Caused by: java.lang.IllegalStateException: RecyclerView has no LayoutManager androidx.recyclerview.

Caused by: java.lang.IllegalStateException: RecyclerView has no LayoutManager androidx.recyclerview.widget.RecyclerView
............
        at com.cfsuman.me.batterychargemefast.AdapterRecyclerSystemApps.onCreateViewHolder(AdapterRecyclerSystemApps.java:52)
        at com.cfsuman.me.batterychargemefast.AdapterRecyclerSystemApps.onCreateViewHolder(AdapterRecyclerSystemApps.java:22)

遇到一个这样子的错误，网络上没有查出来原因。于是我分析代码。

public class FragmentSystemApps extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_system_apps, container, false);
        mRecyclerView = (RecyclerView) view.findViewById(R.id.recycler_view);
        mLayoutManager = new GridLayoutManager(mActivity,1);
        mRecyclerView.setLayoutManager(mLayoutManager);  
        mAdapter = new AdapterRecyclerSystemApps(
                mContext,
                mSystemPackagesList
        );
        mRecyclerView.setAdapter(mAdapter);
        return view;
    }
}     

fragment_system_apps.xml这么写的

<?xml version="1.0" encoding="utf-8"?>
<androidx.coordinatorlayout.widget.CoordinatorLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/coordinator_layout"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    >
    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recycler_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:scrollbars="vertical"
        >
    </androidx.recyclerview.widget.RecyclerView>
    <TextView
        android:id="@+id/tv_info"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="16dp"
        android:elevation="3dp"
        android:background="#7dffffff"
        android:padding="8dp"
        android:layout_gravity="top|end"
        />
</androidx.coordinatorlayout.widget.CoordinatorLayout>

上面代码没问题，继续看报错的地方

public class AdapterRecyclerSystemApps extends RecyclerView.Adapter<AdapterRecyclerSystemApps.ViewHolder>{
    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType){
        View v = LayoutInflater.from(mContext).inflate(R.layout.custom_view_system_apps,parent,false);
        ViewHolder vh = new ViewHolder(v);
        return vh;
    }

报错就是这行代码

        View v = LayoutInflater.from(mContext).inflate(R.layout.custom_view_system_apps,parent,false);

再看custom_view_system_apps.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.recyclerview.widget.RecyclerView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:card_view="http://schemas.android.com/apk/res-auto"
    android:id="@+id/card_view"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    card_view:cardCornerRadius="@dimen/cardCornerRadius"
    card_view:cardElevation="@dimen/cardElevation"
    card_view:cardMaxElevation="@dimen/cardMaxElevation"
    card_view:contentPadding="@dimen/cardContentPadding"
    android:layout_margin="@dimen/cardLayoutMargin">
    <RelativeLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        >
        <ImageView
            android:id="@+id/app_icon"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            style="@style/AppIconStyle"
            />
        <TextView
            android:id="@+id/app_label"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_toRightOf="@id/app_icon"
            android:layout_toEndOf="@id/app_icon"
            style="@style/AppLabelStyle"
            />
        <TextView
            android:id="@+id/app_package"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@id/app_label"
            android:layout_toRightOf="@id/app_icon"
            android:layout_toEndOf="@id/app_icon"
            style="@style/AppPackageStyle"
            />
        <TextView
            android:id="@+id/app_size"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@id/app_package"
            android:layout_toRightOf="@id/app_icon"
            android:layout_toEndOf="@id/app_icon"
            style="@style/AppSizeStyle"
            />
    </RelativeLayout>
</androidx.recyclerview.widget.RecyclerView>

后来发现是这custom_view_system_apps.xml这里搞错了，就是说本来要写CardView，但是我搞错了。于是就报上面的错误了。正确代码如下

<?xml version="1.0" encoding="utf-8"?>
<androidx.cardview.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:card_view="http://schemas.android.com/apk/res-auto"
    android:id="@+id/card_view"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    card_view:cardCornerRadius="@dimen/cardCornerRadius"
    card_view:cardElevation="@dimen/cardElevation"
    card_view:cardMaxElevation="@dimen/cardMaxElevation"
    card_view:contentPadding="@dimen/cardContentPadding"
    android:layout_margin="@dimen/cardLayoutMargin">
    <RelativeLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        >
        <ImageView
            android:id="@+id/app_icon"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            style="@style/AppIconStyle"
            />
        <TextView
            android:id="@+id/app_label"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_toRightOf="@id/app_icon"
            android:layout_toEndOf="@id/app_icon"
            style="@style/AppLabelStyle"
            />
        <TextView
            android:id="@+id/app_package"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@id/app_label"
            android:layout_toRightOf="@id/app_icon"
            android:layout_toEndOf="@id/app_icon"
            style="@style/AppPackageStyle"
            />
        <TextView
            android:id="@+id/app_size"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@id/app_package"
            android:layout_toRightOf="@id/app_icon"
            android:layout_toEndOf="@id/app_icon"
            style="@style/AppSizeStyle"
            />
    </RelativeLayout>
</androidx.cardview.widget.CardView>