Android applications are, at least on the T-Mobile G1, limited to 16 MB of heap. It's both a lot of memory for a phone and yet very little for what some developers want to achieve. Even if you do not plan on using all of this memory, you should use as little as possible to let other applications run without getting them killed. The more applications Android can keep in memory, the faster it will be for the user to switch between his apps. As part of my job, I ran into memory leaks issues in Android applications and they are most of the time due to the same mistake: keeping a long-lived reference to a Context.
On Android, a Context is used for many operations but mostly to load and access resources. This is why all the widgets receive a Context parameter in their constructor. In a regular Android application, you usually have two kinds of Context, Activity and Application. It's usually the first one that the developer passes to classes and methods that need a Context:
Context
@Override protected void onCreate(Bundle state) { super.onCreate(state); TextView label = new TextView(this); label.setText("Leaks are bad"); setContentView(label); }
This means that views have a reference to the entire activity and therefore to anything your activity is holding onto; usually the entire View hierarchy and all its resources. Therefore, if you leak the Context ("leak" meaning you keep a reference to it thus preventing the GC from collecting it), you leak a lot of memory. Leaking an entire activity can be really easy if you're not careful.
When the screen orientation changes the system will, by default, destroy the current activity and create a new one while preserving its state. In doing so, Android will reload the application's UI from the resources. Now imagine you wrote an application with a large bitmap that you don't want to load on every rotation. The easiest way to keep it around and not having to reload it on every rotation is to keep in a static field:
private static Drawable sBackground; @Override protected void onCreate(Bundle state) { super.onCreate(state); TextView label = new TextView(this); label.setText("Leaks are bad"); if (sBackground == null) { sBackground = getDrawable(R.drawable.large_bitmap); } label.setBackgroundDrawable(sBackground); setContentView(label); }
This code is very fast and also very wrong; it leaks the first activity created upon the first screen orientation change. When a Drawable is attached to a view, the view is set as a callback on the drawable. In the code snippet above, this means the drawable has a reference to the TextView which itself has a reference to the activity (the Context) which in turns has references to pretty much anything (depending on your code.)
TextView
This example is one of the simplest cases of leaking the Context and you can see how we worked around it in the Home screen's source code (look for the unbindDrawables() method) by setting the stored drawables' callbacks to null when the activity is destroyed. Interestingly enough, there are cases where you can create a chain of leaked contexts, and they are bad. They make you run out of memory rather quickly.
unbindDrawables()
There are two easy ways to avoid context-related memory leaks. The most obvious one is to avoid escaping the context outside of its own scope. The example above showed the case of a static reference but inner classes and their implicit reference to the outer class can be equally dangerous. The second solution is to use the Application context. This context will live as long as your application is alive and does not depend on the activities life cycle. If you plan on keeping long-lived objects that need a context, remember the application object. You can obtain it easily by calling Context.getApplicationContext() or Activity.getApplication().
Application
In summary, to avoid context-related memory leaks, remember the following:
Note: This article was originally posted on my personal blog.
ListView is one of Android's most widely used widgets. It is rather easy to use, very flexible and incredibly powerful. ListView can also be difficult to understand at times.
ListView
One of the most common issues with ListView happens when you try to use a custom background. By default, like many Android widgets, ListView has a transparent background which means yo can see through the default window's background, a very dark gray (#FF191919 with the current dark theme.) Additionally, ListView enables the fading edges by default, as you can see at the top of the following screenshot; the first text item gradually fades to black. This technique is used throughout the system to indicate that the container can be scrolled.
#FF191919
The fade effect is implemented using a combination of Canvas.saveLayerAlpha() and the Porter-Duff Destination Out blending mode. This technique is similar to the one explained in Filthy Rich Clients and various presentations. Unfortunately, things start to get ugly when you try to use a custom background on the ListView or when you change the window's background. The following two screenshots show what happens in an application when you change the window's background. The left image shows what the list looks like by default and the right image shows what the list looks like during a scroll initiated with a touch gesture:
This rendering issue is caused by an optimization of the Android framework enabled by default on all instances of ListView (for some reason, I forgot to enable it by default on GridView.) I mentioned earlier that the fade effect is implemented using a Porter-Duff blending mode. This implementation works really well but is unfortunately very costly and can bring down drawing performance by quite a bit as it requires to capture a portion of the rendering in an offscreen bitmap and then requires extra blending (which implies readbacks from memory.)
GridView
Since ListView is most of the time displayed on a solid background, there is no reason to go down that expensive route. That's why we introduced an optimization called the "cache color hint." The cache color hint is an RGB color set by default to the window's background color, that is #191919 in Android's dark theme. When this hint is set, ListView (actually, its base class View) knows it will draw on a solid background and therefore replaces th expensive saveLayerAlpha()/Porter-Duff rendering with a simple gradient. This gradient goes from fully transparent to the cache color hint value and this is exactly what you see on the image above, with the dark gradient at the bottom of the list. However, this still does not explain why the entire list turns black during a scroll.
View
saveLayerAlpha()/Porter-Duff
As I said before, ListView has a transparent/translucent background by default, and so all default Android widgets. This implies that when ListView redraws its children, it has to blend the children with the window's background. Once again, this requires costly readbacks from memory that are particularly painful during a scroll or a fling when drawing happens dozen of times per second. To improve drawing performance during scrolling operations, the Android framework reuses the cache color hint. When this hint is set, the framework copies each child of the list in a Bitmap filled with the hint value (this assumes that another optimization, called scrolling cache, is not turned off.) ListView then blits these bitmaps directly on screen and because these bitmaps are known to be opaque, no blending is required. And since the default cache color hint is #191919, you get a dark background behind each item during a scroll.
Bitmap
#191919
To fix this issue, all you have to do is either disable the cache color hint optimization, if you use a non-solid color background, or set the hint to the appropriate solid color value. This can be dome from code or preferably from XML, by using the android:cacheColorHint attribute. To disable the optimization, simply use the transparent color #00000000. The following screenshot shows a list with android:cacheColorHint="#00000000" set in the XML layout file:
android:cacheColorHint
#00000000
android:cacheColorHint="#00000000"
As you can see, the fade works perfectly against the custom wooden background. I find the cache color hint feature interesting because it shows how optimizations can make developers' life more difficult in some situations. In this particular case, however, the benefit of the default behavior outweighs the added complexity for the developer.
Note: this article was originally posted on my personal blog.
Android offers a very powerful and yet easy to use tool called intents. An intent can be use to turn applications into high-level libraries and make code re-use something even better than before. The Android Home screen and AnyCut use intents extensively to create shortcuts for instance. While it is nice to be able to make use of a loosely coupled API, there is no guarantee that the intent you send will be received by another application. This happens in particular with 3rd party apps, like Panoramio and its RADAR intent.
While working on a new application, I came up with a very simple way to find out whether the system contains any application capable of responding to the intent you want to use. I implemented this technique in my application to gray out the menu item that the user would normally click to trigger the intent. The code is pretty simple and easy to follow:
/** * Indicates whether the specified action can be used as an intent. This * method queries the package manager for installed packages that can * respond to an intent with the specified action. If no suitable package is * found, this method returns false. * * @param context The application's environment. * @param action The Intent action to check for availability. * * @return True if an Intent with the specified action can be sent and * responded to, false otherwise. */ public static boolean isIntentAvailable(Context context, String action) { final PackageManager packageManager = context.getPackageManager(); final Intent intent = new Intent(action); List<ResolveInfo> list = packageManager.queryIntentActivities(intent, PackageManager.MATCH_DEFAULT_ONLY); return list.size() > 0; }
Here is how I use it:
@Override public boolean onPrepareOptionsMenu(Menu menu) { final boolean scanAvailable = isIntentAvailable(this, "com.google.zxing.client.android.SCAN"); MenuItem item; item = menu.findItem(R.id.menu_item_add); item.setEnabled(scanAvailable); return super.onPrepareOptionsMenu(menu); }
In this example, the menu is grayed out if the Barcode Scanner application is not installed. Another, simpler, way to do this is to catch the ActivityNotFoundException when calling startActivity() but it only lets you react to the problem, you cannot predict it and update the UI accordingly to prevent the user from doing something that won't work. The technique described here can also be used at startup time to ask the user whether he'd like to install the missing package, you can then simply redirect him to the Android Market by using the appropriate URI.
ActivityNotFoundException
startActivity()
We're back in action after a Thanksgiving break filled with turkey, stuffing, and pumpkin pie. Now it's the holiday season (at least, here in the U.S.) and we're filled with good will toward developers. Today I wanted to talk about a couple things we just finished polishing up.
First, the Android 1.0 SDK, release 2 is now available. Like the previous 1.0_r1 release, this new 1.0_r2 build creates applications that are compatible with Android 1.0 devices, such as the T-Mobile G1. This new release fixes a few bugs. In 1.0_r1, it was possible for developers to write technically-illegal code by using the Java Reflection APIs to access private or protected fields and methods. 1.0_r2 fixes that problem by enforcing private/protected visibility of items accessed via Reflection. Meanwhile, the class android.R.styleable was included in 1.0_r1 primarily for documentation purposes as a way for developers to look up the style attributes available to them to use. However, actually referring to that class via source code would result in applications that might break when run on future versions of the Android platform, so 1.0_r2 corrects the oversight and removes access to the class from the android.jar file. (The class remains in the documentation for reference purposes, though.)
Both of these problems are obscure "future-proofing" issues, and I'd be quite surprised if they actually caused problems for anyone, but now they're fixed. 1.0_r2 also includes a few other smaller changes; check out the release notes for all the details.
Second, many of you have asked if developer devices will be available. We've worked with our partners to create a program for developers to purchase devices that enable them to test and debug applications more easily.
I think these new tools will be quite helpful to developers, and I'm looking forward to seeing what people do with Android, next.
Designing and developing user interfaces for Android is very different from doing so in a regular desktop environment. Because Android runs applications on mobile devices, application designers and developers must deal with numerous constraints that are not always obvious. To help you design and develop better applications, we are publishing a new series of posts focusing on Android user interfaces. In this series, we will give you design guides and tools, development tips, and explain the fundamental principles of the Android UI toolkit. The goal here is simple: we want to help you design and develop a great user experience. To start off this series, I'd like to introduce touch mode, one of the most important principles of the UI toolkit.
The touch mode is a state of the view hierarchy that depends solely on the user interaction with the phone. By itself, the touch mode is something very easy to understand as it simply indicates whether the last user interaction was performed with the touch screen. For example, if you are using a G1 phone, selecting a widget with the trackball will take you out of touch mode; however, if you touch a button on the screen with your finger, you will enter touch mode. When the user is not in touch mode, we talk about the trackball mode, navigation mode or keyboard navigation, so do not be surprised if you encounter these terms. Finally, there is only one API directly related to touch mode, View.isInTouchMode().
Sounds easy enough right? Oddly enough, touch mode is deceivingly simple and the consequences of entering touch mode are far greater than you might think. Let's look at some of the reasons why.
Designing a UI toolkit for mobile devices is difficult because of the various interaction mechanisms they provide. Some devices offer only 12 keys, some have a touch screen, some require a stylus, some have both a touch screen and a keyboard. In that regard, it is a great benefit for the Android development community that the first commercially available device, the G1, offers multiple forms of input using a touch screen, a trackball, and a keyboard. Because the user can interact with applications using three different mechanisms, we had to think very hard about all the possible issues that could arise. One issue led us to create the touch mode.
Imagine a simple application, ApiDemos for example, that shows a list of text items. The user can freely navigate through the list using the trackball and they can also scroll and fling the list using their finger. The issue in this scenario is the selection. If I select an item at the top of the list and then fling the list towards the bottom, what should happen to the selection? Should it remain on the item and scroll off the screen? In this case, what would happen if I then decide to move the selection with the trackball? Or worse, if I press the trackball to act upon the currently selected item, which is not shown on screen anymore. After careful considerations, we decided to remove the selection altogether.
In touch mode, there is no focus and no selection. Any selected item in a list of in a grid becomes unselected as soon as the user enters touch mode. Similarly, any focused widgets become unfocused when the user enters touch mode. The image below illustrates what happens when the user touches a list after selecting an item with the trackball.
To make things more natural for the user, the framework knows how to resurrect the selection/focus whenever the user leaves touch mode. For instance, in the example above, if the user were to use the trackball again, the selection would reappear on the previously-selected item. This is why some developers are confused when they create a custom view and start receiving key events only after moving the trackball once: their application is in touch mode, and they need to use the trackball to exit touch mode and resurrect the focus.
The relationship between touch mode, selection, and focus means you must not rely on selection and/or focus to exist in your application. A very common problem with new Android developers is to rely on ListView.getSelectedItemPosition(). In touch mode, this method will return INVALID_POSITION. You should instead use click listeners or the choice mode.
Now that you know focus doesn't exist in touch mode, I must explain that it's not entirely true. Focus can exist in touch mode but in a very special way we call focusable in touch mode. This special mode was created for widgets that receive text input, like EditText or, when filtering is enabled, ListView. This is why the user can type text inside a text field without first selecting it with the trackball or their finger. When a user touches the screen, the application will enter touch mode if it wasn't in touch mode already. What happens during the transition to touch mode depends on what the user touched, and what currently has focus. If the user touches a widget that is focusable in touch mode, that widget will receive focus. Otherwise, any currently focused widget will not retain focus unless it is focusable in touch mode. For instance, in the picture below, when the user touches the screen, the input text field receives the focus.
Focusable in touch mode is a property that you can set yourself either from code or XML. However, it should be used sparingly and only in very specific situations as it breaks consistency with Android normal behavior. A game is a good example of an application that can make good use of the focusable in touch mode property. MapView, if used in fullscreen as in Google Maps, is another good example of where you can use focusable in touch mode correctly.
Below is another example of a focusable in touch mode widget. When the user taps an AutoCompleteTextView's suggestion with his finger, the focus remains on the input text field:
New Android developers often think that focusable in touch mode is the solution they need to "fix" the problem of disappearing selection/focus. We really encourage you to think very hard before using it. If used incorrectly, it can make your application behave differently from the rest of the system and simply throw off the user's habits. The Android framework contains all the tools you need to handle user interactions without using "focusable in touch mode". For example, instead of trying to make ListView always keep its selection, simply use the appropriate choice mode. And if you feel that the framework does not suit all your need, feel free to let us know or contribute a patch.
Do:
Don't:
This week's developer video features Jason Tomlinson of Hands-On Mobile. He wrote Amazed, an application open sourced in the apps-for-android project. Things Jason mentions in the videos include:
This and other Android developer videos can be found here.
Last week we introduced a couple Android developers who shared how they built their Android apps and gave their insight into Android app development. This week, we have videos of two developers who've built music-related apps.
The first is of Allan Hsu—he wrote imeem's Android app. A couple of things he mentions in his videos:
The second video features Casey Langen—he wrote the Amazon MP3 for Android app. Things he mentions in the videos include:
Check out other Android developer videos here: Android App Developers.
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