Bring MVVM To Your Android Application With Data Binding Library

Google I/O 2015 has come and gone now, only leaving in its tracks one developer tool that really gets me excited.

We saw an array of nice incremental improvements announced. Like Android M and its various user-centric features, NDK (C/C++) support in Android Studio (if you’re into that kinda thing), image generation from vector files, heap analysis, improved theme and layout editors, Gradle performance improvements, etc. I am pleased we finally have a Design Support Library so we can implement the Material Design UI patterns we’ve been guided toward for about a year now. But most of these things were already being done in one form or another by leveraging community tools and libraries.

One thing however that the community’s been craving, but hasn’t come to a good set of patterns or tools on, is how to improve the code that coordinates between the model and the views inside our projects. Until now, Activities and Fragments have typically contained a ton of fragile, untestable and uninteresting code to work with views. But that all changes with the Data Binding Library.

Goals and Benefits

We should all be interested in this library because it will allow us to be more declarative in the way we work with our views. Going declarative should help remove a lot of the code that’s not very fun to write, and along with it, a lot of pesky UI orchestration bugs that result. Less code means less bugs, right? Right.

Another big goal of mine and something the community needs, is lower friction unit testing for our view and application logic. It’s always been possible to have tests here, but it’s been so hard and required so much additional work that a lot of us (not me of course) just skip right over them. This is our opportunity to do better.


In the official docs for this library, they give you an example of directly binding a domain entity properties from User to attributes in the layout. And they give you the idea that you can do fancy things like this:

    android:visibility="@{age < 13 ? View.GONE : View.VISIBLE}"

But let’s be really clear here: I don’t recommend binding directly to domain entities or putting logic into those bindings in the layout file. If you do either of those things, it will make it harder to test your view logic and harder to debug. What we’re after is the opposite: easier to test and debug.

That’s where the MVVM pattern comes into the picture. To over-simplify things, this will decouple our Views from the Model by introducing a ViewModel layer in between that binds to the View and reacts to events. This ViewModel will be a POJO and contain all the logic for our view, making it easy to test and debug. With this pattern, the binding will only be a one-to-one mapping from the result of a ViewModel method into the setter of that View property. Again, this makes testing and debugging our view logic easy and possible inside of a JUnit test.

Project Setup

Let’s get to it then. NOTE: I’ll probably skip over some useful information here in the interest of brevity, so I recommend referencing the official docs.

Start off by adding these dependencies to your project’s build.gradle:

    classpath ''
    classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'

These dependencies must be added to project level build.gradle complete file content:

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    repositories {
    dependencies {
        classpath ''
        classpath ''
        classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files

allprojects {
    repositories {

And then add this to your app module’s build.gradle:

    apply plugin: ''
    apply plugin: ''
    apply plugin: ''

    dependencies {
        apt ''

NOTE: if you have any provided dependencies like dagger-compiler, you will now need to change the provided keyword to apt to prevent them from being added to your classpath and if you’r using kotlin like me you need to change apt to kapt as follow:

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar','*.so'])
    compile ''
    compile ''
    compile ''
    compile ''
    compile ''
    compile ''
    compile 'eu.chainfire:libsuperuser:1.0.0.+'
    compile 'io.reactivex:rxandroid:1.0.1'
    compile 'io.reactivex:rxjava:1.0.16'
    compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    compile ''
    // Dagger 2
    compile ''
    compile 'javax.inject:javax.inject:1'
    compile 'javax.annotation:javax.annotation-api:1.2'
    apt ''
    provided ""
    provided 'javax.annotation:jsr250-api:1.0'
    provided 'org.glassfish:javax.annotation:10.0-b28'

    // Data Binding
    kapt ''

kapt {
    generateStubs = true

The official docs don’t mention android-apt anywhere, but you will want it. The android-apt plugin will make Android Studio aware of classes generated during the build process. This becomes crucial when trying to debug issues and learn more about how the binding mechanism works.

Binding Setup

The mechanism by which your View will receive it’s initial values and updates, and by which your ViewModel will handle events from the View, is through bindings. The Data Binding Library will automatically generate a binding class that will do all most of the hard work for you. Let’s look at the pieces required for this binding to occur.

Variable Declarations

In your layout files, you will need to add a new top level layout wrapper element around your existing layout structure. The first element inside of this will be a data element which will contain the types you will be working with in your layout bindings.

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android=""
    <data class="FragmentCustomerBinding">
        <variable name="viewModel" type="com.example.viewmodels.CustomerViewModel" />
    <!-- the rest of your original layout here -->

Here, we declared a viewModel variable that we will later set to a specific instance inside our Fragment.

Binding Declarations

We can now use this viewModel variable to do lots of interesting things by binding its properties to our layout widget attributes.

    app:onFocusChangeListener="@{viewModel.nameFocusListener}" />

Here, we’re binding the text value, enabled state, error message, a text changed listener and a focus change listener.

NOTE: the android namespace can be used for any standard xml attribute on a view, but the app namespace must be used to map to setters that do not have a corresponding xml attribute. Also, using the app namespace instead of android for standard attributes seems to remove error highlighting in the IDE.

WARNING: due to the order in which the binding code is generated, you will want to use the android namespace for the text attribute to prevent ordering issues inside the generated binding code. Otherwise, the setText() will happen after the setError() and clear the error.

ViewModel Implementation

Now, let’s look at the corresponding methods on the ViewModel that will be bound to the view properties. The ViewModel extends BaseObservable (it doesn’t have to, but it saves you a lot of work), exposes public methods whose name matches the name in the layout binding and the return type matches the type expected by the view setter method being bound to.

public class CustomerViewModel extends BaseObservable {

    public String getCustomerName() {
        return customer.getName();

    public boolean isPrimaryInfoEnabled() {
        return editMode && !customer.isVerified();

    public String getNameError() {
        if (customer.getName().isEmpty()) {
            return "Must enter a customer name";
        return null;

    public TextWatcher getNameWatcher() {
        return new SimpleTextWatcher() {
            public void onTextChanged(String text) {

    public EditText.OnFocusChangeListener getNameFocusListener() {
        return (v, hasFocus) -> {
            if (!hasFocus) notifyPropertyChanged(BR.nameError);

The first method is just doing a simple delegation to a domain entity to get the return value. The second and third are performing some logic to determine the return value. The rest are returning watchers or listeners to react to changes in the view. The great thing here is that this EditText will automatically get populated with the value from the ViewModel, show an error if it doesn’t pass validation rules and send updates back to the ViewModel as things change.


Notice in the validate() method above, the listener calls notifyPropertyChanged(…). This will trigger the view to rebind the property and potentially show an error if one is then returned. The BR class is generated for you, much like the R file, to allow you to reference bindable properties in code. This granular notification isn’t possible unless you annotate the property with @Bindable. Since we only specified the viewModel variable in the layout, it’s the only “bindable” value it creates by default.

You can also trigger the view to rebind all of its properties by using the more generic notifyChange() method.

Be careful here. You can get into situations where you have a TextWatcher that calls notifyChange() which causes the text to be rebound, which triggers the TextWatcher, which causes a notifyChange() , which… you see where this is going?

It seems like best practices here will be one of the following:

  • Short circuit the notification cycle by checking to see if the value actually changed before notifying.
  • Avoid notifying the views that changed inside their own change listeners. If other views need to be notified in this situation, you will need to bind and notify at a more granular level.

Bringing it all together

So far we’ve set up the declarative pieces that will all react to each other and do the right thing. The only thing left is to bootstrap the bind mechanism. This will happen inside your Activity or Fragment. Since I use Fragments for all my views, I’ll show what that looks like.

    FragmentCustomerBinding binding = FragmentCustomerBinding.bind(view);
    CustomerViewModel viewModel = new CustomerViewModel();

Taking it further

We looked at the basic building blocks of creating a UI that reacts to changes in the ViewModel as they are changing. Since you aren’t on the hook for writing the code that updates the UI, you can spend your time creating:

  • Buttons that enable/disable based on the validity of the ViewModel
  • Loading indicators that show/hide based on work being done in the ViewModel
  • Unit tests that exercise every aspect of your view’s logic


There are still some things that don’t seem to be handled well in this new binding world. For example, you can’t easily bind an ActionBar to a ViewModel. (Maybe forgoing the old ActionBar interface and just using a Toolbar directly could help?)

You will also need to delegate back to the Activity for framework-specific things that require the Activity Context. (Which is a lot!) You could inject interface implementations into your ViewModels or set the Activity/Fragment as a listener on your ViewModel, or just use the ViewModel inside the fragment and call methods on it. Either way, you can still use a ViewModel to house all your view logic and delegate out as needed.

Just think of the Fragment now as the place where you have to write your manual binding code – which is what it always was before, except now with all the time you save not writing most of that code, you can spend on the writing automated tests for your ViewModel!

What’s Missing

This library works very well but is still in beta, and you can tell when you use it. I look forward to seeing it mature and provide a better developer experience. Some of the things I look forward to seeing:

  • CTRL+B navigation from method in Layout to the method in the ViewModel
  • Clearer error messages when something goes wrong
  • Auto complete and type checking inside the layout file
  • Reduced boilerplate by combining standard two-way binding functionality
  • Binding support for going from collections to AdapterViews

Run node.js On Older Android Device

TL;DR node.js crashes when ran on Android API level 15 and below due to libuv use of pthread_sigmask which is broken on older versions of Android. If libuv is patched with the fix for that function everything works fine.

As part of the journey to try and run node.js everywhere, I’ve recently came across an interesting issue of running node.js on Android devices with API level 15 and below. (Or, Android versions 4.0.4 and below, which apperently account for more than 10% of Android’s market share).

The ability to build and run node.js on the Android platform has been around for quite some time now, and given the node.js source code, a Linux machine and an NDK copy, it should be pretty straight forward.

However, when trying to run node.js on older Android devices, it seems to immediately crash with the following cryptic error message:

    I/DEBUG﹕ signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr deadbaad
    I/DEBUG﹕ r0 deadbaad  r1 00000001  r2 40000000  r3 00000000
    I/DEBUG﹕ r4 00000000  r5 00000027  r6 0000000a  r7 4aae8bf8
    I/DEBUG﹕ r8 00000004  r9 00000003  10 0000004d  fp 4b51c964
    I/DEBUG﹕ ip ffffffff  sp 4b51c930  lr 4001f121  pc 4001b880  cpsr 60000030
    I/DEBUG﹕ d0  0000000000000000  d1  0000000000000000
    I/DEBUG﹕ d2  0000000000000000  d3  4370000043708000
    I/DEBUG﹕ d4  0000000041c00000  d5  3f80000000000000
    I/DEBUG﹕ d6  0000000000000000  d7  0000000000000000
    I/DEBUG﹕ d8  0000000000000000  d9  0000000000000000
    I/DEBUG﹕ d10 0000000000000000  d11 0000000000000000
    I/DEBUG﹕ d12 0000000000000000  d13 0000000000000000
    I/DEBUG﹕ d14 0000000000000000  d15 0000000000000000
    I/DEBUG﹕ scr 60000012
    I/DEBUG﹕ #00  pc 00017880  /system/lib/
    I/DEBUG﹕ #01  lr 4001f121  /system/lib/
    I/DEBUG﹕ code around pc:
    I/DEBUG﹕ 4001b860 4623b15c 2c006824 e026d1fb b12368db
    I/DEBUG﹕ 4001b870 21014a17 6011447a 48124798 24002527
    I/DEBUG﹕ 4001b880 f7f47005 2106ee60 eeeef7f5 460aa901
    I/DEBUG﹕ 4001b890 f04f2006 94015380 94029303 eab8f7f5
    I/DEBUG﹕ 4001b8a0 4622a905 f7f52002 f7f4eac2 2106ee4c
    I/DEBUG﹕ code around lr:
    I/DEBUG﹕ 4001f100 41f0e92d 46804c0c 447c2600 68a56824
    I/DEBUG﹕ 4001f110 e0076867 300cf9b5 dd022b00 47c04628
    I/DEBUG﹕ 4001f120 35544306 37fff117 6824d5f4 d1ee2c00
    I/DEBUG﹕ 4001f130 e8bd4630 bf0081f0 000283da 41f0e92d
    I/DEBUG﹕ 4001f140 fb01b086 9004f602 461f4815 4615460c
    I/DEBUG﹕ stack:
    I/DEBUG﹕ 4b51c8f0  002d8448
    I/DEBUG﹕ 4b51c8f4  4004c568
    I/DEBUG﹕ 4b51c8f8  000000d0
    I/DEBUG﹕ 4b51c8fc  4004c5a8
    I/DEBUG﹕ 4b51c900  4004770c
    I/DEBUG﹕ 4b51c904  4004c85c
    I/DEBUG﹕ 4b51c908  00000000
    I/DEBUG﹕ 4b51c90c  4001f121  /system/lib/

Unfortunately, the log doesn’t seem to give any information on the source of the error, just a reference to the standard c library (libc) and there’s not a lot we can do with it.
In such cases, there are basically 2 things I try to do:

  1. Try to debug the thing
  2. Add logs everywhere

Since node.js’s source code is pretty big, the first option seemed more promising.
It took some twisting and turning, but after 1-2 days, I was able to make ndk-gdb work with node.js on android, which means that I can now set breakpoints, and inspect local variable values, among other things.

There is plenty of documentation out there on how to get ndk-gdb working,so we’re not gonna spend any time on this part, but the main advice I can tell you about running ndk-gsb is that you should pay close attention carefully to its error messages and don’t be afraid to change the script in order to make it specifically work for your app.

After spending some time on setting up some breakpoints in various code paths in node, I was able to narrow down the source of the SIGSEGV signal to line 103 in libuv’s signal.c:

static void uv__signal_block_and_lock(sigset_t* saved_sigmask) {
    sigset_t new_mask;
    if (sigfillset(&new_mask))
    if (pthread_sigmask(SIG_SETMASK, &new_mask, saved_sigmask))
        abort();  // line 103
    if (uv__signal_lock())

After inspecting the return value of the call to pthread_sigmask it seems that it always fails with the return value of 22, or EINVAL, which causes the 2nd if clause to call abort, which results with the SIGSEGV we were seeing earlier.

Some more digging up, and apparently, pthread_sigmask not working on Android API <=15 is a known issue!

Looking at the change set that fixed this issue for API level 16, it seems like it’s a rather small change that we can try and incorporate into libuv’s signal.c.

We start by adding the fix from the android source base above and a new pthread_sigmask_patched method in which we will first try to call to the system’s pthread_sigmask function, and if it fails with an EINVAL, we’ll try to call the fixed pthread_sigmask version.

/* signal.c code here... */
// --- Start of Android platform fix --
/* Despite the fact that our kernel headers define sigset_t explicitly
 * as a 32-bit integer, the kernel system call really expects a 64-bit
 * bitmap for the signal set, or more exactly an array of two-32-bit
 * values (see $KERNEL/arch/$ARCH/include/asm/signal.h for details).
 * Unfortunately, we cannot fix the sigset_t definition without breaking
 * the C library ABI, so perform a little runtime translation here.
typedef union {
    sigset_t   bionic;
    uint32_t   kernel[2];
} kernel_sigset_t;
/* this is a private syscall stub */
extern int __rt_sigprocmask(int, const kernel_sigset_t *, kernel_sigset_t *, size_t);
int pthread_sigmask_android16(int how, const sigset_t *set, sigset_t *oset)
    int ret, old_errno = errno;
    /* We must convert *set into a kernel_sigset_t */
    kernel_sigset_t  in_set, *in_set_ptr;
    kernel_sigset_t  out_set;
    in_set.kernel[0]  = in_set.kernel[1]  =  0;
    out_set.kernel[0] = out_set.kernel[1] = 0;
    /* 'in_set_ptr' is the second parameter to __rt_sigprocmask. It must be NULL
        * if 'set' is NULL to ensure correct semantics (which in this case would
        * be to ignore 'how' and return the current signal set into 'oset'.
    if (set == NULL) {
        in_set_ptr = NULL;
    } else {
        in_set.bionic = *set;
        in_set_ptr = &in_set;
    ret = __rt_sigprocmask(how, in_set_ptr, &out_set, sizeof(kernel_sigset_t));
    if (ret < 0)
        ret = errno;
    if (oset)
        *oset = out_set.bionic;
    errno = old_errno;
    return ret;
// --- End of Android platform fix --
// first try to call pthread_sigmask, in case of failure try again with the API 16 fix
int pthread_sigmask_patched(int how, const sigset_t *set, sigset_t *oset) {
    int ret = pthread_sigmask(how, set, oset);
    if (ret == EINVAL) {
        return pthread_sigmask_android16(how, set, oset);
/* more signal.c code here... */

Additionally, we also change the 2 methods in signal.c that uses pthread_sigmask to use the patched version instead:

static void uv__signal_block_and_lock(sigset_t* saved_sigmask) {
    sigset_t new_mask;
    if (sigfillset(&new_mask))
    // Code was changed here in order to fix android API <= 15 broken pthread_sigmask issue
    // original code called directly pthread_sigmask
    if (pthread_sigmask_patched(SIG_SETMASK, &new_mask, saved_sigmask))
    if (uv__signal_lock())
static void uv__signal_unlock_and_unblock(sigset_t* saved_sigmask) {
    if (uv__signal_unlock())
    // Code was changed here in order to fix android API <= 15 broken pthread_sigmask issue
    // original code called directly pthread_sigmask
    if (pthread_sigmask_patched(SIG_SETMASK, saved_sigmask, NULL))

Compiling and trying again to run node.js…and guess what? node starts as expected, no crashes, and everything seems to work fine!

Pretty miraculously, this was everything needed in order to make node.js run on older Android versions!

Building Node.js for Android

The good news is that Node.js does run on Android. The bad news is that at least at the time I’m writing this the build process requires a few extra steps. Nothing too scary though. See below for details.

Building Node.js for Android

  1. Go find a Linux machine or maybe a Mac.

    These instructions don’t currently work on Windows due to issues with the sh scripts being used. Yes, I did try the scripts in MINGW32 and no it didn’t work.

  2. Go download the Android NDK.

    Which NDK to download does take a bit of attention. Most Android devices today are 32 bit so I want the Platform (32-bit target). But my Linux OS (Elementary OS) is 64 bit so I want Linux 64-bit (x86) under Platform (32-bit target).

  3. After downloading the NDK unzip it.

    Let’s assume you put the NDK into ~/android-ndk-r10b.

  4. Go clone node.

    Let’s assume you put that into ~/node. I am running these instructions off master branch.

  5. Check that you have all of node’s dependencies as listed here

    I believe any modern Linux distro will have all of these already but just in case I decided to include the link.

  6. Go edit ~/node/android-configure and change ’arm-linux-androideabi-4.7’ to instead be ’arm-linux-androideabi-4.8.

    This is the pull request that added basic Android support to Node. It contains some instructions. The first instruction will set up the build environment for Android. But the set up script is designed for an older version of the Android NDK. So we need to update it. Specifically 4.7 is apparently not supported by NDK 10 so I switched it to 4.8 which is. I decided to leave platform=android-9 for no particularly good reason.

  7. Run from inside of ~/node directory the command “source ./android-configure ~/android-ndk-r10b”
  8. Now go to ~/node/android-toolchain/bin and issue the command “mv python2.7 oldpython2.7 && ln -s /usr/bin/python2.7 python2.7”

    The NDK appears to ship with its own version of Python 2.7 that doesn’t support a library (bz2) that is needed by files in the NDK. In any sane world this just means that the NDK is broken but I’m sure there is some logic here. This bug was reported to Node (since it breaks Node’s support of Android) but they responded that this is an NDK issue so Google should deal with it. But if we want to build we have to get connected to a version of Python that does support bz2. That’s what we did above. We linked the main version of Python (which any sane Linux distro will use) with the NDK so it will use that and hence support bz2.

  9. Now go to ~/node and issue ’make’

    The actual instructions from the checkin say to run ’make -j8’ which enables parallel capabilities in Make. Apparently the rule of thumb is to set the value after j to 2x the number of hardware threads available on the machine.

Using Node.js on Android via ADB

Eventually I’ll write up an AAR that just wraps all the Node stuff and provides a standard API for launching node and feeding it a script. But that isn’t my current priority so instead I need to just get node onto my device and play with it.

  1. Issue the command “adb push ~/node/out/Release /data/local/tmp/Release”
    • There is a step I’m skipping here. I actually do my development on Windows. So I copy the Release folder from my Linux VM (via Virtualbox) and then use the linked drive to move it to my Windows box. So in fact my adb push command above isn’t from the Linux location but my Windows location.
    • The out/Release folder contains all the build artifacts for Node. Of this mess I suspect only the node executable is actually needed. But for the moment I’m going to play it safe and just move everything over.
    • The reason for putting the node materials into /data/local/tmp/Release is because /data/local/tmp is one of the few areas where we can execute the chmod command in the next step and make Node executable. But when we wrap this thing up in an AAR we can actually use the setExecutable function instead.
  2. Issue “adb shell”. Once in the shell issue “chmod 700 /data/local/tmp/Release/node”
  3. I then issued an ’adb push’ for a simple hello world node program I have that I put inside of /data/local/tmp
    • I used “Hello HTTP” from
  4. Then I went in via “adb shell” and ran “/data/local/tmp/Release/node helloworld.js”
    • And yes, it worked! I even tested it by going to the browser on the phone and navigating to http://localhost:8000.
  5. To kill things I just ctrl-c which does kill the adb shell but also the node app. Good enough for now.

What about NPM?

In theory one should be able to use NPM on the Linux box and then just move the whole thing over to Android and run it there. But this only works if none of the dependencies use an add-on. An add-on requires compiling C code into a form Android can handle. It looks like NPM wants to support making this happen but so far I haven’t found the right voodoo. So I’m still investigating.

An Open Source API For The Android Market


The Android Market, now “Play”, is getting bigger and bigger. The result is there are too many applications that couldn’t be easily discovered by all the users. It could be a good idea to have an application which matches our interests and recommend applications that could be interested for us.

The problem behind is there is not an official API which the developers can use to access the Android Market. The good news is there is an open-source API, which seems to work.

This API offers methods to search an App by name, to get the comments, to get the screencasts and even the icon of a specific application.

This post will demonstrate how to install a ruby wrapper over this project, which allow the users to use this API from the terminal. The unique problem is that not all the functions has been ported.

Install the jRuby

Actually the stable version of jRuby is, which could be found here:
Download the file depending on your Operation System and install it. In my case I have install it in a Mac OS, which the installation process was open the file and press several time “Next”.

Install json-jruby

The json-jruby gem must be downloaded, even when the result has to be in XML format.
Download the last json-jruby file from:
Open the terminal
Go to the folder where the file has been downloaded
Type the follow command to install it:

sudo gem install json-jruby

The terminal will ask for the password of your user. Enter it.

Install Supermarket

The project supermarket is the Android Market API Ruby Wrapper project created by jberkel which could be found here:
Open the terminal
Download the project using git

git clone

Go to the folder where the project is
Type the follow command to install it

sudo jruby -S gem install supermarket

The terminal will ask for the password of your user. Enter it.

Install jsonpretty (Optional)

Json Pretty is a program which by give a json string, it format the string and returns a legible output. Because the result from Supermarket could be quite hard to read, it is recommendable to install it.

sudo gem install jsonpretty

The terminal will ask for the password of your user. Enter it.

Host Widgets In Android Application

Tutorial on how to using android widgets in your application, We’re going to learn how to add/remove and reattach widgets after reboot.


You start by creating two objects. The first is an AppWidgetManager, which will give you the data you need about installed widgets. The second one is an AppWidgetHost, which will keep in memory your widget instances. Latter, your app will handle only the view that will draw the widget:

_appWidgetManager = AppWidgetManager.getInstance(_activity);
_homeWidgetHost = new AppWidgetHost(_activity,;

Selecting the Widget

You start by asking to the AppWidgetHost to allocate resources for a widget instance. It will return an ID for that. Then, you need to start an activity to let the user select which widget he wants to add to your app. You need to give this ID to the activity and stored in persistent storage to add selected widget again to host after system reboot.

// Class implements View.OnLongClickListener
private final Activity _activity;
private final AppWidgetManager _appWidgetManager;
private final AppWidgetHost _homeWidgetHost;
private final ViewGroup _widgetContainer;
private int _widgetId;

public HomeWidgetManager(Activity activity){
    _widgetId = -1;
    _activity = activity;
    _widgetContainer = (ViewGroup) _activity.findViewById(;
    _homeWidgetHost = new AppWidgetHost(_activity,;
    _appWidgetManager = AppWidgetManager.getInstance(_activity);
public boolean onLongClick(View v) {
    return true;
public void selectWidget() {
    int appWidgetId = _homeWidgetHost.allocateAppWidgetId();
    Intent pickIntent = new Intent(AppWidgetManager.ACTION_APPWIDGET_PICK);
    pickIntent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, appWidgetId);
private void addEmptyData(Intent pickIntent) {
    ArrayList<AppWidgetProviderInfo> customInfo = new ArrayList<AppWidgetProviderInfo>();
    pickIntent.putParcelableArrayListExtra(AppWidgetManager.EXTRA_CUSTOM_INFO, customInfo);
    ArrayList<Bundle> customExtras = new ArrayList<Bundle>();
    pickIntent.putParcelableArrayListExtra(AppWidgetManager.EXTRA_CUSTOM_EXTRAS, customExtras);

Unfortunately, any kind of software has bugs, and here is one of the Android SDK. The Widget API supports that you merge custom widgets of your application with the installed ones. But if you don’t add anything, the Activity that shows the list of widgets to the user crashes with a NullPointerException. The addEmptyData() method above adds some dummy data to avoid this bug. More on this bug here. If you want to add a custom widget, start looking at this point of the AHSSC.

Configuring the Widget

If the user successfully selects a widget from the list (he didn’t pressed “back”), it will return an OK to you as an activity result. The data for this result contains the widget ID. Use it to retrieve the AppWidgetProviderInfo to check if the widget requires any configuration (some widgets does need). If it requires, you need to launch the activity to configure the widget. If not, jump to the next step.

// Your Activity Class
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    if(requestCode == || requestCode =={
        _homeWidgetManager.onActivityResult(requestCode, resultCode, data);

// Class
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    if (resultCode == Activity.RESULT_OK ) {
        if (requestCode == {
        else if (requestCode == {
    else if (resultCode == Activity.RESULT_CANCELED && data != null) {
        int appWidgetId = data.getIntExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, -1);
        if (appWidgetId != -1) {

private void configureWidget(Intent data) {
    Bundle extras = data.getExtras();
    int appWidgetId = extras.getInt(AppWidgetManager.EXTRA_APPWIDGET_ID, -1);
    AppWidgetProviderInfo appWidgetInfo = _appWidgetManager.getAppWidgetInfo(appWidgetId);
    if (appWidgetInfo.configure != null) {
        Intent intent = new Intent(AppWidgetManager.ACTION_APPWIDGET_CONFIGURE);
        intent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, appWidgetId);
    } else {

Creating and Adding it to Your Views

Now is time to create the widget itself. You will use the Widget ID and the AppWidgetProviderInfo to ask to the AppWidgetHost “could you please create a view of this widget for me?“. It will return an AppWidgetHostView which is a derived class from View. This one you can handle as any other view from the Framework.

// Class
public void createWidget(Intent data) {
    Bundle extras = data.getExtras();
    int appWidgetId = extras.getInt(AppWidgetManager.EXTRA_APPWIDGET_ID, -1);
    if(appWidgetId < 0){

    // Note : Here You Must Save appWidgetId For Future Use Otherwise You Cant Reattach Widget After Reboot

    AppWidgetProviderInfo appWidgetInfo = _appWidgetManager.getAppWidgetInfo(appWidgetId);
    AppWidgetHostView hostView = _homeWidgetHost.createView(_activity, appWidgetId, appWidgetInfo);
    hostView.setAppWidget(appWidgetId, appWidgetInfo);


The widget is now working, but is not being updated by your app. If the widget is a clock, it will be stuck at the time you added it. To register the widget to receive the events it needs, call startListening() on the AppWidgetHost. To avoid wasting battery with unnecessary updates while your app is not visible, call it during the onStart() method of your activity, and call stopListening() during the onStop() method.

// Class
public void startListener(){

public void stopListener(){

// Activity Class
protected void onStart() {
protected void onStop() {

Releasing the Widget

The widget should be working now. But if you want to remove the widget, you need to ask to the AppWidgetHost to release it. If you do not release it, you’ll get a memory leak (your app will consume unnecessary memory).

// Class
public void removeWidget(AppWidgetHostView hostView) {

Reattaching the Widgets

To reattach selected widget all you need is appWidgetId assigned to the widget.

public void restoreWidget(){
    if(_widgetId < 0){
    AppWidgetProviderInfo appWidgetInfo = _appWidgetManager.getAppWidgetInfo(_widgetId);
    AppWidgetHostView hostView = _homeWidgetHost.createView(_activity, _widgetId, appWidgetInfo);
    hostView.setAppWidget(_widgetId, appWidgetInfo);