ListView使用及性能优化

一、ListView是Android中使用最广泛的一类控件，首先介绍其基本使用：

1、布局文件activity_main.xml中添加一个ListView组件

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >

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

</LinearLayout>

2、主活动MainActivity中操作ListView，创建并设置适配器adapter，泛型类型为数据类型，形参为上下文context、列表项布局、列表数据（此处列表数据为数组类型）

public class MainActivity extends Activity {
private String[] data = { "Apple", "Banana", "Orange", "Watermelon",
"Pear", "Grape", "Pineapple", "Strawberry", "Cherry", "Mango" };

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ArrayAdapter<String> adapter = new ArrayAdapter<String>(
MainActivity.this, android.R.layout.simple_list_item_1, data);
ListView listView = (ListView) findViewById(R.id.list_view);
listView.setAdapter(adapter);
}
}

以上式ListView的简单用法，显示列表项为字符串

二、下面自定义列表项，实现复杂界面

1、自定义列表项布局fruit_item.xml文件，显示一个图片和一个文本

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >

<ImageView
android:id="@+id/fruit_image"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />

<TextView
android:id="@+id/fruit_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:layout_marginLeft="10dip" />

</LinearLayout>

2、定义列表项类--即适配器的适配类型

public class Fruit {
private String name;
private int imageId;

public Fruit(String name, int imageId) {
this.name = name;
this.imageId = imageId;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public int getImageId() {
return imageId;
}

public void setImageId(int imageId) {
this.imageId = imageId;
}
}

3、以下步骤为关键点，创建一个自定义的适配器FruitAdapter，继承自ArrayAdapter，并将泛型--即适配器的适配类型指定为fruit 类

public class FruitAdapter extends ArrayAdapter<Fruit> {
private int resourceId;

public FruitAdapter(Context context, int textViewResourceId,
List<Fruit> objects) {
super(context, textViewResourceId, objects);
resourceId = textViewResourceId;
}

@Override
//getView（）方法在子项被滚动到屏幕的时候会被调用
public View getView(int position, View convertView, ViewGroup parent) {
Fruit fruit = getItem(position); // 获取当前项的Fruit实例
View view = LayoutInflater.from(getContext()).inflate(resourceId, null);
ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);
fruitImage.setImageResource(fruit.getImageId());
fruitName.setText(fruit.getName());
return view;
}
}

4、主活动中操作，此时数据格式为列表类型，注意数据列表一定要初始化。

public class MainActivity extends Activity {
private List<Fruit> fruitList = new ArrayList<Fruit>();

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initFruits(); // 初始化水果数据
FruitAdapter adapter = new FruitAdapter(MainActivity.this,
R.layout.fruit_item, fruitList);
ListView listView = (ListView) findViewById(R.id.list_view);
listView.setAdapter(adapter);
}

private void initFruits() {
Fruit apple = new Fruit("Apple", R.drawable.apple);
fruitList.add(apple);
Fruit banana = new Fruit("Banana", R.drawable.banana);
fruitList.add(banana);
Fruit caomei = new Fruit("caomei", R.drawable.caomei);
fruitList.add(caomei);
Fruit chengzi = new Fruit("chengzi", R.drawable.chengzi);
fruitList.add(chengzi);
Fruit fanqie = new Fruit("fanqie", R.drawable.fanqie);
fruitList.add(fanqie);
Fruit li = new Fruit("li", R.drawable.li);
fruitList.add(li);
Fruit lizi = new Fruit("lizi", R.drawable.lizi);
fruitList.add(lizi);
Fruit mangguo = new Fruit("mangguo", R.drawable.mangguo);
fruitList.add(mangguo);
Fruit mihoutao = new Fruit("mihoutao", R.drawable.mihoutao);
fruitList.add(mihoutao);
Fruit orange = new Fruit("orange", R.drawable.orange);
fruitList.add(orange);
Fruit xinzi = new Fruit("xinzi", R.drawable.xinzi);
fruitList.add(xinzi);
Fruit yintao = new Fruit("yintao", R.drawable.yintao);
fruitList.add(yintao);

}
}

5、为列表项添加点击事件

.......
listView.setAdapter(adapter);
//设置列表项点击事件
listView.setOnItemClickListener(new OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view,
int position, long id) {
Fruit fruit = fruitList.get(position);
Toast.makeText(MainActivity.this, fruit.getName(),
Toast.LENGTH_SHORT).show();
}
});
......

三、性能优化：主活动中，getView()方法每次都会将布局重新加载一遍，使得ListView在快速滑动时产生瓶颈，并且要调用View 的findViewById()方法来获取一次控件的实例。现在使用getView()方法的convertView参数缓存布局，新建ViewHolder类缓存控件的实例以优化性能

public class FruitAdapter extends ArrayAdapter<Fruit> {
private int resourceId;

public FruitAdapter(Context context, int textViewResourceId,
List<Fruit> objects) {
super(context, textViewResourceId, objects);
resourceId = textViewResourceId;
}

@Override
// 在子项被滚动到屏幕的时候会被调用
public View getView(int position, View convertView, ViewGroup parent) {
Fruit fruit = getItem(position);
View view;
ViewHolder viewHolder;
if (convertView == null) {
view = LayoutInflater.from(getContext()).inflate(resourceId, null);
viewHolder = new ViewHolder();
viewHolder.fruitImage = (ImageView) view
.findViewById(R.id.fruit_image);
viewHolder.fruitName = (TextView) view
.findViewById(R.id.fruit_name);
view.setTag(viewHolder); // 将ViewHolder存储在View中
} else {
view = convertView;
viewHolder = (ViewHolder) view.getTag(); // 重新获取ViewHolder
}
viewHolder.fruitImage.setImageResource(fruit.getImageId());
viewHolder.fruitName.setText(fruit.getName());
return view;
}

class ViewHolder {
ImageView fruitImage;
TextView fruitName;
}
}