Android自定义数字图标向上、向下按钮背景

我要换号码。乘客的上下按钮

。。。我试过numberpicker.getchildat

但它返回空值

xml

我怎样才能上下改变numberpicker背景？

---

请帮帮我：

你为什么不使用android time picker？因为我不知道如何更改android numberpicker的上下btn背景。。

<LinearLayout
    android:layout_width="0dip"
    android:layout_height="match_parent"
    android:layout_weight="10"
    android:gravity="center">

    <NumberPicker
        android:id="@+id/mypage_hoursetting_np"
        android:layout_width="wrap_content" 
        android:layout_height="wrap_content"
        android:divider="@null"/>

</LinearLayout>

mypage_hoursetting_np = (NumberPicker)view.findViewById(R.id.mypage_hoursetting_np);
mypage_minutesetting_np = (NumberPicker)view.findViewById(R.id.mypage_minutesetting_np);

...

public void setNumberpicker(){

    View hour_upbutton = mypage_hoursetting_np.getChildAt(0);
    View hour_text = mypage_hoursetting_np.getChildAt(1);          // null
    View hour_downbutton = mypage_hoursetting_np.getChildAt(2);    // null

    View minute_upbutton = mypage_minutesetting_np.getChildAt(0);
    View minute_text = mypage_minutesetting_np.getChildAt(1);      // null
    View minute_downView = mypage_minutesetting_np.getChildAt(2);  // null

    mypage_hoursetting_np.setMaxValue(12);
    mypage_hoursetting_np.setMinValue(1);
    mypage_hoursetting_np.setWrapSelectorWheel(true);

    mypage_minutesetting_np.setMaxValue(60);
    mypage_minutesetting_np.setMinValue(1);
    mypage_minutesetting_np.setWrapSelectorWheel(true);

    Drawable alarmup_image = new BitmapDrawable(getActivity().getResources(), ImageDecoder.decodeSampledBitmapFromResource(getActivity().getResources(), R.drawable.mypage_alarmset_up_image, 200, 200));
    Drawable alarmdown_image = new BitmapDrawable(getActivity().getResources(), ImageDecoder.decodeSampledBitmapFromResource(getActivity().getResources(), R.drawable.mypage_alarmset_down_image, 200, 200));

    hour_upbutton.setBackground(alarmup_image);
    //hour_downbutton.setBackground(alarmdown_image);

    minute_upbutton.setBackground(alarmup_image);
    //minute_downView.setBackground(alarmdown_image);
}