一.最常用和最难用的控件(ListView)

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ListView允许用户通过手指上下滑动的方式将屏幕外的数据滚动到屏幕内,同时屏幕上原有的数据则会滚动出屏幕.

1. ListView的简单用法

首先新建一个ListViewTest项目,然后修改activity_main.xml代码.

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<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_height="match_parent"
     android:layout_width="match_parent">
    <ListView
        android:id="@+id/list_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"></ListView>
</LinearLayout>

为ListView指定一个id,然后将宽度和高度都修改为match_parent,这样ListView就占据了整个布局的空间.
ListView布局

接下来修改MainActivity中的代码.

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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);
        //先创建适配器,并且把内容放入去.
        ArrayAdapter<String> adapter = new ArrayAdapter<String>
        (MainActivity.this, android.R.layout.simple_list_item_1,data);
        ListView listView = (ListView) findViewById(R.id.list_view);
        //调用ListView的对象把适配器传进去.
        listView.setAdapter(adapter);
    }
}

数组中的数据是无法直接传递给ListView的,我们需要借助适配器来完成,其中最好用的是ArrayAdapter它可以通过泛型来指定要添加的数据类型,然后在构造函数中把要适配的数据传入即可.注意我们使用了android.R.layout.simple_list_item_1作为ListView的子项布局的id,以及要适配的数据.

最后,我们要调用ListView的SetAdapter()方法,将构造好的适配器对象传递进去,这样ListView和数据之间的关联就建立完成了.
ListView与数据关联

2. 定制ListView的界面

接着定义一个实体类,作为ListView适配器的适配类型,新建类Fruit,需要准备一组图片.

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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 int getImageId() {
		return imageId;
	}
}

Fruit类中只有两个字段,name表示水果的名字,imageId表示水果对应图片的资源id.

然后需要为ListView的子项指定一个我们自定义的布局,在layout目录下新建fruit_item.xml

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<?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/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>

在这个布局里,我们定义了一个ImageView用于显示水果的图片,又定义了一个TextView用于显示水果的名称.

接下来需要创建一个自定义的适配器,这个适配器继承自ArrayAdapter,并将泛型指定为Fruit类.新建类FruitAdapter,代码如下:

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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
	public View getView(int position, View convertView, ViewGroup parent) {
		Fruit fruit = getItem(position);
		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;
	}
}

FruitAdapter重写了父类的一组构造函数,用于将上下文,ListView子项布局的id和数据都传递进来.另外又重写了getView()方法,首先通过getItem()方法得到当前项的Fruit的实例,然后使用LayoutInflater来为这个子项加载我们传入的布局,接着调用View的fndViewById()方法分别获取到ImageView和TextView的实例,并分别调用它们的setImageResource和setText方法来设置显示的图片和文字,最好将布局返回.

下面修改MainActivity中的代码,如下所示:

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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();
		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_pic);
		fruitList.add(apple);
		Fruit banana = new Fruit("Banana", R.drawable.banana_pic);
		fruitList.add(banana);
		Fruit orange = new Fruit("Orange", R.drawable.orange_pic);
		fruitList.add(orange);
		Fruit watermelon = new Fruit("Watermelon", R.drawable.watermelon_pic);
		fruitList.add(watermelon);
		Fruit pear = new Fruit("Pear", R.drawable.pear_pic);
		fruitList.add(pear);
		Fruit grape = new Fruit("Grape", R.drawable.grape_pic);
		fruitList.add(grape);
		Fruit pineapple = new Fruit("Pineapple", R.drawable.pineapple_pic);
		fruitList.add(pineapple);
		Fruit strawberry = new Fruit("Strawberry", R.drawable.strawberry_pic);
		fruitList.add(strawberry);
		Fruit cherry = new Fruit("Cherry", R.drawable.cherry_pic);
		fruitList.add(cherry);
		Fruit mango = new Fruit("Mango", R.drawable.mango_pic);
		fruitList.add(mango);
	}
}

可以看到,这里添加了一个initFruits()方法,用于初始化所有水果的数据,在Fruit类构造函数将水果的名字和对应图片id传入,然后把创建好的对象添加到水果列表中,接着我们再onCreate()方法中创建了FruitAdapter对象,并将FruitAdapter作为适配器传递给ListView.

3. 提升ListView的运行效率

因为在FruitAdapter的getView()方法中每次都将布局重新加载了一次,当ListView快速滚动的时候就会成为性能的阻碍.

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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
	public View getView(int position, View convertView, ViewGroup parent) {
		Fruit fruit = getItem(position); // 获取当前项的Fruit实例
		View view;
		if (convertView == null) {
			view = LayoutInflater.from(getContext()).inflate(resourceId, null);
		} else {
			view = convertView;
		}
		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;
	}
}

所以,我们再getView()方法中进行了判断,如果convertView为空,则使用LayoutInflater去加载布局,如果不为空则直接对convertView进行重用.

每次在getView()方法中还是会调用View的findViewById()方法来获取一次控件的实例,我们还可以借助一个ViewHolder来对这部分性能进行优化,修改FruitAdapter`中的代码,如下所示:

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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
	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);
		} else {
			view = convertView;
			viewHolder = (ViewHolder) view.getTag();
		}
		viewHolder.fruitImage.setImageResource(fruit.getImageId());
		viewHolder.fruitName.setText(fruit.getName());
		return view;
	}
	class ViewHolder {
		ImageView fruitImage;
		TextView fruitName;
	}
}

我们新建了一个内部类ViewHolder,用于对控件的实例进行缓存.当convertView为空的时候,创建一个ViewHolder对象,并将控件的实例都存放在ViewHolder里,然后调用View的SetTag()方法,将ViewHolder对象存储在View中.当convertView不为空的时候则调用View的getTag()方法,把ViewHolder重新取出.这样所有控件的实例都缓存在ViewHolder里,就没有必要每次都通过findViewById()方法来获取控件实例了.

4. ListView的点击事件

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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();
	}
});

最终效果图

5. 总结

  • 先在布局中加入ListView控件
  • 然后自定义适配器,这个适配器继承自ArrayAdapter
  • 初始化数据,把数据传入自定义适配器
  • 然后将适配器传递给ListView.