Android ListView 滑动背景为黑色的解决方法

2012-07-08

Android ListView 滑动背景为黑色的解决办法?1）点击Item时无背景颜色变化在xml文件中的ListView控件中加入

Android ListView 滑动背景为黑色的解决办法

1）点击Item时无背景颜色变化
在xml文件中的ListView控件中加入如下属性：
android:listSelector="@drawable/timer_list_selector"
在drawable中定义timer_list_selector的属性值
timer_list_selector.xml中定义如下：
<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_selected="true" android:drawable="@android:color/transparent" />
</selector>
在values文件夹下的colors.xml中定义transparent如下：
<color name="transparent">#50000000</color>

2）设置Item之间无间隙
在xml文件中ListView控件中加入如下属性：
android:divider="#00000000"
或者在javaCode中如下定义：
listView.setDividerHeight(0);

3)自定义的BaseAdapter中调用notifyDataSetChanged()方法会重新调用BaseAdapter的getView()方法。

用心的朋友应该会发现，listview中在设置了背景之后。会有些问题。

1.、listview在拖动的时候背景图片消失变成黑色背景。等到拖动完毕我们自己的背景图片才显示出来。

2 、listview的上边和下边有黑色的阴影。

3、lsitview的每一项之间需要设置一个图片做为间隔。

针对以上问题在listview的xml文件中设置一下语句。

问题1 有如下代码结解决 android:scrollingCache="false"

问题2 用如下代码解决：android:fadingEdge="none"
问题3 用如下代码解决： android:divider="@drawable/list_driver" 其中 @drawable/list_driver 是一个图片资源

总体如下

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.)
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:
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.

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