How to save data in an android app

问题:

I recently coded an Android app. It\'s just a simple app that allows you to keep score of a basketball game with a few simple counter intervals. I\'m getting demand to add a save feature, so you can save your scores and then load them back up. Currently, when you stop the app, your data is lost. So what I was wondering is what I would have to add to have the app save a label (score) and then load it back up. Thanks guys sorry I don\'t know much about this stuff.

回答1:

You have two options, and I\'ll leave selection up to you.

1. Shared Preferences

   This is a framework unique to Android that allows you to store primitive values (such as int, boolean, and String, although strictly speaking String isn\'t a primitive) in a key-value framework. This means that you give a value a name, say, \"homeScore\" and store the value to this key.

   SharedPreferences settings = getApplicationContext().getSharedPreferences(PREFS_NAME, 0);
   SharedPreferences.Editor editor = settings.edit();
   editor.putInt(\"homeScore\", YOUR_HOME_SCORE);
   
   // Apply the edits!
   editor.apply();
   
   // Get from the SharedPreferences
   SharedPreferences settings = getApplicationContext().getSharedPreferences(PREFS_NAME, 0);
   int homeScore = settings.getInt(\"homeScore\", 0);
   

2. Internal Storage

   This, in my opinion, is what you might be looking for. You can store anything you want to a file, so this gives you more flexibility. However, the process can be trickier because everything will be stored as bytes, and that means you have to be careful to keep your read and write processes working together.

   int homeScore;
   byte[] homeScoreBytes;
   
   homeScoreBytes[0] = (byte) homeScore;
   homeScoreBytes[1] = (byte) (homeScore >> 8);  //you can probably skip these two 
   homeScoreBytes[2] = (byte) (homeScore >> 16); //lines, because I\'ve never seen a                   
                                                 //basketball score above 128, it\'s
                                                 //such a rare occurance.
   
   FileOutputStream outputStream = getApplicationContext().openFileOutput(FILENAME, Context.MODE_PRIVATE);
   outputStream.write(homeScoreBytes);
   outputStream.close();
   

Now, you can also look into External Storage, but I don\'t recommend that in this particular case, because the external storage might not be there later. (Note that if you pick this, it requires a permission)

回答2:

OP is asking for a \"save\" function, which is more than just preserving data across executions of the program (which you must do for the app to be worth anything.)

I recommend saving the data in a file on the sdcard which allows you to not only recall it later, but allows the user to mount the device as an external drive on their own computer and grab the data for use in other places.

So you really need a multi-point system:

1) Implement onSaveInstanceState(). In this method, you\'re passed a Bundle, which is basically like a dictionary. Store as much information in the bundle as would be needed to restart the app exactly where it left off. In your onCreate() method, check for the passed-in bundle to be non-null, and if so, restore the state from the bundle.

2) Implement onPause(). In this method, create a SharedPreferences editor and use it to save whatever state you need to start the app up next time. This mainly consists of the users\' preferences (hence the name), but anything else relavent to the app\'s start-up state should go here as well. I would not store scores here, just the stuff you need to restart the app. Then, in onCreate(), whenever there\'s no bundle object, use the SharedPreferences interface to recall those settings.

3a) As for things like scores, you could follow Mathias\'s advice above and store the scores in the directory returned in getFilesDir(), using openFileOutput(), etc. I think this directory is private to the app and lives in main storage, meaning that other apps and the user would not be able to access the data. If that\'s ok with you, then this is probably the way to go.

3b) If you do want other apps or the user to have direct access to the data, or if the data is going to be very large, then the sdcard is the way to go. Pick a directory name like com/user1446371/basketballapp/ to avoid collisions with other applications (unless you\'re sure that your app name is reasonably unique) and create that directory on the sdcard. As Mathias pointed out, you should first confirm that the sdcard is mounted.

File sdcard = Environment.getExternalStorageDirectory();
if( sdcard == null || !sdcard.isDirectory()) {
    fail(\"sdcard not available\");
}
File datadir = new File(sdcard, \"com/user1446371/basketballapp/\");
if( !datadir.exists() && !datadir.mkdirs() ) {
    fail(\"unable to create data directory\");
}
if( !datadir.isDirectory() ) {
    fail(\"exists, but is not a directory\");
}
// Now use regular java I/O to read and write files to data directory

I recommend simple CSV files for your data, so that other applications can read them easily.

Obviously, you\'ll have to write activities that allow \"save\" and \"open\" dialogs. I generally just make calls to the openintents file manager and let it do the work. This requires that your users install the openintents file manager to make use of these features, however.

回答3:

In onCreate:

SharedPreferences sharedPref = getSharedPreferences(\"mySettings\", MODE_PRIVATE);

    String mySetting = sharedPref.getString(\"mySetting\", null);

In onDestroy or equivalent:

SharedPreferences sharedPref = getSharedPreferences(\"mySettings\", MODE_PRIVATE);

    SharedPreferences.Editor editor = sharedPref.edit();
    editor.putString(\"mySetting\", \"Hello Android\");
    editor.commit();

回答4:

Use SharedPreferences, http://developer.android.com/reference/android/content/SharedPreferences.html

Here\'s a sample: http://developer.android.com/guide/topics/data/data-storage.html#pref

If the data structure is more complex or the data is large, use an Sqlite database; but for small amount of data and with a very simple data structure, I\'d say, SharedPrefs will do and a DB might be overhead.

回答5:

There is a lot of options to store your data and Android offers you to chose anyone Your data storage options are the following:

Shared Preferences Store private primitive data in key-value pairs. Internal Storage Store private data on the device memory. External Storage Store public data on the shared external storage. SQLite Databases Store structured data in a private database. Network Connection Store data on the web with your own network server

Check here for examples and tuto

回答6:

1. Shared preferences: android shared preferences example for high scores?

2. Does your application has an access to the \"external Storage Media\". If it does then you can simply write the value (store it with timestamp) in a file and save it. The timestamp will help you in showing progress if thats what you are looking for. {not a smart solution.}

回答7:

In my opinion db4o is the easiest way to go. Here you can find a tutorial: http://community.versant.com/documentation/reference/db4o-7.12/java/tutorial/

And here you can download the library:

http://www.db4o.com/community/download.aspx?file=db4o-8.0-java.zip

(Just put the db4o-8.0...-all-java5.jar in the lib directory into your project\'s libs folder. If there is no libs folder in you project create it)

As db4o is a object oriented database system you can directly save you objects into the database and later get them back.

回答8:

Please don\'t forget one thing - Internal Storage data are deleted when you uninstall the app. In some cases it can be \"unexpected feature\". Then it\'s good to use external storage.

Google docs about storage - Please look in particular at getExternalStoragePublicDirectory