Android第一行代码(第二版)

3.5最常用和最难用的控件----ListView

ListView非常常见，也是Android中最常见的空间之一，几乎所有程序都会用到，QQ聊天，刷微博，诸如此类。
ListView用法也是比较复杂。下面以一个小项目来练习ListView。
新建一个ListViewTest项目，默认设置。

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

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ArrayAdapter adapter = new ArrayAdapter(
                MainActivity.this,android.R.layout.simple_list_item_1,data);
        ListView listView = (ListView) findViewById(R.id.list_view);
        listView.setAdapter(adapter);

    }
}

如图所示，先提供好数据，String类型的数组数据，数组中的数据无法直接传递给ListView的，要借助适配器来完成。这里用ArrayAdapter。它可通过泛型来指定要适配的实现类，然后在构造函数中把适配的数据传入。
ArrayAdapter有多个构造函数重载，这里把ArrayAdapter泛型指定为String，再在ArrayAdapter构造函数中依次传入当前上下文、ListView子项布局的id、以及要适配的数据。使用了android.R.layout.simple_list_item_1作为ListView的子项布局id，这是Android内部的布局文件，里面只有一个TextView，可用于简单显示一段文本。这样适配器对象就构建好了。
3.5.2定制ListView界面
准备好图片，对应上面提供的每一种水果。让水果名称的旁边都有一个图样。

定义一个实体类，作为List适配器的适配类型。新建Fruit类，代码如下：

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;
    }
}

然后为了ListView的子项指定一个自定义的布局，新建fruit_item.xml，

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" 
    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_vertical"
        android:layout_marginLeft="10dp"/>

LinearLayout>

创建一个自定义适配器，继承自ArrayAdapter，将泛型指定为Fruit类。新建类FruitAdapter。

public class FruitAdapter extends ArrayAdapter {
    private int resourcedId;

    public FruitAdapter(Context context, int textViewResourceId, List objects) {

        super(context,textViewResourceId,objects);
        resourcedId = textViewResourceId;

    }

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView,  ViewGroup parent) {
        Fruit fruit = getItem(position);
        View view = LayoutInflater.from(getContext()).inflate(resourcedId,parent,
                false);
        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;
    }
}

修改MainActivity代码：

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

    @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() {
        for (int i = 0;i<2;i++) {
            Fruit apple = new Fruit("Apple",R.drawable.apple_pic);
            fruitList.add(apple);
            Fruit banana = new Fruit("Apple",R.drawable.banana_pic);
            fruitList.add(banana);
            Fruit orange = new Fruit("Apple",R.drawable.orange_pic);
            fruitList.add(orange);
            Fruit watermelon = new Fruit("Apple",R.drawable.watermelon_pic);
            fruitList.add(watermelon);
            Fruit pear = new Fruit("Apple",R.drawable.pear_pic);
            fruitList.add(pear);
            Fruit grape = new Fruit("Apple",R.drawable.grape_pic);
            fruitList.add(grape);
            Fruit pineapple = new Fruit("Apple",R.drawable.pineapple_pic);
            fruitList.add(pineapple);
            Fruit strawberry = new Fruit("Apple",R.drawable.strawberry_pic);
            fruitList.add(strawberry);
            Fruit cherry = new Fruit("Apple",R.drawable.cherry_pic);
            fruitList.add(cherry);
            Fruit mango = new Fruit("Apple",R.drawable.mango_pic);
            fruitList.add(mango);
        }
    }
}