Data Storage in Android: SharedPreferences

SharedPreferences
- How to Use?
EncryptedSharedPreferences

We have several data storage solutions for Android app development.

`SharedPreferences`

SharedPreferences is a light-weight storage for Android to read and store key-value pairs, which is suitable for simple data like app configs, user preferences, login state.

SharedPreferences is, in essence, an .xml file under app-private folder /data/data/<package_name>/shared_prefs.

How to Use?

Retrieval

We mainly have 2 ways to retrieve the SharedPreferences object.

getSharedPreferences(String name, int mode)

This method is suitable for scenarios like multiple configurations or the config file should be shared among different components.

Here, the name argument tells the file name of the config file. The mode argument is usually Context.MODE_PRIVATE, which means this file can only be accessed by current app.
```
  SharedPreferences userPrefs = getSharedPreferences("user_settings", Context.MODE_PRIVATE);
```
getPreferences(int mode)

This method is a shortcut for activities to create default, private configuration file, whose name is default to activity’s class name.

This method is only suitable for preference configs within same activity.
```
  SharedPreferences activityPrefs = getPreferences(Context.MODE_PRIVATE);
```

Writing Data

Writing data is proxied by SharedPreferences.Editor object and follows 3 steps.

invoke edit() method to get Editor instance
invoke putXXX() method to modify or add data, e.g., putString(), putInt(), putBoolean()
invoke apply() or commit() to submit modification.

  // 1. get SharedPreferences.Editor
  SharedPreferences.Editor editor = getSharedPreferences("app_config", Context.MODE_PRIVATE).edit();

  // 2. put data
  editor.putString("username", "AndroidDev");
  editor.putInt("login_count", 5);

  // 3. submit change
  editor.apply();

The key difference between apply() and commit() is about async or sync, which drastically affect speed.

Feature	`apply()`	`commit()`
Execution	Async	Sync
Return value	/	`boolean` to indicate success
Blocking	Non-Blocking	Blocking
Performance	Higher	Lower
Data consistency	No guarantee	Immediate ensure

In most cases, we should use apply()

Reading Data

We just simply invoke getXXX() of SharedPreferences objects.

`EncryptedSharedPreferences`

Standard SharedPreferences stores data in bear xml files. If we want to store passwords, tokens, we should use EncryptedSharedPreferences, which automatically encrypts keys and values.

First, we have to add dependency in build.gradle of AndroidX Security library.

  dependencies {
    implementation "androidx.security:security-crypto:1.1.0-alpha06"
  }