提高ListView的性能

ListView优化一直是一个老生常谈的问题，不管是面试还是平常的开发中，ListView永远不会被忽略掉，那么这篇文章我们来看看如何最大化的优化ListView的性能。

1.在adapter中的getView方法中尽量少使用逻辑
2.尽最大可能避免GC
3.滑动的时候不加载图片
4.将ListView的scrollingCache和animateCache设置为false
5.item的布局层级越烧越好
6.使用ViewHolder

1.在adapter中的getView方法中尽量少使用逻辑

不要在你的getView()中写过多的逻辑代码，我们可以将这些代码放在别的地方，例如：

优化前的getView（）：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> View <span class="hljs-title" style="box-sizing: border-box;">getView</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> position, View convertView, ViewGroup paramViewGroup) {
Object current_event = mObjects.get(position);
ViewHolder holder = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (convertView == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
holder = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ViewHolder();
convertView = inflater.inflate(R.layout.row_event, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>);
holder.ThreeDimension = (ImageView) convertView.findViewById(R.id.ThreeDim);
holder.EventPoster = (ImageView) convertView.findViewById(R.id.EventPoster);
convertView.setTag(holder);

} <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
holder = (ViewHolder) convertView.getTag();
}

<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//在这里进行逻辑判断，这是有问题的 </span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (doesSomeComplexChecking()) {
holder.ThreeDimention.setVisibility(View.VISIBLE);
} <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
holder.ThreeDimention.setVisibility(View.GONE);
}

<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 这是设置image的参数，每次getView方法执行时都会执行这段代码，这显然是有问题的</span>
RelativeLayout.LayoutParams imageParams = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> RelativeLayout.LayoutParams(measuredwidth, rowHeight);
holder.EventPoster.setLayoutParams(imageParams);

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> convertView;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li></ul>

优化后的getView（）：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> View <span class="hljs-title" style="box-sizing: border-box;">getView</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> position, View convertView, ViewGroup paramViewGroup) {
Object object = mObjects.get(position);
ViewHolder holder = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>;

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (convertView == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
holder = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ViewHolder();
convertView = inflater.inflate(R.layout.row_event, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>);
holder.ThreeDimension = (ImageView) convertView.findViewById(R.id.ThreeDim);
holder.EventPoster = (ImageView) convertView.findViewById(R.id.EventPoster);
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//设置参数提到这里，只有第一次的时候会执行，之后会复用 </span>
RelativeLayout.LayoutParams imageParams = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> RelativeLayout.LayoutParams(measuredwidth, rowHeight);
holder.EventPoster.setLayoutParams(imageParams);
convertView.setTag(holder);
} <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
holder = (ViewHolder) convertView.getTag();
}

<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 我们直接通过对象的getter方法代替刚才那些逻辑判断，那些逻辑判断放到别的地方去执行了</span>
holder.ThreeDimension.setVisibility(object.getVisibility());

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> convertView;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li></ul>

2.GC 垃圾回收器

当你创建了大量的对象的时候，GC就会频繁的执行，所以在getView（）方法中不要创建很多的对象，最好的优化是，不要在ViewHolder以外创建任何对象，如果你的你的log里面发现“GC has freed some memory”频繁出现的话，那你的程序肯定有问题了。你可以检查一下：

a) item布局的层级是否太深

b) getView（）方法中是否有大量对象存在

c) ListView的布局属性

3.加载图片

如果你的ListView中需要显示从网络上下载的图片的话，我们不要在ListView滑动的时候加载图片，那样会使ListView变得卡顿，所以我们需要再监听器里面监听ListView的状态，如果滑动的时候，停止加载图片，如果没有滑动，则开始加载图片

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">listView.setOnScrollListener(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> OnScrollListener() {

<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onScrollStateChanged</span>(AbsListView listView, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> scrollState) {
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//停止加载图片 </span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (scrollState == AbsListView.OnScrollListener.SCROLL_STATE_FLING) {
imageLoader.stopProcessingQueue();
} <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//开始加载图片</span>
imageLoader.startProcessingQueue();
}
}

<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onScroll</span>(AbsListView view, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> firstVisibleItem, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> visibleItemCount, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> totalItemCount) {
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// TODO Auto-generated method stub</span>

}
});</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li></ul>

4.将ListView的scrollingCache和animateCache设置为false

scrollingCache: scrollingCache本质上是drawing cache，你可以让一个View将他自己的drawing保存在cache中（保存为一个bitmap），这样下次再显示View的时候就不用重画了，而是从cache中取出。默认情况下drawing cahce是禁用的，因为它太耗内存了，但是它确实比重画来的更加平滑。而在ListView中，scrollingCache是默认开启的，我们可以手动将它关闭。

animateCache: ListView默认开启了animateCache，这会消耗大量的内存，因此会频繁调用GC，我们可以手动将它关闭掉

优化前的ListView

<code class="hljs perl has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><ListView
android:id=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@android</span>:id/list"</span>
android:layout_width=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"match_parent"</span>
android:layout_height=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"wrap_content"</span>
android:cacheColorHint=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"#00000000"</span>
android:divider=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@color</span>/list_background_color"</span>
android:dividerHeight=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"0dp"</span>
android:listSelector=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"#00000000"</span>
android:smoothScrollbar=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"true"</span>
android:visibility=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"gone"</span> />

</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>

优化后的ListView

<code class="hljs perl has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><ListView
android:id=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@android</span>:id/list"</span>
android:layout_width=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"match_parent"</span>
android:layout_height=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"wrap_content"</span>
android:divider=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@color</span>/list_background_color"</span>
android:dividerHeight=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"0dp"</span>
android:listSelector=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"#00000000"</span>
android:scrollingCache=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"false"</span>
android:animationCache=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"false"</span>
android:smoothScrollbar=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"true"</span>
android:visibility=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"gone"</span> /></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>

5.减少item的布局的深度

我们应该尽量减少item布局深度，因为当滑动ListView的时候，这回直接导致测量与绘制，因此会浪费大量的时间，所以我们应该将一些不必要的布局嵌套关系去掉。减少item布局深度

6.使用ViewHolder

这个大家应该非常熟悉了，但是不要小看这个ViewHolder，它可以大大提高我们ListView的性能

ListView的优化我们已经讲完了，如果在你的项目中，这些基本优化你还没有做到的话，那么你的ListView是有问题的，还有很大的提升潜力，以后再使用ListView的时候，一定要将这几点考虑进去，发挥它的最大的性能。

android:descendantFocusability用法简析

开发中很常见的一个问题，项目中的listview不仅仅是简单的文字，常常需要自己定义listview，自己的Adapter去继承BaseAdapter，在adapter中按照需求进行编写，问题就出现了，可能会发生点击每一个item的时候没有反应，无法获取的焦点。原因多半是由于在你自己定义的Item中存在诸如ImageButton，Button，CheckBox等子控件(也可以说是Button或者Checkable的子类控件)，此时这些子控件会将焦点获取到，所以常常当点击item时变化的是子控件，item本身的点击没有响应。
这时候就可以使用descendantFocusability来解决啦，API描述如下：

android:descendantFocusability

Defines the relationship between the ViewGroup and its descendants when looking for a View to take focus.
Must be one of the following constant values.



该属性是当一个为view获取焦点时，定义viewGroup和其子控件两者之间的关系。
属性的值有三种：
beforeDescendants：viewgroup会优先其子类控件而获取到焦点
afterDescendants：viewgroup只有当其子类控件不需要获取焦点时才获取焦点
blocksDescendants：viewgroup会覆盖子类控件而直接获得焦点

通常我们用到的是第三种，即在Item布局的根布局加上android:descendantFocusability=”blocksDescendants”的属性就好了，至此listview点击的灵异事件告一段落。心得：遇到不会不懂的地方除了网上查询资料之外，也可以多多去尝试每种属性的作用，多阅读官方文档(我始终觉得还是读原文的比翻译的理解的会更好)。