1page.title=ListView Backgrounds: An Optimization 2@jd:body 3 4<p>{@link android.widget.ListView} is one of Android's most widely used widgets. 5It is rather easy to use, very flexible, and incredibly powerful. 6<code>ListView</code> can also be difficult to understand at times.</p> 7 8<p>One of the most common issues with <code>ListView</code> happens when you try 9to use a custom background. By default, like many Android widgets, 10<code>ListView</code> has a transparent background which means that you can see 11through the default window's background, a very dark gray 12(<code>#FF191919</code> with the current <code>dark</code> theme.) Additionally, 13<code>ListView</code> enables the <em>fading edges</em> by default, as you can 14see at the top of the following screenshot — the first text item gradually 15fades to black. This technique is used throughout the system to indicate that 16the container can be scrolled.</p> 17 18<div style="text-align: center;"><img src="images/list_fade_1.png" alt="Android's default ListView"></div> 19 20<p>The fade effect is implemented using a combination of 21{@link android.graphics.Canvas#saveLayerAlpha(float, float, float, float, int, int) Canvas.saveLayerAlpha()} 22and the {@link android.graphics.PorterDuff.Mode#DST_OUT Porter-Duff Destination Out blending mode}. </p> 23 24<p>Unfortunately, things start to get ugly when you try to use a custom 25background on the <code>ListView</code> or when you change the window's 26background. The following two screenshots show what happens in an application 27when you change the window's background. The left image shows what the list 28looks like by default and the right image shows what the list looks like during 29a scroll initiated with a touch gesture:</p> 30 31<div style="text-align: center;"> 32<img style="margin-right: 12px;" src="images/list_fade_2.png" alt="Dark fade"> 33<img src="images/list_fade_3.png" alt="Dark list"></div> 34 35<p>This rendering issue is caused by an optimization of the Android framework 36enabled by default on all instances of <code>ListView</code>. I mentioned 37earlier that the fade effect is implemented using a Porter-Duff blending mode. 38This implementation works really well but is unfortunately very costly and can 39bring down drawing performance by quite a bit as it requires to capture a 40portion of the rendering in an offscreen bitmap and then requires extra blending 41(which implies readbacks from memory.)</p> 42 43<p>Since <code>ListView</code> is most of the time displayed on a solid 44background, there is no reason to go down that expensive route. That's why we 45introduced an optimization called the "cache color hint." The cache color hint 46is an RGB color set by default to the window's background color, that is #191919 47in Android's dark theme. When this hint is set, <code>ListView</code> (actually, 48its base class <code>View</code>) knows it will draw on a solid background and 49therefore replaces th expensive <code>saveLayerAlpha()/Porter-Duff</code> 50rendering with a simple gradient. This gradient goes from fully transparent to 51the cache color hint value and this is exactly what you see on the image above, 52with the dark gradient at the bottom of the list. However, this still does not 53explain why the entire list turns black during a scroll.</p> 54 55<p>As mentioned before, <code>ListView</code> has a transparent/translucent 56background by default, and so all default widgets in the Android UI toolkit. 57This implies that when <code>ListView</code> redraws its children, it has to 58blend the children with the window's background. Once again, this requires 59costly readbacks from memory that are particularly painful during a scroll or a 60fling when drawing happens dozen of times per second. </p> 61 62<p>To improve drawing performance during scrolling operations, the Android 63framework reuses the cache color hint. When this hint is set, the framework 64copies each child of the list in a <code>Bitmap</code> filled with the hint 65value (assuming that another optimization, called <em>scrolling cache</em>, is 66not turned off). <code>ListView</code> then blits these bitmaps directly on 67screen and because these bitmaps are known to be opaque, no blending is 68required. Also, since the default cache color hint is <code>#191919</code>, you 69get a dark background behind each item during a scroll.</p> 70 71<p>To fix this issue, all you have to do is either disable the cache color hint 72optimization, if you use a non-solid color background, or set the hint to the 73appropriate solid color value. You can do this from code (see 74{@link android.widget.AbsListView#setCacheColorHint(int)}) or preferably from 75XML, by using the <code>android:cacheColorHint</code> attribute. To disable the 76optimization, simply use the transparent color <code>#00000000</code>. The 77following screenshot shows a list with 78<code>android:cacheColorHint="#00000000"</code> set in the XML layout file:</p> 79 80<div style="text-align: center;"><img src="images/list_fade_4.png" alt="Fade on a custom background"></div> 81 82<p>As you can see, the fade works perfectly against the custom wooden 83background. The cache color hint feature is interesting because it 84shows how optimizations can make your life more difficult in 85some situations. In this particular case, however, the benefit of the 86default behavior outweighs the added complexity..</p> 87
