在GridLayout中水平居中项目

我尝试在GridLayout中居中我的所有项目,我尝试了很多事情(使用RelativeLayout作为父级,然后使用layout_centerInParent,使用基本的android:layout_gravity=“center”,尝试添加一些空间元素…),但我没有成功地将项目居中。作为一个形象,他胜过文字,以下是我所拥有的:

这就是我想要的:

我现在的代码是:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/mainContainer"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="MainActivity">

    <LinearLayout
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="#66000000">

        <TextView
            android:id="@+id/textView4"
            android:layout_width="match_parent"
            android:layout_height="0dp" ... />

        <RelativeLayout
            android:layout_width="match_parent"
            android:layout_height="0dp"
            android:layout_weight="0.50">

        <android.support.v7.widget.GridLayout
            xmlns:android="http://schemas.android.com/apk/res/android"
            xmlns:grid="http://schemas.android.com/apk/res-auto"
            android:id="@+id/droparea"
            android:layout_width="match_parent"
            android:layout_height="0dp"
            android:layout_weight="0.50"
            grid:columnCount="3"
            grid:orientation="horizontal">

            <SquareRelativeLayout
                android:id="@+id/relativelayout"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                grid:layout_columnWeight="1"
                grid:layout_gravity="fill"
                android:background="@drawable/round_layout"
                android:gravity="center_vertical|center_horizontal">
                ...
            </SquareRelativeLayout>

            <SquareRelativeLayout
                android:id="@+id/relativelayout2"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                grid:layout_columnWeight="1"
                grid:layout_gravity="fill"
                android:background="@drawable/round_layout"
                android:gravity="center_vertical|center_horizontal">
                ...
            </SquareRelativeLayout>

            <SquareRelativeLayout
                android:id="@+id/relativelayout3"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                grid:layout_columnWeight="1"
                grid:layout_gravity="fill"
                android:background="@drawable/round_layout"
                android:gravity="center_vertical|center_horizontal"> 
                ...
            </SquareRelativeLayout>

            <SquareRelativeLayout
                android:id="@+id/relativelayout4"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                grid:layout_columnWeight="1"
                grid:layout_gravity="fill"
                android:background="@drawable/round_layout"
                android:gravity="center_vertical|center_horizontal">
                ....
            </SquareRelativeLayout>

            <SquareRelativeLayout
                android:id="@+id/relativelayout5"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                grid:layout_columnWeight="1"
                grid:layout_gravity="fill"
                android:background="@drawable/round_layout"
                android:gravity="center_vertical|center_horizontal">
                ...
            </SquareRelativeLayout>

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

        <include layout="@layout/draggable_linearlayout"/>

    </LinearLayout>

</RelativeLayout>

你有什么想法吗,我如何用GridLayout或类似的东西得到这样的结果?

注意:SquareRelativeLayout只是一个简单的CustomLayout,它强制我的RelativeLayout为正方形(宽度=高度)。