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。 （如果你有一个有效的例子，请随意跳过其余部分并将其附在下面。我会非常感激） 我第一次打开一个小组，一切正常。如果我然后关闭并重新打开该组，则会出现溢出错误。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第3组。如果我关闭第3组并再次打开它会发生什么：

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

因此出于某种原因onLoadFinished（）在重新打开组时会反复调用子游标。

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auto requery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，但仍然没有正确答案（现在已经存在）。

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;
}

该方法使用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：

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

它通过调用CursorTreeAdapter.getChildrenCursorHelper来检索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还不存在，并且该方法现在调用getChildrenCursor（Cursor），这是启动整个调用链的方法。从逻辑上讲，我们将遇到StackOverflowError。

这是CursorTreeAdapter类中的错误。虽然其他人建议使用BaseExpandableAdapter，但我建议创建一个自定义的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，然后在retrieveChildCursor设置为true（新参数）时检索子游标。从setChildrenCursor调用方法意味着我们传递false以不进入无限递归。

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;
}

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

Answer 2

好的，所以在谷歌与某人交谈时，扩展列表可能很久以前就已被弃用了。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。如果你真的不需要它，请继续使用游标加载器。

随意纠正我:)

Answer 3

在destroyLoader()上致电onGroupCollapsed()。但它并没有完全解决问题。查看CursorTreeAdapter实现，请确保不要将其与CursorLoader一起使用。

扩展BaseExpandableAdapter真的更好。这种方式允许避免不必要的（在使用加载器的情况下）内容观察者的使用。在我的自定义适配器中，我保持稀疏数组中的创建加载器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，它会使堆栈满， 尝试使用restartLoader。

@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);
        }
    });
 }