ListView 不显示内容

Question

我尝试从firebase 检索数据并使用adapter 在listview 中显示它们。 检索数据一切正常，我通过调试器进行了检查，但没有显示任何内容。 这是更新的resource_layout.xml：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/borders">

    <ListView
        android:id="@+id/listViewBooks"
        android:layout_height="match_parent"
        android:layout_width="match_parent" />
</RelativeLayout>

更新了 resource_item.xml：

    <?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="wrap_content">

    <TextView
        android:id="@+id/name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView" />

    <TextView
        android:id="@+id/author"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView" />

    <TextView
        android:id="@+id/date"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView" />

</LinearLayout>

ResourceAdapter.java：

public class ResourceAdapter extends ArrayAdapter<Books> {

    private Activity context;
    private List<Books> bookList;

    public ResourceAdapter(Activity context, List<Books> bookList){
        super(context, R.layout.resource_item, bookList);
        this.context = context;
        this.bookList = bookList;
    }

    @NonNull
    @Override
    public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {

        LayoutInflater inflater = context.getLayoutInflater();
        View listViewItem = inflater.inflate(R.layout.resource_item, null, true);

        TextView textViewName = (TextView) listViewItem.findViewById(R.id.name);
        TextView textViewAuthor = (TextView) listViewItem.findViewById(R.id.author);
        TextView textViewDate = (TextView) listViewItem.findViewById(R.id.date);

        Books book = bookList.get(position);
        textViewName.setText(book.getName());
        textViewAuthor.setText(book.getAuthor());
        textViewDate.setText(String.valueOf(book.getDate()));

        return listViewItem;
    }
}

ResourceActivity.Java：

public class Resource extends AppCompatActivity {
    DatabaseReference databaseBooks;
    ListView listViewBooks;
    List<Books> bookList;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.resource_layout);

        databaseBooks = FirebaseDatabase.getInstance().getReference("books");

        bookList = new ArrayList<>();
        listViewBooks = (ListView) findViewById(R.id.listViewBooks);
    }

    @Override
    protected void onStart() {
        super.onStart();

        databaseBooks.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                bookList.clear();

                for (DataSnapshot bookSnapshot : dataSnapshot.getChildren()){
                    Books book = bookSnapshot.getValue(Books.class);
                    bookList.add(book);
                }

                ResourceAdapter adapter = new ResourceAdapter(Resource.this, bookList);
                listViewBooks.setAdapter(adapter);

            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
            }
        });
    }
}

Answer 1

您的ListView 高度是0dp，它有一个重量。它不适用于RelativeLayout。就这样改吧。

<ListView
    android:id="@+id/listViewBooks"
    android:layout_height="match_parent"
    android:layout_width="match_parent"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_alignParentTop="true" />

更新：

像这样更改行布局。将TextViews中的weight参数去掉，设置LinearLayout的layout_height为wrap_content。高度计算可能会导致问题。

<?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="wrap_content">

        <TextView
            android:id="@+id/name"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="TextView" />

        <TextView
            android:id="@+id/author"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="TextView" />

        <TextView
            android:id="@+id/date"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="TextView" />

</LinearLayout>

更新 2： 尝试如下更改适配器的 getView() 方法。

@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {

    if(convertView==null){
        // inflate the layout
        LayoutInflater inflater = context.getLayoutInflater();
        convertView = inflater.inflate(R.layout.resource_item, null, false);
    }


    TextView textViewName = (TextView) convertView.findViewById(R.id.name);
    TextView textViewAuthor = (TextView) convertView.findViewById(R.id.author);
    TextView textViewDate = (TextView) convertView.findViewById(R.id.date);

    Books book = bookList.get(position);
    textViewName.setText(book.getName());
    textViewAuthor.setText(book.getAuthor());
    textViewDate.setText(String.valueOf(book.getDate()));

    return convertView;
}

Answer 2

问题在于您的XML。你的父布局是RelativeLayout，你正在使用android:layout_weight="1"和android:layout_height="0dp"。

您可以在RelativeLayout 中使用ListView，如下所示：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/borders">

    <ListView
        android:id="@+id/listViewBooks"
        android:layout_height="match_parent"
        android:layout_width="match_parent" />

</RelativeLayout>

您也可以使用LinearLayout，如下：

<?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:background="@drawable/borders"
    android:orientation="vertical">

    <ListView
        android:id="@+id/listViewBooks"
        android:layout_weight="1"
        android:layout_height="0dp"
        android:layout_width="match_parent" />
</<LinearLayout>