Android：片段，SQLite和加载器

Question

因此，我到了需要为我的应用程序实现SQLite数据库的地步。遵循“
Android开发的繁忙程序员指南”，我创建了一个Extended的DatabaseHelper类SQLiteOpenHelper。

我的用例之一是对数据库运行查询，并ListView在内显示结果Fragment（我使用支持库中的片段）。

据我了解，使用managedQuery()并不是真正合适的方法，即使不建议使用，因为实际上封装在该方法中的某些逻辑实际上是在主线程上执行的，据reQuery()我所知具体是在Activity重新启动。

因此，我一直试图Loader第一次熟悉该类，只是看到了这一点：

"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自己的内容提供程序，并可能阻止其他应用程序访问它，然后我ContentProvider通过developer.android.com
阅读了文档中的以下内容：

"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

但是我对这个项目并不熟悉，并且不确定是否可以在生产环境中使用它。

因此，现在我所能想到的就是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，确保在需要时取消它们，而不会取消它们。

还有其他选择吗？

Answer 1

您可以扩展Loader类，以执行其他异步工作，例如直接从数据库中加载。

这是一个例子

编辑： 添加了一个更好的Loader使用示例。

最终设法找到了真正帮助我了解事情原理的教程。

通过扩展加载程序类，您可以避免与内容观察者打交道，并且它相当容易实现（最后），需要在代码中进行的修改是

• 添加的实现LoaderManager.LoaderCallbacks<D>，D您的数据列表在哪里（代码段1）
• 创建您的加载程序类，复制代码段2，并添加来自数据库的数据加载
• 最后，调用加载程序1进行初始化，然后刷新重启调用。（摘要2和3）

片段1： 如何将加载程序与片段“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);
    // ... 
  }

  /* ... */
}

片段2：
您的自定义加载程序的架构：（我对观察者的观点进行了两次评论，因为我认为从一开始就很难实现它，您可以简单地调用加载程序而不会弄乱自动刷新）

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;
}

片段3： 如何首次调用加载程序（仅）

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

片段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：

• 片段1：从此处提取的加载程序的用法
• 片段2：此处提供更多信息和逻辑，贯穿整篇文章
• 片段3和4：只是装载程序的用法。

创建者还将这些的完整代码上传到github