Question

我在我的活动的一个片段中创建了一个recyclerView。我将它绑定到数据集。我已经设置了LinearLayoutManager和RecyclerView Adapter。我的数据集也正确创建，我的Viewholder也是如此。我的Logcat中没有编译警告，也没有例外。我检查了片段和单个项目的XML布局，并且都将所有项目都设置为Visible。

然而我的recyclerView没有显示在片段中。

导致recyclerView不可见的常见疏忽是什么？感谢我能就这件事得到的任何指导。

初始化我的recylerView的相关方法如下：

private void loadMyPlacardView(View view){
        List<Post> postList = new ArrayList<>();
        RecyclerView placardsRecyclerView = view.findViewById(R.id.fp_placards_recyclerView);
        LinearLayoutManager postsLayoutManager = new LinearLayoutManager(view.getContext());
        placardsRecyclerView.setHasFixedSize(true);
        placardsRecyclerView.setLayoutManager(postsLayoutManager);
        DataSnapshot dataSnapshot = FirebaseStorageUtils.postDataSnapshot;
        for (DataSnapshot singleSnapShot: dataSnapshot.getChildren()) {
            Post thisPost = singleSnapShot.getValue(Post.class);
            postList.add(thisPost);
        }
        PlacardsRecyclerViewAdapter placardsRecyclerViewAdapter = new PlacardsRecyclerViewAdapter(postList);
        placardsRecyclerView.setAdapter(placardsRecyclerViewAdapter);
    }

RecyclerviewAdapter的代码在这里

public class PlacardsRecyclerViewAdapter 
extends RecyclerView.Adapter<PlacardHolder> {
private static final String TAG = "PlacardsViewAdapter";
private  List<Post> listofPosts;

public PlacardsRecyclerViewAdapter(List <Post> list) {
    listofPosts = list;
}
@Override
    public PlacardHolder onCreateViewHolder(ViewGroup parent, int viewType){ 
View view = LayoutInflater.from(parent.getContext())
                        .inflate(R.layout.placard_item, parent, false);
                return new PlacardHolder(view);
           }
@Override
public void onBindViewHolder(PlacardHolder placard, int position) {
    Post currentPost = listofPosts.get(position);
    String username = currentPost.getUserName();
    placard.userName.setText(username);
}

@Override
public int getItemCount() {
    return listofPosts.size();
    }
}

Recyclerviewholder的代码为PlacardHolder.java类，位于

之下

public class PlacardHolder extends RecyclerView.ViewHolder {
private static final String TAG = "PlacardsHolder";

public TextView userName;

public PlacardHolder(View view){
    super(view);
        this.userName = view.findViewById(R.id.pi_profile_name);
}
}

Placard Item的XML是下面的placard_item.xml

    <?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <TextView
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:id="@+id/pi_profile_name"
        android:text="@string/default_username"
        android:textAlignment="center"
        android:textSize="25sp"
        android:textStyle="bold"
        android:layout_weight="7"/></android.support.constraint.ConstraintLayout>

和fragment_placard.xml的代码是

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

    <android.support.v7.widget.RecyclerView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/fp_placards_recyclerView"
        android:scrollbars="vertical"
        android:visibility="visible"
        android:layout_margin="10dp">
    </android.support.v7.widget.RecyclerView>
</LinearLayout>

fragment_placard.xml中的textview正在加载，但不是recyclelerview。

Answer 1

好！它与 Firebase 有关。将数据添加到数据集后，您不会通知适配器。做

if(placardsRecyclerViewAdapter != null)
    placardsRecyclerViewAdapter.notifyDataSetChanged();

将数据添加到postList后

。

已更新

完整结构：

private ValueEventListener postListener = null;

private void loadMyPlacardView(View view) {

    List<Post> postList = new ArrayList<>();
    RecyclerView placardsRecyclerView = view.findViewById(R.id.fp_placards_recyclerView);
    LinearLayoutManager postsLayoutManager = new LinearLayoutManager(view.getContext());
    placardsRecyclerView.setHasFixedSize(true);
    placardsRecyclerView.setLayoutManager(postsLayoutManager);
    PlacardsRecyclerViewAdapter placardsRecyclerViewAdapter = new PlacardsRecyclerViewAdapter(postList);
    placardsRecyclerView.setAdapter(placardsRecyclerViewAdapter);

    //Fetching data from Firebase
    postListener = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                if (dataSnapshot.hasChildren()) {
                    for (DataSnapshot ds : dataSnapshot.getChildren()) {
                        Post thisPost = ds.getValue(Post.class);
                        postList.add(thisPost);
                    }
                    if(placardsRecyclerViewAdapter != null)
                        placardsRecyclerViewAdapter.notifyDataSetChanged(); //Now postList has data so notify adapter.
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                // Getting Post failed, log a message
                Log.e("DB error", "loadPost:onCancelled", databaseError.toException());
            }
    };
    databaseReference.addListenerForSingleValueEvent(valueEventListener);
}

Answer 2

我找到了问题的答案。有2个错误。第一个问题出现在placard_item.xml中，我在其中声明了Constraintview，如下所示：

android:layout_height ="match parent"

因此布局占据了屏幕的整个空间。第二个错误是我将TextView定义如下

android:layout_width = "0dp"

这导致TextView在运行时在recyclelerview中消失，尽管它在AndroidStudio IDE中的xml的“Design”视图中表现得非常好。我在这里发帖回答以防其他人面临同样的问题。在使用Recyclerview时，“ConstraintView”通常难以处理。所以回到更简单的LinearLayout。

Recylerview在Android App中不可见

2 个答案: