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;
	}
}