写一个用Android 的日历源码

好的，下面是一个简单的 Android 日历应用的源代码示例，你可以参考它来编写你自己的应用：

首先是布局文件 activity_main.xml，用来显示日历和日期选择器：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <!-- 日历 -->
    <com.example.calendarview.CalendarView
        android:id="@+id/calendar_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp" />

    <!-- 日期选择器 -->
    <NumberPicker
        android:id="@+id/date_picker"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/calendar_view"
        android:layout_centerHorizontal="true" />

</RelativeLayout>

接下来是 CalendarView.java 文件，实现了日历的显示和点击事件：

public class CalendarView extends GridView {

    private static final int DAYS_COUNT = 42;

    private Context mContext;
    private Calendar mCalendar;
    private CalendarAdapter mAdapter;

    public CalendarView(Context context, AttributeSet attrs) {
        super(context, attrs);
        mContext = context;
        mCalendar = Calendar.getInstance();
        mAdapter = new CalendarAdapter(mContext, mCalendar);
        setAdapter(mAdapter);
        setOnItemClickListener(new OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                // 处理点击事件
                mAdapter.setSelected(position);
                mAdapter.notifyDataSetChanged();
            }
        });
    }

    private class CalendarAdapter extends BaseAdapter {

        private LayoutInflater mInflater;
        private Calendar mCalendar;
        private int mSelectedPosition;

        public CalendarAdapter(Context context, Calendar calendar) {
            mInflater = LayoutInflater.from(context);
            mCalendar = calendar;
            mSelectedPosition = -1;
        }

        public void setSelected(int position) {
            mSelectedPosition = position;
        }

        @Override
        public int getCount() {
            return DAYS_COUNT;
        }

        @Override
        public Object getItem(int position) {
            return null;
        }

        @Override
        public long getItemId(int position) {
            return 0;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            if (convertView == null) {
                convertView = mInflater.inflate(R.layout.calendar_item, parent, false);
            }
            TextView textView = (TextView) convertView.findViewById(R.id.calendar_text);
            int day = position - mCalendar.get(Calendar.DAY_OF_WEEK) + 2;
            if (day < 1 || day > mCalendar.getActualMaximum(Calendar.DAY_OF_MONTH)) {
                textView.setText("");
            } else {
                textView.setText(String.valueOf(day));
            }
            if (position == mSelectedPosition) {
                textView.setTextColor(Color.RED);
            } else {
                textView.setTextColor(Color.BLACK);
            }
            return convertView;
        }
    }
}

接下来是 calendar_item.xml 文件，用来显示日历中的每一天：

<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/calendar_text"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:textSize="20sp" />

最后是 MainActivity.java 文件，实现了日期选择器的显示和选择事件：

public class MainActivity extends AppCompatActivity {

    private CalendarView mCalendarView;
    private NumberPicker mDatePicker;

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

        mCalendarView = (CalendarView) findViewById(R.id.calendar_view);
        mDatePicker = (NumberPicker) findViewById(R.id.date_picker);
        mDatePicker.setMinValue(1);
        mDatePicker.setMaxValue(31);
        mDatePicker.setValue(1);
        mDatePicker.setOnValueChangedListener(new NumberPicker.OnValueChangeListener() {
            @Override
            public void onValueChange(NumberPicker picker, int oldVal, int newVal) {
                // 处理日期选择事件
                mCalendarView.setSelection(newVal + mCalendarView.getFirstVisiblePosition() - 2);
            }
        });
    }
}

这就是一个简单的 Android 日历应用的源代码示例，希望能对你有所帮助。