ListView的性能优化之convertView和viewHolder

ListView优化大致从以下几个角度:
1.复用已经生成的convertView；
2.添加viewHolder类；
3.缓存数据（图片缓存）；
4.分页加载。

一、复用convertView
首先讲下ListView的原理：ListView中的每一个Item显示都需要Adapter调用一次getView的方法，这个方法会传入一个convertView的参数，返回的View就是这个Item显示的View。如果当Item的数量足够大，再为每一个Item都创建一个View对象，必将占用很多内存，创建View对象（mInflater.inflate(R.layout.lv_item, null);从xml中生成View，这是属于IO操作）也是耗时操作，所以必将影响性能。Android提供了一个叫做Recycler(反复循环器)的构件，就是当ListView的Item从上方滚出屏幕视角之外，对应Item的View会被缓存到Recycler中，相应的会从下方生成一个Item，而此时调用的getView中的convertView参数就是滚出屏幕的Item的View，所以说如果能重用这个convertView，就会大大改善性能。

这连个超链接需要看看 ，写的比较好
http://www.cnblogs.com/itstudent/p/3729117.html
http://blog.csdn.net/jacman/article/details/7087995

这是我做的程序测试：

看代码：

package com.example.mypic;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ListView;

public class MainActivity extends Activity {
    private ListView listView;
    private MyAdapter adapter;
    private int[] items = new int[] { R.drawable.pc, R.drawable.pc,
            R.drawable.pc, R.drawable.pc, R.drawable.pc, R.drawable.pc,
            R.drawable.pc, R.drawable.pc, R.drawable.pc, R.drawable.pc,
            R.drawable.pc, R.drawable.pc, R.drawable.pc, R.drawable.pc,
            R.drawable.pc, R.drawable.pc, R.drawable.pc, R.drawable.pc,
            R.drawable.pc, R.drawable.pc, R.drawable.pc, R.drawable.pc,
            R.drawable.pc, R.drawable.pc, R.drawable.pc, R.drawable.pc,
            R.drawable.pc, R.drawable.pc, R.drawable.pc, R.drawable.pc,
            R.drawable.pc, R.drawable.pc, R.drawable.pc, R.drawable.pc,
            R.drawable.pc, R.drawable.pc };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        listView = (ListView) findViewById(R.id.listView);
        Log.v("aa", "--显示图片-1-");
        adapter = new MyAdapter(this, items);
        Log.v("aa", "--显示图片-2-");
        listView.setAdapter(adapter);
        Log.v("aa", "--显示图片-3-");
        adapter.notifyDataSetChanged();
    }
}

package com.example.mypic;
import android.content.Context;
import android.graphics.Bitmap;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.TextView;

public class MyAdapter extends BaseAdapter {
private int[] items = new int[] {};
private Context context;
private String TAG = "aa";
private Bitmap bitmap = null;

public MyAdapter(Context context, int[] items) {
this.context = context;
this.items = items;
    }

@Override
public int getCount() {
return items.length;
    }

@Override
public Object getItem(int position) {
return items[position];
    }

@Override
public long getItemId(int position) {
return position;
    }

@Override
public View getView(int position, View convertView, ViewGroup parent) {

//Log.v(TAG, "position=="+position);
//Log.v(TAG, "convertView=="+convertView);
//Log.v(TAG, "ViewGroup=="+parent);

        ViewHolder holder;
if (convertView == null) {
            convertView = LayoutInflater.from(context).inflate(R.layout.list_item, null);
            holder = new ViewHolder();
            holder.imageView = (ImageView) convertView.findViewById(R.id.imageView);
            holder.textView = (TextView) convertView.findViewById(R.id.textView);

            Log.v(TAG, position+"----"+holder);

            convertView.setTag(holder);
        }else {
            holder = (ViewHolder) convertView.getTag();

            Log.v(TAG, position+"--1--"+holder);
        }

        bitmap = BitmapCompressTools.decodeSampledBitmapFromResource(
                    context.getResources(), R.drawable.pc, 100, 100);

//Log.d(TAG, "压缩之后的图片大小为:" + bitmap.getByteCount());
        holder.imageView.setImageBitmap(bitmap);
        holder.textView.setText("图片"+position);

return convertView;
    }

    class ViewHolder {
        ImageView imageView;
        TextView textView;
    }
}

package com.example.mypic;

import android.content.res.Resources;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;

public class BitmapCompressTools {
public static Bitmap decodeSampledBitmapFromResource(Resources res,
int resId, int reqWidth, int reqHeight) {

// 给定的BitmapFactory设置解码的参数
final BitmapFactory.Options options = new BitmapFactory.Options();
// 从解码器中获取原始图片的宽高，这样避免了直接申请内存空间
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(res, resId, options);

// Calculate inSampleSize
        options.inSampleSize = calculateInSampleSize(options, reqWidth,
                reqHeight);

// 压缩完后便可以将inJustDecodeBounds设置为false了。
        options.inJustDecodeBounds = false;
return BitmapFactory.decodeResource(res, resId, options);
    }

/**
 * 指定图片的缩放比例
 * 
 * @param options
 * @param reqWidth
 * @param reqHeight
 * @return
 */

public static int calculateInSampleSize(BitmapFactory.Options options,
int reqWidth, int reqHeight) {
// 原始图片的宽、高
final int height = options.outHeight;
final int width = options.outWidth;
int inSampleSize = 1;

// if (height > reqHeight || width > reqWidth) {
// //这里有两种压缩方式，可供选择。
// /**
// * 压缩方式二
// */
// // final int halfHeight = height / 2;
// // final int halfWidth = width / 2;
// // while ((halfHeight / inSampleSize) > reqHeight
// // && (halfWidth / inSampleSize) > reqWidth) {
// // inSampleSize *= 2;
// // }
// 
/**
 * 压缩方式一
 */
// 计算压缩的比例：分为宽高比例
final int heightRatio = Math.round((float) height
                    / (float) reqHeight);
final int widthRatio = Math.round((float) width / (float) reqWidth);
            inSampleSize = heightRatio < widthRatio ? heightRatio : widthRatio;
// }

return inSampleSize;
    }
}

main.ml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".MainActivity" >

<ListView
 android:id="@+id/listView"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true" >
</ListView>

</RelativeLayout>

list_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal" >

<ImageView
 android:id="@+id/imageView"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />

<TextView
 android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content" 
android:layout_marginLeft="20dp"/>

</LinearLayout>

这是运行结果图：