android - ListView 根本不可见

Question

我遇到 ListView 问题，有一个自定义卡片布局 View 应该出现在 ListView 中。这是包含适配器类的主要 FirstActivity.java 文件，后面是 activity_first.xml、card_layout.xml 和一个屏幕截图。我的问题是，当我运行这个应用程序时，它安装成功但 ListView 根本不可见:

public class FirstActivity extends AppCompatActivity {

    private Toolbar toolbar;
    private ListView listView;


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

            setupUIViews();
            toolbarSettings();
            setupListView();
        }
        catch (java.lang.NullPointerException exception) {
            exception.printStackTrace();
        }

    }

    private void setupUIViews () {

        try {
            toolbar = (Toolbar) findViewById(R.id.ToolbarMain);
            listView = (ListView) findViewById(R.id.lv_main);
        }
        catch (java.lang.NullPointerException exception) {
            exception.printStackTrace();
        }
    }

    private void toolbarSettings() {

        setSupportActionBar(toolbar);
        getSupportActionBar().setTitle("TimeTable");

    }

        private void setupListView () {

            try {

                String[] title = getResources().getStringArray(R.array.Main);
                String[] description = getResources().getStringArray(R.array.Desc);

                adapter adapter = new adapter(this, title, description);
                listView.setAdapter(adapter);

            } catch (java.lang.NullPointerException exception) {
                exception.printStackTrace();
            }
        }

    public class adapter extends BaseAdapter {

        private Context mContext;
        private LayoutInflater layoutInflater;
        private TextView title, description;
        private String[] titleArray;
        private String[] descriptionArray;
        private ImageView imageView;

        public adapter(Context context, String[] titleArray, String[] descriptionArray) {

            mContext = context;
            titleArray = titleArray;
            descriptionArray = descriptionArray;
            layoutInflater = LayoutInflater.from(context);

        }


        @Override
        public int getCount() {
            int length = 0;
            try {
                length = titleArray.length;
            }
            catch (java.lang.NullPointerException exception) {
                exception.printStackTrace();
            }

            return length;
        }

        @Override
        public Object getItem(int position) {
            return titleArray[position];
        }

        @Override
        public long getItemId(int position) {
            return position;
        }

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

            if(convertView == null ) {
                convertView = layoutInflater.inflate(R.layout.card_layout, null);
            }

            title = (TextView)convertView.findViewById(R.id.tv_heading);
            description = (TextView)convertView.findViewById(R.id.tv_desc);
            imageView = (ImageView)convertView.findViewById(R.id.iv_main);

            title.setText(titleArray[position]);
            description.setText(descriptionArray[position]);

            if (titleArray[position].equalsIgnoreCase("TimeTable")) {
                imageView.setImageResource(R.drawable.timetable);
            }
            else if (titleArray[position].equalsIgnoreCase("Subjects")) {
                imageView.setImageResource(R.drawable.calender);
            }
            else if (titleArray[position].equalsIgnoreCase("Faculty")) {
                imageView.setImageResource(R.drawable.contact);
            }
            else
                imageView.setImageResource(R.drawable.settings);

            return convertView;
        }

    }

}

这是 first_activity.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    tools:context=".FirstActivity">

    <android.support.v7.widget.Toolbar
        android:layout_width="match_parent"
        android:layout_height="?android:attr/actionBarSize"
        android:background="@color/colorPrimary"
        android:id="@+id/ToolbarMain">
    </android.support.v7.widget.Toolbar>

    <ListView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginLeft="10dp"
        android:layout_marginRight="10dp"
        android:layout_marginTop="10dp"
        android:layout_marginBottom="10dp"
        android:layout_below="@+id/ToolbarMain"
        android:divider="@null"
        android:id="@+id/lv_main">
    </ListView>
</RelativeLayout>

这是 card_layout.xml 文件:-

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@color/colorAccent"
    android:elevation="4dp">

    <RelativeLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content">

        <ImageView
            android:layout_width="85dp"
            android:layout_height="105dp"
            android:layout_marginStart="10dp"
            android:id="@+id/iv_main"/>

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_toEndOf="@id/iv_main"
            android:textStyle="bold"
            android:layout_marginBottom="5dp"
            android:text="IshaanNagwani"
            android:textSize="20sp"
            android:id="@+id/tv_heading"/>

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="IshaanNagwani"
            android:textSize="16sp"
            android:layout_below="@id/tv_heading"
            android:layout_toEndOf="@id/iv_main"
            android:id="@+id/tv_desc"/>

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@id/tv_desc"
            android:layout_alignBottom="@id/iv_main"
            android:layout_alignParentRight="true"
            android:layout_marginTop="29sp"
            android:text="IshaanNagwani"
            android:textAlignment="center"
            android:textSize="12sp"
            android:id="@+id/tv_third"/>

    </RelativeLayout>

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

最后，这是我的 strings.xml:-

<resources>
    <string name="app_name">TimeTable</string>

    <string-array name="Main">
        <item>TimeTable</item>
        <item>Subjects</item>
        <item>Faculty</item>
        <item>Resources</item>
    </string-array>

    <string-array name="Desc">
        <item>desc_1</item>
        <item>desc_2</item>
        <item>desc_3</item>
        <item>desc_4</item>
    </string-array>

</resources>

我是 android 新手，这是我的第一篇文章。我正在使用 ListView。显然我做错了什么，但我不知道是什么。这是应用程序完全安装后的屏幕截图:

Answer 1

在adapter 构造函数中，一些类变量的名称与参数的名称相同。因此，所有操作都将仅针对参数进行，它使 this.titleArray 的大小将为零，然后 getCount() 方法始终返回 0 然后没有项目出现在您的 Activity 中.所以你必须使用 this 来引用类变量。修改您的代码如下:

public adapter(Context context, String[] titleArray, String[] descriptionArray) {

        mContext = context;
        this.titleArray = titleArray;
        this.descriptionArray = descriptionArray;
        layoutInflater = LayoutInflater.from(context);

}