android - CardView 未显示在 RecyclerView 中

Question

我遇到了一个让我抓狂的问题。基本上它是一个显示卡片 View 的简单 RecyclerView。已经有很多帖子了，我查了一下。我已经按照此处 (RecyclerView Not Displaying Any CardView Items) 的指示分配了 LayoutManager，我已经按照此处 (card view not showing up) 的说明实现了 getItemCount 方法，并且更改为以下版本(此处建议更改版本 RecyclerView of cards not showing anything):

compile 'com.android.support:support-v4:24.0.0'
compile 'com.android.support:cardview-v7:24.0.0'
compile 'com.android.support:recyclerview-v7:24.0.0'

我还按照此处的说明执行了 notifyDataSetChanged() (RecyclerView Not Displaying Any CardView Items)。但是，没有任何东西为我修复它。

我有一个具有以下代码的 Activity (仅相关部分，getDummyData 返回一个包含 1 个虚拟数据的列表):

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    DatabaseHelper databaseHelper = new DatabaseHelper(getApplicationContext());

    setContentView(R.layout.activity_overview);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    RecyclerView rv = (RecyclerView) findViewById(R.id.rv);
    rv.setHasFixedSize(true);
    LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
    linearLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
    rv.setLayoutManager(linearLayoutManager);

    RVAdapter adapter = new RVAdapter(getDummyData());
    rv.setAdapter(adapter);
    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });
    adapter.notifyDataSetChanged();

}


public class RVAdapter extends RecyclerView.Adapter<RVAdapter.DocumentViewHolder> {

    List<Document> DocumentList;

    public RVAdapter(List<Document> Documents) {
        this.DocumentList = Documents;
    }

    @Override
    public DocumentViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.document_item, parent, false);
        DocumentViewHolder pvh = new DocumentViewHolder(v);
        return pvh;
    }

    @Override
    public void onBindViewHolder(DocumentViewHolder DocumentViewHolder, int i) {
        DocumentViewHolder.Date.setText(DocumentList.get(i).getCreatedFormatted());
        DocumentViewHolder.participants.setText(DocumentList.get(i).getParticipants(getApplicationContext()));
        DocumentViewHolder.counterOfItems.setText(DocumentList.get(i).getcounterOfItems(getApplicationContext()) + StringUtils.EMPTY);
    }

    @Override
    public void onAttachedToRecyclerView(RecyclerView recyclerView) {
        super.onAttachedToRecyclerView(recyclerView);
    }

    @Override
    public int getItemCount() {

        //return DocumentList.size();
    return 1;
    }

    public class DocumentViewHolder extends RecyclerView.ViewHolder {
        TextView Date;
        TextView participants;
        TextView ideasPreview;
        TextView counterOfItems;
        ImageView Photo;

        public DocumentViewHolder(View itemView) {
            super(itemView);
            Date = (TextView) itemView.findViewById(R.id._date);
            participants = (TextView) itemView.findViewById(R.id.participants);
            ideasPreview = (TextView) itemView.findViewById(R.id.ideas_preview);
            counterOfItems = (TextView) itemView.findViewById(R.id.counter_of_items);
            Photo = (ImageView) itemView.findViewById(R.id._photo);
        }
    }


}

我有一个 Document_item - xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">

<android.support.v7.widget.CardView 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"
    android:layout_gravity="center"
    card_view:cardCornerRadius="@dimen/_card_corner">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <ImageView
            android:id="@+id/_photo"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="0.25" />

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:gravity="center_horizontal"
            android:orientation="vertical"
            android:paddingBottom="@dimen/activity_vertical_margin"
            android:paddingEnd="@dimen/activity_horizontal_margin"
            android:paddingStart="@dimen/activity_horizontal_margin"
            android:paddingTop="@dimen/activity_vertical_margin">

            <TextView
                android:id="@+id/_date"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="21st June 2016" />

            <TextView
                android:id="@+id/participants"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="Thomas, Christian and Johann" />

            <TextView
                android:id="@+id/counter_of_items"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="Thomas, Christian and Johann" />

            <TextView
                android:id="@+id/ideas_preview"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="Hello, Test, Test, Test" />

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

Activity 的 xml 如下所示:

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout     
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"
android:fitsSystemWindows="true"
tools:context="com.chmaurer.idea.OverviewActivity">

<android.support.design.widget.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/AppTheme.AppBarOverlay">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary"
        app:popupTheme="@style/AppTheme.PopupOverlay" />

</android.support.design.widget.AppBarLayout>

<android.support.v7.widget.RecyclerView
    android:id="@+id/rv"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
</android.support.v7.widget.RecyclerView>

<android.support.design.widget.FloatingActionButton
    android:id="@+id/fab"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="bottom|end"
    android:layout_margin="@dimen/fab_margin"
    android:src="@drawable/ic_playlist_add_white_24dp" />

</android.support.design.widget.CoordinatorLayout>

代码编译执行正常，但没有显示任何卡片。我现在已经为自己尝试了几个小时，但这绝对是我很乐意得到提示的地方......

Answer 1

您的代码很好，问题出在布局文件:document_item.xml:

    <LinearLayout
        android:layout_width="match_parent"  
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <ImageView
            android:id="@+id/_photo"
            android:layout_width="wrap_content"  -->mistake
            android:layout_height="wrap_content"
            android:layout_weight="0.25" />

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent" -->mistake
            android:gravity="center_horizontal"
            android:orientation="vertical"
            ...

第二个 LinearLayout 从父级(第一个 LinearLayout)获取高度，第一个从其唯一的子级获取高度:ImageView (所以如果没有设置 drawable，第二个布局的高度将为零!)，如果你为 ImageView 设置一个 drawable(在 onBindViewHolder 方法或在布局 xml 文件中)它可能会裁剪一些 TextView 项目，此外，当您将 android:layout_width 设置为 "wrap_content"，实际上你忽略了 layout_weight。

所以像这样编辑布局文件:

          ...
          <ImageView
            android:id="@+id/_photo"
            android:layout_width="0dp"
            android:layout_gravity="center_vertical"
            android:layout_height="wrap_content"
            android:layout_weight="0.25" />

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            ...