java - 寻求一些了解 simpleCursorAdapter 工作原理的见解

Question

大家好，我只是希望有人能阐明这段代码的工作原理，更具体地说是 simpleCursorAdapter。完整的程序是一个待办事项列表应用程序，它是一个非常简单的教程，用户可以输入数据或“注释”并使用游标和加载器保存到 sqlite 数据库。

所以我的问题是有一种特定的方法我无法理解它是如何工作的，因此我无法操纵数据的显示方式。我认为问题在于我只是不明白适配器如何采用与显示内容不同的布局并将其全部显示在 ListView 中。

  private void fillData() {

    // Fields from the database (projection)
    // Must include the _id column for the adapter to work
    String[] from = new String[] { TodoTable.COLUMN_SUMMARY };


    // Fields on the UI to which we map
    int[] to = new int[] { R.id.label }; //I don't quite understand but I know it's just a value for the adapter

    getLoaderManager().initLoader(0, null, this);

    adapter = new SimpleCursorAdapter(this, R.layout.todo_row, null, from,  
        to, 0); /*This line specifically I don't understand how it is working. 
            R.layout.todo_row is a near blank xml, used when there are no "todos"
            with no listviews.  R.layout.todo_list has the listview's but when 
            assigned in the adapter it doesn't work.


    setListAdapter(adapter);

  }

总的来说，我正在尝试并排制作 3 个 ListView ，以从数据库中读取数据并四处游玩。如果有人可以帮助我，我将不胜感激，谢谢。

R.layout.todo_row

<?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="wrap_content" >

    <ImageView
        android:id="@+id/icon"
        android:layout_width="30dp"
        android:layout_height="24dp"
        android:layout_marginLeft="4dp"
        android:layout_marginRight="8dp"
        android:layout_marginTop="8dp"
        android:src="@drawable/reminder" >
    </ImageView>

    <TextView
        android:id="@+id/label"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="6dp"
        android:lines="1"
        android:text="@+id/TextView01"
        android:textSize="24dp" 
        >
    </TextView>


</LinearLayout> 

和R.layout.todo_list

<?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:orientation="vertical" >

        <ListView
           android:id="@android:id/list"
            android:layout_width="110dp"
            android:layout_height="200dp" >

        </ListView>

        <ListView
            android:id="@+id/listMiddle"
            android:layout_width="110dp"
            android:layout_height="200dp"
            android:layout_toRightOf="@android:id/list" >
        </ListView>

       <ListView
            android:id="@+id/listRight"
            android:layout_width="110dp"
            android:layout_height="200dp"
            android:layout_toRightOf="@id/listMiddle" >
        </ListView>

    <TextView
        android:id="@android:id/empty"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/no_todos" />

</RelativeLayout> 

全类在下面

package de.vogella.android.todos;

import android.app.ListActivity;
import android.app.LoaderManager;
import android.content.CursorLoader;
import android.content.Intent;
import android.content.Loader;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.view.ContextMenu;
import android.view.ContextMenu.ContextMenuInfo;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView.AdapterContextMenuInfo;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
import de.vogella.android.todos.contentprovider.MyTodoContentProvider;
import de.vogella.android.todos.database.TodoTable;

/*
 * TodosOverviewActivity displays the existing todo items
 * in a list
 * 
 * You can create new ones via the ActionBar entry "Insert"
 * You can delete existing ones via a long press on the item
 */

public class TodosOverviewActivity extends ListActivity implements
    LoaderManager.LoaderCallbacks<Cursor> {
  private static final int ACTIVITY_CREATE = 0;
  private static final int ACTIVITY_EDIT = 1;
  private static final int DELETE_ID = Menu.FIRST + 1;
  // private Cursor cursor;
  private SimpleCursorAdapter adapter;
  private SimpleCursorAdapter middleAdapter;
  private SimpleCursorAdapter rightAdapter;



/** Called when the activity is first created. */

  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.todo_list);
    this.getListView().setDividerHeight(2);
    fillData();
    registerForContextMenu(getListView());
  }

  // Create the menu based on the XML defintion
  @Override
  public boolean onCreateOptionsMenu(Menu menu) {
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.listmenu, menu);
    return true;
  }

  // Reaction to the menu selection
  @Override
  public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    case R.id.insert:
      createTodo();
      return true;
    }
    return super.onOptionsItemSelected(item);
  }

  @Override
  public boolean onContextItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    case DELETE_ID:
      AdapterContextMenuInfo info = (AdapterContextMenuInfo) item
          .getMenuInfo();
      Uri uri = Uri.parse(MyTodoContentProvider.CONTENT_URI + "/"
          + info.id);
      getContentResolver().delete(uri, null, null);
      fillData();
      return true;
    }
    return super.onContextItemSelected(item);
  }

  private void createTodo() {
    Intent i = new Intent(this, TodoDetailActivity.class);
    startActivity(i);
  }

  // Opens the second activity if an entry is clicked
  @Override
  protected void onListItemClick(ListView l, View v, int position, long id) {
    super.onListItemClick(l, v, position, id);
    Intent i = new Intent(this, TodoDetailActivity.class);
    Uri todoUri = Uri.parse(MyTodoContentProvider.CONTENT_URI + "/" + id);
    i.putExtra(MyTodoContentProvider.CONTENT_ITEM_TYPE, todoUri);

    startActivity(i);
  }



  private void fillData() {

    // Fields from the database (projection)
    // Must include the _id column for the adapter to work
    String[] from = new String[] { TodoTable.COLUMN_SUMMARY };
    String[] middleId = new String[] { TodoTable.COLUMN_ID };

    // Fields on the UI to which we map
    int[] to = new int[] { R.id.label };
    int[] two = new int[] { R.id.label };

    getLoaderManager().initLoader(0, null, this);
    adapter = new SimpleCursorAdapter(this, R.layout.todo_row, null, from,
        to, 0);

    middleAdapter = new SimpleCursorAdapter(this, R.layout.todo_row, null, middleId,
            two, 0);


    setListAdapter(adapter);
   // setListAdapter(middleAdapter);

  }

  @Override
  public void onCreateContextMenu(ContextMenu menu, View v,
      ContextMenuInfo menuInfo) {
    super.onCreateContextMenu(menu, v, menuInfo);
    menu.add(0, DELETE_ID, 0, R.string.menu_delete);
  }

  // Creates a new loader after the initLoader () call
  @Override
  public Loader<Cursor> onCreateLoader(int id, Bundle args) {
    String[] projection = { TodoTable.COLUMN_ID, TodoTable.COLUMN_SUMMARY };
    CursorLoader cursorLoader = new CursorLoader(this,
        MyTodoContentProvider.CONTENT_URI, projection, null, null, null);
    return cursorLoader;
  }

  @Override
  public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
    adapter.swapCursor(data);
  }

  @Override
  public void onLoaderReset(Loader<Cursor> loader) {
    // data is not available anymore, delete reference
    adapter.swapCursor(null);
  }

} 

Answer 1

So my problem is that there is a specific method that I'm having trouble grasping how it works and as a result I cannot manipulate the way the data is displayed.

方法:

adapter = new SimpleCursorAdapter(this, R.layout.todo_row, null, from, to, 0);

好吧，让我们按每个参数分解这个构造函数:

1. this，一个上下文。适配器需要一个上下文来扩充每一行的布局。
2. R.layout.todo_row，行的布局。 Cursor 中的每条记录都将显示在此布局中。 (具体如何显示 Cursor 取决于 from 和 to。)
3. null，一个游标。这包含将显示在您的 ListView 中的所有数据。
4. from，行布局中的基本 View 数组。
5. 到，您的 Cursor 中的基本列的数组。
6. 0，何时以及为何应刷新数据的标志。

每件事背后的诀窍是:第四个 (from) 中的每个 ID 都必须与第二个参数 (R.layout.todo_row) 中的一个 View 相匹配。第五个参数中的每个字符串都必须与 Cursor 中的列名相匹配。第四个(from)和第五个参数(to)必须一一匹配，因为每一列都显示在一个View中。原来如此。

---

你现在可能已经意识到，这个注释:

R.layout.todo_row is a near blank xml, used when there are no "todos" with no listviews.

错了，抱歉。如果您想在 Cursor 为空时显示注释，请添加:

 <TextView android:id="@android:id/empty"
           android:layout_width="match_parent"
           android:layout_height="match_parent"
           android:text="No data"/>

到 todo_list.xml，如 ListActivity's documenation 中所述.通过在您的 TextView 中使用这个“魔术 id”，注释应该会在适当的时候自动显示或隐藏。

---

所有这些只与第一个 ListView 交互(id:`android:id="@android:id/list")，您需要创建新的 Cursors 和 Adapters 才能使用其他 ListView。希望对您有所帮助!