微调框下拉背景矢量图像问题

Question

当我尝试为微调器添加背景但在bg_spinner_border.xml中未显示向下箭头时，请进行修复。

预览bg_spinner_border.xml

main.xml文件

<Spinner
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:background="@drawable/bg_dropdown"
                android:layout_marginEnd="20dp"
                android:layout_marginStart="20dp"
                android:paddingBottom="@dimen/layout_10_margin"
                android:paddingTop="@dimen/layout_10_margin"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                android:id="@+id/dealer_spinner"
                android:spinnerMode="dropdown" />

我被更改为矢量图像时也会发生同样的问题

bg_spinner_border.xml

<?xml version="1.0" encoding="utf-8"?>
    <layer-list xmlns:android="http://schemas.android.com/apk/res/android"
                xmlns:app="http://schemas.android.com/apk/res-auto">
        <item>
            <layer-list>
                <item>
                    <shape>
                        <stroke
                                android:width="1dip"
                                android:color="@android:color/black" />
                        <corners android:radius="3dp" />
                        <solid android:color="@android:color/transparent"></solid>
                        <padding
                                android:bottom="5.0dip"
                                android:left="5.0dip"
                                android:right="5.0dip"
                                android:top="5.0dip" />
                    </shape>
                </item>
                <item>
                    <bitmap
                            android:gravity="center_vertical|right"
                            android:src="@drawable/ic_down" />
                </item>
            </layer-list>
        </item>
    </layer-list>

谢谢..