2.3 自定义BaseAdapter

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：

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：

@Override
public 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：

@Override
public 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：

@Override
public 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;
}