CursorLoader 和 CursorTreeAdapter

Question

我正在尝试将 CursorLoader 与 CursorTreeAdapter 一起使用，但遇到了一个我无法弄清楚的问题。 （如果您有这方面的工作示例，请随意跳过其余部分并将其附在下面。我将非常感激） 第一次开群，一切正常。如果我随后关闭并重新打开该组，则会收到溢出错误。这是错误：

V/SpellBook(29520): Activity being created 
D/SpellBook(29520): onCreateLoader for id 123456 
V/SpellBook(29520): Resuming 
V/SpellBook(29520): Processing query for uri content://com.zalzala.spellbook.SpellProvider/levels/bard 
D/SpellBook(29520): onLoadFinished() for id 123456 
D/SpellBook(29520): onCreateLoader for id 3 
V/SpellBook(29520): Processing query for uri content://com.zalzala.spellbook.SpellProvider/class/bard/3 
D/SpellBook(29520): onLoadFinished() for id 3 

这里一切都很好。到目前为止，我已经打开了活动并打开了组号 3。 如果我关闭组 3 并再次打开它，会发生以下情况：

D/SpellBook(29520): onLoadFinished() for id 3 
D/SpellBook(29520): onLoadFinished() for id 3 
D/SpellBook(29520): onLoadFinished() for id 3 
D/SpellBook(29520): onLoadFinished() for id 3 
D/SpellBook(29520): onLoadFinished() for id 3 
(... and a lot more of these) 
    E/AndroidRuntime(29520): FATAL EXCEPTION: main 
E/AndroidRuntime(29520): java.lang.StackOverflowError 
E/AndroidRuntime(29520):    at java.util.HashMap.get(HashMap.java:302) 
E/AndroidRuntime(29520):    at android.database.sqlite.SQLiteCursor.getColumnIndex(SQLiteCursor.java:355)
E/AndroidRuntime(29520):    at android.database.CursorWrapper.getColumnIndex(CursorWrapper.java:67) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment$MyExpandableListAdapter.getChildrenCursor(SpellListView.java:216) 
E/AndroidRuntime(29520):    at android.widget.CursorTreeAdapter.getChildrenCursorHelper(CursorTreeAdapter.java:106) 
E/AndroidRuntime(29520):    at android.widget.CursorTreeAdapter.setChildrenCursor(CursorTreeAdapter.java:159) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment.onLoadFinished(SpellListView.java:183) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment.onLoadFinished(SpellListView.java:1) 
E/AndroidRuntime(29520):    at android.support.v4.app.LoaderManagerImpl$LoaderInfo.callOnLoadFinished(LoaderManager.java:413) 
E/AndroidRuntime(29520):    at android.support.v4.app.LoaderManagerImpl.initLoader(LoaderManager.java:547) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment$MyExpandableListAdapter.getChildrenCursor(SpellListView.java:217) 
E/AndroidRuntime(29520):    at android.widget.CursorTreeAdapter.getChildrenCursorHelper(CursorTreeAdapter.java:106) 
E/AndroidRuntime(29520):    at android.widget.CursorTreeAdapter.setChildrenCursor(CursorTreeAdapter.java:159) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment.onLoadFinished(SpellListView.java:183) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment.onLoadFinished(SpellListView.java:1) 
E/AndroidRuntime(29520):    at android.support.v4.app.LoaderManagerImpl$LoaderInfo.callOnLoadFinished(LoaderManager.java:413) 
E/AndroidRuntime(29520):    at android.support.v4.app.LoaderManagerImpl.initLoader(LoaderManager.java:547) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment$MyExpandableListAdapter.getChildrenCursor(SpellListView.java:217) 
E/AndroidRuntime(29520):    at android.widget.CursorTreeAdapter.getChildrenCursorHelper(CursorTreeAdapter.java:106) 
E/AndroidRuntime(29520):    at android.widget.CursorTreeAdapter.setChildrenCursor(CursorTreeAdapter.java:159) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment.onLoadFinished(SpellListView.java:183) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment.onLoadFinished(SpellListView.java:1) 
E/AndroidRuntime(29520):    at android.support.v4.app.LoaderManagerImpl$LoaderInfo.callOnLoadFinished(LoaderManager.java:413) 
E/AndroidRuntime(29520):    at android.support.v4.app.LoaderManagerImpl.initLoader(LoaderManager.java:547) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment$MyExpandableListAdapter.getChildrenCursor(SpellListView.java:217) 
E/AndroidRuntime(29520):    at android.widget.CursorTreeAdapter.getChildrenCursorHelper(CursorTreeAdapter.java:106) 
E/AndroidRuntime(29520):    at android.widget.CursorTreeAdapter.setChildrenCursor(CursorTreeAdapter.java:159) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment.onLoadFinished(SpellListView.java:183) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment.onLoadFinished(SpellListView.java:1) 
E/AndroidRuntime(29520):    at android.support.v4.app.LoaderManagerImpl$LoaderInfo.callOnLoadFinished(LoaderManager.java:413) 
E/AndroidRuntime(29520):    at android.support.v4.app.LoaderManagerImpl.initLoader(LoaderManager.java:547) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment$MyExpandableListAdapter.getChildrenCursor(SpellListView.java:217) 
E/AndroidRuntime(29520):    at android.widget.CursorTreeAdapter.getChildrenCursorHelper(CursorTreeAdapter.java:106) 
E/AndroidRuntime(29520):    at android.widget.CursorTreeAdapter.setChildrenCursor(CursorTreeAdapter.java:159) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment.onLoadFinished(SpellListView.java:183) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment.onLoadFinished(SpellListView.java:1) 
E/AndroidRuntime(29520):    at android.support.v4.app.LoaderManagerImpl$LoaderInfo.callOnLoadFinished(LoaderManager.java:413) 
E/AndroidRuntime(29520):    at android.support.v4.app.LoaderManagerImpl.initLoader(LoaderManager.java:547) 
E/AndroidRuntime(29520):    at com.zalzala.spellbook.SpellListView$ExpandableListCursorLoaderFragment$MyExpandableListAdapter.getChildrenCursor(SpellListView.java:217) 
E/AndroidRuntime(29520):    at android.widget.CursorTreeAdapter.getChildrenCursorHelper(CursorTreeAdapter.ja 
W/ActivityManager( 6887):   Force finishing activity com.zalzala.spellbook/.SpellListView

因此，由于某种原因，在重新打开组时，子光标会一遍又一遍地调用 onLoadFinished() 。

这是我的加载器实现：

    @Override
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        // TODO Auto-generated method stub
        Log.d(Spellbook.TAG,"onCreateLoader for id "+id);
        if (id <123456){
            Uri spellUri = Uri.withAppendedPath(SpellProvider.CONTENT_URI, "class");
            spellUri = Uri.withAppendedPath(spellUri, mCLASS);
            spellUri = ContentUris.withAppendedId(spellUri, id);
            return new CursorLoader(getActivity(), spellUri, null, null, null, null);
        }else {
            //get group cursor
            Uri groupUri = Uri.withAppendedPath(SpellProvider.CONTENT_URI, "levels");
            groupUri = Uri.withAppendedPath(groupUri, mCLASS);
            return new CursorLoader(getActivity(), groupUri, null, null, null, null);
        }
    }

    @Override
    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
        // TODO Auto-generated method stub
        int id = loader.getId();
        Log.d(Spellbook.TAG,"onLoadFinished() for id "+id);
        if (id < 123456){
            //child cursor
            ((CursorTreeAdapter) mAdapter).setChildrenCursor(id, data);
        } else {
            ((CursorTreeAdapter) mAdapter).setGroupCursor(data);
        }

    }

    @Override
    public void onLoaderReset(Loader<Cursor> loader) {
        // TODO Auto-generated method stub
        int id = loader.getId();
        Log.d(Spellbook.TAG,"onLoaderReset() for id "+id);
        if (id < 123456){
            //child cursor
            ((CursorTreeAdapter) mAdapter).setChildrenCursor(id, null);
        } else {
            ((CursorTreeAdapter) mAdapter).setGroupCursor(null);
        }

    }

我使用构造函数调用 CursorTreeAdapter，该构造函数将自动重新查询指定为 false。

    public class MyExpandableListAdapter extends CursorTreeAdapter {

        public MyExpandableListAdapter(Cursor cursor, Context context) {
            super(cursor, context, false); //do not auto requery. pretty sure CursorLoader needs this.
        }

        @Override
        protected Cursor getChildrenCursor(Cursor groupCursor) {
            // Given the group, we return a cursor for all the children within that group 
            int id = groupCursor.getInt(groupCursor.getColumnIndex(SpellDbAdapter.KEY_LEVEL));
            getLoaderManager().initLoader(id, null, ExpandableListCursorLoaderFragment.this);
            return null;
        }

感谢您的帮助！

Answer 1

这是一个老问题，但仍然没有正确的答案（现在有了）。

MyExpandableListAdapter 有一个 getChildrenCursor(Cursor) 方法：

@Override
protected Cursor getChildrenCursor(Cursor groupCursor) {
    int id = groupCursor.getInt(groupCursor.getColumnIndex(SpellDbAdapter.KEY_LEVEL));
    getLoaderManager().initLoader(id, null, ExpandableListCursorLoaderFragment.this);
    return null;
}

该方法通过 initLoader 调用启动 Loader。加载数据后，将调用 onLoadFinished 并设置子游标：

@Override
public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
    // some code
    ((CursorTreeAdapter) mAdapter).setChildrenCursor(id, data);
    // some more code
}

CursorTreeAdapter.setChildrenCursor 方法如下所示：

public void setChildrenCursor(int groupPosition, Cursor childrenCursor) {
    MyCursorHelper childrenCursorHelper = getChildrenCursorHelper(groupPosition);
    childrenCursorHelper.changeCursor(childrenCursor, false);
}

它通过调用 CursorTreeAdapter.getChildrenCursorHelper 检索childrenCursorHelper：

synchronized MyCursorHelper getChildrenCursorHelper(int groupPosition, boolean requestCursor) {
    MyCursorHelper cursorHelper = mChildrenCursorHelpers.get(groupPosition);

    if (cursorHelper == null) {
        if (mGroupCursorHelper.moveTo(groupPosition) == null) return null;

        final Cursor cursor = getChildrenCursor(mGroupCursorHelper.getCursor());
        cursorHelper = new MyCursorHelper(cursor);
        mChildrenCursorHelpers.put(groupPosition, cursorHelper);
    }

    return cursorHelper;
}

在某些情况下，childrenCursorHelper 尚不存在，并且方法现在调用 getChildrenCursor(Cursor)，这是启动整个调用链的方法。从逻辑上讲，我们会遇到 StackOverflowError。

这是 CursorTreeAdapter 类中的一个错误。虽然其他人建议改用 BaseExpandableAdapter，但我建议创建自定义 CursorTreeAdapter 类并进行必要的更改和修复。

通过更改 getChildrenCursorHelper 方法可以轻松修复 StackOverflowError。首先创建MyCursorHelper，将其添加到mChildrenCursorHelpers，然后如果retrieveChildCursor设置为true（新参数），则检索子光标。从 setChildrenCursor 调用该方法意味着我们传递 false 以避免陷入无限递归。

synchronized MyCursorHelper getChildrenCursorHelper(int groupPosition, boolean retrieveChildCursor) {
    MyCursorHelper cursorHelper = mChildrenCursorHelpers.get(groupPosition);

    if (cursorHelper == null && mGroupCursorHelper.moveTo(groupPosition) != null) {
        cursorHelper = new MyCursorHelper(null);
        mChildrenCursorHelpers.put(groupPosition, cursorHelper);

        if (retrieveChildCursor) {
            final Cursor cursor = getChildrenCursor(mGroupCursorHelper.getCursor());
            if (cursor != null) {
                cursorHelper.changeCursor(cursor, false);
            }
        }
    }

    return cursorHelper;
}

还要删除 MyCursorHelper.deactivate 方法中的代码，因为 CursorLoader 负责关闭过时的游标。

Answer 2

好吧，所以与 Google 的某人交谈，扩展列表可能早就应该被弃用了。如果您确实需要使用游标加载器和扩展列表，请扩展基本适配器。如果您确实不需要它，请继续而不使用游标加载器。

请随时纠正我:)

Answer 3

在 onGroupCollapsed() 上调用 destroyLoader()。但这并不能完全解决问题。查看 CursorTreeAdapter 实现，确保我不要将它与 CursorLoader 一起使用。

扩展BaseExpandableAdapter确实更好。这种方式可以避免不必要地（在使用加载器的情况下）使用内容观察器。在我的自定义适配器中，我将创建的加载器保留在稀疏数组中，并在组崩溃时销毁它们。效果很好:)

Answer 4

我认为当我们使用 initLoader 时，它会使堆栈变满，
尝试使用 restartLoader 代替。

@Override
    protected Cursor getChildrenCursor(Cursor groupCursor) {
        // TODO Auto-generated method stub          
        Bundle b = new Bundle();
        //b.putString("Artist", "\""+groupCursor.getString(groupCursor.getColumnIndex(artist.ARTIST))+"\"");
        b.putString("Artist", groupCursor.getString(groupCursor.getColumnIndex(artist.ARTIST)));
        getLoaderManager().restartLoader(groupCursor.getPosition(), b, MpArtist.this);
        return null;
    }

Answer 5

试试这个：

@Override
public void onLoadFinished(Loader<Cursor> loader, final Cursor data) {
    // ...
    final int id = loader.getId();
    new Handler().post(new Runnable() {
        @Override
        public void run() {
            ((CursorTreeAdapter) mAdapter).setChildrenCursor(id, data);
        }
    });
 }