2.3 自定义BaseAdapter

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一、布局文件

1、在系统自动创建的activity_main.xml中添加一个ListView的布局代码：

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

2、自己新建一个线性布局文件，命名为list_item，添加如下布局代码：

<?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">    <ImageView        android:id="@+id/image"        android:layout_width="60dp"        android:layout_height="60dp"        android:paddingTop="10dp"        android:paddingBottom="10dp"        />    <LinearLayout        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:orientation="vertical"        >        <TextView            android:id="@+id/text1"            android:layout_width="wrap_content"            android:layout_height="wrap_content"            android:textSize="18sp"            />        <TextView            android:id="@+id/text2"            android:layout_width="wrap_content"            android:layout_height="wrap_content"            android:textSize="15sp"            />    </LinearLayout>></LinearLayout>

二、Java代码

1、创建一个Java类，用于处理数据，命名为Animal，添加如下代码：

package com.example.yuancan.test0919002;/** * Created by yuancan on 16/9/19. */public class Animal {    private String aName;    private String aSpeak;    private int aIcon;    public Animal () {    }    public Animal(String aName, String aSpeak, int aIcon) {        this.aName = aName;        this.aSpeak = aSpeak;        this.aIcon = aIcon;    }    public String getaName() {        return aName;    }    public String getaSpeak() {        return aSpeak;    }    public int getaIcon() {        return aIcon;    }    public void setaName(String aName) {        this.aName = aName;    }    public void setaSpeak(String aSpeak) {        this.aSpeak = aSpeak;    }    public void setaIcon(int aIcon) {        this.aIcon = aIcon;    }}

2、创建一个继承BaseAdapter的类，命名为：AnimalAdapter,添加如下代码：

package com.example.yuancan.test0919002;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.LinkedList;/** * Created by yuancan on 16/9/20. */public class AnimalAdapter extends BaseAdapter {    private LinkedList<Animal> mData;    private Context mContext;    public AnimalAdapter(LinkedList<Animal> mData, Context mContext) {        this.mData = mData;        this.mContext = mContext;    }    @Override    public int getCount() {        return mData.size();    }    @Override    public Object getItem(int position) {        return null;    }    @Override    public long getItemId(int position) {        return position;    }    @Override    public View getView(int position, View convertView, ViewGroup parent) {        convertView = LayoutInflater.from(mContext).inflate(R.layout.list_item,parent,false);        ImageView img_icon = (ImageView)convertView.findViewById(R.id.image);        TextView txt_aName = (TextView)convertView.findViewById(R.id.text1);        TextView txt_aSpeak = (TextView)convertView.findViewById(R.id.text2);        img_icon.setBackgroundResource(mData.get(position).getaIcon());        txt_aName.setText(mData.get(position).getaName());        txt_aSpeak.setText(mData.get(position).getaSpeak());        return convertView;    }}

3、在MainActivity中添加代码：

package com.example.yuancan.test0919002;import android.content.Context;import android.support.v7.app.AppCompatActivity;import android.os.Bundle;import android.widget.ListView;import java.util.HashMap;import java.util.LinkedList;import java.util.List;public class MainActivity extends AppCompatActivity {    private List<Animal> mData = null;    private Context mContext;    private AnimalAdapter mAdapter = null;    private ListView list_animal;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        mContext = MainActivity.this;        list_animal = (ListView)findViewById(R.id.listView);        mData = new LinkedList<Animal>();        for (int i=0; i<10; i++) {            mData.add(new Animal("熟悉","兴趣",R.drawable.avator));        }        mAdapter = new AnimalAdapter((LinkedList<Animal>) mData, mContext);        list_animal.setAdapter(mAdapter);    }}

ok ,写完了，最后来看下效果：

这里写图片描述

三、BaseAdapter的优化

我们从代码中可以看出比较重要的两个方法:getCount()和getView()，界面上有多少列就会调用多少次getView，这个时候可能看出一些端倪，每次都是新inflate一个View，都要进行这个XML的解析，这样会很浪费资源，当然，几十列或者几百列的列表并不能体现什么问题，但假如更多或者布局更加复杂？所以学习ListView的优化很重要，而本节针对的是BaseAdapter的优化，优化的两点有，复用convertView 以及使用ViewHolder重用组件，不用每次都findViewById，我们具体通过代码来体会吧！

1、复用ConvertView

界面上有多少个Item，那么getView方法就会被调用多少次！我们来看看上一节我们写的getView()部分的代码：

@Overridepublic View getView(int position, View convertView, ViewGroup parent) {    convertView = LayoutInflater.from(mContext).inflate(R.layout.item_list_animal,parent,false);    ImageView img_icon = (ImageView) convertView.findViewById(R.id.img_icon);    TextView txt_aName = (TextView) convertView.findViewById(R.id.txt_aName);    TextView txt_aSpeak = (TextView) convertView.findViewById(R.id.txt_aSpeak);    img_icon.setBackgroundResource(mData.get(position).getaIcon());    txt_aName.setText(mData.get(position).getaName());    txt_aSpeak.setText(mData.get(position).getaSpeak());    return convertView;}

inflate()每次都要加载一次xml，其实这个convertView是系统提供给我们的可供服用的View 的缓存对象，那就坐下判断咯，修改下，优化后的代码：

@Overridepublic View getView(int position, View convertView, ViewGroup parent) {    if(convertView == null){        convertView = LayoutInflater.from(mContext).inflate(R.layout.item_list_animal,parent,false);    }    ImageView img_icon = (ImageView) convertView.findViewById(R.id.img_icon);    TextView txt_aName = (TextView) convertView.findViewById(R.id.txt_aName);    TextView txt_aSpeak = (TextView) convertView.findViewById(R.id.txt_aSpeak);    img_icon.setBackgroundResource(mData.get(position).getaIcon());    txt_aName.setText(mData.get(position).getaName());    txt_aSpeak.setText(mData.get(position).getaSpeak());    return convertView;}

二、ViewHolder重用组件

getView()会被调用多次，那么findViewById不一样得调用多次，而我们的ListView的Item 一般都是一样的布局，我们可以对这里在优化下，我们可以自己定义一个ViewHolder类来对这一部分进行性能优化！修改后的代码如下：

@Overridepublic View getView(int position, View convertView, ViewGroup parent) {    ViewHolder holder = null;    if(convertView == null){        convertView = LayoutInflater.from(mContext).inflate(R.layout.item_list_animal,parent,false);        holder = new ViewHolder();        holder.img_icon = (ImageView) convertView.findViewById(R.id.img_icon);        holder.txt_aName = (TextView) convertView.findViewById(R.id.txt_aName);        holder.txt_aSpeak = (TextView) convertView.findViewById(R.id.txt_aSpeak);        convertView.setTag(holder);   //将Holder存储到convertView中    }else{        holder = (ViewHolder) convertView.getTag();    }    holder.img_icon.setBackgroundResource(mData.get(position).getaIcon());    holder.txt_aName.setText(mData.get(position).getaName());    holder.txt_aSpeak.setText(mData.get(position).getaSpeak());    return convertView;}static class ViewHolder{    ImageView img_icon;    TextView txt_aName;    TextView txt_aSpeak;}

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