Hello, Stack Overflow!

Over the past year, an Android presence has been growing on a relatively new technical Q&A web site called Stack Overflow. The site was designed specifically for programmers, with features like syntax highlighting, tagging, user reputation, and community editing. It's attracted a loyal software developer community, and developers continue to express great praise for this new tool. Well, the Android team has been listening...and we agree.

Today, I'm happy to announce that we're working with Stack Overflow to improve developer support, especially for developers new to Android. In essence, the Android tag on Stack Overflow will become an official Android app development Q&A medium. We encourage you to post your beginner-level technical questions there. It's also important to point out that we don't plan to change the android-developers group, so intermediate and expert users should still feel free to post there.

I think that this will be a great new resource for novice Android developers, and our team is really excited to participate in the growth of the Android developer community on Stack Overflow. I hope to see you all there!

Back and other hard keys: three stories

Android 2.0 introduces new behavior and support for handling hard keys such as BACK and MENU, including some special features to support the virtual hard keys that are appearing on recent devices such as Droid.

This article will give you three stories on these changes: from the most simple to the gory details. Pick the one you prefer.

Story 1: Making things easier for developers

If you were to survey the base applications in the Android platform, you would notice a fairly common pattern: add a little bit of magic to intercept the BACK key and do something different. To do this right, the magic needs to look something like this:

public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_BACK && event.getRepeatCount() == 0) {
// do something on back.
return true;

return super.onKeyDown(keyCode, event);

How to intercept the BACK key in an Activity is also one of the common questions we see developers ask, so as of 2.0 we have a new little API to make this more simple and easier to discover and get right:

public void onBackPressed() {
// do something on back.

If this is all you care about doing, and you're not worried about supporting versions of the platform before 2.0, then you can stop here. Otherwise, read on.

Story 2: Embracing long press

One of the fairly late addition to the Android platform was the use of long press on hard keys to perform alternative actions. In 1.0 this was long press on HOME for the recent apps switcher and long press on CALL for the voice dialer. In 1.1 we introduced long press on SEARCH for voice search, and 1.5 introduced long press on MENU to force the soft keyboard to be displayed as a backwards compatibility feature for applications that were not yet IME-aware.

(As an aside: long press on MENU was only intended for backwards compatibility, and thus has some perhaps surprising behavior in how strongly the soft keyboard stays up when it is used. This is not intended to be a standard way to access the soft keyboards, and all apps written today should have a more standard and visible way to bring up the IME if they need it.)

Unfortunately the evolution of this feature resulted in a less than optimal implementation: all of the long press detection was implemented in the client-side framework's default key handling code, using timed messages. This resulted in a lot of duplication of code and some behavior problems; since the actual event dispatching code had no concept of long presses and all timing for them was done on the main thread of the application, the application could be slow enough to not update within the long press timeout.

In Android 2.0 this all changes, with a real KeyEvent API and callback functions for long presses. These greatly simplify long press handling for applications, and allow them to interact correctly with the framework. For example: you can override Activity.onKeyLongPress() to supply your own action for a long press on one of the hard keys, overriding the default action provided by the framework.

Perhaps most significant for developers is a corresponding change in the semantics of the BACK key. Previously the default key handling executed the action for this key when it was pressed, unlike the other hard keys. In 2.0 the BACK key is now execute on key up. However, for existing apps, the framework will continue to execute the action on key down for compatibility reasons. To enable the new behavior in your app you must set android:targetSdkVersion in your manifest to 5 or greater.

Here is an example of code an Activity subclass can use to implement special actions for a long press and short press of the CALL key:

public boolean onKeyLongPress(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_CALL) {
// a long press of the call key.
// do our work, returning true to consume it. by
// returning true, the framework knows an action has
// been performed on the long press, so will set the
// canceled flag for the following up event.
return true;
return super.onKeyLongPress(keyCode, event);
public boolean onKeyUp(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_CALL && event.isTracking()
&& !event.isCanceled()) {
// if the call key is being released, AND we are tracking
// it from an initial key down, AND it is not canceled,
// then handle it.
return true;
return super.onKeyUp(keyCode, event);

Note that the above code assumes we are implementing different behavior for a key that is normally processed by the framework. If you want to implement long presses for another key, you will also need to override onKeyDown to have the framework track it:

public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_0) {
// this tells the framework to start tracking for
// a long press and eventual key up. it will only
// do so if this is the first down (not a repeat).
return true;
return super.onKeyDown(keyCode, event);

Story 3: Making a mess with virtual keys

Now we come to the story of our original motivation for all of these changes: support for virtual hard keys, as seen on the Droid and other upcoming devices. Instead of physical buttons, these devices have a touch sensor that extends outside of the visible screen, creating an area for the "hard" keys to live as touch sensitive areas. The low-level input system looks for touches on the screen in this area, and turns these into "virtual" hard key events as appropriate.

To applications these basically look like real hard keys, though the generated events will have a new FLAG_VIRTUAL_HARD_KEY bit set to identify them. Regardless of that flag, in nearly all cases an application can handle these "hard" key events in the same way it has always done for real hard keys.

However, these keys introduce some wrinkles in user interaction. Most important is that the keys exist on the same surface as the rest of the user interface, and they can be easily pressed with the same kind of touches. This can become an issue, for example, when the virtual keys are along the bottom of the screen: a common gesture is to swipe up the screen for scrolling, and it can be very easy to accidentally touch a virtual key at the bottom when doing this.

The solution for this in 2.0 is to introduce a concept of a "canceled" key event. We've already seen this in the previous story, where handling a long press would cancel the following up event. In a similar way, moving from a virtual key press on to the screen will cause the virtual key to be canceled when it goes up.

In fact the previous code already takes care of this — by checking isCanceled() on the key up, canceled virtual keys and long presses will be ignored. There are also individual flags for these two cases, but they should rarely be used by applications and always with the understanding that in the future there may be more reasons for a key event to be canceled.

For existing application, where BACK key compatibility is turned on to execute the action on down, there is still the problem of accidentally detecting a back press when intending to perform a swipe. Though there is no solution for this except to update an application to specify it targets SDK version 5 or later, fortunately the back key is generally positioned on a far side of the virtual key area, so the user is much less likely to accidentally hit it than some of the other keys.

Writing an application that works well on pre-2.0 as well as 2.0 and later versions of the platform is also fairly easy for most common cases. For example, here is code that allows you to handle the back key in an activity correctly on all versions of the platform:

public boolean onKeyDown(int keyCode, KeyEvent event) {
if (android.os.Build.VERSION.SDK_INT < android.os.Build.VERSION_CODES.ECLAIR
&& keyCode == KeyEvent.KEYCODE_BACK
&& event.getRepeatCount() == 0) {
// Take care of calling this method on earlier versions of
// the platform where it doesn't exist.

return super.onKeyDown(keyCode, event);

public void onBackPressed() {
// This will be called either automatically for you on 2.0
// or later, or by the code above on earlier versions of the
// platform.

For the hard core: correctly dispatching events

One final topic that is worth covering is how to correctly handle events in the raw dispatch functions such as onDispatchEvent() or onPreIme(). These require a little more care, since you can't rely on some of the help the framework provides when it calls the higher-level functions such as onKeyDown(). The code below shows how you can intercept the dispatching of the BACK key such that you correctly execute your action when it is release.

public boolean dispatchKeyEvent(KeyEvent event) {
if (event.getKeyCode() == KeyEvent.KEYCODE_BACK) {
if (event.getAction() == KeyEvent.ACTION_DOWN
&& event.getRepeatCount() == 0) {

// Tell the framework to start tracking this event.
getKeyDispatcherState().startTracking(event, this);
return true;

} else if (event.getAction() == KeyEvent.ACTION_UP) {
if (event.isTracking() && !event.isCanceled()) {

return true;

return super.dispatchKeyEvent(event);
} else {
return super.dispatchKeyEvent(event);

The call to getKeyDispatcherState() returns an object that is used to track the current key state in your window. It is generally available on the View class, and an Activity can use any of its views to retrieve the object if needed.

New resources and sample code on developer.android.com

Hey Android developers—if you've visited the online Android SDK documentation recently, you may have noticed a few changes. That's right, there's a new Resources tab, which was designed to take some of the load off the Developer's Guide. We've moved a number of existing resources to the Resources tab, including tutorials, sample code, and FAQs. We've also formalized a few of our most popular developer blog posts into technical articles; watch for more of these to appear in the future.

In addition, we just released a new batch of sample code, available now as a ZIP file download on the samples index page. And we're working on updating the way in which we distribute official sample code; more on that some other time.

New sample screenshots

The new sample code includes:

  • Multiple Resolutions: a simple example showing how to use resource directory qualifiers to support multiple screen configurations and Android SDK versions.
  • Wiktionary and WiktionarySimple: sample applications that illustrate how to create an interactive home screen widget.
  • Contact Manager: an example on using the new ContactsContract interface to query and manipulate a user's various accounts and contact providers.
  • Bluetooth Chat: a fun little demo that allows two users to have a 1 on 1 chat over Bluetooth. It demonstrates how to discover devices, initiate a connection, and transfer data.
  • API Demos > App > Activity > QuickContactsDemo: a demo showing how to use the android.widget.QuickContactsBadge class, new in Android 2.0.
  • API Demos > App > Activity > SetWallpaper: a demo showing how to use the new android.app.WallpaperManager class to allow users to change the system wallpaper.
  • API Demos > App > Text-To-Speech: a sample using Text-To-Speech (speech synthesis) to make your application talk.
  • NotePad (now with Live Folders): this sample now includes code for creating Live Folders.

We hope these new samples can be a valuable resource for learning some of the newer features in Android 1.6 and 2.0. Let us know in the android-developers Google Group if you have any questions about these new samples or about the new Resources tab.

Thanks for tuning in, and 'til next time, happy coding!

Knowing is half the battle

As a developer, I often wonder which Android platforms my applications should support,especially as the number of Android-powered devices grows. Should my application only focus on the latest version of the platform or should it support older ones as well?

To help with this kind of decision, I am excited to announce the new
device dashboard. It provides information about deployed Android-powered devices that is helpful to developers as they build and update their apps. The dashboard provides the relative distribution of Android platform versions on devices running Android Market.

Android PlatformPercentage of Devices

The above graph shows the relative number of Android devices that have accessed Android Market during the first 14 days of December 2009.

From a developer's perspective, there are a number of interesting points on this graph:

  • At this point, there's little incentive to make sure a new application is
    backward compatible with Android 1.0 and Android 1.1.
  • Close to 30% of the devices are running Android 1.5. To take advantage of this significant install base, you may consider support for Android 1.5.
  • Starting with Android 1.6, devices can have different screen densities & sizes. There are several devices out there that fall in this category, so make sure to adapt your application to support different screen sizes and take advantage of devices with small, low density (e.g QVGA) and normal, high density (e.g. WVGA) screens. Note that Android Market will not list your application on small screen devices unless its manifest explicitly indicates support for "small" screen sizes. Make sure you properly configure the emulator and test your application on different screen sizes before uploading to Market.
  • A new SDK for Android 2.0.1 was released two weeks ago. All Android 2.0 devices will be updated to 2.0.1 before the end of the year, so if your application uses features specific to Android 2.0, you are encouraged to update it to take advantage of the latest Android 2.0.1 API instead.

In summary, Android 1.5, 1.6, and 2.0.1 are the 3 versions of the platform that are deployed in volume. Our goal is to provide you with the tools and information to make it easy for you to target specific versions of the platform or all the versions that are deployed in volume.

We plan to update the dashboard regularly to reflect deployment of new Android platforms. We also plan to expand the dashboard to include other information like devices per screen size and so on.

Come to Our Virtual Office Hours

Starting this week, we're going to be holding regular IRC office hours for Android app developers in the #android-dev channel on irc.freenode.net. Members of the Android team will be on hand to answer your technical questions. (Note that we will not be able to provide customer support for the phones themselves.)

We've arranged our office hours to accommodate as many different schedules as possible, for folks around the world. We will initially hold two sessions each week:

  • 12/15/09 Tuesday, 9 a.m. to 10 a.m. PST
  • 12/17/09, Thursday 5 p.m. to 6 p.m. PST
  • 12/22/09, Tuesday 9 a.m. to 10 a.m. PST
  • 01/06/10 Wednesday 9 a.m. to 10 a.m. PST
  • 01/07/10 Thursday 5 p.m. to 6 p.m. PST

Check Wikipedia for a helpful list of IRC clients. Alternatively, you could use a web interface such as the one at freenode.net. We will try to answer as many as we can get through in the hour.

We hope to see you there!

Optimize your layouts

Writing user interface layouts for Android applications is easy, but it can sometimes be difficult to optimize them. Most often, heavy modifications made to existing XML layouts, like shuffling views around or changing the type of a container, lead to inefficiencies that go unnoticed.

Starting with the SDK Tools Revision 3 you can use a tool called layoutopt to automatically detect common problems. This tool is currently only available from the command line and its use is very simple - just open a terminal and launch the layoutopt command with a list of directories or XML files to analyze:

$ layoutopt samples/
7:23 The root-level <FrameLayout/> can be replaced with <merge/>
11:21 This LinearLayout layout or its FrameLayout parent is useless samples/simple.xml
7:7 The root-level <FrameLayout/> can be replaced with <merge/>
-1:-1 This layout has too many nested layouts: 13 levels, it should have <= 10!
20:81 This LinearLayout layout or its LinearLayout parent is useless
24:79 This LinearLayout layout or its LinearLayout parent is useless
28:77 This LinearLayout layout or its LinearLayout parent is useless
32:75 This LinearLayout layout or its LinearLayout parent is useless
36:73 This LinearLayout layout or its LinearLayout parent is useless
40:71 This LinearLayout layout or its LinearLayout parent is useless
44:69 This LinearLayout layout or its LinearLayout parent is useless
48:67 This LinearLayout layout or its LinearLayout parent is useless
52:65 This LinearLayout layout or its LinearLayout parent is useless
56:63 This LinearLayout layout or its LinearLayout parent is useless
7:413 The root-level <FrameLayout/> can be replaced with <merge/>
-1:-1 This layout has too many views: 81 views, it should have <= 80! samples/useless.xml
7:19 The root-level <FrameLayout/> can be replaced with <merge/>
11:17 This LinearLayout layout or its FrameLayout parent is useless
For each analyzed file, the tool will indicate the line numbers of each tag that could potentially be optimized. In some cases, layoutopt will also offer a possible solution.

The current version of layoutopt contains a dozen rules used to analyze your layout files and future versions will contain more. Future plans for this tool also include the ability to create and use your own analysis rules, to automatically modify the layouts with optimized XML, and to use it from within Eclipse and/or a standalone user interface.

Windows users: to start layoutopt, open the file called layoutopt.bat in the tools directory of the SDK and on the last line, replace %jarpath% with -jar %jarpath%.

Android SDK Updates

Today we are releasing updates to multiple components of the Android SDK:

  • Android 2.0.1, revision 1
  • Android 1.6, revision 2
  • SDK Tools, revision 4

Android 2.0.1 is a minor update to Android 2.0. This update includes several bug fixes and behavior changes, such as application resource selection based on API level and changes to the value of some Bluetooth-related constants. For more detailed information, please see the Android 2.0.1 release notes.

To differentiate its behavior from Android 2.0, the API level of Android 2.0.1 is 6. All Android 2.0 devices will be updated to 2.0.1 before the end of the year, so developers will no longer need to support Android 2.0 at that time. Of course, developers of applications affected by the behavior changes should start compiling and testing their apps immediately.

We are also providing an update to the Android 1.6 SDK component. Revision 2 includes fixes to the compatibility mode for applications that don't support multiple screen sizes, as well as SDK fixes. Please see the Android 1.6, revision 2 release notes for the full list of changes.

Finally, we are also releasing an update to the SDK Tools, now in revision 4. This is a minor update with mostly bug fixes in the SDK Manager. A new version of the Eclipse plug-in that embeds those fixes is also available. For complete details, please see the SDK Tools, revision 4 and ADT 0.9.5 release notes.

One more thing: you can now follow us on twitter @AndroidDev.