Android ListView那些事
时间: 2019-01-31 来源: admin

Android 英皇娱乐网站ListView那些事

ListView是我们在开发Android程序时用得比较多的一种widget,通常用来展示多条数据,这里,我对ListView的一些功能点作一个简单介绍。

1. Cache color hint 默认情况下,Android中的View的背景都是透明的,这是AAAAAAAA但是,当渲染到屏幕上时,这会引入许多的计算,因为所有的child的背景是透明的,这就意味着当ListView绘制它的child时,会导致child与window的背景色相混合,当ListView上下滑动或Fling时,性能就会下降。当ListView上下滑动或者是Fling时,为了避免这种情况发生或提高渲染速度,ListView引入了"scrolling cache"机制。这个机制简单地说来就是ListView把可见的child绘制在一个bitmap上面(系统将每一个child拷贝到由cache color hint填充的bitmap上面),然后再直接把这个bitmap绘制到屏幕上,由于这个bitmap是不透明的,所以就不会有混合计算,这样就提高了绘制的效率。

但是,由于系统默认的cache color hint的颜色是#191919,当你滑动ListView时,你就会看到一个黑色的背景。

出现这种情况肯定是不正确的,那么怎么解决呢?

最简单的方法是将cache color hint设置为透明。

android:cacheColorHint="#00000000" 或 setCacheColorHint(Color.TRANSPARENT)

以下是官方文档的原文:
As mentioned before, ListView has a transparent/translucent background by default, and so all default widgets in the Android UI toolkit. 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 (assuming that another optimization, called scrolling cache,AAAAAAAAAArned off). ListView then blits these bitmaps directly on screen and because these bitmaps are known to be opaque, no blending is required. Also, 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. You can do this from code (see setCacheColorHint(int)) 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.

2. Divider 通过android:divider XML属性来指定。 Divider可以颜色或者Drawable,当绘制内容时,ListView会自动地计算出divider的位置。由于divider接收一个drawable,它可以是ColorDrawable,也可以是BitmapDrawable,但是,由于ColorDrawable,它是没有高度的,所以Drawable#getIntrinsicHeight()方法返回-1,因此,为了避免这个问题, 我们可以通过android:dividerHeight属性来设计其高度。
3. ListView绘制顺序 对于ListView的绘制顺序,我们很有必要了解一下。先看一下下面的图片。
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"blocksDescendants"即可,这样Item Layout就屏蔽了所有子控件获取Focus的权限,不需要针对Item Layout中的每一个控件重新设置focusable属性了,如此就可以顺利的响应onItemClickListener中的onItemClick()方法了。
我看一下ListView处理touch事件的方法,AbsListView#onTouchEvent()方法中,代码如下: [java] view plaincopy
  1. ...
  2. case MotionEvent.ACTION_UP: {
  3. switch (mTouchMode) {
  4. case TOUCH_MODE_DOWN:
  5. case TOUCH_MODE_TAP:
  6. case TOUCH_MODE_DONE_WAITING:
  7. final int motionPosition = mMotionPosition;
  8. final View child = getChildAt(motionPosition - mFirstPosition);
  9. final float x = ev.getX();
  10. final boolean inList = x > mListPadding.left
  11. && x < getWidth() - mListPadding.right;
  12. if (child != null && !child.hasFocusable() && inList) {
  13. if (mTouchMode != TOUCH_MODE_DOWN) {
  14. child.setPressed(false);
  15. }
  16. if (mPerformClick == null) {
  17. mPerformClick = new PerformClick();
  18. }
  19. ...
    总结如下: 原因: ListView中的Item内部的View获得了焦点,如Button, Checkbox等。
    解决办法: 不要让ListView中的Item内部的View获得焦点就OK了,这样做:android:descendantFocusability="blocksDescendants"
    public static final int descendantFocusability Defines the relationship between the ViewGroup and its descendants when looking for a View to take focus.
    Constant Value Description
    beforeDescendants 0 The ViewGroup will get focus before any of its descendants.
    afterDescendants 1 The ViewGroup will get focus only if none of its descendants want it.
    blocksDescendants 2 The ViewGroup will block its descendants from receiving focus.


    注意:
    还有一种情况也会导致OnItemClickListener或OnItemLongClickListener回调不会执行,那就是ListView的child设置了onClickListener或onLongClickListener。我们可以通过源代码看出,在你调用setOnClickListener()方法后,它会调用setClickable(true),在onTouchEvent里面的实现如下: [java] view plaincopy
    1. if (((viewFlags & CLICKABLE) == CLICKABLE ||
    2. (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
    3. // ....
    4. return true;
    5. } 当一个View在onTouchEvent里面返回true后,ListView就不会正常接收到事件。

      5. ListView不要做的事 1)android:layout_height="warp_content"? 有时候我们很容易让ListView的高度是wrap_content,这样做,很容易导致性能问题,wrap_content意味着as big as my children,这会导致1)测量大量的children;2)当添加child时,容易引起ListView重新发生layout,这又可能引发child的layout。我曾经遇到过一个问题,就是在维护别的写的代码时,ListView上下滑动速度特别慢,我把adapter等地方都优化完后,仍然一样,最终才发现ListView的高度是wrap_content,我把其改为match_parent之后,性能一下就正常了。
      2)ListView套在ScrollView中,这种用法实在蛋痛,不解释。

      http://www.bkjia.com/Androidjc/856261.htmlwww.bkjia.comtruehttp://www.bkjia.com/Androidjc/856261.htmlTechArticleAndroid ListView那些事 ListView是我们在开发Android程序时用得比较多的一种widget,通常用来展示多条数据,这里,我对ListView的一些功能点作一个...

      本文源自: 环亚国际



      上一篇:iOS数据存储的几种方式,ios数据存储几种
      下一篇:没有了