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how can i persist a model between multiple activities in android?

I'm a beginner to android development, and I'm trying to write my code in an MVC pattern, but I'm having trouble understanding how a model would work. As far as I can tell every time you start a new activity with an intent you are not able to pass a model along with it. As far as i can tell you'd have to reinitialize it each time you start a new activity. Am I missing something? I looked into Parcelable, but it seems that you loose your methods if you make your model Parcelable. right now I'm building a log in system, which checks my local sqllite db on start up if the user has already logged in, and if so it passes to another activity, otherwise it passes to the log in activity, but I wan't to keep that user model alive through all the activities. Is thee a way to do that?

You might want to also consider keeping a static reference around to the model data that you want to share across activities so that you don't have to keep serializing/deserializing the model when switching between activities. You can get away with using Parcelable if your models are small, but at some point, performance may become an issue.
I'm working on a project where we keep the models in a Singleton that we can access throughout the app, and although I generally hate Singleton's for how they can make unit testing more difficult, I have found this approach to perform better with larger models than trying to rely on Android's serialization mechanism.
Here's is a very rough example of what I mean (disclaimer: I have not actually run tested this code, but I hope this illustrates the concept):
You might have a singleton class that I terribly called Models
public class Models {
private static Models instance;
private boolean isInitialized = false;
private User user;
private OtherInterestingModel otherInterestingModel;
private Models() {
}
public static synchronized Models getInstance() {
if (instance == null) {
instance = new Models();
}
return instance;
}
public void loadModels() {
if (!isInitialized) {
/*
* One-time model initialization here.
*/
isInitialized = true;
}
}
public User getUser() {
return user;
}
public OtherInterestingModel getOtherInterestingModel() {
return otherInterestingModel;
}
}
In your LoginActivity, you can initialize the Models class, say, in your onCreate():
public class LoginActivity extends Activity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Models.getInstance().loadModels();
User user = Models.getInstance().getUser();
OtherInterestingModelData otherData = Models.getInstance().getOtherInterestingModel();
// Do something with the model data...
}
/*
* This might be called after the user enters data and clicks a login button...
*/
private void login() {
startActivity(new Intent(this, AwesomeLoggedInActivity.class));
}
}
Once the user successfully logs into your app, you could have basically the same code in your main activity:
public class AwesomeLoggedInActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Models.getInstance().loadModels();
User user = Models.getInstance().getUser();
OtherInterestingModelData otherData = Models.getInstance().getOtherInterestingModel();
// Do something with the model data...
}
}
Notice that by having a Singleton, you avoided having to serialize the model data by passing it through the intent that started the main activity.

Yes, you can do that with the Parcelable interface.
You do not lose your class's methods when you implement the Parcelable interface. The interface simply defines a method for writing your member variables to a Parcel object when you need to pass the object around.
Once you retrieve the data from your Intent via getParcelableExtra(), the object is recreated from the Parcel and you can once again treat it as an instance of whatever class it is.
For example, if you have a User class that extends Parcelable, you can bundle it with an Intent by calling putExtra("user", myUser). myUser is then (behind the scenes) packed into a Parcel and attached to the Intent. In your next Activity, you can retrieve that User object with User myUser = (User) getParcelableExtra("user");, and the Parcel will be unpacked and returned to you. You wil once again have a fully functioning User object.

android new activity can't load singleton

I am trying to use a Singleton to share a large data object between Activities. But when I open the new Activity, the singleton comes up as empty. It seems to me that the Singleton should be the same no matter where in the Application I call if from.
It seems like the Scope of the Singleton is being limited to the individual Activity. Working around this is making my App very complicated. I must be doing something wrong. I even tried instantiating them in an extended Application class... Google says I should not have to use that though...
Can someone please point out where I am going wrong? i.e. Why does this singletom not contain the same data in each Activity?
I call it from an Activity with...
DataLog dataLog = DataLog.getInstance(this);
I have...
public class DataLog extends ArrayList<String> implements Serializable {
private static final long serialVersionUID = 0L;
private static DataLog sInstance;
private static Context mContext;
public static DataLog getInstance(Context context) {
mContext = context.getApplicationContext();
prefs = PreferenceManager.getDefaultSharedPreferences(mContext);
if (sInstance == null) {
sInstance = new DataLog();
}
return sInstance;
}
private DataLog() {
}
public boolean add(String entry) {
super.add(entry);
return true;
}
public void add(int index, String entry) {
if (index > 0)
super.add(index, entry);
else
super.add(entry);
}
public void clear() {
super.clear();
}
...
}

Its highly advisable to avoid singleton for sharing large data sets in android.
Singletons are used for short life-cycle objects.
Switch to SharedPrefferences, SQLite DB's or file storing. You are not the only to have experienced this behavior, and the reason lies in the nature of android Activities and the system itself(managing activities and its data).
Here is an example why singleton is bad for your case:
You stored important data in it. The user knows that he can close the app on home button to call someone or whatever)maybe someone called him when he was in your app), and that when he opens your app he will come back at the same place with everything in order. (this is expected behavior from users and android apps). The system can easily kill your process and all static variables in it for memory maintenance, app inactivity etc...result=data lost. Thus its not safe to use it.

Keep application state on android

I am having trouble saving the state/singleton of my application.
When the application starts a loading screen (activity) is shown and a singleton is initialized with values from a webservice call (note that network access cannot run on the main thread).
After the singleton is created I open my main activity. Note that values from the singleton are required to build the layout.
Now assume the app goes in the background and is killed there (e.g. because of low memory). My singleton instance is deleted as the app is killed. When I switch back to my app it tries to recreate the main activity. As I mentioned earlier the values from the singleton are required to build the layout, so this leads to a NullPointerException (when I try to access members of the singleton, as it is not there anymore).
Can I somehow tell android to start the first loading activity after the app was killed? It would be great if I could refresh the singleton before the layout is recreated, but this seems to be a problem as network calls can not be on the main thread and therefore not block until the refresh is finished.
I assume that I could save the singleton in all activities onStop and recreate it in the onCreate methods, but this seems a bit too unpredictable and would probably lead to a inconsistent state...
Another way could be to just always finish my activity onStop, but this would lead to losing on which tab the user last and so on, even if the app is not killed, so this is not a good option.
Any ideas on how to solve this?

Why not just use a SharedPreferences instead of a singleton?
Anytime you want to save some global state, commit it to preferences. Anytime you want to read the global state, read it back from preferences.
Then you don't have to concern yourself with application lifecycle at all, as your data will always be preserved regardless of what the phone is doing.

For something like that I used a pseudo singelton object as a Application class. This object will be created on the beginning and will be in the memory. But note that the system will terminate the application if the memory is needed by other applications. However this object is persitent even if all activities are temporally terminated.
To use that you need to declare that in your android manifest like here:
<application android:label="#string/app_name"
android:icon="#drawable/icon"
android:description="#string/desc"
android:name=".MySingeltonClass"
...
Here is a code example:
public abstract class MySingeltonClass extends Application {
// ...
public void informClientOnline() {
clientOnline=true;
Log.v(LOG_TAG, "Client is online!");
}
public void informClientShutdown() {
clientOnline=false;
Log.v(LOG_TAG, "Client is going offline. Waiting for restart...");
Timer t=new Timer("shutdowntimer", false);
t.schedule(new TimerTask() {
#Override
public void run() {
if(!clientOnline) {
Log.v(LOG_TAG, "Client has not restartet! Shutting down framework.");
shutdown();
System.exit(0);
}
}
}, 5000);
}
}
this two functions are called like this:
((MySingeltonClass)getApplicationContext()).informClientOnline();

You could save your Singleton when onSaveInstanceState() in the Activity gets called. All you need to do is to make it implement Parcelable (it's Androids own form of serialization), then you can put it in the outState Bundle in onSaveInstanceState() which will allow you to retrieve it laver in onCreate() or onRestoreInstanceState() in the Activity, whichever you like.
I've included an example for you:
public class TestActivity extends Activity {
private MySingleton singleton;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if(savedInstanceState.containsKey("singleton")) {
singleton = savedInstanceState.getParcelable("singleton");
} else {
singleton = MySingleton.getInstance(5);
}
}
#Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
outState.putParcelable("singleton", singleton);
}
public static class MySingleton implements Parcelable {
private static MySingleton instance;
private int myData;
private MySingleton(int data) {
myData = data;
}
public static MySingleton getInstance(int initdata) {
if(instance == null) {
instance = new MySingleton(initdata);
}
return instance;
}
public static final Parcelable.Creator<MySingleton> CREATOR = new Creator<TestActivity.MySingleton>() {
#Override
public MySingleton[] newArray(int size) {
return new MySingleton[size];
}
#Override
public MySingleton createFromParcel(Parcel source) {
return new MySingleton(source.readInt());
}
};
#Override
public int describeContents() {
return 0;
}
#Override
public void writeToParcel(Parcel parcel, int flags) {
parcel.writeInt(myData);
}
}
}

Can I have a multi-process global variable?

I have a real problem using my app that involve 2 processes. One process its executing a Service (p1) and the other the GUI (p2).
I have a class in p2 that implements the use of an object (iThing) that is custom memory managed (and its static). It has to be like this bacause of Android OS implementation of destroying the views whenever he wants.
public class Connections{
public static int iGlobalCounter=0;
public static Object iThing;
public static void start(){
iGlobalCounter++;
Log.d("PROCESS", "UP: "+iGlobalCounter);
if (iGlobalCounter<=1){
//Create the object "iThing"
}
}
public static int stop(){
iGlobalCounter--;
Log.d("PROCESS", "DOWN: "+iGlobalCounter);
if (iGlobalCounter<=0){
//Destroy the object "iThing"
}
}
}
The main GUI (in p2), starts and stops the variable on the onCreate / onDestroy (for all views in my app)
public class MyMainClass extends Activity{
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Connections.start();
}
#Override
public void onDestroy(){
super.onDestroy();
Connections.stop();
}
}
Finally in p1 I have the service, which also needs the variable, so, it does the same
public class MyMainService extends Service{
#Override
public void onCreate() {
super.onCreate();
Connections.start();
}
#Override
public void onDestroy(){
super.onDestroy();
Connections.stop();
}
}
The problem is that if I use only p2 (GUI), it goes all well, but when I execute the service (in p1), the counter doesn't increment from the last state, but from 0, resulting in destroying the object when leaving the service, not the app.
if do this navigation, I get the following counters:
MyMainClass (1) --> OtherClass (2) --> AnotherClass (3) --> MyMainService (1)
My question is if there is a way of having a multi-process global variable? As it seems that every process takes its own static variables and are not "real static". A solution could be using SharedPreferences to save the state, but not really nice solution, as it hasn't to be saved when leaving the app.
Thanks,
PAU

I think that you should extend Application class and put your globalVariable there.

You can store your global data in shared memory (see MemoryFile).
To synchronize access to the file, I think the best approach is to implement some sort of spinlock using the same memory file.
In and case, I don't know a simply way of doing this.

You have the following options which you can look into for sharing data between different processes,
Message Queue,
Named Pipes,
Memory mapped files
WCF on Named Pipes or MSMQ

Store Objects in ApplicationContext

When my application goes to background , my (static and singleton) objects are cleared.
So I tried to store these objects in Applicaton Context . I am using the following code.
Accounts.create(getApplicationContext()) will be called once to store the accounts instance.
Is that possible(reliable) to store objects in Application Context ? I am not sure the following way is correct or not . please guide ..
public class Init extends Application {
private Hashtable<Object, Object> globalStore = new Hashtable<Object, Object>();
public void putToGlobalStore(Object key, Object value) {
globalStore.put(key, value);
}
public Object takeFromGlobalStore(Object key) {
return this.globalStore.get(key);
}
public void removeFromGlobalStore(Object key) {
this.globalStore.remove(key);
}
public boolean containsInGlobalStore(Object key) {
return this.globalStore.containsKey(key);
}
}
public class Accounts {
protected Accounts(String name, Context context) {
Init init = (Init) applicationContext;
init.putToGlobalStore(name, this);
}
private static Init applicationContext;
public static void create(Context context) {
if (context instanceof Application)
applicationContext = (Init) context;
else
applicationContext = (Init) context.getApplicationContext();
if (applicationContext.containsInGlobalStore(GLOBAL_NAME))
Logger.log("Warning " + GLOBAL_NAME
+ " is already created. This will remove all old datas");
new Accounts(GLOBAL_NAME, applicationContext);
}
private static final String GLOBAL_NAME = "accounts";
public static Accounts getInstance() {
try {
return (Accounts) applicationContext
.takeFromGlobalStore(GLOBAL_NAME);
} catch (Exception e) {
Logger.log("GLOBAL_NAME Lost");
return null;
}
}
Please help.

You should know that the application context itself gets destroyed if left unused for a long time in the background. So there is no guarantee that your static and singleton objects will not be cleared when the app is in background. Instead what you can do is persist your objects from time to time (either in a flat-file or shared preference or database) and restore them in the onCreate method of the Application class

I have been using this method in my application and i didn't see any problem unless my process gets killed by the OS or if there is a crash in my application and my app gets restarted.
If you think whatever data you are storing is valid for only life time of a program why don't you override OnCreate of Application object and create all your singletons there. This way you can always make sure your application has all singletons before your app starts functioning.

Application class is not permanent.
If App process killed, Application class private member variable data loss.
Using Shared Preferences.

I know this question was asked a long time ago, but here's a good article that suggests using the Application object to store data is generally not a sound design methodology.