Sending broadcasts within your app with a local BroadcastReceiver

In this tutorial I will explain how to send broadcasts within your own app using the LocalBroadcastManager.

To begin with, let’s start with a FAQ.

What is a BroadcastReceiver?

An Android app is made of components, including Application, Activity, Service, ContentProvider and BroacastReceiver, each of which used for a particular purpose.

So a BroadcastReceiver is a component (optional) of an Android app.

What are BroadcastReceivers generally used for?

BroadcastReceiver is a component running in the background and listening to “broacasts” sent by the Android system or by an Android app.

For example, when the user turns on or off the screen, when the level of the battery changes, when wifi is connected/disconnected, etc., the Android system sends a “broacast” with the relevant information about the event that has occured. You can set up a BroadcastReceiver that intercepts this event and performs a certain action in response (for example, starting a Service).

In your own app you can also send broacasts containing information and set up a BroacastReceiver that listens to these broadcasts and performs certain actions in response. We already saw an example in a previous tutorial about the IntentService class, with which you can use a Service to perform slow or long running operations in a background thread.

When the IntentService finishes its task you may want to communicate the result back to the calling Activity: to do this you have to send a broadcast within your IntentService containing the relevant information, and set up a BroadcastReceiver within you Activity to listen to the result.

So, what is a local BroadcastReceiver?

BroadcastReceiver is generally used to send/receive broacasts between different processes (for example, different apps) because it implements an inter process communication system.

But if the “broadcaster” and the BroadcastReceiver belong to the same process (for example, sending broacasts within your own app), a local BroadcastReceiver, which is managed by the LocalBroadcastManager class, is much more efficient.

In addition to that a local BroadcastReceiver is more secure because all relevant information stays within your own app and is not visible outside.

So a local BroadcastReceiver is a more efficient and secure BroacastReceiver that works only within your own app.

Now let’s see how to implement a local BroadcastReceiver within your Activity (here you can download the code in a more readable format):

public class YourClass extends Activity {

//the main code of the Activity is omitted for brevity

// used for filtering broadcasts (the receiver listens only to Intents with “myBroacast” set as action in the constructor)

public static final String MY_ACTION = “myBroacast”;

@Override

protected void onResume() {

super.onResume(); //call the superclass first

LocalBroadcastManager myLocalBroadcastManager = LocalBroadcastManager.getInstance(this);

IntentFilter myIntentFilter = new IntentFilter(MY_ACTION);

//set up the local LocalBroadcastReceiver

myLocalBroadcastReceiver = new BroadcastReceiver() {

@Override

public void onReceive(Context context, Intent intent) {

//handle the broadcast here

}

};

//register the local BroadcastReceiver with the specified intent filter

myLocalBroadcastManager.registerReceiver(myLocalBroadcastReceiver, myIntentFilter);

}

@Override

protected void onPause() {

super.onPause();

LocalBroadcastManager myLocalBroadcastManager = LocalBroadcastManager.getInstance(this);

//always unregister the receiver when it’s no longer needed to save resources

myLocalBroadcastManager.unregisterReceiver(myLocalBroadcastReceiver);

}

//instance variables

private BroadcastReceiver myLocalBroadcastReceiver;

}

When you want to send a local broadcast, you just have to use the following code:

//create an Intent with the action “myBroadcast” (the filter used in our local receiver)

Intent localBroadcastIntent = new Intent(“myBroacast”);

//you can add additional info to the Intent

localBroadcastIntent.putExtra(“risult”, risult);

LocalBroadcastManager myLocalBroadcastManager = LocalBroadcastManager.getInstance(this);

myLocalBroadcastManager.sendBroacast(localBroadcastIntent);

Using directed broadcasts

Another way of sending broacasts within you own app is by using directed broadcasts, explicitly specifying the BroadcastReceiver that has to intercept them.

You just have to set the ComponentName in the Intent:

Intent myIntent = new Intent(action);

myIntent.setComponent(new ComponentName(packageName, className));

sendBroadcast(myIntent);

action = String representing the filter used by our BroadcastReceiver

packageName = String representing the name of the package of our BroadcastReceiver (like “com.androidthetechnicalblog.example”)

className = String representing the name of the class of our BroadcastReceiver

This way our broadcast is received only by the specified BroadcastReceiver.

Directed broadcasts can also be used with different processes/applications.

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Exporting SQLite Database into .csv format

While developing my next app I came across an interesting problem: how do you export an SQLite database into .csv format? There are of course third party libraries that will do the job for you, but this is Android - The Technical Blog, so we want to understand "how stuff works" under the hood.

So let's start with Wikipedia...
According to Wikipedia csv stands for "comma separated values". CSV files are basically text files where values are separated by commas. There is not a formal standard defining .csv files, but the easiest way to convert an SQLite database into a .csv file is the following:

1. first you create a text file with the .csv extension. This way you can easily read the file with Excel or OpenOffice Calc, but you can also use .txt suffix for reading the file with Notepad;
2. you read the database one record after the other;
3. different records in the database are represented by different lines in the .csv file, while the fields of each record are separeted by commas.

In the following example we use the java.io.FileWriter class, because we want to create a text file writing one character at a time (2 bytes), and the java.io.PrintWriter class as a Decorator (remember the Decorator pattern in Java?), because we want to write one line at a time in the file.

Here you can download the code in a more readable format.

public boolean exportDatabase() {
DateFormat df = DateFormat.getDateInstance(DateFormat.SHORT, Locale.getDefault());

/**First of all we check if the external storage of the device is available for writing.
* Remember that the external storage is not necessarily the sd card. Very often it is
* the device storage.
*/
String state = Environment.getExternalStorageState();
if (!Environment.MEDIA_MOUNTED.equals(state)) { 
return false;
}
else {
//We use the Download directory for saving our .csv file.
   File exportDir = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);      
   if (!exportDir.exists()) 
   {
       exportDir.mkdirs();
   }
   
   File file;
   PrintWriter printWriter = null;
   try 
   {
    file = new File(exportDir, "MyCSVFile.csv");
       file.createNewFile();                
       printWriter = new PrintWriter(new FileWriter(file));
                  
       /**This is our database connector class that reads the data from the database.
        * The code of this class is omitted for brevity.
        */
    DBCOurDatabaseConnector dbcOurDatabaseConnector = new DBCOurDatabaseConnector(myContext);
    dbcOurDatabaseConnector.openForReading(); //open the database for reading
   
    /**Let's read the first table of the database.
    * getFirstTable() is a method in our DBCOurDatabaseConnector class which retrieves a Cursor
    * containing all records of the table (all fields).
    * The code of this class is omitted for brevity.
    */
    Cursor curCSV = dbcOurDatabaseConnector.getFirstTable();
    //Write the name of the table and the name of the columns (comma separated values) in the .csv file.
    printWriter.println("FIRST TABLE OF THE DATABASE");
    printWriter.println("DATE,ITEM,AMOUNT,CURRENCY");
       while(curCSV.moveToNext())
       {
        Long date = curCSV.getLong(curCSV.getColumnIndex("date"));
        String item = curCSV.getString(curCSV.getColumnIndex("item"));
        Double amount = curCSV.getDouble(curCSV.getColumnIndex("amount"));
        String currency = curCSV.getString(curCSV.getColumnIndex("currency"));
       
        /**Create the line to write in the .csv file. 
        * We need a String where values are comma separated.
        * The field date (Long) is formatted in a readable text. The amount field
        * is converted into String.
        */
        String record = df.format(new Date(date)) + "," + item + "," + importo.toString() + "," + currency;
   printWriter.println(record); //write the record in the .csv file
}
 
curCSV.close();
dbcOurDatabaseConnector.close();
   }
  
  catch(Exception exc) {
  //if there are any exceptions, return false
  return false;
  }
  finally {
  if(printWriter != null) printWriter.close();
  }
  
  //If there are no errors, return true.
  return true;
}
}

Alternative main Activity

Let's take a little break from Services and cover an easier topic...

In this tutorial I will explain how you can create an alternative main Activity using the attribute android:enabled in the AndroidManifest.xml file.

An alternative main Activity could be useful for different purposes: a brief description or tutorial covering the main features of your app, or for a setup process for example. Of course, you want the alternative main Activity to be started only once (the first time the user launches the app), while afterwards the regular main Activity should be used.

To begin with, we declare the two Activities in the AndroidManifest.xml file, like in the following example:

<activity

   android:name=”.ActivityTutorial”

   android:label=”@string/activity_tutorial_label”

   android:icon=”@drawable/activity_tutorial_icon”

   android:enabled=”true”>

   <intent-filter>

      <action android:name=”android.intent.action.MAIN”/>

      <category android:name=”android.intent.category.LAUNCHER”/>

   </intent-filter>

</activity>

<activity

   android:name=”.ActivityMain”

   android:label=”@string/activity_main_label”

   android:icon=”@drawable/activity_main_icon”

   android:enabled=”false”>

   <intent-filter>

      <action android:name=”android.intent.action.MAIN”/>

      <category android:name=”android.intent.category.LAUNCHER”/>

   </intent-filter>

</activity>

There are two things to notice in the previous example: the enabled attribute is set to true for the tutorial Activity (this is the Activity that starts when the user launches the app for the first time) and false for the usual main Activity.

Notice also the intent filter of the two Activities: it is the same because both Activities are used as the main Activity of your app: the ActivityTutorial only the first time, the ActivityMain from the second time onwards.

In order to toggle the enabled attribute of the two Activities (from true to false for ActivityTutorial, from false to true for ActivityMain) we use the following code within the ActivityTutorial:

PackageManager myPackageManager = getPackageManager();

//let’s disable the ActivityTutorial

myPackageManager.setComponentEnabledSetting(new ComponentName(this, ActivityTutorial.class), PackageManager.COMPONENT_ENABLED_STATE_DISABLED, PackageManager.DONT_KILL_APP);

//let’s enable the ActivityMain

myPackageManager.setComponentEnabledSetting(new ComponentName(this, ActivityMain.class), PackageManager.COMPONENT_ENABLED_STATE_ENABLED, PackageManager.DONT_KILL_APP);

We used the DONT_KILL_APP flag because we don’t want to kill the app containing the component.

The class PackageManager belongs to the following package: android.content.pm.

IntentService in Android

In this tutorial I will explain briefly what an IntentService is and how it should be used in an Android app.
To begin with, let's start with a FAQ...

What is a Service?
An Android app is made of components. The most important components are the following: Activity, Service, BroadcastReceiver and ContentProvider.
An IntentService is a special kind of Service and therefore a component of an Android app.

What are Services used for?
Services in Android are generally used for operations that are not directly involved with the user interface (UI). For example: network operations, writing and reading data to/from the local storage (a database, file, etc.), playing music, and other tasks like these.
It is generally better to use one Service for each different task: one Service for network operations, one different Service for managing the database and so on.
Some ot these tasks can also be managed within a normal Activity, using a dedicated thread (AsyncTask or Handler, as I explained in a different tutorial), especially if the data involved comes from the user interface (for example: writing to a local database the data entered by the user).
But there are other tasks (like network operations or playing music) that are bettere managed by Services.
Generally speaking Services allow you to efficiently separate the logic of background tasks from the code that handles the user interface.

Are Services running in a different thread?
No, the code in a Service runs in the main thread (the thread of the GUI). So it is very important to create a different thread within the Service (using an AsyncTask or Handler) to manage long running or slow operations, otherwise they could slow down or block the user interface.

So, what is an IntentService in the end?
An IntentService is a special utility class which represents a Service whose operations run in a different thread than the thread of the GUI.
This is managed automatically by the IntentService class, so you don't have to implement an AsynckTask or a Handler.

Let's see an example.
The following IntentService is used to upload a text or a photo to a remote server in a different thread.

public class MyIntentService extends IntentService {

    //constants
    private static final String CLASSNAME = "MyIntentService";

    //the following 2 actions have to be added in the intent filter for the
    //IntentService in the manifest file
    public static final String ACTION_UPLOAD_TEXT = "com.tutorial.UPLOADTEXT";
    public static final String ACTION_UPLOAD_PHOTO = "com.tutorial.UPLOADPHOTO";
    
    //extras to put in the Intent that starts this IntentService
    public static final String TEXT = "text";
    public static final String PHOTO= "photo";

    public MyIntentService() {
        super(CLASSNAME);

        //if the system shuts down the service, the Intent is not redelivered
        //and therefore the Service won't start again
        setIntentRedelivery(false);
    }

    //this method runs in a different thread (not the main thread)
    @Override
    protected void onHandleIntent(Intent intent) {
        String action = intent.getAction();

        if(action.equals(ACTION_UPLOAD_TEXT)) {
            String text = intent.getStringExtra(TEXT);
            uploadText(text);
        }
        else if(action.equals(ACTION_UPLOAD_PHOTO)) {
            Bitmap photo = itent.getParcelableExtra(PHOTO);
            uploadPhoto(photo);
        }
    }

    private void uploadText(String text) {
        //in this method you upload the text to the server: omitted for brevity
    }

    private void uploadPhoto(Bitmap photo) {
        //in this method you upload the photo to the server: omitted for brevity
    }
}

If you want to use this IntentService from your Activity to upload a text or a photo to the remote server in a dedicated thread, you just have to use the command Context.startService() with the Itent carrying the specific action and extras.

There is only one disadvantage in this approach: if you have multiple calls to the IntentService, these will be put in a queue and processed one at a time (there is no parallel execution). If you need parallel execution, to make sure that each background operation is executed as soon as possibile, you have to use a different approach, but this will be the object of a different tutorial.

Formatted strings and plurals

If you have been programming in Android for a while, you probably know that string resources should be put in a centralized file, strings.xml in the res/values directory.
This way you can easily change string resources by changing the corresponding value in the strings.xml file (even people who can't program in Android would be able to do this). You can also create different strings.xml files for different languages in order to internationalize your app.

But probably not everyone knows that you can do more than this. Let's see what...

1) FORMATTED STRINGS

You can of course format strings in the strings.xml file. The syntax is the same as that of the java.util.Formatter class (see for more details).
Here is an example:

<?xml version="1.0" encoding="utf-8"?>
<resources>
<string name="login_message">Login %s!</string>
...
</resources>

and here is how you can use this string with a paramter:

getString(R.string.login_message, name)

2) PLURALS

With plurals you can obtain different strings depending on the amount of input. Here is a complete example of what you can do:

<?xml version="1.0" encoding="utf-8"?>
<resources>
<plurals name="score_count">
    <item quantity="zero">Your score is zero! Very bad!</item>
    <item quantity="one">Your score is one! This is not very good!</item>
    <item quantity="two">Your score is two! Come on!</item>
    <item quantity="few">Your score is %d!</item>
    <item quantity="many">Your score %1$d! %2$s, well done!</item>
    <item quantity="other">Your score is %1$d!! %2$s, try again!</item>
</plurals>
...
</resources>

and here is how you can use this plurals resource with paramters: the system will select the correct string (using and formatting the parameters if necessary) depending on the amount of input:

getQuantityString(R.plurals.score_count, score, score, yourname);

As you can see, the secondo paramter (score) is used to determine which string has to be chosen (it is the amount), while the other two parameters (score and yourname) are used for formatting the chosen string.