ListView的优化和自定义适配器继承于BaseAdapter

                          ListView的优化和万能适配器继承于BaseAdapter

         在android开发中Listview是一个很重要的组件，它以列表的形式根据数据的长自适应展示具体内容,用户可以自由的定义listview每一列的布局，但当listview有大量的数据需要加载的时候，会占据大量内存，影响性能，这时候就需要按需填充并重新使用view来减少对象的创建。

     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

     二,ViewHolder类

     避免了就是每次在getVIew的时候，都需要重新的findViewById，重新找到控件，然后进行控件的赋值以及事件相应设置。这样其实在做重复的事情)

     三 自定义适配器BaseAdapter

      抽取BaseAdapter类里面的方法写一个自定义适配器,因为一个项目中会有多次使用到适配器,其实每次使用,我们主要是写getView方法和传入数据,所以把其他的方法抽取出来写到一个自定义类里面,供多次使用,看看我的代码就知道了.

     看看我写的自定义适配器

    

   MyBaseAdapter.java

package com.gaomin.myBaseAdapter;

import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;

import java.util.List;

/**
 * Created by gaomin on 2016/3/21.
 */
public abstract class MyBaseAdapter extends BaseAdapter{
    public   List T;

    public MyBaseAdapter(List t){
          T=t;
    }
    @Override
    public int getCount() {
        return T.size();
    }

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

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

    @Override
    public abstract View getView(int position, View convertView, ViewGroup parent);
}

等会在程序中会使用的,先把布局文件贴出来吧

activity_main.xml

每一项的布局

listview_item.xml 

 

   最后就是将要实现程序的主要java代码了

package com.gaomin.listViewDemo;

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

import com.gaomin.myBaseAdapter.MyBaseAdapter;

import java.util.ArrayList;
import java.util.HashMap;

public class MainActivity extends Activity {
    private SongAdapter songAdapter;
    private ListView mListView;
    private ArrayList songArrayList;
    private HashMap songMap;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mListView =(ListView)findViewById(R.id.mListView);

        songArrayList = new ArrayList();
        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","林心如");
        songMap.put("songInfo","一万个舍不得");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","宋慧乔");
        songMap.put("songInfo","太阳的后裔");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","庄心妍");
        songMap.put("songInfo","一直还想着她");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","林心如");
        songMap.put("songInfo","一万个舍不得");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","宋慧乔");
        songMap.put("songInfo","太阳的后裔");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","庄心妍");
        songMap.put("songInfo","一直还想着她");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","林心如");
        songMap.put("songInfo","一万个舍不得");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","宋慧乔");
        songMap.put("songInfo","太阳的后裔");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","庄心妍");
        songMap.put("songInfo","一直还想着她");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","林心如");
        songMap.put("songInfo","一万个舍不得");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","宋慧乔");
        songMap.put("songInfo","太阳的后裔");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","庄心妍");
        songMap.put("songInfo","一直还想着她");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","林心如");
        songMap.put("songInfo","一万个舍不得");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","宋慧乔");
        songMap.put("songInfo","太阳的后裔");
        songArrayList.add(songMap);

        songMap = new HashMap();
        songMap.put("image",R.mipmap.lyf);
        songMap.put("singer","庄心妍");
        songMap.put("songInfo","一直还想着她");
        songArrayList.add(songMap);



        if(songAdapter==null){
            songAdapter = new SongAdapter(this,songArrayList);
        }else {
            //刷新适配器,不用每次都new SongAdapter(this,songArrayList)
            songAdapter.notifyDataSetChanged();
        }
        mListView.setAdapter(songAdapter);

   }
    class SongAdapter extends MyBaseAdapter{
        private Context context;
        SongAdapter(Context c,ArrayList arrayList){
            super(arrayList);
            context=c;
        }
        //重用了convertView，很大程度上的减少了内存的消耗。通过判断convertView是否为null，
        // 是的话就需要产生一个视图出来，然后给这个视图数据，最后将这个视图返回给底层，呈献给用户。
        @Override
        public View getView(final int position, View convertView, ViewGroup parent) {
            ViewHolder viewHolder;
            if(convertView==null){
                LayoutInflater inflater = LayoutInflater.from(context);
                convertView =inflater.inflate(R.layout.listview_item,null);

                viewHolder = new ViewHolder();
                viewHolder.singerImageView =(ImageView)convertView.findViewById(R.id.singerImageView);
                viewHolder.songInfoTextView =(TextView)convertView.findViewById(R.id.songInfoTextView);
                viewHolder.singerTextView =(TextView)convertView.findViewById(R.id.singerTextView);
                viewHolder.deleteSongBtn =(Button)convertView.findViewById(R.id.deleteSongBtn);
                viewHolder.downLoadSongBtn=(Button)convertView.findViewById(R.id.downLoadSongBtn);

                convertView.setTag(viewHolder);
                viewHolder.deleteSongBtn.setTag(position);
                viewHolder.downLoadSongBtn.setTag(position);
            }else {
                viewHolder =(ViewHolder)convertView.getTag();
            }
                viewHolder.singerImageView.setImageResource((Integer)
                                   ((HashMap)songArrayList.get(position)).get("image"));
                viewHolder.singerTextView.setText((String)((HashMap)songArrayList.get(position)).get("singer"));
                viewHolder.songInfoTextView.setText((String)((HashMap)songArrayList.get(position)).get("songInfo"));

                viewHolder.deleteSongBtn.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        int dePosition = (Integer)v.getTag();
                        songArrayList.remove(dePosition);
                        songAdapter.notifyDataSetChanged();
                    }
                });
                viewHolder.downLoadSongBtn.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        int downLoadPosition =(Integer)v.getTag();
                        String singerStr =(String)((HashMap)songArrayList.get(downLoadPosition)).get("singer");
                        String songStr =(String)((HashMap)songArrayList.get(downLoadPosition)).get("songInfo");
                        String toastStr =singerStr+songStr;
                        Toast.makeText(getApplicationContext(),"你准备下载"+toastStr,Toast.LENGTH_SHORT).show();
                    }
                });
            return convertView;
        }
    }
    //避免了就是每次在getVIew的时候，都需要重新的findViewById，
    // 重新找到控件，然后进行控件的赋值以及事件相应设置。这样其实在做重复的事情)
    class ViewHolder{
        ImageView singerImageView;
        TextView songInfoTextView;
        TextView singerTextView;
        Button deleteSongBtn;
        Button downLoadSongBtn;
    }
}

运行截图