BaseAdapter数据适配器

Posted 2023-02-07 故意的是吧

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));