//0，为唯一的ID，可以为任意整数，为Loader的唯一标识
//null，为Bundle类型，可以向Loader传递构造参数
//LoaderCallbacks，LoaderManager对Loader各事件的调用，参考下面讲到的 LoaderManager.LoaderCallbacks
getSupportLoaderManager().initLoader(0, null, new LoaderCallbacks<D>());

...
new LoaderManager.LoaderCallbacks<String>() {
      @Override
      public Loader<String> onCreateLoader(int id, Bundle args) {
        return new MyLoader();
      }
      ...
}

new LoaderManager.LoaderCallbacks<String>() {
      ...
      @Override
      public void onLoadFinished(Loader<String> loader, String data) {
          show(data);
      }
      ...
}

new LoaderManager.LoaderCallbacks<String>() {
      ...
      @Override
      public void onLoaderReset(Loader<String> loader) {
          show(null);
      }
      ...
}

//继承AsyncTaskLoader类，里面的泛型为返回的数据的类型，这里设为String
public class MyAsyncTaskLoader extends AsyncTaskLoader<String>{

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

  @Override
   public String loadInBackground() {
    //模拟加载
    try {
      Thread.sleep(3000);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    //返回获取到的数据
    return new String("MyAsyncTaskLoader Test Result");
  }  
}

public class BaseActivity extends AppCompatActivity implements LoaderManager.LoaderCallbacks{
  @Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.base_activity_layout);
//    addFragment();
    log("onCreate");
    loadData();
  }

  protected void loadData(){
    Log.e(getClassName(),"call");
    getSupportLoaderManager().initLoader(0, null, this);
  }

  protected String getClassName(){
    return getClass().getSimpleName();
  }

  @Override
  public Loader onCreateLoader(int id, Bundle args) {
    Log.e(getClassName(),"onCreateLoader");
    return new MyAsyncTaskLoader(BaseActivity.this);
  }

  @Override
  public void onLoadFinished(Loader loader, Object data) {
    Log.e(getClassName(),"data:"+data);
  }

  @Override
  public void onLoaderReset(Loader loader) {

  }
}

//可以把 ModernAsyncTask 看做 AsyncTask

final class LoadTask extends ModernAsyncTask<Void, Void, D> implements Runnable {
    ....
    @Override
    protected D doInBackground(Void... params) {
       ...
        D data = AsyncTaskLoader.this.onLoadInBackground();
       ...
    }
   .....
  }

public abstract class AsyncTaskLoader<D> extends Loader<D> {
....
volatile LoadTask mTask;
....
}

...
  @Override
  protected void onForceLoad() {
    ...
    mTask = new LoadTask();
    ...
    executePendingTask();
  }
  ...
  void executePendingTask() {
    ...
      if (mUpdateThrottle > 0) {
        ...
          mHandler.postAtTime(mTask, mLastLoadCompleteTime+mUpdateThrottle);
          return;
        }
      }
      ...
      mTask.executeOnExecutor(mExecutor, (Void[]) null);
    }
  }

public class Loader{
...
  public void forceLoad() {
    onForceLoad();
  }
...
}

public class CursorLoader extends AsyncTaskLoader<Cursor> {
  ...
  @Override
  protected void onStartLoading() {
    if (mCursor != null) {
      deliverResult(mCursor);
    }
    if (takeContentChanged() || mCursor == null) {
      forceLoad();
    }
  }
  ...
}

//继承AsyncTaskLoader类，里面的泛型为返回的数据的类型，这里设为String
public class MyAsyncTaskLoader extends AsyncTaskLoader<String>{

  public MyAsyncTaskLoader(Context context) {
    super(context);
  }
  
  //添加了这段代码
  @Override
  protected void onStartLoading() {
    forceLoad();
  }

  @Override
   public String loadInBackground() {
    //模拟加载
    try {
      Thread.sleep(3000);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    //返回获取到的数据
    return new String("MyAsyncTaskLoader Test Result");
  }  
}

LoaderManager的实现类为LoaderManagerImpl
init()方法里面创建 LoaderInfo info
info = createAndInstallLoader(id, args, (LoaderManager.LoaderCallbacks<Object>)callback); 进入 createAndInstallLoader 方法
mCallbacks.onLoadFinished(loader, data); 进入 onLoadFinished 方法
createLoader(id, args, callback) 进入 createLoader 方法
installLoader(info); 进入 installLoader 方法
info.start(); 进入 start 方法
mLoader.startLoading(); 进入 startLoading 方法
onStartLoading();