android 下拉树

在 Android 中实现下拉树可以使用 ExpandableListView 控件。ExpandableListView 是 ListView 的扩展，它可以展开和折叠子项，类似于树形结构。

以下是实现下拉树的基本步骤：

1. 准备数据：定义一个包含父项和子项的数据结构。

2. 创建适配器：继承 BaseExpandableListAdapter 类，实现其方法，将数据绑定到 ExpandableListView 上。

3. 初始化 ExpandableListView：在布局文件中定义 ExpandableListView 控件，并在代码中获取它的实例。

4. 设置适配器：将适配器设置给 ExpandableListView 控件。

5. 添加事件监听器：处理父项和子项的点击事件，实现展开和折叠子项的功能。

下面是一个简单的示例代码：

public class MainActivity extends AppCompatActivity {

    private ExpandableListView mExpListView;
    private List<String> mGroupList;
    private Map<String, List<String>> mChildMap;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        initData();

        mExpListView = findViewById(R.id.exp_list_view);
        MyExpandableListAdapter adapter = new MyExpandableListAdapter(this, mGroupList, mChildMap);
        mExpListView.setAdapter(adapter);

        mExpListView.setOnGroupClickListener(new ExpandableListView.OnGroupClickListener() {
            @Override
            public boolean onGroupClick(ExpandableListView parent, View v, int groupPosition, long id) {
                // 处理父项点击事件
                if (mExpListView.isGroupExpanded(groupPosition)) {
                    mExpListView.collapseGroup(groupPosition);
                } else {
                    mExpListView.expandGroup(groupPosition);
                }
                return true;
            }
        });

        mExpListView.setOnChildClickListener(new ExpandableListView.OnChildClickListener() {
            @Override
            public boolean onChildClick(ExpandableListView parent, View v, int groupPosition, int childPosition, long id) {
                // 处理子项点击事件
                String childText = mChildMap.get(mGroupList.get(groupPosition)).get(childPosition);
                Toast.makeText(MainActivity.this, childText, Toast.LENGTH_SHORT).show();
                return true;
            }
        });
    }

    private void initData() {
        mGroupList = new ArrayList<>();
        mChildMap = new HashMap<>();

        mGroupList.add("水果");
        mGroupList.add("蔬菜");

        List<String> fruitList = new ArrayList<>();
        fruitList.add("苹果");
        fruitList.add("香蕉");
        fruitList.add("橙子");
        mChildMap.put("水果", fruitList);

        List<String> vegetableList = new ArrayList<>();
        vegetableList.add("青菜");
        vegetableList.add("白菜");
        vegetableList.add("西红柿");
        mChildMap.put("蔬菜", vegetableList);
    }
}

在布局文件中定义 ExpandableListView 控件：

<ExpandableListView
    android:id="@+id/exp_list_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>

在适配器中实现数据绑定：

public class MyExpandableListAdapter extends BaseExpandableListAdapter {

    private Context mContext;
    private List<String> mGroupList;
    private Map<String, List<String>> mChildMap;

    public MyExpandableListAdapter(Context context, List<String> groupList, Map<String, List<String>> childMap) {
        mContext = context;
        mGroupList = groupList;
        mChildMap = childMap;
    }

    @Override
    public int getGroupCount() {
        return mGroupList.size();
    }

    @Override
    public int getChildrenCount(int groupPosition) {
        return mChildMap.get(mGroupList.get(groupPosition)).size();
    }

    @Override
    public Object getGroup(int groupPosition) {
        return mGroupList.get(groupPosition);
    }

    @Override
    public Object getChild(int groupPosition, int childPosition) {
        return mChildMap.get(mGroupList.get(groupPosition)).get(childPosition);
    }

    @Override
    public long getGroupId(int groupPosition) {
        return groupPosition;
    }

    @Override
    public long getChildId(int groupPosition, int childPosition) {
        return childPosition;
    }

    @Override
    public boolean hasStableIds() {
        return false;
    }

    @Override
    public View getGroupView(int groupPosition, boolean isExpanded, View convertView, ViewGroup parent) {
        String groupText = mGroupList.get(groupPosition);
        if (convertView == null) {
            convertView = LayoutInflater.from(mContext).inflate(R.layout.group_item, parent, false);
        }
        TextView groupTextView = convertView.findViewById(R.id.group_text_view);
        groupTextView.setText(groupText);
        return convertView;
    }

    @Override
    public View getChildView(int groupPosition, int childPosition, boolean isLastChild, View convertView, ViewGroup parent) {
        String childText = mChildMap.get(mGroupList.get(groupPosition)).get(childPosition);
        if (convertView == null) {
            convertView = LayoutInflater.from(mContext).inflate(R.layout.child_item, parent, false);
        }
        TextView childTextView = convertView.findViewById(R.id.child_text_view);
        childTextView.setText(childText);
        return convertView;
    }

    @Override
    public boolean isChildSelectable(int groupPosition, int childPosition) {
        return true;
    }
}

group_item.xml：

<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/group_text_view"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:padding="16dp"
    android:textSize="18sp"/>

child_item.xml：

<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/child_text_view"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:padding="16dp"
    android:textSize="16sp"/>