AsyncTaskLoader onLoadFinished 有一个挂起的任务和配置更改

Question

我正在尝试使用AsyncTaskLoader 在后台加载数据以填充详细视图以响应选择的列表项。我已经让它大部分工作了，但我仍然有一个问题。如果我在列表中选择第二个项目，然后在第一个选定项目的加载完成之前旋转设备，那么onLoadFinished() 调用将报告正在停止的活动而不是新活动.当只选择一个项目然后旋转时，这很好用。

这是我正在使用的代码。活动：

public final class DemoActivity extends Activity
        implements NumberListFragment.RowTappedListener,
                   LoaderManager.LoaderCallbacks<String> {

    private static final AtomicInteger activityCounter = new AtomicInteger(0);

    private int myActivityId;

    private ResultFragment resultFragment;

    private Integer selectedNumber;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        myActivityId = activityCounter.incrementAndGet();
        Log.d("DemoActivity", "onCreate for " + myActivityId);

        setContentView(R.layout.demo);

        resultFragment = (ResultFragment) getFragmentManager().findFragmentById(R.id.result_fragment);

        getLoaderManager().initLoader(0, null, this);

    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        Log.d("DemoActivity", "onDestroy for " + myActivityId);
    }

    @Override
    public void onRowTapped(Integer number) {
        selectedNumber = number;
        resultFragment.setResultText("Fetching details for item " + number + "...");
        getLoaderManager().restartLoader(0, null, this);
    }

    @Override
    public Loader<String> onCreateLoader(int id, Bundle args) {
        return new ResultLoader(this, selectedNumber);
    }

    @Override
    public void onLoadFinished(Loader<String> loader, String data) {
        Log.d("DemoActivity", "onLoadFinished reporting to activity " + myActivityId);
        resultFragment.setResultText(data);
    }

    @Override
    public void onLoaderReset(Loader<String> loader) {

    }

    static final class ResultLoader extends AsyncTaskLoader<String> {

        private static final Random random = new Random();

        private final Integer number;

        private String result;

        ResultLoader(Context context, Integer number) {
            super(context);
            this.number = number;
        }

        @Override
        public String loadInBackground() {
            // Simulate expensive Web call
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            return "Item " + number + " - Price: $" + random.nextInt(500) + ".00, Number in stock: " + random.nextInt(10000);
        }

        @Override
        public void deliverResult(String data) {
            if (isReset()) {
                // An async query came in while the loader is stopped
                return;
            }

            result = data;

            if (isStarted()) {
                super.deliverResult(data);
            }
        }

        @Override
        protected void onStartLoading() {
            if (result != null) {
                deliverResult(result);
            }

            // Only do a load if we have a source to load from
            if (number != null) {
                forceLoad();
            }
        }

        @Override
        protected void onStopLoading() {
            // Attempt to cancel the current load task if possible.
            cancelLoad();
        }

        @Override
        protected void onReset() {
            super.onReset();

            // Ensure the loader is stopped
            onStopLoading();

            result = null;
        }

    }

}

列表片段：

public final class NumberListFragment extends ListFragment {

    interface RowTappedListener {

        void onRowTapped(Integer number);

    }

    private RowTappedListener rowTappedListener;

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);

        rowTappedListener = (RowTappedListener) activity;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        ArrayAdapter<Integer> adapter = new ArrayAdapter<Integer>(getActivity(),
                                                                  R.layout.simple_list_item_1,
                                                                  Arrays.asList(1, 2, 3, 4, 5, 6));
        setListAdapter(adapter);

    }

    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        ArrayAdapter<Integer> adapter = (ArrayAdapter<Integer>) getListAdapter();
        rowTappedListener.onRowTapped(adapter.getItem(position));
    }

}

结果片段：

public final class ResultFragment extends Fragment {

    private TextView resultLabel;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View root = inflater.inflate(R.layout.result_fragment, container, false);

        resultLabel = (TextView) root.findViewById(R.id.result_label);
        if (savedInstanceState != null) {
            resultLabel.setText(savedInstanceState.getString("labelText", ""));
        }

        return root;
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);

        outState.putString("labelText", resultLabel.getText().toString());
    }

    void setResultText(String resultText) {
        resultLabel.setText(resultText);
    }

}

我已经能够使用普通的 AsyncTasks 来完成这项工作，但我正在尝试了解更多关于 Loaders 的信息，因为它们会自动处理配置更改。

---

编辑：我想我可能已经通过查看LoaderManager 的来源找到了问题所在。在配置更改后调用initLoader 时，LoaderInfo 对象的mCallbacks 字段将使用新活动更新为LoaderCallbacks 的实现，正如我所料。

public <D> Loader<D> initLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<D> callback) {
    if (mCreatingLoader) {
        throw new IllegalStateException("Called while creating a loader");
    }

    LoaderInfo info = mLoaders.get(id);

    if (DEBUG) Log.v(TAG, "initLoader in " + this + ": args=" + args);

    if (info == null) {
        // Loader doesn't already exist; create.
        info = createAndInstallLoader(id, args,  (LoaderManager.LoaderCallbacks<Object>)callback);
        if (DEBUG) Log.v(TAG, "  Created new loader " + info);
    } else {
        if (DEBUG) Log.v(TAG, "  Re-using existing loader " + info);
        info.mCallbacks = (LoaderManager.LoaderCallbacks<Object>)callback;
    }

    if (info.mHaveData && mStarted) {
        // If the loader has already generated its data, report it now.
        info.callOnLoadFinished(info.mLoader, info.mData);
    }

    return (Loader<D>)info.mLoader;
}

但是，当有一个挂起的加载器时，主 LoaderInfo 对象也有一个 mPendingLoader 字段并引用了 LoaderCallbacks，并且这个对象永远不会随着 @987654339 中的新活动而更新@ 场地。我希望看到代码看起来像这样：

// This line was already there
info.mCallbacks = (LoaderManager.LoaderCallbacks<Object>)callback;
// This line is not currently there
info.mPendingLoader.mCallbacks = (LoaderManager.LoaderCallbacks<Object>)callback;

似乎正因为如此，挂起的加载程序在旧的活动实例上调用onLoadFinished。如果我在此方法中设置断点并使用调试器进行我觉得缺少的调用，那么一切都会按我的预期进行。

新问题是：我是否发现了错误，或者这是预期的行为？

Answer 1

在大多数情况下，如果 Activity 已经销毁，您应该忽略此类报告。

public void onLoadFinished(Loader<String> loader, String data) {
    Log.d("DemoActivity", "onLoadFinished reporting to activity " + myActivityId);
    if (isDestroyed()) {
       Log.i("DemoActivity", "Activity already destroyed, report ignored: " + data);
       return;
    }
    resultFragment.setResultText(data);
}

您还应该在任何内部类中插入检查isDestroyed()。 Runnable - 是最常用的情况。

例如：

// UI thread
final Handler handler = new Handler();
Executor someExecutorService = ... ;
someExecutorService.execute(new Runnable() {
    public void run() {
        // some heavy operations
        ...
        // notification to UI thread
        handler.post(new Runnable() {
            // this runnable can link to 'dead' activity or any outer instance
            if (isDestroyed()) {
                return;
            }

            // we are alive
            onSomeHeavyOperationFinished();
        });
    }
});

但在这种情况下最好的方法是避免将 Activity 的强引用传递给另一个线程（AsynkTask、Loader、Executor 等）。

最可靠的解决方案在这里：

// BackgroundExecutor.java
public class BackgroundExecutor {
    private static final Executor instance = Executors.newSingleThreadExecutor();

    public static void execute(Runnable command) {
        instance.execute(command);
    }
}

// MyActivity.java
public class MyActivity extends Activity {
    // Some callback method from any button you want
    public void onSomeButtonClicked() {
        // Show toast or progress bar if needed

        // Start your heavy operation
        BackgroundExecutor.execute(new SomeHeavyOperation(this));
    }

    public void onSomeHeavyOperationFinished() {
        if (isDestroyed()) {
            return;
        }

        // Hide progress bar, update UI
    }
}

// SomeHeavyOperation.java
public class SomeHeavyOperation implements Runnable {
    private final WeakReference<MyActivity> ref;

    public SomeHeavyOperation(MyActivity owner) {
        // Unlike inner class we do not store strong reference to Activity here
        this.ref = new WeakReference<MyActivity>(owner);
    }

    public void run() {
        // Perform your heavy operation
        // ...
        // Done!

        // It's time to notify Activity
        final MyActivity owner = ref.get();
        // Already died reference
        if (owner == null) return;

        // Perform notification in UI thread
        owner.runOnUiThread(new Runnable() {
            public void run() {
                owner.onSomeHeavyOperationFinished();
            }
        });
    }
}

Answer 2

也许不是最好的解决方案，但...... 此代码每次都重新启动加载器，这很糟糕，但只能解决这个问题 - 如果您想使用加载器。

Loader l = getLoaderManager().getLoader(MY_LOADER);
if (l != null) {
    getLoaderManager().restartLoader(MY_LOADER, null, this);
} else {
    getLoaderManager().initLoader(MY_LOADER, null, this);
}

顺便说一句。我正在使用 Cursorloader ...

Answer 3

一种可能的解决方案是在自定义单例对象中启动 AsyncTask，并从 Activity 中的单例访问 onFinished() 结果。每次旋转屏幕时，执行 onPause() 或 onResume()，都会使用/访问最新的结果。如果您的单例对象中仍然没有结果，则您知道它仍然很忙，或者您可以重新启动任务。

另一种方法是使用像 Otto 这样的服务总线，或者使用 Service。

Answer 4

好的，如果我误解了任何内容，请原谅我试图理解这一点，但是当设备旋转时您会丢失对某些内容的引用。

试一试……

会添加

android:configChanges="orientation|keyboardHidden|screenSize"

在该活动的清单中修复您的错误？或阻止onLoadFinished() 说活动已停止？