BaseAdapter数据适配器

ListView的显示和缓存机制

需要才显示，显示完就回收到缓存

基本结构

• public int getCount() 适配器中数据集中数据的个数
• public Object getItem(int position) 获取数据集中与指定索引对应的数据项
• public long getItemId(int positon) 获取指定行对应的ID
• public View getView(int position, View convertView, ViewGroup parent) 获取每一个Item的内容

ViewHolder优化BaseAdapter的思路

• 创建Bean对象，用于封装数据
• 在构造方法中初始化用于映射的数据List
• 创建ViewHolder类，创建布局映射关系
• 判断convertView，为空则创建，并设置tag，否则通过tag来取出ViewHolder
• 给ViewHolder中的控件设置数据

代码示例：

主界面新建ListView

<ListView    android:id="@+id/lv_main"          android:layout_width="match_parent"    android:layout_height="match_parent"></ListView>

新建item.xml

<?xml version="1.0" encoding="utf-8"?><RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:orientation="vertical" android:layout_width="match_parent"    android:layout_height="match_parent">    <ImageView        android:id="@+id/iv_image"        android:layout_width="60dp"        android:layout_height="60dp"        android:src="@drawable/banshee"/>    <TextView        android:id="@+id/tv_title"        android:layout_width="match_parent"        android:layout_height="30dp"        android:layout_toRightOf="@+id/iv_image"        android:text="Title"        android:gravity="center"        android:textSize="16sp"/>    <TextView        android:id="@+id/tv_text"        android:layout_width="match_parent"        android:layout_height="30dp"        android:layout_toRightOf="@+id/iv_image"        android:layout_below="@+id/tv_title"        android:text="Hello World"        android:gravity="center"        android:textSize="12sp"/></RelativeLayout>

新建ItemBean.java

package com.bcp.galleryandimageswitcher;import android.widget.ImageView;import android.widget.TextView;public class ItemBean {    public int ItemImageResid;    public String ItemTitle, ItemContent;    public ItemBean(int itemImageResid,String itemTitle,String itemContent){        ItemImageResid=itemImageResid;        ItemTitle=itemTitle;        ItemContent=itemContent;    }}

新建Myadapter.java

package com.bcp.galleryandimageswitcher;import android.content.Context;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;import android.widget.BaseAdapter;import android.widget.ImageView;import android.widget.TextView;import java.util.List;public class MyAdapter extends BaseAdapter{    private List<ItemBean> mList;    private LayoutInflater mInflater;    public MyAdapter(Context context,List<ItemBean> list){        mList=list;        mInflater=LayoutInflater.from(context);//context：要使用当前adapter的界面对象        //mInflater 布局装载器对象    }    @Override    public int getCount() {        return mList.size();//返回ListView需要显示的数据量    }    @Override    public Object getItem(int position) {        return mList.get(position);    }    @Override    public long getItemId(int position) {        return position;    }    //返回每一项的显示内容    @Override    public View getView(int position, View convertView, ViewGroup parent) {        //效率低下的方式,每次都创建新的view，设置控件，效率很低//        View view=mInflater.inflate(R.layout.item,null);//        ImageView imageView= (ImageView) view.findViewById(R.id.iv_image);//        TextView title= (TextView) view.findViewById(R.id.tv_title);//        TextView content= (TextView) view.findViewById(R.id.tv_text);//        ItemBean bean=mList.get(position);//        imageView.setImageResource(bean.ItemImageResid);//        title.setText(bean.ItemTitle);//        content.setText(bean.ItemContent);//        return view;        //普通式 利用了ListView的缓存特性，如果没有缓存才创建新的View        //但findViewById还是会很费时//        if (convertView==null){//view未被实例化，缓存池中无缓存//            convertView=mInflater.inflate(R.layout.item,null);//        }//        ImageView imageView= (ImageView) convertView.findViewById(R.id.iv_image);//        TextView title= (TextView) convertView.findViewById(R.id.tv_title);//        TextView content= (TextView) convertView.findViewById(R.id.tv_text);//        ItemBean bean=mList.get(position);//        imageView.setImageResource(bean.ItemImageResid);//        title.setText(bean.ItemTitle);//        content.setText(bean.ItemContent);//        return convertView;        //最终方式        //利用了ListView的缓存，并且通过ViewHolder类实现显示数据的视图缓存，避免多次findViewById        ViewHolder viewHolder;//声明一个对象，包含所需的三个控件，避免重复findViewById操作        if (convertView==null){            viewHolder=new ViewHolder();            convertView=mInflater.inflate(R.layout.item,null);            viewHolder.imageView=(ImageView) convertView.findViewById(R.id.iv_image);            viewHolder.title=(TextView) convertView.findViewById(R.id.tv_title);            viewHolder.content=(TextView) convertView.findViewById(R.id.tv_text);            convertView.setTag(viewHolder);        }else{            viewHolder= (ViewHolder) convertView.getTag();        }        ItemBean bean=mList.get(position);        viewHolder.imageView.setImageResource(bean.ItemImageResid);        viewHolder.title.setText(bean.ItemTitle);        viewHolder.content.setText(bean.ItemContent);        return convertView;    }    //最终方式使用的内部类    class ViewHolder{        public ImageView imageView;        public TextView title,content;    }}

修改MainActivity.java

package com.bcp.galleryandimageswitcher;import android.support.v7.app.AppCompatActivity;import android.os.Bundle;import android.widget.Gallery;import android.widget.ListView;import android.widget.SeekBar;import android.widget.TextView;import java.util.ArrayList;import java.util.List;public class MainActivity extends AppCompatActivity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        List<ItemBean> itemBeanList=new ArrayList<>();        for (int i=0;i<20;i++){            itemBeanList.add(new ItemBean(                    R.drawable.banshee,"我是标题i","我是内容i"            ));        }        ListView listView= (ListView) findViewById(R.id.lv_main);        listView.setAdapter(new MyAdapter(this,itemBeanList));    }}