android - 使用 ExpandableListView/SimpleCursorTreeAdapter 在 UI 线程上运行 SQLite 查询

Question

我正在开发一个用于显示大量 RSS 提要的 Android 应用程序(是的，我知道已经有很多这样的应用程序了)。要显示的数据由内容提供者提供支持，我希望向后兼容 API 级别 4。

我正在使用 ExpandableListView 来显示三个不同 RSS 提要的内容。 ExpandableListView 的适配器是作为 SimpleCursorTreeAdapter 的子类实现的:

private class RssFeedLatestListAdapter extends SimpleCursorTreeAdapter {

    public static final String FEED_NAME_COLUMN = "feedName";

    public RssFeedLatestListAdapter(Context ctx, Cursor groupCursor, int groupLayout,
            String[] groupFrom, int[] groupTo, int childLayout, String[] childFrom,
            int[] childTo) {
        super(ctx, groupCursor, groupLayout, groupLayout, groupFrom, groupTo, childLayout, childFrom, childTo);
    }

    @Override
    protected Cursor getChildrenCursor(final Cursor groupCursor) {
        final String feedName = groupCursor.getString(groupCursor.getColumnIndex(FEED_NAME_COLUMN));
        return managedQuery(LATEST_URI, null,
                FeedItemColumns.CATEGORY + " = ? AND " + FeedItemColumns.FEED + " = ?",
                new String[] { mCategory.getId(), feedName }, null);
    }

    @Override
    protected void bindGroupView(View view, Context context, Cursor cursor, boolean isExpanded) {
        super.bindGroupView(view, context, cursor, isExpanded);

        // Bind group view (impl details not important) ...
    }

    @Override
    protected void bindChildView(View view, Context context, Cursor cursor, boolean isLastChild) {
        super.bindChildView(view, context, cursor, isLastChild);

        // Bind child view (impl details not important) ...
    }
}

使用此设置，所有内容都按预期加载。但是，在加载列表内容期间，UI 会随机挂起/断断续续。通常不足以获得 ANR，但仍然很明显且非常烦人!

为了调试这个问题，我启用了 android.os.StrictMode(很棒的新功能/工具 btw!)，并启动了模拟器(在 Android 2.3 上)。加载列表内容后，我得到以下 StrictMode 输出:

D/StrictMode(  395): StrictMode policy violation; ~duration=1522 ms: android.os.StrictMode$StrictModeDiskReadViolation: policy=23 violation=2D/StrictMode(  395):  at android.os.StrictMode$AndroidBlockGuardPolicy.onReadFromDisk(StrictMode.java:745)D/StrictMode(  395):  at android.database.sqlite.SQLiteDatabase.rawQueryWithFactory(SQLiteDatabase.java:1345)D/StrictMode(  395):  at android.database.sqlite.SQLiteQueryBuilder.query(SQLiteQueryBuilder.java:330)D/StrictMode(  395):  at com.mycompany.myapp.provider.RSSFeedProvider.query(RSSFeedProvider.java:128)D/StrictMode(  395):  at android.content.ContentProvider$Transport.query(ContentProvider.java:187)D/StrictMode(  395):  at android.content.ContentResolver.query(ContentResolver.java:262)D/StrictMode(  395):  at android.app.Activity.managedQuery(Activity.java:1550)D/StrictMode(  395):  at com.mycompany.myapp.activity.MultipleFeedsActivity$RssFeedLatestListAdapter.getChildrenCursor(MultipleFeedsActivity.java:388)D/StrictMode(  395):  at android.widget.CursorTreeAdapter.getChildrenCursorHelper(CursorTreeAdapter.java:106)D/StrictMode(  395):  at android.widget.CursorTreeAdapter.getChildrenCount(CursorTreeAdapter.java:178)D/StrictMode(  395):  at android.widget.ExpandableListConnector.refreshExpGroupMetadataList(ExpandableListConnector.java:561)D/StrictMode(  395):  at android.widget.ExpandableListConnector.expandGroup(ExpandableListConnector.java:682)D/StrictMode(  395):  at android.widget.ExpandableListConnector.expandGroup(ExpandableListConnector.java:636)D/StrictMode(  395):  at android.widget.ExpandableListView.expandGroup(ExpandableListView.java:608)D/StrictMode(  395):  at com.mycompany.myapp.activity.MultipleFeedsActivity.onReceiveResult(MultipleFeedsActivity.java:335)D/StrictMode(  395):  at com.mycompany.myapp.service.FeedResultReceiver.onReceiveResult(FeedResultReceiver.java:40)D/StrictMode(  395):  at android.os.ResultReceiver$MyRunnable.run(ResultReceiver.java:43)D/StrictMode(  395):  at android.os.Handler.handleCallback(Handler.java:587)D/StrictMode(  395):  at android.os.Handler.dispatchMessage(Handler.java:92)D/StrictMode(  395):  at android.os.Looper.loop(Looper.java:123)D/StrictMode(  395):  at android.app.ActivityThread.main(ActivityThread.java:3647)D/StrictMode(  395):  at java.lang.reflect.Method.invokeNative(Native Method)D/StrictMode(  395):  at java.lang.reflect.Method.invoke(Method.java:507)D/StrictMode(  395):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)D/StrictMode(  395):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)D/StrictMode(  395):  at dalvik.system.NativeStart.main(Native Method)

This seems reasonable! In SimpleCursorTreeAdapter.getChildrenCursor(), the content provider is queried on the UI thread, which of course is a bad idea that will most definitely hang the UI!

I had a look at the JavaDoc (API level 4) of CursorTreeAdapter (a super-class of SimpleCursorTreeAdapter) and for getChildrenCursor() it says:

"If you want to asynchronously query a provider to prevent blocking the UI, it is possible to return null and at a later time call setChildrenCursor(int, Cursor).".

Great! Let's do that! I change my implementation of getChildrenCursor() to start a new task responsible for performing the query and setting the children cursor on the adapter. After having started the task, I just return null in getChildrenCursor():

@Override
protected Cursor getChildrenCursor(final Cursor groupCursor) {
    final String feedName = groupCursor.getString(groupCursor.getColumnIndex(FEED_NAME_COLUMN));
    new RefreshChildrenCursorTask(groupCursor.getPosition()).execute(feedName);
    return null;
}

，其中 RefreshChildrenCursorTask 实现为:

private class RefreshChildrenCursorTask extends AsyncTask<String, Void, Cursor> {

    private int mGroupPosition;

    public RefreshChildrenCursorTask(int groupPosition) {
        this.mGroupPosition = groupPosition;
    }

    @Override
    protected Cursor doInBackground(String... params) {
        String feedName = params[0];
        return managedQuery(LATEST_URI, null,
                FeedItemColumns.CATEGORY + " = ? AND " + FeedItemColumns.FEED + " = ?",
                new String[] { mCategory.getId(), feedName }, null);
        }

        @Override
        protected void onPostExecute(Cursor childrenCursor) {
            mLatestListAdapter.setChildrenCursor(mGroupPosition, childrenCursor);
        }
    }
}

我在 2.3 模拟器上重新安装了该应用程序并启动了它。它就像一个魅力，再见无响应的用户界面!但喜悦并没有持续多久……接下来要做的是在目标设备上运行相同的代码(在本例中是运行 Android 2.2 的三星 Galaxy S)。然后我在加载列表提要内容期间得到以下 Exception:

E/AndroidRuntime(23191): FATAL EXCEPTION: mainE/AndroidRuntime(23191): java.lang.NullPointerExceptionE/AndroidRuntime(23191):  at android.widget.SimpleCursorTreeAdapter.initFromColumns(SimpleCursorTreeAdapter.java:194)E/AndroidRuntime(23191):  at android.widget.SimpleCursorTreeAdapter.initChildrenFromColumns(SimpleCursorTreeAdapter.java:205)E/AndroidRuntime(23191):  at android.widget.SimpleCursorTreeAdapter.init(SimpleCursorTreeAdapter.java:186)E/AndroidRuntime(23191):  at android.widget.SimpleCursorTreeAdapter.(SimpleCursorTreeAdapter.java:136)E/AndroidRuntime(23191):  at com.mycompany.myapp.activity.MultipleFeedsActivity$RssFeedLatestListAdapter.(MultipleFeedsActivity.java:378)E/AndroidRuntime(23191):  at com.mycompany.myapp.activity.MultipleFeedsActivity$2.run(MultipleFeedsActivity.java:150)E/AndroidRuntime(23191):  at android.os.Handler.handleCallback(Handler.java:587)E/AndroidRuntime(23191):  at android.os.Handler.dispatchMessage(Handler.java:92)E/AndroidRuntime(23191):  at android.os.Looper.loop(Looper.java:123)E/AndroidRuntime(23191):  at android.app.ActivityThread.main(ActivityThread.java:4627)E/AndroidRuntime(23191):  at java.lang.reflect.Method.invokeNative(Native Method)E/AndroidRuntime(23191):  at java.lang.reflect.Method.invoke(Method.java:521)E/AndroidRuntime(23191):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:871)E/AndroidRuntime(23191):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:629)E/AndroidRuntime(23191):  at dalvik.system.NativeStart.main(Native Method)

快速查看 SimpleCursorTreeAdapter 的源代码告诉我，它无法处理从 getChildrenCursor() 异步返回的 null。他们似乎已经在 Android 2.3 中解决了这个问题，但是你应该如何在早期版本的 Android 中绕过它呢？我真的必须编写自己的 CursorTreeAdapter 实现才能异步刷新子游标吗？

有没有人遇到过同样的问题，甚至可能找到了(可行的)解决方法？如果没有，任何人都可以提示我如何进行!？ 如果能得到任何帮助，我将不胜感激!

提前致谢!

问候，雅各布

Answer 1

如果他们在 2.3 SimpleCursorTreeAdapter 中修复了它，您为什么不直接下载 java 文件并将其包含在您的项目中并使用该版本而不是手机上的版本？

我没有看过代码，但只要他们没有进行任何新的 2.3 API 调用，它就应该可以工作。