AsyncTaskLoader基本示例.(机器人)

Question

我在我的应用程序中使用Loader,并根据我使用此Loader在COntacts上执行的查询得到的结果,我执行一些计算并将它们存储回Sqlite DB中.我希望这个操作是异步的,但我在使用Async任务之间感到困惑,因为我要返回很多不同的数据类型,或者我应该使用简单的处理程序或AsyncTaskLoader,我希望它很简单,因为我是新手装载机.我试图搜索AsyncTaskLoader的例子,但它似乎是火箭科学,在我的场景的上下文中的三个中的任何一个的基本和简单的功能示例将是非常有帮助的.

Answer 1

如果你想使用AsyncTaskLoader,这里有一个很好的样本.

编辑:我决定做一个更简单的解决方案(基于这个回购):

public abstract class AsyncTaskLoaderEx<T> extends AsyncTaskLoader<T> {
    private static final AtomicInteger sCurrentUniqueId = new AtomicInteger(0);
    private T mData;
    public boolean hasResult = false;

    public static int getNewUniqueLoaderId() {
        return sCurrentUniqueId.getAndIncrement();
    }

    public AsyncTaskLoaderEx(final Context context) {
        super(context);
        onContentChanged();
    }

    @Override
    protected void onStartLoading() {
        if (takeContentChanged())
            forceLoad();
        //this part should be removed from support library 27.1.0 :
        //else if (hasResult)
        //    deliverResult(mData);
    }

    @Override
    public void deliverResult(final T data) {
        mData = data;
        hasResult = true;
        super.deliverResult(data);
    }

    @Override
    protected void onReset() {
        super.onReset();
        onStopLoading();
        if (hasResult) {
            onReleaseResources(mData);
            mData = null;
            hasResult = false;
        }
    }

    protected void onReleaseResources(T data) {
        //nothing to do.
    }

    public T getResult() {
        return mData;
    }
}

用法:

在你的活动中:

        getSupportLoaderManager().initLoader(TASK_ID, TASK_BUNDLE, new LoaderManager.LoaderCallbacks<Bitmap>() {
            @Override
            public Loader<Bitmap> onCreateLoader(final int id, final Bundle args) {
                return new ImageLoadingTask(MainActivity.this);
            }

            @Override
            public void onLoadFinished(final Loader<Bitmap> loader, final Bitmap result) {
                if (result == null)
                    return;
                //TODO use result
            }

            @Override
            public void onLoaderReset(final Loader<Bitmap> loader) {
            }
        });

内部静态类或普通类:

private static class ImageLoadingTask extends AsyncTaskLoaderEx<Bitmap> {

    public ImageLoadingTask (Context context) {
        super(context);
    }

    @Override
    public Bitmap loadInBackground() {
        //TODO load and return bitmap
    }
}

更新:从支持库27.1.0开始,事情发生了一些变化(链接在这里):

在版本27.1.0中,每次启动Activity时都会调用onStartLoading().由于在onStartLoading()中调用了deliverResult(),因此触发onLoadFinished().这是按预期工作的.

您应该从onStartLoading()中删除对deliverResult()的调用,因为它不需要(加载器已经提供了在loadInBackground()中计算的结果,而您无需任何额外的工作).

我已经更新了上面的代码以进行此更改.

Answer 2

从Honeycomb和v4兼容性库可以使用AsyncTaskLoader.据我所知,它AsyncTaskLoader可以通过配置更改生存,如屏幕翻转.但使用AsyncTask你可能会搞乱配置更改.

关键信息:AsyncTaskLoader是子类Loader.此类执行与AsyncTask相同的功能,但更好一点,它在处理配置更改(屏幕方向)时也很有用.

这里给出了一个很好的例子和解释. http://www.javacodegeeks.com/2013/01/android-loaders-versus-asynctask.html

谷歌直接在API文档中有一个很好的例子.Android设计模式提供了更多细节和Loaders背后的原因.

本教程将帮助您.http://www.javacodegeeks.com/2013/08/android-custom-loader-to-load-data-directly-from-sqlite-database.html

Answer 3

这是一步一步的实施教程AsyncTaskLoader.或者在Medium上查看同一篇文章

1. LoaderManager.LoaderCallbacks<String>在MainActivity上实现并创建一个static int唯一标识您的加载器并创建一个String键以将字符串url传递给您的加载器

   public class MainActivity extends AppCompatActivity 
                implements LoaderManager.LoaderCallbacks<String>{
       public static final int OPERATION_SEARCH_LOADER = 22;
       public static final String OPERATION_QUERY_URL_EXTRA = "query";
   //...}
   

2. 覆盖onCreateLoader,onLoadFinished并且onLoaderReset内MainActivity功能

   @Override
   public Loader<String> onCreateLoader(int id, final Bundle args) {
       //Here we will initiate AsyncTaskLoader
       return null;
   }
   
   @Override
   public void onLoadFinished(Loader<String> loader, String operationResult) {
       //Think of this as AsyncTask onPostExecute method, the result from onCreateLoader will be available in operationResult variable and here you can update UI with the data fetched.
       Log.d("MAINACTIVITY","result : "+ operationResult);
   }
   
   @Override
   public void onLoaderReset(Loader<String> loader) {
       //Don't bother about it, Android Studio will override it for you
   }
   

3. inside onCreateLoader()返回一个new AsyncTaskLoader<String>作为匿名内部类,this作为构造函数的参数并覆盖loadInBackground&onStartLoading在匿名内部类中

   @Override
   public Loader<String> onCreateLoader(int id, final Bundle args) {
       return new AsyncTaskLoader<String>(this) {
           @Override
           public String loadInBackground() {
               //Think of this as AsyncTask doInBackground() method, here you will actually initiate Network call
               return null;
           }
   
           @Override
           protected void onStartLoading() {
              //Think of this as AsyncTask onPreExecute() method,start your progress bar,and at the end call forceLoad(); 
              forceLoad();
           }
       };
   }
   

4. 内部loadInBackground使用HTTPUrlConnection或OKHttp或您使用的任何内容进行网络调用.

    @Override
       public String loadInBackground() {
           String url = args.getString(OPERATION_QUERY_URL_EXTRA);//This is a url in string form 
           if (url!=null&&"".equals(url)) {
               return null;//if url is null, return
           }
           String operationResult="";
           try {
               operationResult = NetworkUtils.getResponseFromHttpUrl(url);//This just create a HTTPUrlConnection and return result in strings
           } catch (IOException e) {
               e.printStackTrace();
           }
           return operationResult;
       }
   

5. 在内部onCreate使用OPERATION_SEARCH_LOADER作为ID初始化加载程序,对于bundle初始化为null,并为上下文初始化

   getSupportLoaderManager().initLoader(OPERATION_SEARCH_LOADER, null, this);
   

6. 现在,无论何时何地想要触发加载器,都要调用此方法

   private void makeOperationSearchQuery(String url) {
   
       // Create a bundle called queryBundle
       Bundle queryBundle = new Bundle();
       // Use putString with OPERATION_QUERY_URL_EXTRA as the key and the String value of the URL as the value
       queryBundle.putString(OPERATION_QUERY_URL_EXTRA,url);
       // Call getSupportLoaderManager and store it in a LoaderManager variable
       LoaderManager loaderManager = getSupportLoaderManager();
       // Get our Loader by calling getLoader and passing the ID we specified
       Loader<String> loader = loaderManager.getLoader(OPERATION_SEARCH_LOADER);
       // If the Loader was null, initialize it. Else, restart it.
       if(loader==null){
           loaderManager.initLoader(OPERATION_SEARCH_LOADER, queryBundle, this);
       }else{
           loaderManager.restartLoader(OPERATION_SEARCH_LOADER, queryBundle, this);
       }
   }
   

Walla,你已经完成了,只是为了提醒你NetworkUtils.getResponseFromHttpUrl(url);是我的自定义函数,它将字符串转换成URL反过来用来创建它HTTPUrlConnection

Answer 4

我喜欢这个简短的例子 AsyncTask和AsyncTaskLoader.

class FooLoader extends AsyncTaskLoader {
   public FooLoader(Context context, Bundle args) {
      super(context);
      // do some initializations here
   }
   public String loadInBackground() {
      String result = "";
      // ...
      // do long running tasks here
      // ...
      return result;
   }
} 


class FooLoaderClient implements LoaderManager.LoaderCallbacks {
   Activity context;
   // to be used for support library:
   // FragmentActivity context2;
   public Loader onCreateLoader(int id, Bundle args) {
      // init loader depending on id
      return new FooLoader(context, args);
   }
   public void onLoadFinished(Loader loader, String data) {
      // ...
      // update UI here
      //
   }
   public void onLoaderReset(Loader loader) {
      // ...
   }
   public void useLoader() {
      Bundle args = new Bundle();
      // ...
      // fill in args
      // ...
      Loader loader = 
         context.getLoaderManager().initLoader(0, args, this);
      // with support library: 
      // Loader loader = 
      //    context2.getSupportLoaderManager().initLoader(0, args, this);
      // call forceLoad() to start processing
      loader.forceLoad();
   }
}