Save State in my Android application

I am developing an android application in which,the first activity is that for Login. After successful authentication, user can see an activity with 4 Tabs. My problem is that when user taps on the home button,the application should terminate and when the user again starts my application, he 开发者_如何学Pythonshould be able to see the Login screen again and after successful Login, user should be able to see the tab on which he was working before application terminated(i.e., save state for all the tabs). Please help me out in this. Thanks in advance.

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Many applications may provide a way to capture user preferences on the settings of a specific application or an activity. For supporting this, Android provides a simple set of APIs.

Preferences are typically name value pairs. They can be stored as “Shared Preferences” across various activities in an application (note currently it cannot be shared across processes). Or it can be something that needs to be stored specific to an activity.

1. Shared Preferences: The shared preferences can be used by all the components (activities, services etc) off the applications.

2. Activity handled preferences: These preferences can only be used with in the activity and can not be used by other components of the application.

Shared Preferences:

The shared preferences are managed with the help of getSharedPreferences method of the Context class. The preferences are stored in a default file(1) or you can specify a file name(2) to be used to refer to the preferences.

(1) Here is how you get the instance when you specify the file name

public static final String PREF_FILE_NAME = "PrefFile";
   SharedPreferences preferences = getSharedPreferences(PREF_FILE_NAME, MODE_PRIVATE);

MODE_PRIVATE is the operating mode for the preferences. It is the default mode and means the created file will be accessed by only the calling application. Other two mode supported are MODE_WORLD_READABLE and MODE_WORLD_WRITEABLE. In MODE_WORLD_READABLE other application can read the created file but can not modify it. In case of MODE_WORLD_WRITEABLE other applications also have write permissions for the created file.

(2) The recommended way is to use the default mode on the PreferenceManager, without specifying the file name

SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(context);

Finally, once you have the preferences instance, here is how you can retrieve the stored values from the preferences:

 int storedPreference = preferences.getInt("storedInt", 0);

To store values in the preference file SharedPreference.Editor object has to be used. Editor is the nested interface of the SharedPreference class.

SharedPreferences.Editor editor = preferences.edit();
editor.putInt("storedInt", storedPreference); // value to store
editor.commit();

Editor also support methods like remove() and clear() to delete the preference value from the file.

Activity Preferences:

The shared preferences can be used by other application components. But if you do not need to share the preferences with other components and want to have activities private preferences. You can do that with the help of getPreferences() method of the activity. The getPreference method uses the getSharedPreferences() method with the name of the activity class for the preference file name.

Following is the code to get preferences

SharedPreferences preferences = getPreferences(MODE_PRIVATE);
int storedPreference = preferences.getInt("storedInt", 0);

The code to store values is also same as in case of shared preferences.

SharedPreferences preferences = getPreference(MODE_PRIVATE);
SharedPreferences.Editor editor = preferences.edit();
editor.putInt("storedInt", storedPreference); // value to store
editor.commit();

You can also use other methods like storing the activity state in database. Note Android also contains a package called android.preference. The package defines classes to implement application preferences UI.

To see some more examples check Android's Data Storage post on developers site.

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You can save state in three ways in Android.

1. Preferences : used to store app preferences.
2. File system : using Android IO.
3. Sql lite DB: save data to database

You can store the tab in the DB and select the saved state every time the user comes back to your app.

The android dev site has lots of info on these storage methods. data storage

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use SharedPreferences

SharedPreferences Example (from webworld):

     /**
       * get if this is the first run
       *
       * @return returns true, if this is the first run
       */
          public boolean getFirstRun() {
          return mPrefs.getBoolean("firstRun", true);
       }

       /**
       * store the first run
       */

       public void setRunned() {
          SharedPreferences.Editor edit = mPrefs.edit();
          edit.putBoolean("firstRun", false);
          edit.commit();
       }

       SharedPreferences mPrefs;

   /**
   * setting up preferences storage
   */
   public void firstRunPreferences() {
      Context mContext = this.getApplicationContext();
      mPrefs = mContext.getSharedPreferences("myAppPrefs", 0); //0 = mode private. only this app can read these preferences
   }