范例HelloDatePicker

Android docs中的范例，简单翻译整理如下。
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1、创建新项目，名称HelloDatePicker；

2、修改res/layout/main.xml，内容如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="vertical">
    <TextView android:id="@+id/dateDisplay"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text=""/>
    <Button android:id="@+id/pickDate"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Change the date"/>
</LinearLayout>

3、修改HelloDatePicker.java，增加成员变量定义，代码如下：

    private TextView mDateDisplay;
    private Button mPickDate;
    private int mYear;
    private int mMonth;
    private int mDay;

    static final int DATE_DIALOG_ID = 0;

4、修改HelloDatePicker.java的onCreate方法，内容如下：

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        // capture our View elements
        mDateDisplay = (TextView) findViewById(R.id.dateDisplay);
        mPickDate = (Button) findViewById(R.id.pickDate);

        // add a click listener to the button
        mPickDate.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                showDialog(DATE_DIALOG_ID);
            }
        });

        // get the current date
        final Calendar c = Calendar.getInstance();
        mYear = c.get(Calendar.YEAR);
        mMonth = c.get(Calendar.MONTH);
        mDay = c.get(Calendar.DAY_OF_MONTH);

        // display the current date (this method is below)
        updateDisplay();
    }

5、修改HelloDatePicker.java增加updateDisplay方法，内容如下：

    // updates the date in the TextView
    private void updateDisplay() {
        mDateDisplay.setText(
            new StringBuilder()
                    // Month is 0 based so add 1
                    .append(mMonth + 1).append("-")
                    .append(mDay).append("-")
                    .append(mYear).append(" "));
    }

6、修改HelloDatePicker.java增加日期选择对话框，内容如下：

    // the callback received when the user "sets" the date in the dialog
    private DatePickerDialog.OnDateSetListener mDateSetListener =
            new DatePickerDialog.OnDateSetListener() {

                public void onDateSet(DatePicker view, int year, 
                                      int monthOfYear, int dayOfMonth) {
                    mYear = year;
                    mMonth = monthOfYear;
                    mDay = dayOfMonth;
                    updateDisplay();
                }
            };

7、修改HelloDatePicker.java增加onCreateDialog方法，内容如下：

@Override
protected Dialog onCreateDialog(int id) {
    switch (id) {
    case DATE_DIALOG_ID:
        return new DatePickerDialog(this,
                    mDateSetListener,
                    mYear, mMonth, mDay);
    }
    return null;
}

8、运行项目，即可看到运行效果，如下图所示：