防止AsyncTask在bindView中多次运行

Question

I have a custom CursorAdapter that is using multiple AsyncTask s in its bindView method to load images into a grid.

When bindView runs my AsyncTask s get launched multiple times. This has the effect of pushing up my heap size and can cause Out of Memory errors.

What is the best approach to take, to get AsyncTask s to run just once?

Answer 1

You can cache the results of the Asynctask. There is a very useful project on Github you might want to look into. https://github.com/nostra13/Android-Universal-Image-Loader

Answer 2

I think you are looking for AsyncTaskLoader or its support package implementation. It handles everything for you.

An example of how to use it:

public 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 garbag ecollected.
    // 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
    }

    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;
}

The source: androiddesignpatterns.com - Implementing Loaders (part 3)