Android控件学习（九）——ListView

新建一个空项目，命名为UIListView

一、简单用法

修改主布局：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">
    
    <ListView
        android:id="@+id/list_view"
        android:layout_height="match_parent"
        android:layout_width="match_parent"
        />

</LinearLayout>

修改主活动：

public class MainActivity extends AppCompatActivity {
    private String[] Data = {"苹果", "香蕉", "橙子", "西瓜", "梨", "葡萄", "菠萝", "草莓", "樱桃", "芒果", "苹果", "香蕉", "橙子", "西瓜", "梨", "葡萄", "菠萝", "草莓", "樱桃", "芒果"};

    @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);
    }
}

使用了一个Data数组存放水果名，然后用适配器将数据传递给ListView。
ArrayAdapter用泛型指定适配的数据类型，然后在构造函数中把要适配的数据传入：

• 传递的都是字符串，所以泛型指定为String
• 参数一：当前上下文
• 参数二：子项布局的ID，android.R.layout.simple_list_item_1是Android内置布局，里面只有一段TextView，用于简单地显示一段文本
• 参数三：要适配的数据

最后用setAdapter()将数据传递进去

运行：

二、定制ListView界面

定义个Fruit类

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;
    }
}

接着自定义一个fruit_item布局

<?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="wrap_content">

    <ImageView
        android:id="@+id/fruite_image"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        />
    <TextView
        android:id="@+id/fruite_name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_vertical"
        android:layout_marginLeft="10dp"
        />

</LinearLayout>

其中涉及三个知识点：

• 《Android布局学习（一）——线性布局LinearLayout》
• 《Android控件学习（四）——ImageView》
• 《Android控件学习（一）——TextView》

接下来需要自定义一个适配器FruitAdaper，继承自ArrayAdapter，并将泛型指向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;
    }

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
        Fruit fruit = getItem(position); // 获取当前项的Fruit实例
        @SuppressLint("ViewHolder") View view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
        ImageView fruitImage = view.findViewById(R.id.fruite_image);
        TextView fruitName = view.findViewById(R.id.fruite_name);
        fruitImage.setImageResource(fruit.getImageId());
        fruitName.setText(fruit.getName());
        return view;
    }
}

FruitAdaper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。另外有又重写了getView()，这个方法在每个子贤那个被滚动到屏幕内时会被调用。在这个方法中，先用getItem()得到当前项的Fruit实例，然后使用LayoutInflater加载传入的布局。
关于LayoutInflater的前两个参数和两个方法参考这里
这里LayoutInflater的inflate方法接收三个参数，第三个参数指定False，表示只让我们在父布局中声明的layout属性生效。但不会为这个View添加父布局，因为一旦View有了父布局后，他就不能再添加到ListView中了。

接着继续修改MainActivity：

public class MainActivity extends AppCompatActivity {
    
    private List<Fruit> 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 = findViewById(R.id.list_view);
        listView.setAdapter(adapter);
    }

    private void initFruits(){
        for (int i = 0; i < 2; i++){
            Fruit apple = new Fruit("苹果", R.drawable.apple);
            fruitList.add(apple);
            Fruit banana = new Fruit("香蕉", R.drawable.banana);
            fruitList.add(banana);
            Fruit orange = new Fruit("橙子", R.drawable.orange);
            fruitList.add(orange);
            Fruit watermelon = new Fruit("西瓜", R.drawable.watermelon);
            fruitList.add(watermelon);
            Fruit pear = new Fruit("梨", R.drawable.pear);
            fruitList.add(pear);
            Fruit grape = new Fruit("葡萄", R.drawable.grape);
            fruitList.add(grape);
            Fruit pineapple = new Fruit("菠萝", R.drawable.pineapple);
            fruitList.add(pineapple);
            Fruit strawberry = new Fruit("草莓", R.drawable.strawberry);
            fruitList.add(strawberry);
            Fruit cherry = new Fruit("樱桃", R.drawable.cherry);
            fruitList.add(cherry);
            Fruit mango = new Fruit("芒果", R.drawable.mango);
            fruitList.add(mango);
        }
    }
}

添加了一个initFruits()方法，用来初始化水果数据，将图片id和水果名传入，再把构造好的水果对象添加到水果列表中。最后将水果列表转换到FruitAdapter中，定制的界面就完成了：

三、提升 ListView的运行效率

之前自定义的适配器FruitAdaper中重写了getView()，这个方法在每个子贤那个被滚动到屏幕内时会被调用。当ListView快速滚动时，就会很卡顿。
那么怎么提升效率呢？

1、convertView缓存

getView()方法有个convertView参数，用来将之前加载好的布局进行缓存，以便之后重用：

@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull 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, parent, false);
    }else{
        view = convertView;
    }
    ImageView fruitImage = view.findViewById(R.id.fruite_image);
    TextView fruitName = view.findViewById(R.id.fruite_name);
    fruitImage.setImageResource(fruit.getImageId());
    fruitName.setText(fruit.getName());
    return view;
    }

2、ViewHolder缓存

现在，虽然不会重复加载布局，但是每次还是会findViewById()来获取一次控件的实例，所以要借助ViewHolder来优化：

@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
    Fruit fruit = getItem(position); // 获取当前项的Fruit实例
    View view;
    ViewHolder viewHolder;
    if (convertView == null) {
        view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
        viewHolder = new ViewHolder();
        viewHolder.fruitImage = (ImageView) view.findViewById(R.id.fruite_image);
        viewHolder.fruitName = (TextView) view.findViewById(R.id.fruite_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;
}

新增一个内部类ViewHolder，用于对实例进行缓存，将实例对象通过View.setTag()存在View中，这样就没必要每次都findViewById()了

四、点击事件

通过setOnItemClickListener()方法注册监听器：

@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 = findViewById(R.id.list_view);
    listView.setAdapter(adapter);
    
    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            Fruit fruit = fruitList.get(position);
            Toast.makeText(MainActivity.this, "你点击了 "+fruit.getName(), Toast.LENGTH_SHORT).show();
        }
    });
}

运行：