Android studio 如何使mRecyclerView控件填充空闲空间？

我使用代码A设计带有约束布局的UI，UI的顶部是google广告，底部是工具栏

我希望名为mRecyclerView的RecyclerView控件填充所有其他空间，我该怎么办

顺便说一句，当我将RecyclerView控件拖放到UI时，代码B由Android Studio 3.01自动生成。这是布局宽度和布局高度的硬代码

代码A

<?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">

    <com.google.android.gms.ads.AdView xmlns:ads="http://schemas.android.com/apk/res-auto"
        android:id="@+id/adView"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        ads:layout_constraintTop_toTopOf="parent"
        ads:adSize="SMART_BANNER"
        ads:adUnitId="@string/ad_unit_id"
     />

    <LinearLayout
        android:id="@+id/linearLayoutToolBar"
        style="@style/myToolbar"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:weightSum="4"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent">

        <Button
            android:id="@+id/btnBackup"
            style="@style/myTextMedium"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:layout_weight="1"
            android:text="@string/BtnBackup" />

        <Button
            android:id="@+id/btnExit"
            style="@style/myTextMedium"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:layout_weight="1"
            android:text="@string/BtnExit" />
    </LinearLayout>

    <android.support.v7.widget.RecyclerView
       android:id="@+id/mRecyclerView"
        android:layout_width="368dp"
        android:layout_height="396dp"
        tools:layout_editor_absoluteX="8dp"
        tools:layout_editor_absoluteY="58dp" />

</android.support.constraint.ConstraintLayout>

代码B

<android.support.v7.widget.RecyclerView
       android:id="@+id/mRecyclerView"
        android:layout_width="368dp"
        android:layout_height="396dp"
        tools:layout_editor_absoluteX="8dp"
        tools:layout_editor_absoluteY="58dp" />

---

替换您的RecyclerView代码，如下所示，以填充可用空间：

<android.support.v7.widget.RecyclerView
        android:id="@+id/mRecyclerView"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layout_constraintTop_toBottomOf="@id/adView"
        app:layout_constraintBottom_toTopOf="@id/linearLayoutToolBar"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"/>