Sticky Broadcast

Sticky Broadcast(s) are a special kind of Broadcast(s) that remain active for awhile after being sent. You can send a sticky Broadcast with the method:

Context.sendStickyBroadcast()

If you register a BroadcastReceiver that listens to sticky Broadcast(s), it can receive two kind of Broadcast(s):

1. Broadcast(s) that have just been sent
2. Broadcast(s) that have been sent previously and are still active

You can find out what kind of Broadcast you have just received with the method:

isInitialStickyBroadcast()

returning false if the Broadcast represents an event that has just occured.

Sticky Broadcast(s) are generally used for events lasting for a certain period of time. Examples of such Broadcast(s) sent by the system are:

• Intent.ACTION_BATTERY_CHANGED: indicates changes in the battery level. BroadcastReceiver(s) must be programmatically registered with Context.registerReceiver()
• Intent.ACTION_DOCK_EVENT: indicates when a device is placed on a dock
• Intent.ACTION_DEVICE_STORAGE_LOW: indicates low memory condition

public class MyBroadcastReceiver extends BroadcastReceiver {
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        if(action.equals(Intent.ACTION_DOCK_EVENT)) {
            if(!isInitialStickyBroadcast()) {
                // this is a new broadcast
            } 
            else {
                // this is an old broadcast
            }
        }
    }
}

In order to send sticky Broadcast(s) you must include the permission android.permission.BROADCAST_STICKY in the Manifest file.

If you want to remove a sticky Broadcast you can use the method:

Context.removeStickyBroadcast(Intent intent)

passing the Intent that was previously broadcast.

NB. Sticky Broadcast(s) consume system resources, so use them sparingly when really needed!

API level 21

Starting from API level 21 sticky Broadcast(s) are deprecated.

"Sticky broadcasts should not be used. They provide no security (anyone can access them), no protection (anyone can modify them), and many other problems. The recommended pattern is to use a non-sticky broadcast to report that something has changed, with another mechanism for apps to retrieve the current value whenever desired".

Reverse engineering part 2: apktool

After getting the apk file (see article: Reverse engineering part 1: getting the apk file) it's time to disassemble it and analyze its content.
The tool I will talk about is called apktool and can be downloaded from https://ibotpeaches.github.io/Apktool/, where you can also read the complete documentation. Apktool is especially useful to analyze resource files (xml layout files, Manifest, drawables, xml animation files, strings).

Apktool

Apktool is a tool for reverse engineering 3rd party, closed, binary Android apps. It can decode resources to nearly original form and rebuild them after making some modifications. It also makes working with an app easier because of the project like file structure and automation of some repetitive tasks like building apk, etc.

Reverse engineering

After installing apktool you'll find the apktool.bat file that can be run from the command line.
You just have to type:

apktool d filename.apk

(replace filename.apk with the actual file name of the apk file)

and apktool disassembles the apk file in a folder where you can find all the resource files perfectly readable.

For example, I ran apktool for Whatsapp and this is the result:

In the "res" folder you can have a look at almost every resource file of the app:

This is the layout file settings_security.xml of Whatsapp. As you can see the file is completely readable.

This is another example: the animation defined in design_fab_out.xml, that fades out a FloatingActionButton:

Analyzing the file you can find out how the animation is achieved: animating alpha from 1 to 0 and the scale of the FAB from 1 to 0. Very easy!

Conclusion

Apktool is a very powerful utility especially if you want to analyze the resource files of an app. If you have found an app with a beautiful design, or a fancy animation, and want to find out how they are made, apktool is the tool to use.

Reverse engineering part 1: getting the apk file

Reverse engineering, also called back engineering, is the processes of extracting knowledge or design information from anything man-made and re-producing it or re-producing anything based on the extracted information. The process often involves disassembling something (a mechanical device, electronic component, computer program, or biological, chemical, or organic matter) and analyzing its components and workings in detail.

As far as Android is concerned, suppose we have full-working Android app and we would like to find out how a particular layout is structured, how a specific animation is coded, how a visual effect is made. We can do that by reverse-engineering the apk file and extract useful information from it.

There are different tools for doing that and in the following articles we'll have a look at some of the most widely used.

Reverse engineering

Getting the apk file

The first step to reverse-engineer an Android app is getting the apk file, in order to disassemble and analyze it using the different tools available.

From the web

The easiest way to obtain the apk file of a particular app is using one the many online resources available.

For example, using this website:

http://apk-dl.com/

you can paste the Google Play Store Url of the app you're interested in, and the service lets you download the apk file to your pc.

If you download an apk file from the web, like the online service previously mentioned, be aware of the risk of possible viruses or other malware.

From the smartphone

You can also very easily extract the apk file from an app installed on your smartphone.

To do that we can use the adb.exe utility of the Android Sdk platform (you can find it in the folder: Android\sdk\platform-tools).

Just connect the device to your pc and type the following commands in command line...

adb shell pm list packages 

to obtain a list of the apps (packages) installed and find the package you are interested in

adb shell pm path your-package-name

replace "your-package-name" with the name of the desired package

to get the full package name and the name of the app

adb pull full_path_of_the.apk

replace "full_path_of_the.apk" with the full package name of the apk file obtained with the previous command

to get the apk file in the folder where the command is executed

How to blur an image using Renderscript

RenderScript is a framework for running computationally intensive tasks at high performance on Android. RenderScript is primarily oriented for use with data-parallel computation, although serial workloads can benefit as well. The RenderScript runtime parallelizes work across processors available on a device, such as multi-core CPUs and GPUs.
RenderScript is especially useful for applications performing image processing, computational photography, or computer vision.
Let's see how you can use Renderscript to effectively blur an image.

Blurred image

Integrating Renderscript with Gradle

To add the Renderscript support library to your Android Studio project just add the following lines to build.gradle (module: app).

android {
    ...
    defaultConfig {
        renderscriptTargetApi 19
        renderscriptSupportModeEnabled true
    }
    ...
}

Code to blur the image

Now create the utility class BlurBuilder that uses RenderScript to blur the image:

/*
 * Copyright (c) - Software developed by iClaude.
 */

package com.flingsoftware.personalbudget.utilita;


import android.content.Context;
import android.graphics.Bitmap;
import android.support.v8.renderscript.Allocation;
import android.support.v8.renderscript.Element;
import android.support.v8.renderscript.RenderScript;
import android.support.v8.renderscript.ScriptIntrinsicBlur;


/**
 * This class is used to blur a Bitmap.
 * The static method blur takes a Bitmap as input e returns a blurred version of if.
 */
public class BlurBuilder {
    private static final float BITMAP_SCALE = 0.4f;
    private static final float BLUR_RADIUS = 7.5f;


    public static Bitmap blur(Context context, Bitmap image) {
        int width = Math.round(image.getWidth() * BITMAP_SCALE);
        int height = Math.round(image.getHeight() * BITMAP_SCALE);

        Bitmap inputBitmap = Bitmap.createScaledBitmap(image, width, height, false);
        Bitmap outputBitmap = Bitmap.createBitmap(inputBitmap);

        RenderScript rs = RenderScript.create(context);
        ScriptIntrinsicBlur theIntrinsic = ScriptIntrinsicBlur.create(rs, Element.U8_4(rs));
        Allocation tmpIn = Allocation.createFromBitmap(rs, inputBitmap);
        Allocation tmpOut = Allocation.createFromBitmap(rs, outputBitmap);
        theIntrinsic.setRadius(BLUR_RADIUS);
        theIntrinsic.setInput(tmpIn);
        theIntrinsic.forEach(tmpOut);
        tmpOut.copyTo(outputBitmap);

        return outputBitmap;
    }
}

As you can see the class exposes a static method, "blur", that takes the original Bitmap as a parameter and returns the blurred version of the image.
You can adjust the constants BITMAP_SCALE and BLUR_RADIUS to control the amount of blurriness.

Blurring the image

Now, if you have a Bitmap you can blur it using the following code (remeber to do this in a separate thread to avoid locking the UI):

Bitmap origBitmap = ...
Bitmap blurredBitmap = BlurBuilder.blur(mContext, origBitmap);
myImageView.setImageBitmap(blurredBitmap );

Managing AsyncTasks on configuration changes

The problem

Suppose you have an AsyncTask performing some task in the background (downloading data from a web service, updating an online database, etc.). If the user rotates the device, as you probably know, the current Activity is destroyed and recreated (the callback methods onDestroy and onCreate are invoked).

As a result, when the AsyncTask finishes its task and should deliver its results, the original Activity may no longer exist or it could have been replaced by a brand new Activity. However the AsyncTask is tied to the original Activity and its onPostExecute method won’t work with the newly created Activity. The newly created Activity may even start another AsyncTask (like in our example, see below), thus wasting system resources.

That’s why, as a general rule, you should always perform network operations in a Service: the Service’s lifecycle isn’t directly tied to the Activity lifecycle; a Service is not destroyed and recreated on configuration changes, like AsyncTasks do.

However AsyncTasks are generally easier and quicker to program than Services, so let’s examine some possibile solutions to our “configuration changes problem” using AsyncTasks.

Code

You can download the code for this tutorial from the Android – The Techinal Blog Githubrepository.

The easy solution

A very easy solution to our problem is preventing the Activity from being destroyed and recreated on configuration changes. For example, if you use the same layout for portrait and landscape orientation there is no need to destroy and recreate the Activity when the user rotates the device.

To achieve this, just insert the following line in the AndroidManifest.xml file under the Activity tag:

android:configChanges="orientation|screenSize|keyboardHidden”

This way, when the user rotates the device, the Activity is not destroyed and recreated anymore; conversely the method onConfigurationChanged gets called and you can override it if you need it.

The correct solution

The correct solution to the “configuration change problem” is using a so called “retained Fragment”.

A Fragment represents a behavior or a portion of the user interface within an Activity. Fragment’s life cycle is generally coordinated with the lifecycle of its containing Activity: when the Activity is destroyed on configuration changes all the contained Fragments get destroyed too.

However if you call setRetainInstance(true) within the Fragment class the destroy-and-recreated cycle of the Fragment is bypassed. As a result you can use a “retained Fragment” to hold active objects, like the AsyncTasks, Sockets, etc.

Let’s start with the retained Fragment:

/**
 * This Fragment performs a specific background task and is not
 * destroyed on configuration changes.
 */
public class RetainedFragment extends Fragment {
    /**
     * Interface used by the retained Fragment to communicate with
     * the Activity.
     */
    interface RetainedFragmentCallbacks {
        void onPreExecute();

        void onProgressUpdate(int progress);

        void onCancelled();

        void onPostExecute();
    }

    private RetainedFragmentCallbacks retainedFragmentCallbacks;
    private MyAsyncTask myAsyncTask;

    /**
     * Hold a reference to the parent Activity so we can report the
     * task's current progress and results (the Activity must implement
     * the RetainedFragmentCallbacks interface to receive the data sent
     * by this Fragment).
     * On configuration changes the Android framework passes to this
     * method a reference to the newly created Activity. This way
     * the AsyncTask can deliver its results to the new Activity.
     */
    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        retainedFragmentCallbacks = (RetainedFragmentCallbacks) activity;
    }

    /**
     * This method gets invoked when the Fragment is created for
     * first time.
     */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // This Fragment won’t be destroyed on configuration changes.
        setRetainInstance(true);

        // Create and execute the background task.
        myAsyncTask = new MyAsyncTask();
        myAsyncTask.execute();
    }

    /**
     * Set the callback to null so we don't accidentally leak the
     * Activity instance.
     */
    @Override
    public void onDetach() {
        super.onDetach();
        retainedFragmentCallbacks = null;
    }

    /**
     * This AsyncTask performs a long task in the background
     * (we fake it with a sleep) and communicates progress and
     * results to the containing Activity.
     * In each method we check that retainedFragmentCallbacks is
     * not null because the Activity may have been destroyed on
     * configuration changes or because the user exits the Activity
     * pressing the back button.
     */
    private class MyAsyncTask extends AsyncTask {

        @Override
        protected void onPreExecute() {
            if (retainedFragmentCallbacks != null) {
                retainedFragmentCallbacks.onPreExecute();
            }
        }

        /**
         * Note that we do NOT call the callback object's methods
         * directly from the background thread, as this could result
         * in a race condition.
         */
        @Override
        protected Void doInBackground(Void... ignore) {
            for (int i = 0; !isCancelled() && i < 10; i++) {
                SystemClock.sleep(1000);
                publishProgress(i);
            }
            return null;
        }

        @Override
        protected void onProgressUpdate(Integer... progress) {
            if (retainedFragmentCallbacks != null) {
                retainedFragmentCallbacks.onProgressUpdate(progress[0]);
            }
        }

        @Override
        protected void onCancelled() {
            if (retainedFragmentCallbacks != null) {
                retainedFragmentCallbacks.onCancelled();
            }
        }

        @Override
        protected void onPostExecute(Void ignore) {
            if (retainedFragmentCallbacks != null) {
                retainedFragmentCallbacks.onPostExecute();
            }
        }
    }
}

And now the code of the main Activity:

/**
 * This Activity creates a RetainedFragment to perform a background
 * task, and receives progress updates and results from the
 * Fragment when they occur.
 */
public class MainActivity extends AppCompatActivity implements RetainedFragment. RetainedFragmentCallbacks {

    private static final String TAG_RETAINED_FRAGMENT = "RETAINED FRAGMENT";
    private RetainedFragment retainedFragment;
    private TextView textView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        textView = (TextView) findViewById(R.id.textView);

        FragmentManager fm = getFragmentManager();
        retainedFragment = (RetainedFragment) fm.findFragmentByTag(TAG_RETAINED_FRAGMENT);

        // If the Fragment is not null, then it is currently being
        // retained across a configuration change.
        if (retainedFragment == null) {
            retainedFragment = new RetainedFragment();
            fm.beginTransaction().add(retainedFragment, TAG_RETAINED_FRAGMENT).commit();
            Log.d(TAG_RETAINED_FRAGMENT, "Retained Fragment created anew");
        }
        else {
            Log.d(TAG_RETAINED_FRAGMENT, "Retained Fragment retained on configuration change");
        }
    }

    // The four methods below are called by the RetainedFragment when new
    // progress updates or results are available. The MainActivity
    // should respond by updating its UI to indicate the change.

    @Override
    public void onPreExecute() {
        Log.d(TAG_RETAINED_FRAGMENT, "onPreExecute() received...");
        textView.setText("onPreExecute() received...");
    }

    @Override
    public void onProgressUpdate(int progress) {
        Log.d(TAG_RETAINED_FRAGMENT, "onProgressUpdate received with progress: " + progress);
        textView.setText("onProgressUpdate received with progress: " + progress);
    }

    @Override
    public void onCancelled() {
        Log.d(TAG_RETAINED_FRAGMENT, "onCancelled() received...");
        textView.setText("onCancelled() received...");
    }

    @Override
    public void onPostExecute() {
        Log.d(TAG_RETAINED_FRAGMENT, "onPostExecute() received...");
        textView.setText("onPostExecute() received...");
    }


}

The code is quite self-explanatory.

When the Activity is created for the first time the method findFragmentByTag returns null, since no Fragment has already been created. As a result a new instance of the RetainedFragment class is created and attached to the main Activity. The main Activity implements RetainedFragment.RetainedFragmentCallbacks interface and is registered to receive updates from the Fragment in the method onAttach.

When the Fragment is created setRetainInstance(true) ensures that it won’t be destroyed on configuration changes. In addition to that the Fragment creates and starts an AsyncTask, defined and managed inside the Fragment class, to perform a long lasting task in the background (for example downloading data from a web service).

Progress updates and final results are delivered to the main Activity through the RetainedFragment.RetainedFragmentCallbacks interface, which is implemented in the Activity (see the logs).

Now let’s suppose that the user rotates the device. This is what happens:

• the RetainedFragment is not destroyed (because we have called setRetainInstance(true) when the Fragment was created) and the AsyncTask continues to work in the background;
• the main Activity is destroyed and recreated as usual. When it’s destroyed it’s also detached from the RetainedFragment and the retainedFragmentCallbacks variable inside the Fragment class is set to null as a result. When the Activity is recreated it’s re-attached to the existing Fragment and the retainedFragmentCallbacks variable inside the Fragment is now set to point to the newly created Activity: results and progress updates are therefore delivered to this new Activity, and not to the old one (which doesn’t exist anymore). Notice that the RetainedFragment itself is not recreated because the method findFragmentByTag in onCreate now returns the existing Fragment.

Ordered Broadcasts

There are two types of broadcasts in Android: normal and ordered. The following table shows the main differences:

	Normal	Ordered
Way of delivery	Normal broadcasts are delivered to the available receivers asynchronously, in an unspecified order	Ordered broadcasts are delivered to the available receivers one at a time, in a specified order (the order depends on the android:priority attribute of the different receivers in the AndroidManifest file)
Feedback	With normal broadcasts no feedback can be sent to the broadcaster	With ordered broadcasts the receiver can send information to the broadcaster (using the methods abortBroadcast(), seResultCode(), setResultData()

Tip: in a receiver (handling ordered broadcasts) you can call the method abortBroadcast() to make sure that the broadcast is not sent to other receivers. If you combine this method with a high value for the android:priority attribute, you can make sure that your receiver is the only one to handle that broadcast (SMS apps can use this technique).

BroadcastReceiver in Android

BroadcastReceiver

Let’s see how a BroadcastReceiver can handle ordered broadcasts (se the comments in the code for more information):

public class MyReceiver extends BroadcastReceiver {
    [...]
    public void onReceive(Context context, Intent intent) {
        //is this an ordered broadcast or not?
        if(isOrderedBroadcast() {
            //here you can handle the broadcast    
            [...]

            //here you can send information back to the broadcaster
            setResultCode(Activity.RESULT_OK);
            setResultData(“here you can pass a String”);

            //you can also create complex data to send to the broadcaster
            Bundle myBundle = getResultExtras(true);
            myBundle.put [...] //code omitted for brevity
        }
    }
}

Broadcaster

Sending an ordered broadcast is quite simple. Let’s look at the code:

Intent int = new Intent(YOUR_FILTER);
sendOrderedBroadcast(int, responseReceiver, null, RESULT_OK, null, null);

As you can see you just have to create an Intent with the filter registered for the BroadcastReceiver MyReceiver and call the method sendOrderedBroadcast. This method takes a BroadcastReceiver (responseReceiver) as a parameter: this is the receiver that will receive the information sent back by the BroadcastReceiver MyReceiver.

So you finally have to register the BroadcastReceiver responseReceiver (to handle the results) in the following way:

BroadcastReceiver responseReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        //here you can retrieve the results sent by MyReceiver
        String str = getResultData();

        //here you can retrieve the complex data sent by MyReceiver
        Bundle myBundle = getResultExtras(false);
        if(myBundle != null) {
            //here you can retrieve the data using myBundle.get... methods
        }
    }
}

Creating a pie chart with aChart Engine

In many Android apps you may need to show data in graphical format (pie charts, bar charts, time charts, etc.). To do this you can write the code from scratch, or use one of the many third-party libraries available. One of the most used is certainly aChart Engine, and there are several good reasons for that:

1. it is leight;
2. it is free;
3. it is relatively complete.

In this brief tutorial I will explain the most important features you must know to create a pie chart using aChart Engine.

Download and install the library

First of all you have to download the library (version 1.1.0) and set up your Eclipse project to use it. The steps are the following:

1. donwload the library (jar file, version 1.1.0) from this link. You can find the complete documentation of the package here;
2. open Eclipse; right click on the main project folder; select "Build Path" and "Configure Build Path..."; select "Libraries" tab; click on "Add external Jars..." and select the library Jar file you have downloaded.

Now your Eclipse project is ready to use the library. Of course, if you want to use the different classes, you first have to import them in your Activity (import org.achartengine.model.CategorySeries ... etc.). 

Source data

Let’s suppose we want to create a pie chart showing the distribution of the different Android versions, using the following data:

Kit Kat – 7.8%

Jelly Bean – 33%

Ice Cream Sandwich – 25%

Honeycomb – 17%

Gingerbread – 11%

Older – 6.2%

Each slice of the pie chart is represented by a different color.

The source data in Java is represented by three arrays, each with six elements:

String[] labels = new String[] {“Kit Kat”, “Jelly Bean”, “Ice Cream Sandwich”, “Honeycomb”, “Gingerbread”, “Older”};

double[] values = {7.8, 33, 25, 17, 11, 6.2};

int[] colors = {Color.RED, Color.BLUE, Color.GREEN, Color.MAGENTA, Color.YELLOW, Color.GRAY};

CategorySeries

The class CategorySeries of aChart Engine (org.achartengine.model.CategorySeries) is used to represent a series for the category charts like the pie ones.

We create an instance of this class and populate it with the previously showed source data (labels and values):

CategorySeries categorySeries = new CategorySeries(“Android versions”);

for(int i=0; i<labels.length; i++) {

    categorySeries.add(labels[i], values[i]);

}

DefaultRenderer

The class DefaultRenderer (org.achartengine.renderer.DefaultRenderer) is an abstract renderer used to format the charts.

We can create an instance of this class and specify how we want our pie chart to be rendered:

DefaultRenderer defaultRenderer = new DefaultRenderer();

defaultRenderer.setChartTitle(“Android versions”);

defaultRenderer.setChartTitleTextSize(30);

defaultRenderer.setZoomButtonsVisible(false);

...

NB: It is important to set the zoom buttons visibility to false, otherwise you may get a NullPointerException in the onDraw() method of the class GraphicalView (as far as I know, this should be a bug in the library).

The are of course many other formattings you can control using a DefaultRenderer: you can read the official documentation for more information.

To control the rendering of each slice of the pie chart (color, showing or not showing values, etc.) you can use the SimpleSeriesRenderer class (org.achartengine.renderer.SimpleSeriesRenderer): you just have to create an instance of this class for each slice, set the relevant data and add it to the previously defined DefaultRenderer:

for(int i=0; i<labels.length; i++) {

    SimpleSeriesRenderer simpleSeriesRenderer = new SimpleSeriesRenderer();

    simpleSeriesRenderer.setColor(colors[i]);

    simpleSeriesRenderer.setDisplayChartValues(true);

    ...

    defaultRenderer.addSeriesRenderer(simpleSeriesRenderer);

}

Starting a new Activity

To show our pie chart we first have to retrieve an Intent, populated with the relevant data, using the ChartFactory class:

Intent intPiechart = ChartFactory.getPieChartIntent(context, categorySeries, defaultRenderer, “Android versions”);

startActivity(intPieChart);

The new Activity must be declared in the AndroidManifest.xml file as well:

<activity android:name=”org.achartengine.GraphicalActivity”/>