【android学习随笔】android列表组件ListView

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添加xml布局

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

定义item实体类

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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;
    }

}

定义item布局

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<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="wrap_content">
    <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listview自定义适配器

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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 = LayoutInflater.from(getContext()).inflate(resourceId, 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;
    } 
}

引入activity活动

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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(); // 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() {
        for (int i = 0; i < 2; i++) {
            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);
        } 
    }
}

优化listview

通过对convertView缓存是否为空进行判断是否重载

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public class FruitAdapter extends ArrayAdapter<Fruit> {
    ...
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        Fruit fruit = getItem(position);
        View view;
        if (convertView == null) {
            view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
        } 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;
    }
}

消除findViewById重复获取组件实例的开销

  1. 新增内部类ViewHolder，用于对控件的实例进行缓存。
  2. 当 convertView 为 null 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
  3. 当 convertView 不为 null 的时候， 则调用 View 的 getTag()方法，把 ViewHolder 重新取出。
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public class FruitAdapter extends ArrayAdapter<Fruit> {
    ...
    @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, parent, false);
            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;
    }
}

添加点击事件

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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(); // 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);
        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, fruit.getName(), Toast.LENGTH_SHORT).show();
            }
        });
    }
    ...
}