[移动开发]Android基础——ListView

ListView是什么？

ListView以列表的形式展现数据，用户可通过对屏幕的上下滑动加载数据。

纯列表ListView

在activity_main.xml中加入ListView：

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

修改MainActivity，用系统自带的ArrayAdapter，参数分别为Context、适配布局id和要适配的数据，这里适配布局采用内置布局android.R.layout.simple_list_item_1

public class MainActivity extends AppCompatActivity {

    private String[] data = {"A", "B", "C", "D", "E", "F", "G", "H"};

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ArrayAdapter<String> adapter = new ArrayAdapter<>(MainActivity.this, android.R.layout.simple_list_item_1, data);
        ListView listView = findViewById(R.id.list_view);
        listView.setAdapter(adapter);
    }
}

效果图：

定制ListView

准备数据，创建名为Data的bean类：

public class Data {
    private String name;
    private int imageId;

    public Data(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;
    }
}

准备ListView的适配布局，创建list_item.xml：

<?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/list_iv"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>
    <TextView
        android:id="@+id/list_tv"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>
</LinearLayout>

创建数据适配器，重写构造器暂存数据、重写getView()方法加载布局并获取控件设置数据：

public class DataAdapter extends ArrayAdapter<Data> {

    private int resourceId;

    public DataAdapter(@NonNull Context context, int resource, @NonNull List<Data> objects) {
        super(context, resource, objects);
        resourceId = resource;
    }

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
        Data data = getItem(position);
        View view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
        ImageView dataIv = view.findViewById(R.id.list_iv);
        TextView dataTv = view.findViewById(R.id.list_tv);
        dataIv.setImageResource(data.getImageId());
        dataTv.setText(data.getName());
        return view;
    }
}

找到ListView并为其添加适配器：

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

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initData();
        DataAdapter adapter = new DataAdapter(MainActivity.this, R.layout.list_item, dataList);
        ListView listView = findViewById(R.id.list_view);
        listView.setAdapter(adapter);

    }

    private void initData() {
        for (int i = 0; i < 5; i++) {
            Data data = new Data("data" + i, R.mipmap.ic_launcher);
            dataList.add(data);
        }
    }
}

提升ListView效率

在上面DataAdapter的getView()方法中，每次都会将布局重新加载，当快速滑动时，可能会出现问题。与此同时，每次加载都会调用findViewById获取控件实例，大大影响ListView效率。

解决办法：

1. 通过getView()中的convertView参数，可以缓存加载好的布局，避免重复加载。
2. 创建内部类ViewHolder暂存控件实例

修改后的Adapter如下：

public class DataAdapter extends ArrayAdapter<Data> {

    private int resourceId;

    public DataAdapter(@NonNull Context context, int resource, @NonNull List<Data> objects) {
        super(context, resource, objects);
        resourceId = resource;
    }

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
        Data data = getItem(position);
        View view;
        ViewHolder viewHolder;
        if (convertView == null) {
            view = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
            viewHolder = new ViewHolder();
            viewHolder.dataIv = view.findViewById(R.id.list_iv);
            viewHolder.dataTv = view.findViewById(R.id.list_tv);
            view.setTag(viewHolder);
        } else {
            view = convertView;
            viewHolder = (ViewHolder) view.getTag();
        }

        viewHolder.dataIv.setImageResource(data.getImageId());
        viewHolder.dataTv.setText(data.getName());
        return view;
    }

    private class ViewHolder {
        ImageView dataIv;
        TextView dataTv;
    }
}

ListView点击事件

listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Toast.makeText(MainActivity.this, dataList.get(position).getName(), Toast.LENGTH_SHORT).show();
            }
        });