位置列表项位于center_horizo​​ntal

Question

我无法将列表项水平放置在中心位置。当根视图是layout_gravity时，用center_horizontal值设置LinearLayout无效。如果根是RelativeLayout或ConstraintLayout，则情况相同。

有人可以告诉我这是怎么回事吗？

下面是fragment's layout和list item's layout。

片段布局

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recycler_view"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <ProgressBar
        android:id="@+id/pb_loading_indicator"
        android:layout_width="@dimen/progress_bar_size"
        android:layout_height="@dimen/progress_bar_size"
        android:visibility="invisible"
        style="@style/Progress_Bar"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>

列表项布局

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    style="@style/ListItem_Category_Card">

    <TextView
        android:id="@+id/list_item_category_title"
        style="@style/ListItem_Category_Title"
        android:text="Education"
        android:layout_gravity="center_horizontal"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"/>

    <android.support.v7.widget.CardView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_below="@id/list_item_category_title"
        android:layout_centerHorizontal="true">

        <ImageView
            android:id="@+id/list_item_category_poster"
            style="@style/ListItem_Podcast_Image"
            android:layout_gravity="top"
            android:contentDescription="@string/category_picture_description"
            android:src="@drawable/sappfo" />

        <TextView
            android:id="@+id/list_item_category_explanation"
            style="@style/ListItem_Podcast_Title"
            android:layout_gravity="bottom"
            android:text="@string/lorem_ipsum" />

    </android.support.v7.widget.CardView>
</RelativeLayout>