Question

我使用AsyncTaskLoader从数据库查询加载游标。我关注Android Developers示例： http://developer.android.com/reference/android/content/AsyncTaskLoader.html

但不知何故，在此片段（使用加载器）之后添加到页面适配器的片段不会附加到活动上，并且当它尝试使用需要活动的方法（如getString（））时被抛出并说这个片段没有附加到任何活动。

以下是一些代码：

将片段添加到页面适配器。

mAdapter = new PlaceFragmentPagerAdapter(getSupportFragmentManager());


NewFragment newFrag = new NewFragment();
mAdapter.addFragment(newShiftFrag);

ListFragment listFrag = new ListFragment();
mAdapter.addFragment(listFrag);

SettingsFragment settingsFrag = new SettingsFragment();
mAdapter.addFragment(settingsFrag);

mPager = (ViewPager)findViewById(R.id.pager);
mPager.setAdapter(mAdapter);

AsyncTaskLoader实现：

抽象公共类AbstractCursorLoader扩展了AsyncTaskLoader {

    abstract protected Cursor buildCursor();
    Cursor lastCursor=null;

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

    /** 
     * Runs on a worker thread, loading in our data. Delegates
     * the real work to concrete subclass' buildCursor() method. 
     */
    @Override
    public Cursor loadInBackground() {
        Cursor cursor=buildCursor();

        if (cursor!=null) {
            // Ensure the cursor window is filled
            cursor.getCount();
        }

        return(cursor);
    }

    /**
     * Runs on the UI thread, routing the results from the
     * background thread to whatever is using the Cursor
     * (e.g., a CursorAdapter).
     */
    @Override
    public void deliverResult(Cursor cursor) {
        if (isReset()) {
            // An async query came in while the loader is stopped
            if (cursor!=null) {
                cursor.close();
            }

            return;
        }

        Cursor oldCursor=lastCursor;
        lastCursor=cursor;

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

        if (oldCursor!=null && oldCursor!=cursor && !oldCursor.isClosed()) {
            oldCursor.close();
        }
    }

    /**
     * Starts an asynchronous load of the list data.
     * When the result is ready the callbacks will be called
     * on the UI thread. If a previous load has been completed
     * and is still valid the result may be passed to the
     * callbacks immediately.
     * 
     * Must be called from the UI thread.
     */
    @Override
    protected void onStartLoading() {
        if (lastCursor!=null) {
            deliverResult(lastCursor);
        }
        if (takeContentChanged() || lastCursor==null) {
            forceLoad();
        }
    }

    /**
     * Must be called from the UI thread, triggered by a
     * call to stopLoading().
     */
    @Override
    protected void onStopLoading() {
        // Attempt to cancel the current load task if possible.
        cancelLoad();
    }

    /**
     * Must be called from the UI thread, triggered by a
     * call to cancel(). Here, we make sure our Cursor
     * is closed, if it still exists and is not already closed.
     */
    @Override
    public void onCanceled(Cursor cursor) {
        if (cursor!=null && !cursor.isClosed()) {
            cursor.close();
        }
    }

    /**
     * Must be called from the UI thread, triggered by a
     * call to reset(). Here, we make sure our Cursor
     * is closed, if it still exists and is not already closed.
     */
    @Override
    protected void onReset() {
        super.onReset();

        // Ensure the loader is stopped
        onStopLoading();

        if (lastCursor!=null && !lastCursor.isClosed()) {
            lastCursor.close();
        }

        lastCursor=null;
    }
}

    private static class ListLoader extends AbstractCursorLoader {
        private String mName;

        public ShiftsListLoader(Context context, String name) {
            super(context);
            mName = name;
        }

        @Override
        protected Cursor buildCursor() {
            PlacesHandler wph = new PlacesHandler(this.getContext());
            return wph.GetShifts(mName);
        }
    }

初始化加载程序：

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

    // Give some text to display if there is no data.  In a real
    // application this would come from a resource.
    // TODO change to resource and back
    setEmptyText("Nothing here..");

    // Start out with a progress indicator.
    setListShown(false);



    // Prepare the loader.  Either re-connect with an existing one,
    // or start a new one.
    getLoaderManager().initLoader(0, null, this);

}

    @Override
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        return new ListLoader(getActivity(), mWorkPlaceName);
    }
    @Override
    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
        // Create an empty adapter we will use to display the loaded data.
    mAdapter = new ListCursorAdapter(getActivity(), data, CursorAdapter.FLAG_REGISTER_CONTENT_OBSERVER);
    setListAdapter(mAdapter);

    }
@Override
public void onLoaderReset(Loader<Cursor> loader) {
    // TODO Auto-generated method stub

}

我真的不知道为什么会这样。

P.S。我在评论中制作代码块时遇到了一些问题，我发现它有一个bug，很抱歉。

提前谢谢，Elad。

Answer 1

简短的回答是，在收到适当的生命周期回调信号之前，你永远不应该假设某个片段处于任何特定状态。

您所看到的是在ViewPager利用的ICS期间添加的优化。 FragmentPagerAdapter通过调用setUserVisibleHint特别标记屏幕外片段，因为用户不可见。 FragmentManager使用它来优先处理加载器的执行方式，以便用户首先看到完全加载的可见页面，加载侧页不会减慢加载可见页面的过程。具体来说，它会延迟将片段移动到“已启动”状态，也就是当加载器开始运行时。

如果用户在此过程中滚动到另一个页面，FragmentManager会将片段移动到启动状态并立即开始运行其加载器作为FragmentPagerAdapter＃setPrimaryItem（）的一部分，因为此方法将当前页面标记为现在用户可见。

使用AsyncTaskLoader的片段会影响附加到活动的其他片段

1 个答案: