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13 January 2009

Why is my list black? An Android optimization

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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.

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.

Android's default ListView

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:

Dark fadeDark list

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.)

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.

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.

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:

Fade on a custom background

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.