ListView的优化和自定义适配器继承于BaseAdapter

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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<T> extends BaseAdapter{    public   List<T> T;    public MyBaseAdapter(List<T> 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

<?xml version="1.0" encoding="utf-8"?><RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="match_parent"    android:layout_height="match_parent">    <ListView        android:id="@+id/mListView"        android:layout_width="match_parent"        android:layout_height="match_parent" /></RelativeLayout>

每一项的布局

listview_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/singerImageView"        android:layout_width="100dp"        android:layout_height="50dp" />    <TextView        android:id="@+id/songInfoTextView"        android:layout_width="40dp"        android:layout_height="wrap_content"        android:textColor="#DD017F"        android:textSize="10dp" />    <TextView        android:id="@+id/singerTextView"        android:layout_width="70dp"        android:layout_height="wrap_content"        android:textColor="#DD017F"        android:textSize="13dp" />    <!--     ListView的item中加入Button后 导致ListView对OnItemClick事件无法响应     原因是因为Button的事件响应优先级高于List Item,所以屏蔽了ListItem的单击事件     android:focusableInTouchMode="false"     android:clickable="false"   -->    <Button        android:id="@+id/deleteSongBtn"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:background="@android:color/transparent"        android:focusable="false"        android:text="删除"        android:textColor="#025730" />    <!--@android:color/transparent让按钮变透明-->    <Button        android:id="@+id/downLoadSongBtn"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:background="@android:color/transparent"        android:focusable="false"        android:text="下载"        android:textColor="#025730" /></LinearLayout>

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<ArrayList>{        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

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