Android ListView优化实践

在看了一些vogella的文章之后，发现关于android listview性能优化这一段很有意思，于是实践了一下，经过优化，性能确实提升不少！

先看看优化前和优化后的比较：

优化前的log截图：

优化后的log截图：

并且，在不停滚动ListView的过程中，优化之前会出现ANR现象，在AVD上特别容易复现：

然后，优化后显得很流畅，附上对于的log截图：

下面附上相关代码分析：

ListView中的每一个Item由一个ImageView 和一个TextView组成

Layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:orientation="horizontal" >
        
        <ImageView android:id="@+id/imageView"
            android:layout_width="wrap_content"
            android:layout_height="fill_parent" />"
        <TextView android:id="@+id/textView"
            android:layout_width="wrap_content"
            android:layout_height="fill_parent"
            android:layout_marginLeft="15dp"
            android:gravity="center_vertical" />
</LinearLayout>

Activity继承自ListActivity，我故意增加了Item，方便测试，效果更明显：

public class ListViewDemo extends ListActivity{

	private final String[] mItems = new String[] { "Android", "iPhone",
			"WindowsMobile", "Blackberry", "WebOS", "Ubuntu", "Windows7",
			"Max OS X", "Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X",
			"Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2",
			"Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2", "Ubuntu",
			"Windows7", "Max OS X", "Linux", "OS/2", "Ubuntu", "Windows7",
			"Max OS X", "Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X",
			"Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2",
			"Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2", "Ubuntu",
			"Windows7", "Max OS X", "Linux", "OS/2", "Ubuntu", "Windows7",
			"Max OS X", "Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X",
			"Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2",
			"Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2", "Ubuntu",
			"Windows7", "Max OS X", "Linux", "OS/2", "Ubuntu", "Windows7",
			"Max OS X", "Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X",
			"Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2",
			"Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2", "Ubuntu",
			"Windows7", "Max OS X", "Linux", "OS/2", "Ubuntu", "Windows7",
			"Max OS X", "Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X",
			"Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2",
			"Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2", "Ubuntu",
			"Windows7", "Max OS X", "Linux", "OS/2", "Ubuntu", "Windows7",
			"Max OS X", "Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X",
			"Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2",
			"Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2", "Ubuntu",
			"Windows7", "Max OS X", "Linux", "OS/2", "Ubuntu", "Windows7",
			"Max OS X", "Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X",
			"Linux", "OS/2", "Ubuntu", "Windows7", "Max OS X", "Linux", "OS/2" };
	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		ListViewArrayAdapter adapter = new ListViewArrayAdapter(this, mItems);
		getListView().setAdapter(adapter);
	}
}

然后custom Adapter，优化之前的adapter:

@Override
	public View getView(int position, View convertView, ViewGroup parent) {
		long start = System.currentTimeMillis();
		LayoutInflater inflater = (LayoutInflater) mContext.getLayoutInflater();
		View rowView = inflater.inflate(mViewResourceId, parent, false);
		TextView textView = (TextView) rowView
				.findViewById(mTextViewResourceId);
		ImageView imageView = (ImageView) rowView
				.findViewById(mImageViewResourceId);
		textView.setText(mNames[position]);
		String s = mNames[position];
		if (s.startsWith("Windows7") || s.startsWith("iPhone")) {
			imageView.setImageResource(R.drawable.no);
		} else {
			imageView.setImageResource(R.drawable.yes);
		}
		    
		Log.v("jerikc","cost time = " + (System.currentTimeMillis() - start));
		return rowView;
	}

优化之后的Adapter：

public class ListViewArrayAdapter extends ArrayAdapter<String>{

	private final Activity mContext;
	private final String[] mNames;
	private final static int mViewResourceId = R.layout.text_image_row_layout;
	private final static int mTextViewResourceId = R.id.textView;
	private final static int mImageViewResourceId = R.id.imageView;
	static class ViewHolder {
		public TextView text;
		public ImageView image;
	}
	
	public ListViewArrayAdapter(Activity context, String[] names) {
		super(context, mViewResourceId, names);
		this.mContext = context;
		this.mNames = names;
	}
	
	@Override
	public View getView(int position, View convertView, ViewGroup parent) {
		long start = System.currentTimeMillis();
		View rowView = convertView;
		if (rowView == null) {
			LayoutInflater inflater = mContext.getLayoutInflater();
			rowView = inflater.inflate(mViewResourceId, null);
			ViewHolder viewHolder = new ViewHolder();
			viewHolder.text = (TextView) rowView.findViewById(mTextViewResourceId);
			viewHolder.image = (ImageView) rowView.findViewById(mImageViewResourceId);
			rowView.setTag(viewHolder);
		}
		
		ViewHolder holder = (ViewHolder) rowView.getTag();
		String s = mNames[position];
		holder.text.setText(s);
		if (s.startsWith("Windows7") || s.startsWith("iPhone")) {
			holder.image.setImageResource(R.drawable.no);
		} else {
			holder.image.setImageResource(R.drawable.yes);
		}
		    
		Log.v("jerikc","cost time = " + (System.currentTimeMillis() - start));
		return rowView;
	}
}

优化的大致思想就是：优化之前，每次加载item的时候，都要加载一下布局文件，然后生成一个新的row View对象，然后通过View找到对应的ImageView和TextView，正如我们所知道的那样，加载布局文件时很耗时的，特别是在操作比较频繁情况下，这是不可忍受的，所以会导致ANR现象。

因此，我们可以重复利用已不可见的row View对象。Android中，当它决定让row View对象不可见的时候，它允许通过getView方法中的convertView参数来重复利用刚刚不可见的row View对象。

在优化的过程中，第一次加载的时候，我们需要把相关的数据保存起来，而View有一个方法setTag，该方法可用来保存一些数据结构。我们一个row View对象是由ImageView和TextView空间组成的，因此定义一个ViewHolder来保存ImageView和TextView对象。在重复利用的过程中，只需简单修改它们的值，而不用再次findViewById。

关于findViewById耗时的分析，可参考：

http://www.cnblogs.com/over140/archive/2011/03/23/1991100.html