Persistent Storage with Shared Preferences, and Simple Network Requests

Read about SharedPreferences for simple storage between activities, read about how to perform simple HTTP requests to draw data in via the internet, and read about how to display lists using the RecyclerView.

Share about what sort of applications you imagine you’ll be able to build with these new pieces of Android.

Resources

• Shared Preferences Guide
• Using Shared Preferences
• Sending a Simple Request
• Recycler View

Persistent Storage

It can be hard to persist information while users switch from activity to activity. Although it’s possible to tie activities together passing and receiving data through Intents this can turn out be be quite cumbersome.

Android’s built-in SharedPreferences storage mechanism allows us to store information that persists throughout the entire app. SharedPreferences allows you to create a file that stores primitive values as key-value pairs. It’s especially great for storing information like someone’s username or an auth token – things you’ll need to access all throughout the app.

This custom class, MySharedPreferences wraps around the standard SharedPreferences to create even more easy functionality. This is something we create manually and is not part of Android itself. It creates two methods, storeUsername and getUsername to standardize the way applications interact with the SharedPreferences. Notice that the filename and the PREF_USERNAME values are standardized at the top of the file. Standardizing key names like this prevents typo errors.

Use this class as a pattern to build your own class that adds more get and set methods to store all sorts of primitive data to share between all the Activities in your app.

LoginActivity.java

protected void onCreate(Bundle savedInstanceState) {
  // store the username
  MySharedPreferences.storeUsername(this, "moon");

  // retrieve the username
  String username = MySharedPreferences.getUsername(this);
}

MySharedPreferences.java

public class MySharedPreferences {
    private static final String PREFERENCES_FILENAME = "userinfo";
    private static final String PREF_USERNAME = "username";

    public static void storeUsername(Context context, String username) {
        SharedPreferences.Editor editor = context.getSharedPreferences(PREFERENCES_FILENAME, MODE_PRIVATE).edit();
        editor.putString(PREF_USERNAME, username);
        editor.commit();
    }

    public static String getUsername(Context context) {
        SharedPreferences prefs = context.getSharedPreferences(PREFERENCES_FILENAME, MODE_PRIVATE);
        String username = prefs.getString(PREF_USERNAME, null);
        return username;
    }
}

Here’s a way to hook up an EditText to respond to the enter key.

EditText mUsernameInput = (EditText) findViewById(R.id.usernameInput);
mUsernameInput.setOnEditorActionListener(new TextView.OnEditorActionListener() {
  @Override
  public boolean onEditorAction(TextView textView, int i, KeyEvent keyEvent) {
      // detect if the user presses [enter]
      if (keyEvent.getKeyCode() == KeyEvent.KEYCODE_ENTER) {
          saveUsername();
          return true;
      }
      return false;
  }
});

Network Requests

In order for your app to interact with the internet you need to add the android.permission.INTERNET permission to your app’s manifest. This presents an explicit dialog to the user to make them aware that the app they are using may read their data and transmit information to the internet. Explicitly asking for internet permission from the user is a security and privacy concern. If you download a simple calculator app and it asks for internet permission that may be a clue the app is doing something nefarious on your phone.

Android has a library called Volley that helps manage network requests. Volley builds an object called a RequestQueue to manage multiple network requests. It’s possible to create a request, define how the request should execute on success or on an error, and add the request to the RequestQueue. The StringRequest Response.Listener acts very much like a JavaScript promise. The listener response and error methods will execute when the network request completes.

final TextView mTextView = (TextView) findViewById(R.id.text);

// Instantiate the RequestQueue.
RequestQueue queue = Volley.newRequestQueue(this);
String url ="http://www.google.com";

// Request a string response from the provided URL.
StringRequest stringRequest = new StringRequest(Request.Method.GET, url,
            new Response.Listener<String>() {
    @Override
    public void onResponse(String response) {
        // Display the first 500 characters of the response string.
        mTextView.setText("Response is: "+ response.substring(0,500));
    }
}, new Response.ErrorListener() {
    @Override
    public void onErrorResponse(VolleyError error) {
        mTextView.setText("That didn't work!");
    }
});
// Add the request to the RequestQueue.
queue.add(stringRequest);