Android:Fragments,SQLite和Loaders

Question

所以我已经到了需要为我的应用程序实现SQLite数据库的地步.在"繁忙的编码器Android开发指南"之后,我创建了一个扩展的DatabaseHelper类SQLiteOpenHelper.

我的一个用例是对数据库运行查询并在a ListView内显示结果Fragment(我使用支持库中的片段).

根据我的理解,使用managedQuery()并不是真正合适的,即使它是不推荐的,因为这个方法中封装的一些逻辑实际上是在主线程上执行的,具体reQuery()是我理解的时候执行的Activity重新启动.

所以我Loader第一次尝试熟悉这门课,只看到这个:

"The only supplied concrete implementation of a Loader is CursorLoader, and that is only for use with a ContentProvider"

我最初的想法是实现我自己的内容提供商,并可能阻止其他应用程序访问它,然后我ContentProvider通过developer.android.com 阅读文档中的以下内容:

"You don't need a provider to use an SQLite database if the use is entirely within your own application."

我也一直在玩这个:

https://github.com/commonsguy/cwac-loaderex

但我不熟悉这个项目,也不确定它是否可以在生产环境中使用.

所以,现在我能想到的是AsyncTask在我的内部创建一堆实例Fragment并适当地管理它们的生命周期,确保它们在需要时被取消等等.

还有其他选择吗？

Answer 1

我认为实现内容提供商是一个好主意,无论数据是否都无法在应用程序之外访问.它提供了非常现代化的界面,根据我的经验,您的应用程序容易出现数据库锁定问题和其他特定于数据库的问题.

我已经在我的最新项目中实现了它,我很高兴使用它.

Answer 2

我推荐OrmLite库,一个可以用于Android的轻量级对象关系映射.这个图书馆将让您的生活更轻松.您不需要手动创建或更新数据库,您不需要专注于管理数据库连接,所有查询选择,插入,更新都会更容易用一种DAO方法(通常您不需要编写自己的SQL查询)和很多功能.他们有一些你可以开始的例子.

如果你想使用Loader,有一个ORMLite Extras,ORMLite的附加功能可用github(你可以使用与支持android库兼容的支持包).以下是我之前项目的示例用法:

public class EventsFragment extends Fragment implements LoaderCallbacks<Cursor>{
   private static final int LOADER_ID = EventsFragment.class.getName().hashCode();
   @Override
   public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    getLoaderManager().initLoader(LOADER_ID, null, this);
   }

    @Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    View layoutRoot = inflater.inflate(
            R.layout.fragment_events, null);
    lvEvents = (ListView) layoutRoot.findViewById(R.id.lvEvents);   

    adapter = new EventAdapter(getActivity(), null, null);
    lvEvents.setAdapter(adapter);

    return layoutRoot;
}

    @Override
    public Loader<Cursor> onCreateLoader(int arg0, Bundle arg1) {
       try {
           PreparedQuery<Event> query = getDatabaseHelper().getEventDao().getQuery();
           return getDatabaseHelper().getEventDao().getSQLCursorLoader(query );
        } catch (Exception e) {
        e.printStackTrace();
    }

    return null;
}

@Override
public void onLoadFinished(Loader<Cursor> arg0, Cursor cursor) {
    adapter.swapCursor(cursor);
    try {
        adapter.setQuery(getDatabaseHelper().getEventDao().getQuery());
    } catch (SQLException e) {
        e.printStackTrace();
    }
      }

@Override
public void onLoaderReset(Loader<Cursor> arg0) {
    adapter.swapCursor(null);
}

    private OrmliteDatabaseHelper getDatabaseHelper(){
         return ((MainActivity)getActivity()).getDatabaseHelper();
    }
 }

适配器

 public class EventAdapter extends OrmliteCursorAdapter<Event>{

public EventAdapter(Context context, Cursor c, PreparedQuery<Event> query) {
    super(context, c, query);
}

@Override
public void bindView(View itemView, Context context, Event item) {
    TextView tvEventTitle = (TextView) itemView.findViewById(R.id.tvEventTitle); 
    TextView tvEventStartDate = (TextView) itemView.findViewById(R.id.tvEventStartDate);

    tvEventTitle.setText(item.getTitle());
    tvEventStartDate.setText(item.getFormatStartDate());
}

@Override
public View newView(Context context, Cursor arg1, ViewGroup arg2) {
    LayoutInflater inflater = LayoutInflater.from(context);
    View retView = inflater.inflate(R.layout.event_item_row, arg2, false);
    return retView;
}
 }

和PreparedQuery上面提供游标适配器的自定义Dao :

public interface IEventDao extends Dao<Event, Integer>{
    PreparedQuery<Event> getQuery() throws SQLException;
    OrmliteCursorLoader<Event> getSQLCursorLoader(Context context, PreparedQuery<Event> query) throws SQLException;
}

public class EventDao extends AndroidBaseDaoImpl<Event, Integer> implements IEventDao{

public EventDao(ConnectionSource connectionSource) throws SQLException {
    super(connectionSource, Event.class);
}

public EventDao(ConnectionSource connectionSource,
        DatabaseTableConfig<Event> tableConfig) throws SQLException {
    super(connectionSource, tableConfig);
}

@Override
public PreparedQuery<Event> getQuery() throws SQLException{
    return queryBuilder().prepare();
}
}

希望这可以帮助!

Answer 3

您可以扩展Loader类以执行其他异步工作,例如直接从数据库加载.

这是一个例子

编辑:添加了Loader使用的更好示例.

最后设法找到真正帮助我理解工作原理的教程.

通过扩展loader类,您可以避免弄乱内容观察者,并且很容易实现(最后)代码中需要进行的修改

• 添加实现LoaderManager.LoaderCallbacks<D>,D您的数据列表在哪里(代码段1)
• 创建您的加载器类,复制代码段2,并从数据库添加数据加载
• 最后调用加载器1调用init,然后刷新重启调用.(摘录2和3)

代码段1:如何将加载程序与您的片段"链接":

public static class AppListFragment extends ListFragment implements
      LoaderManager.LoaderCallbacks<List<SampleItem>> {

  public Loader<List<SampleItem>> onCreateLoader(int id, Bundle args) { 
     //...
     return new SampleLoader (getActivity());
  }

  public void onLoadFinished(Loader<List<SampleItem>> loader, List<SampleItem> data) {
    // ... 
     mAdapter.setData(data);

     if (isResumed()) {
       setListShown(true);
     } else {
       setListShownNoAnimation(true);
     }
    // ... 
 }

  public void onLoaderReset(Loader<List<SampleItem>> loader) { 
    // ... 
    mAdapter.setData(null);
    // ... 
  }

  /* ... */
}

代码段2:你的自定义加载器的模式:(我已经对观察者的事情进行了评论,因为我认为从一开始就很难实现它,你可以简单地回想起加载器而不会搞乱自动刷新)

public class SampleLoader extends AsyncTaskLoader<List<SampleItem>> {

  // We hold a reference to the Loader’s data here.
  private List<SampleItem> mData;

  public SampleLoader(Context ctx) {
    // Loaders may be used across multiple Activitys (assuming they aren't
    // bound to the LoaderManager), so NEVER hold a reference to the context
    // directly. Doing so will cause you to leak an entire Activity's context.
    // The superclass constructor will store a reference to the Application
    // Context instead, and can be retrieved with a call to getContext().
    super(ctx);
  }

  /****************************************************/
  /** (1) A task that performs the asynchronous load **/
  /****************************************************/

  @Override
  public List<SampleItem> loadInBackground() {
    // This method is called on a background thread and should generate a
    // new set of data to be delivered back to the client.
    List<SampleItem> data = new ArrayList<SampleItem>();

    // TODO: Perform the query here and add the results to 'data'.

    return data;
  }

  /********************************************************/
  /** (2) Deliver the results to the registered listener **/
  /********************************************************/

  @Override
  public void deliverResult(List<SampleItem> data) {
    if (isReset()) {
      // The Loader has been reset; ignore the result and invalidate the data.
      releaseResources(data);
      return;
    }

    // Hold a reference to the old data so it doesn't get garbage collected.
    // We must protect it until the new data has been delivered.
    List<SampleItem> oldData = mData;
    mData = data;

    if (isStarted()) {
      // If the Loader is in a started state, deliver the results to the
      // client. The superclass method does this for us.
      super.deliverResult(data);
    }

    // Invalidate the old data as we don't need it any more.
    if (oldData != null && oldData != data) {
      releaseResources(oldData);
    }
  }

  /*********************************************************/
  /** (3) Implement the Loader’s state-dependent behavior **/
  /*********************************************************/

  @Override
  protected void onStartLoading() {
    if (mData != null) {
      // Deliver any previously loaded data immediately.
      deliverResult(mData);
    }

    // Begin monitoring the underlying data source.
    ////if (mObserver == null) {
      ////mObserver = new SampleObserver();
      // TODO: register the observer
    ////}

    //// takeContentChanged() can still be implemented if you want 
    ////     to mix your refreshing in that mechanism 
    if (takeContentChanged() || mData == null) {
      // When the observer detects a change, it should call onContentChanged()
      // on the Loader, which will cause the next call to takeContentChanged()
      // to return true. If this is ever the case (or if the current data is
      // null), we force a new load.
      forceLoad();
    }
  }

  @Override
  protected void onStopLoading() {
    // The Loader is in a stopped state, so we should attempt to cancel the 
    // current load (if there is one).
    cancelLoad();

    // Note that we leave the observer as is. Loaders in a stopped state
    // should still monitor the data source for changes so that the Loader
    // will know to force a new load if it is ever started again.
  }

  @Override
  protected void onReset() {
    // Ensure the loader has been stopped.
    onStopLoading();

    // At this point we can release the resources associated with 'mData'.
    if (mData != null) {
      releaseResources(mData);
      mData = null;
    }

    // The Loader is being reset, so we should stop monitoring for changes.
    ////if (mObserver != null) {
      // TODO: unregister the observer
     //// mObserver = null;
    ////}
  }

  @Override
  public void onCanceled(List<SampleItem> data) {
    // Attempt to cancel the current asynchronous load.
    super.onCanceled(data);

    // The load has been canceled, so we should release the resources
    // associated with 'data'.
    releaseResources(data);
  }

  private void releaseResources(List<SampleItem> data) {
    // For a simple List, there is nothing to do. For something like a Cursor, we 
    // would close it in this method. All resources associated with the Loader
    // should be released here.
  }

  /*********************************************************************/
  /** (4) Observer which receives notifications when the data changes **/
  /*********************************************************************/

  // NOTE: Implementing an observer is outside the scope of this post (this example
  // uses a made-up "SampleObserver" to illustrate when/where the observer should 
  // be initialized). 

  // The observer could be anything so long as it is able to detect content changes
  // and report them to the loader with a call to onContentChanged(). For example,
  // if you were writing a Loader which loads a list of all installed applications
  // on the device, the observer could be a BroadcastReceiver that listens for the
  // ACTION_PACKAGE_ADDED intent, and calls onContentChanged() on the particular 
  // Loader whenever the receiver detects that a new application has been installed.
  // Please don’t hesitate to leave a comment if you still find this confusing! :)
  ////private SampleObserver mObserver;
}

代码段3:如何首次调用加载程序(仅限)

  // Initialize a Loader with an id. If the Loader with this id is not 
  // initialized before
  getLoaderManager().initLoader(LOADER_ID, null, this);

代码段4:用于刷新数据(调用查询)

 // Check if the loader exists and then restart it.
 if (getLoaderManager().getLoader(LOADER_ID) != null)
     getLoaderManager().restartLoader(LOADER_ID, null, this);

参考:

• 代码段1:从这里提取的加载程序的用法
• 代码片段2:这里有更多信息和逻辑读取整个孔文章
• 小组件3和4:只是装载机的使用.

这些的完整代码也由创建者在github上传