Android ListView優化實踐

在看了一些vogella的文章以後，發現關於android listview性能優化這一段頗有意思，因而實踐了一下，通過優化，性能確實提高很多！

先看看優化前和優化後的比較：

優化前的log截圖：

優化後的log截圖：

而且，在不停滾動ListView的過程當中，優化以前會出現ANR現象，在AVD上特別容易復現：

而後，優化後顯得很流暢，附上對於的log截圖：

下面附上相關代碼分析：

ListView中的每個Item由一個ImageView 和一個TextView組成

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繼承自ListActivity，我故意增長了Item，方便測試，效果更明顯：

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);
	}
}

而後custom Adapter，優化以前的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;
	}

優化以後的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;
	}
}

優化的大體思想就是：優化以前，每次加載item的時候，都要加載一下佈局文件，而後生成一個新的row View對象，而後經過View找到對應的ImageView和TextView，正如咱們所知道的那樣，加載佈局文件時很耗時的，特別是在操做比較頻繁狀況下，這是不可忍受的，因此會致使ANR現象。

所以，咱們能夠重複利用已不可見的row View對象。Android中，當它決定讓row View對象不可見的時候，它容許經過getView方法中的convertView參數來重複利用剛剛不可見的row View對象。

在優化的過程當中，第一次加載的時候，咱們須要把相關的數據保存起來，而View有一個方法setTag，該方法可用來保存一些數據結構。咱們一個row View對象是由ImageView和TextView空間組成的，所以定義一個ViewHolder來保存ImageView和TextView對象。在重複利用的過程當中，只需簡單修改它們的值，而不用再次findViewById。

關於findViewById耗時的分析，可參考：

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