安卓：处理大型数据集的ContentProvider，以避免内存限制内存、数据、以避免、ContentProvider

2023-09-05 09:52:45 作者：4．月色如浅梦

我使用的是的ContentProvider 来查询数据库，并返回一个光标被用在 CursorLoader ：

I am using a ContentProvider to query a database and return a Cursor that is used in a CursorLoader:

ItemsActivity：

ItemsActivity:

public class ItemsActivity extends SherlockActivity implements LoaderCallbacks<Cursor> {

    @Override
    public void onCreate(Bundle savedInstance) {
        ....
        getSupportLoaderManager().initLoader(LOADER_ID, null, this);
        ...
    }

    @Override
    public Loader<Cursor> onCreateLoader(int loaderId, Bundle bundle) {
        return new CursorLoader(getApplicationContext(), itemsListUri, ...); 
    }

    ...
}

ItemsContentProvider：

ItemsContentProvider:

public Cursor query(Uri uri, String[] projection, String selection, ...) {
    SqliteQueryBuilder builder = new SqliteQueryBuilder();
    builder.setTables(ItemsTable.NAME);
    return builder.query(db, projection, selection, ...);
}

该活动有一个的ListView ，和我使用的是的CursorAdapter （通过 LoaderCallbacks ）重新present数据光标之内。

The activity has a ListView, and I am using a CursorAdapter (updated via the LoaderCallbacks) to represent the data within the cursor.

这是工作的罚款，直到我需要查找在大型数据集中的项目（例如，超过30000行）。观察日志我看到查找超出内存限制和一些行是从游标的最终下降。

This is working fine, until I need to lookup the items in a large data set (for example, over 30,000 rows). Observing the logs I see that the lookup exceeds memory limits and some rows are dropped from the resulting cursor.

我的问题：什么是处理大型数据集使用这样的游标时，最好的办法

My question: what is the best way of handling very large datasets when using cursors like this?

我目前的解决办法是打破在的ContentProvider SQLite的查询与补偿和限制查询的序列，然后用 MergeCursor 类：

My current solution is to break up the SQLite query in the ContentProvider into a sequence of queries with offsets and limits, then combine these queries using the MergeCursor class:

private static final int LIMIT = 5000;

// Ignoring projection, selection, etc for simplicity here
public Cursor query(Uri uri, String projection, String selection, ...) {
  List<Cursor> cursors = newList();
  int offset = 0;
  Cursor c = db.rawQuery("select * from items limit " + LIMIT + " offset " + offset, null);
  while (c.getCount() > 0) {
    cursors.add(c);
    offset += c.getCount();
    c = db.rawQuery("select * from items limit " + LIMIT + " offset " + offset, null);
  }
  return createMergedCursor(cursors);
}

private Cursor createMergedCursors(List<Cursor> cursors) {
    if (cursors.size() == 1) {
        return cursors.get(0);
    }
    return new MergeCursor(toCursorsArray(cursors));
}

这将加载所有的数据，但有一个较长时间的延迟，而这样做的查询首次。列表视图是空的约5秒，同时多个查询被执行。

This will load all the data, but there's a long delay while doing the lookups for the first time. The list view is empty for about 5 seconds while multiple queries are performed.

请注意，当我尝试单查询（而不是批量查找），装载几乎是瞬间，虽然有滚动的内存达到限制列表时略有停顿。

Note that when I try a single lookup (rather than batched lookups), the loading is almost instantaneous, although there are slight pauses when scrolling the list as memory limits are reached.

所以：

使用单个查询：快速列表视图更新，但是滚动暂停，内存限制达到了

Using a single query: fast list view updating, but scrolling pauses and memory limits reached.

使用次查询：缓慢列表视图更新，但是滚动平滑，没有内存限制达到了

Using batched queries: slow list view updating, but scrolling is smooth and no memory limits reached.

我不知道是否有更好的解决方案，将快速更新的列表视图，但由于滚动列表需要的时候也获取更多的数据。

I'm wondering if there's a better solution that will update the list view quickly, but will also fetch more data as required when scrolling the list.

安卓4.2.1，的Nexus 7

Android 4.2.1, Nexus 7