ViewFlipper不显示任何图像

Question

我在ViewFlipper中添加了2个图像视图，但是没有图像显示。设置View Flipper的背景颜色时，会显示颜色，但不会投影任何图像。这是一个非常基础的项目。请告诉我原因。我同时附有Java和XML文件，请看看

MainActivity.java

public class MainActivity extends AppCompatActivity {
        ViewFlipper viewFlipper;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            viewFlipper = findViewById(R.id.viewFlipper);

            viewFlipper.setAutoStart(true);
            viewFlipper.setFlipInterval(3000);
            viewFlipper.startFlipping();
        }
    }

activity_main.xml

    <?xml version="1.0" encoding="utf-8"?>
    <android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity">
        <ViewFlipper
            android:id="@+id/viewFlipper"
            android:layout_width="match_parent"
            android:layout_height="281dp"
            android:inAnimation="@anim/right_to_left"
            android:outAnimation="@anim/right_to_left"
            android:background="@color/colorPrimaryDark"
            android:padding="10dp"
            android:visibility="visible">


            <ImageView
                android:id="@+id/imageView3"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                tools:srcCompat="@tools:sample/backgrounds/scenic" />

            <ImageView
                android:id="@+id/imageView2"
                android:layout_width="wrap_content"
                android:scaleType="fitXY"
                android:layout_height="wrap_content"
                app:srcCompat="@drawable/f2" />
        </ViewFlipper>


    </android.support.constraint.ConstraintLayout>