Activity's data.
However, now that my AsyncTask is in the TabActivity activity, I don't have a direct way to call my Chat's ListAdapter notifyDataSetChanged() from my onPostExecute() method anymore. Is there a way to get a reference to a given Tab's current Activity from the TabHost/TabActivity?
Or, alternatively, can I assume my chat activity will never be destroyed as a child activity of the TabActivity activity (well, never destroyed while the TabActivity is active at least), and then just put the AsyncTask in the Chat Activity?

Really the best way to do this is to have a thread that runs and periodically gets the chat data. Then from your UI you can poll the thread to get new messages. This way you can have the thread running no matter what happens to the Activity.

If you're using the Intent loading mechanism for your tabhost, then you should be safe in assuming the task won't get randomly killed any more than the TabHost itself (only paused), and you can safely modify views in it (as safely as you could from the TabHost, at least).
But beware: Any activity can be killed at any time (example: if a user clicks on a link in the tab that opens a new activity while your task is still running), including the tabhost itself, which can lead to ugly crashes when the task returns and tries to update the UI of the now-dead Activity. So make sure to either cancel your AsyncTasks on destroy, have the asynctasks check that your activity is still active before returning results (see WeakAsyncTask in the Android source tree), or use an Activity-independent (semi)persistent pollable background-thread solution like CaseyB suggests (cwac-bus is a nice premade solution in that vein).
Alternatively you could just have the AsyncTask's UI-updating code catch all exceptions (I've seen a few apps that do this) and fail silently, but that smells funny to me.
Lately I've used a modified version of the WeakAsyncTask in most places that checks that the activity is non-finished on returning, personally.

Handling UI code from a Thread

I know how to use handlers to update UI elements such as progress bars toasts etc.
The problem I am having is when the context goes away such as the user pressing the back button or the Activity finishing for some reason. This causes my application to crash often.
I tried using getApplicationContext() (Thinking that this would be available throughout my entire application) but this did not work, ever - instead my application crashed!
I put try catch blocks around all UI update code, this works but is it necessary?
So...what is the bast way to handle this?

The problem I am having is when the
context goes away such as the user
pressing the back button or the
Activity finishing for some reason.
This causes my application to crash
often.
You will also get this, by default, if the user changes screen orientation, as the original activity is destroyed and a new one created.
I tried using getApplicationContext()
(Thinking that this would be available
throughout my entire application) but
this did not work, ever - instead my
application crashed!
The application context is useless from the standpoint of manipulating the UI.
So...what is the bast way to handle this?
In the end, what you need is for your thread to deliver an event to the right activity. Some techniques that people have used include:
Use a listener pattern (e.g., service manages the thread, activities register and unregister listeners with the service, thread invokes the listeners on key events)
Put the "current" instance of the activity in a static data member, which the thread uses to find out which one should be used (dangerous due to memory leaks and fails if you need multiple instances)
Limit background threads to ones that cache data, which the activity pulls (e.g., via polling) as needed