给我写一个android日历控件的demo

好的，以下是一个简单的Android日历控件的demo：

1. 在布局文件中添加日历控件：

<com.example.calendarview.CalendarView
    android:id="@+id/calendar_view"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />

2. 创建一个CalendarView类，并继承自ViewGroup，实现日历控件的布局和初始化：

public class CalendarView extends ViewGroup {

    private Calendar calendar;
    private int year, month, day;
    private int selectedYear, selectedMonth, selectedDay;
    private OnDateSelectedListener listener;

    public CalendarView(Context context) {
        super(context);
        init();
    }

    public CalendarView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public CalendarView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    private void init() {
        calendar = Calendar.getInstance();
        year = calendar.get(Calendar.YEAR);
        month = calendar.get(Calendar.MONTH);
        day = calendar.get(Calendar.DAY_OF_MONTH);
        selectedYear = year;
        selectedMonth = month;
        selectedDay = day;
        setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                showDatePicker();
            }
        });
    }

    private void showDatePicker() {
        DatePickerDialog dialog = new DatePickerDialog(getContext(), new DatePickerDialog.OnDateSetListener() {
            @Override
            public void onDateSet(DatePicker view, int year, int month, int dayOfMonth) {
                selectedYear = year;
                selectedMonth = month;
                selectedDay = dayOfMonth;
                calendar.set(year, month, dayOfMonth);
                if (listener != null) {
                    listener.onDateSelected(year, month, dayOfMonth);
                }
                invalidate();
            }
        }, year, month, day);
        dialog.show();
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        // 布局日历控件的子View
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // 测量日历控件的宽高
    }

    public void setOnDateSelectedListener(OnDateSelectedListener listener) {
        this.listener = listener;
    }

    public interface OnDateSelectedListener {
        void onDateSelected(int year, int month, int day);
    }
}

3. 在布局文件中添加一个TextView，用于显示选中的日期：

<TextView
    android:id="@+id/date_text"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textSize="20sp" />

4. 在Activity中初始化日历控件和TextView，并设置日期选择监听器：

public class MainActivity extends AppCompatActivity {

    private CalendarView calendarView;
    private TextView dateText;

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

        calendarView = findViewById(R.id.calendar_view);
        dateText = findViewById(R.id.date_text);

        calendarView.setOnDateSelectedListener(new CalendarView.OnDateSelectedListener() {
            @Override
            public void onDateSelected(int year, int month, int day) {
                dateText.setText(year + "-" + (month + 1) + "-" + day);
            }
        });
    }
}

这样就完成了一个简单的Android日历控件的demo。当点击日历控件时，弹出DatePickerDialog供用户选择日期，选择完后更新界面并回调给监听器。