I'm working on an application which has a few Activities. One Activity starts the next one. To share some Values I'm using a custom implementation of Application (I'm talking about android.app.Application) called MyApplication.
As we all know, the Android system kills an app, if it needs more space. But this leads to a problem:
I open my app and use it just like any other app
I close it (Home Button) and use other apps
The system will kill my application because it's wasting memory
When I reopen my App, it wants to open the last activity I used and I get a force close, because the values in MyApplication are null
The strange thing is, that the system destroys my Application, but it seems like it keeps the Activity. I don't really understand why this is so because the Application doesn't seem to have a life cycle.
What I want to have:
When MyApplication (whole Application, not only the activity) gets killed, I want the last activities to be killed too. So when I reopen the App, it starts the main acitvity provided by the manifest.xml.
or
The values in MyApplication are persisted and don't get lost if the Application gets destroyed. (I'm talking about a few objects so i think the shared preferences won't work).
I don't want to use a service to bind my activities to, but is there a similar way to tell the system that my last used activity depends on the application-context?
I hope you understand what my problem is and someone can help me out with this.
The right way to do things would be to save your application state.
Override the onSaveInstanceState(Bundle savedInstanceState) method to save your state and onRestoreInstanceState to retrieve it.
If you need to save large sets of data consider using a SQL database
You should make sure the app closes and restarts the way you want it to in the onPause() , onResume() and onStop() methods. Check out savedInstanceState which can save the state of the app (and restore it, when it's sent as a parameter to onCreate)
In your custom implementation of Application, add a Flag say :
public boolean appContextExist = false;
On your first Activity set the flag to true,
Override onCreate and onResume method on your Activity which need the contexts, add following :
MyApplication myApp = ((MyApplication) getApplicationContext());
if (!myApp.appContextExist) {
// Code to return to start activity here
}
Related
I am not understanding how android activities are managed.
I have an activity and every once in a while i have noticed that, when my app goes into the background, android destroys whatever the current activity is (say Activity3) and several other singletons and objects etc. Thats fine. The problem is when the app is resumed then intuition tells me that since android has destroyed the activity and objects for memory or whatever, then android would just restart the app completely from Activity1 so all the objects and data members would get properly initalized.
NOT SO!
It seems that when my app is resumed, the Activity3 is recreated and onCreate is called with the same parameters as it was the first time (when it was called from Activity2) only this time all the singletons and other objects that were initialized in Activity1 and Activity2 are recreated with their default values and are rendered useless.
How is this a safe policy/technique? How can android just randomly destroy objects and activities and then when the user resumes just call onCreate on the recent activity and expect everything to be hunky doory and NOT have to go through the proper startup procedure/initialization?
UPDATE / SOLUTION
Thanks to the commentors for their excellent info.
ACCORDING TO ANDROID DOCUMENTATION
onCreate
Bundle: If the activity is being re-initialized after previously being shut down then this Bundle contains the data it most recently supplied in onSaveInstanceState(Bundle). Note: Otherwise it is null.
THEREFORE what I ended up doing is I set TWO flags. One in onSaveInstanceState in the Bundle so to know that it is a valid Bundle set by me. The other in the class itself to determine if onCreate was called because of recreation or Auto-Rotation. And so in onCreate I checked to see if onSaveInstanceState is not null, check the Bundle flag, and check bInit (which defaults to false). If both flags are true then it means android dumped and destroyed our apps memory and the safest way to ensure everything is initialized again in a linear-style application is to just restart it and launch the beginning activity.
public class SomeMiddleActivity extends AppCompatActivity
{
private static boolean bInit = false; // only way it will be false again is if android cleared our memory and we are recreating
#Override
public void onSaveInstanceState(Bundle state)
{
// set a flag so that onCreate knows this is valid
state.putBoolean("StateSaved", true);
super.onSaveInstanceState(state);
}
#Override
protected void onCreate(Bundle savedInstanceState)
{
// this must be called first always for some reason
super.onCreate(savedInstanceState);
if (savedInstanceState != null)
{
if (savedInstanceState.getBoolean("StateSaved", false) && !bInit)
{
// we were recreated... start app over
Intent intent = new Intent(getApplicationContext(), Startup.class);
startActivity(intent);
finish();
return;
}
}
bInit = true; // this will stay true until android has cleared our memory
.......
}
Although this has worked thus far, if anyone has a different suggestion let me know. I will be posting another article on this.
And FYI: the onSaveInstanceState(Bundle, PersistableBundle) version of onSaveInstanceState is never called ever so I dont know why they even implement it. (?)
#goldenb #Rishabh Thanks to goldenb and Rishabh for the insight.
Android, if destroys, also gives you tools to handle it.
Mobile devices have limited amount of memory which needs to be shared among Applications running simultaneously. Thus, smart resource allocation is necessary. The Apps running on foreground are used by End-User and gain high priority for better performance and user experience. Thus, applications running in background need to release the resources to suffice the memory requirements for foreground applications. Hence, background applications are destroyed (not completely) sometimes (in case of low memory).
Android Activities have Callbacks likes onSaveInstanceState() and onRestoreInstanceState() which enable you to save your current state of Activity (i.e., values of variables) when it is destroyed and retrieve them when the Activity is recreated.
You can get more information from here: How to save and retrieve the state of Activity using onSaveInstanceState and onRestoreInstanceState.
You can perform validations on the retreived state to ensure the Activity performs exactly as it was doing pre-destruction. You would find it very easy and logical once you get hands-on it.
Just giving my 50 cents on the issue. The correct way to deal with the issue of an activity being killed by the system for its resources in background is a common problem in android and according to Google the solution for this is:
onPause() is where you deal with the user leaving your activity. Most
importantly, any changes made by the user should at this point be
committed (usually to the ContentProvider holding the data).
Emphasis is mine. But what this means is that the Android lifecycles are designed so that under normal conditions onPause should be called as an Activity or Fragment is sent to the background. They hint at this in several of the android documentation pages:
As your activity enters the paused state, the system calls the onPause() method on your Activity, which allows you to stop ongoing actions that should not continue while paused (such as a video) or persist any information that should be permanently saved in case the user continues to leave your app.
Also worthy of your attention: if you wish that views are restored during Activity recreation, you should have set the ID attribute of all views ;)
Note: In order for the Android system to restore the state of the
views in your activity, each view must have a unique ID, supplied by
the android:id attribute.
PS.
You were wondering why onSaveInstanceState(Bundle, PersistableBundle) is not called, one possibility is that you do not have the right activity attribute set
This is the same as onRestoreInstanceState(Bundle) but is called for
activities created with the attribute persistableMode set to
persistAcrossReboots..
I am unable to find out application is active(in background) or not.
For example, if user is in activity A (launcher/main activity) and navigates to activity B, then goes in background after some time or if android needs memory it'll kill application and next time if user wants to bring that app from background, or start it from home screen it will bring him to activity A, despite he sent it to background with activity B on top of activity stack.It would be logical that android returns application with where user left it.
Previously, I tried to handle it with static variable, but I found out that android kills application so that anything that I write to that variable is lost.
It would be nice, if there would be some notification from android OS before it kills applications. I was desperately searching for some workaround solution but didn't found anything that fills my needs.
I would be grateful if you could point me to some kind of solution.
UPDATE :
I have logic for restoring data but I don't know if application is in background or not ?
I want to do something like this
if(inBackground)
{
restore();
}
Try this:
On a BaseActivity that all Activities extend:
protected static boolean isVisible = false;
#Override
public void onResume()
{
super.onResume();
setVisible(true);
}
#Override
public void onPause()
{
super.onPause();
setVisible(false);
}
Whenever you need to check if any of your application activities is in foreground just check isVisible();
you can bundle A's state variables and send it to B, so that B can send it back to A later (if A gets killed) so you can restore A's previous state. This requires the data you want to save to be serilizable.
The suggestion (#Valentin) of using onSaveInstanceState will save the viewHierarchy and data stored in views. It's a bit confusing, because correct use requires calling the super functions in onCreate, OnSavedInstanceState, and not everything gets saved...just ViewHierarchy stuff (text in textViews, scroll positions, etc). You have to manually add/retrieve class fields to the bundle, so like #1, your data has to be serilizable.
If the data can't be serialized, you may try creating a reference to the data object directly in Activity B, so that when Activity A gets killed, the data object is hopefully preserved since it's referenced by Activity B as well, and B can pass the reference back to A.
So I have the following:
A Common class that many of my Activities access in my android application, via setting the class in my manifest:
<application android:name="com.dev.games.phraseparty.Common"... />
Now, within this class I have several objects that are constructed to preserve application state and common services to perform applications that are constructed in the Common constructor
ie
GameStateVO gameState;
public Common()
{
gameState = new GameStateVO();
}
My problem is that my Activity has an Admob ad. When the user clicks on the ad, it calls the webbrowser intent to open the ad URL in a webbrowser.
Now, when I click back from the webbrowser launched by admob, it takes me back to the caling activity and the OnCreate method is called.
This activity then gets a null pointer exception because it will do something like:
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Common common = this.getApplication();
//null pointer here since the game state VO is null since the Common has lost its state.
int score = common.getGameState().getScore();
}
If you have no active foreground activity, then your process is ripe for shutdown by the OS to obtain more resources. The browser app in particular i've noticed uses a lot of resources and can quickly lead to background activities and later processes being killed off. Having a service can help keep your process around, but even then it can still be killed off if needed. I think you'll need to use the activity lifetime cycle method to save & restore your state. See the process lifecycle section of the docs for more info
You might want to look into implementing the onSaveInstanceState method: this lets you store any relevant information before it gets killed off. See http://developer.android.com/reference/android/app/Activity.html#onSaveInstanceState%28android.os.Bundle%29 for the actual call you need to implement, and Saving Android Activity state using Save Instance State for a quite excellent example.
I start my Android app, which in turns initializes some state on the first screen. The app has a many screens, and after randomly navigating through some screens, I minimize the app using the "Home key". Now after running some other apps from the phone, the OS decides that it needs to free up my app and hence kills it.
Now when I again click on the app icon, the OS remembers the history and tries to go back to the screen from where I minimized the app. But, the problem is since the OS purged my app sometime back, all my states are lost and the screen may not have any relevance.
How do I tackle this? How do I ensure that the OS calls the launcher screen, if it has been purged before and not the Activity in the history?
From your above comment about singleton class being initialized I also faced situations similar to you. Since I could not avoid it, what I did was I used the Application class. Whenever the OS decides to purge your app, the next time you launch the app, onCreate on the Application class will be called. Override the onCreate method to initialize the singleton class rather than doing the same in the launcher screen
The code snippet is as follows
public class CellApplication extends Application {
#Override
public void onCreate() {
super.onCreate();
// Do your singleton class initialization here
}
}
You don't make sure the OS calls the launcher screen. The best way to solve this problem ist to save the state of your screens.
Everytime one of your activities is put in background onSaveInstanceState is called. This allows you to save the state of your app to the bundle that is provided in this method. If your app resumes onCreate will be called with the exact same bundle. You can now rebuild the app the same state the user left it.
Just check if the the bundle in onCreate is null. If it is null your activity is new if not rebuild the state from the bundle.
I am using a separate class with only static fields, to store current application data.
It is partly populated from sharedpreferences on application startup. The rest is data like results of some action, used for further browsing these results (multiple activities that use the results).
I can go to the home screen, start other applications etc. and when I return to my own application it just works correctly.
However, since the new Error Reporting feature I get some bug reports all related to a nullreference error. The object that is null is a reference to the static field in the mentioned separate class.
Since I cannot reproduce the bug I am inclined to think this is due to the application getting killed due to low memory, and when it relaunches it calls the oncreate from the activity that the user was currently in. However all the static data in the separate class is not restored and thus it crashes.
I would like to know: Is there a way to force the application to "restart" completely, and not start with the last used activity if it gets killed? Or is that standard behaviour?
Can I do this programmatically? Like when the static fields are null, restart app?
Restarting the activity where the user was is normal behaviour - the idea is to make it look to the user like the app was never closed. There are two things you can look at:
protected void onSaveInstanceState(Bundle outState){
// This gets called by the system when it's about to kill your app
// Put all your data in the outState bundle
}
That bundle is the same one that gets passed to the activity in onCreate(). You can then get any necessary information out of it and restore the values in the static class.
The other way is to simply check the values in the onResume() method of any of your activities. If the values are null or wrong in some way, then you can call start the original activity and finish() the one being started.