SimpleCursorAdapter在ListActivity中不起作用，日志中没有错误

Question

I have a simple listview in my MainActivity that I want to populate with Data form my Database. But it doesn't work. I just get a white activity with no signs of any list.

As far as I understand it I need my Activity that extends ListActivity or Activity. Then get the Listview and set a SimpleCursorAdapter object.

Some tutorials suggest a line of code that goes like this:

setContentView(myListView)

What would that be for?

First here is the (i think-) relevant code:

MainActivity.java

public class MainActivity extends ListActivity {

    ListView list;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        showTasks();
    }
...
...
...
 public void showTasks() {
        SubtaskDbHelper thelper = new SubtaskDbHelper(getApplicationContext());
        SQLiteDatabase db = thelper.getReadableDatabase();

        Cursor c = db.rawQuery("SELECT * FROM task", null);

        CursorAdapter ca = new SimpleCursorAdapter(
                getApplicationContext(),
                R.layout.list_element,
                c,
                new String[] {Task._ID,Task.TITLE,Task.CONTENT, Task.DUEDATE},
                new int[] {R.id.list_item_id, R.id.list_item_title, R.id.list_item_content, R.id.list_item_duedate},
                2 );

        setListAdapter(ca);

        c.close();
        db.close();
    }


}

I also tried to extend the class just with "Activity".

I do not know what I am missing to get this running.

activity_main.xml

<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="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">

    <ListView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@android:id/list"
        android:layout_alignParentStart="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true" />
</RelativeLayout>

list_element.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="match_parent">

    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/list_item_id"/>
    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/list_item_title"/>
    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/list_item_content"/>
    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/list_item_duedate"/>

</LinearLayout>

Answer 1

I found the solution. The problem was that I closed the cursoradapter in the ShowTasks method.

Cursor c = db.rawQuery("SELECT * FROM task", null);
...
...
...
c.close();

Just do not close the Cursoradapter.

public void showTasks() {
    SubtaskDbHelper thelper = new SubtaskDbHelper(getApplicationContext());
    SQLiteDatabase db = thelper.getReadableDatabase();

    Cursor c = db.rawQuery("SELECT * FROM task", null);

    CursorAdapter ca = new SimpleCursorAdapter(
            getApplicationContext(),
            R.layout.list_element,
            c,
            new String[] {Task._ID,Task.TITLE,Task.CONTENT, Task.DUEDATE},
            new int[] {R.id.list_item_id, R.id.list_item_title, R.id.list_item_content, R.id.list_item_duedate},
            2 );

    setListAdapter(ca);

    // remove this -> c.close();
    db.close();
}