ListView的优化

ListView的优化：

（前两点都是利用ListView的自身优化机制优化【缓存优化】）

1.利用ListView自身的缓存机制，他会缓存条目中的一个条目item，当listview第一屏显示完成之后，就会出现一个缓存条目，其实就是BaseAdapter里面的public View getView(int position, View convertView, ViewGroup parent)。

2.减少findViewById()的次数，findViewById是一个相对比较耗性能的操作，因为这个操作每次都需要到布局中去查找文件。把item里面的控件封装成一个javaBean，当item条目被加载的时候就去找到对应的控件。

3.利用时间去换取时间，比如开机优化，把一些重要的程序先启动了，启动完系统之后再启动其他程序。

4.利用空间去换取时间，把要获取的数据先加载到内存里面，再处理数据的时候，直接从内存中获取。

   例如：减少数据库的频繁打开和关闭和减少查询的次数。

Demo：需求： 程序锁，将手机中所有的应用显示出来，是否是带锁的应用用数据库进行存储，通过ListView显示。

import java.util.List;
import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.animation.TranslateAnimation;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.RelativeLayout;
import android.widget.TextView;

import com.android.hzy.mobilesafe.R;
import com.android.hzy.mobilesafe.dao.AppLockDao;
import com.android.hzy.mobilesafe.domain.AppInfo;
import com.android.hzy.mobilesafe.engine.AppInfoService;

public class AppLockManagerActivity extends Activity {

    protected static final int SUCCESS_GET_APPLICATION = 0;
    private ListView lv_applockmanager;
    private RelativeLayout rl_loading;
    private AppInfoService appInfoService;
    private List<AppInfo> appInfos;
    
    private List<String> appLocks; // 程序锁应用集合
    
    private AppLockManagerAdapter mAdapter;
    private AppLockDao appLockDao;
    private Handler mHandler = new Handler(){

        public void handleMessage(Message msg) {
            switch (msg.what) {
            case SUCCESS_GET_APPLICATION:
                appLocks = appLockDao.findAll();
                mAdapter = new AppLockManagerAdapter();
                lv_applockmanager.setAdapter(mAdapter);
                rl_loading.setVisibility(View.GONE);
                break;

            default:
                break;
            }
        }
        
    };
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.app_lock_manager);
        
        lv_applockmanager = (ListView) findViewById(R.id.lv_applockmanager);
        rl_loading = (RelativeLayout) findViewById(R.id.rl_loading);
        appInfoService = new AppInfoService(this);
        
        new Thread() {
            public void run() {
                appInfos = appInfoService.getAppInfos();
                Message msg = new Message();
                msg.what = SUCCESS_GET_APPLICATION;
                mHandler.sendMessage(msg);
            };
        }.start();
        
        appLockDao = new AppLockDao(this);
        
        lv_applockmanager.setOnItemClickListener(new MyOnItemClickListener());
    }
    
    /**********listview条目点击事件*************************/
    private final class MyOnItemClickListener implements OnItemClickListener{

        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position,
                long id) {
            // TODO Auto-generated method stub
            // 通过ViewHolder的iv_lock 减少findViewById
            ImageView iv_lock = ((ViewHolder)view.getTag()).iv_lock;
            AppInfo appInfo = (AppInfo) mAdapter.getItem(position);
//            boolean isLockApp = appLockDao.isLockApp(appInfo.getPackagename()); 减少数据库的查询
            boolean isLockApp = appLocks.contains(appInfo.getPackagename());
            if (isLockApp) {
                // 如果有锁  就解锁
                appLockDao.delete(appInfo.getPackagename());
                // 解锁需要将集合中带锁应用的移除
                appLocks.remove(appInfo.getPackagename());
                iv_lock.setImageResource(R.drawable.unlock);
            } else {
                // 如果没有锁  就加锁
                appLockDao.add(appInfo.getPackagename());
                // 加锁需要将有锁的集合中添加该应用
                appLocks.add(appInfo.getPackagename());
                iv_lock.setImageResource(R.drawable.lock);
            }
            
            // 设置动画
            TranslateAnimation animation = new TranslateAnimation(0, 80, 0, 0);
            animation.setDuration(300);
            view.startAnimation(animation);
        }
        
    }
    
    /**
     * 适配器
     */
    public final class AppLockManagerAdapter extends BaseAdapter{
        
        private LayoutInflater mInflater;
        
        public AppLockManagerAdapter() {
            // TODO Auto-generated constructor stub
            mInflater = getLayoutInflater();
        }
        
        @Override
        public int getCount() {
            // TODO Auto-generated method stub
            return appInfos.size();
        }

        @Override
        public Object getItem(int position) {
            // TODO Auto-generated method stub
            return appInfos.get(position);
        }

        @Override
        public long getItemId(int position) {
            // TODO Auto-generated method stub
            return position;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            // TODO Auto-generated method stub
            View view = null;
            ViewHolder holder = null;
            if (convertView != null) {
                view = convertView;
                holder = (ViewHolder) view.getTag();
                Log.i("i", " convertView != null  缓存不等于空  使用缓存");
                
            } else {
                Log.i("i", " convertView == null  缓存等于空");
                view = mInflater.inflate(R.layout.app_lock_manager_item, null);
                holder = new ViewHolder();
                holder.iv_appicon = (ImageView) view.findViewById(R.id.iv_appicon);
                holder.tv_appname = (TextView) view.findViewById(R.id.tv_appname);
                holder.iv_lock = (ImageView) view.findViewById(R.id.iv_lock);
                view.setTag(holder);
            }
            AppInfo appInfo = appInfos.get(position);
            
            ImageView iv_appicon = holder.iv_appicon;
            TextView tv_appname = holder.tv_appname;
            ImageView iv_lock = holder.iv_lock;
            
            iv_appicon.setImageDrawable(appInfo.getApp_icon());
            tv_appname.setText(appInfo.getApp_name());
            
//            boolean isLockApp = appLockDao.isLockApp(appInfo.getPackagename());
            boolean isLockApp = appLocks.contains(appInfo.getPackagename()); // 此功能优化后  不用每次查询数据库 直接将所有有锁的程序都放入集合中存储
            if(isLockApp){
                iv_lock.setImageResource(R.drawable.lock);
            }else{
                iv_lock.setImageResource(R.drawable.unlock);
            }
            
            return view;
        }
        
    }
    
    static class ViewHolder{
        ImageView iv_appicon;
        TextView tv_appname;
        ImageView iv_lock;
    }
}

效果如上图，当第一次加载ListView里面条目时，我们所能看见的都是没有缓存的，当我们往下滑动后，控件都是使用缓存中的控件，减少了findViewById的次数