虚拟地图图块提供程序不起作用(未调用getTile)

我想使用google maps api v2，但仅在后台显示一个虚拟地图. 这是我正在使用的PNG文件的示例，称为"dummy_map_tile.png".我将其放置在资产文件夹中名为"images"的目录下.它的大小是256x256像素.还尝试了JPG相似文件.

I would like to use the google maps api v2, but show just a dummy map in the background. This is a sample of the PNG file I am using, called "dummy_map_tile.png". I placed it in the asset folder, under a dir named "images". It's size is 256x256 pixels. Tried also a JPG similar file.

这是我的虚拟地图图块提供程序的代码，它当然应该可以脱机工作:

This is the code for my dummy map tile provider, which of course is supposed to work offline:

public class DummyTileProvider implements TileProvider {

    protected Tile mDummyTile = null;

    public DummyTileProvider(Context context) {
        InputStream inputStream = null;
        ByteArrayOutputStream outputStream = null;
        try {
            String tileFilename = "images/dummy_map_tile.png";
            inputStream = context.getResources().getAssets().open(tileFilename);
            outputStream = new ByteArrayOutputStream();
            byte[] buffer = new byte[2048];
            int count;
            while((count = inputStream.read(buffer)) != -1)
                outputStream.write(buffer, 0, count);
            outputStream.flush();
            mDummyTile = new Tile(256, 256, outputStream.toByteArray());
        }
        catch (IOException e) {
            mDummyTile = null;
        }
        finally {
            if (inputStream != null)
                try {inputStream.close();} catch (IOException e) {}
            if (outputStream != null)
                try {outputStream.close();} catch (IOException e) {}
        }
    }

    @Override
    public Tile getTile(int x, int y, int zoom) {
        return mDummyTile;
    }
}

一些日志记录(上面的代码中未显示)使我能够确保虚拟图块提供程序的构造正确，即，没有IOException发生，并且mDummyTile不为空.

Some logging (not shown in the code above) allowed me to make sure that the dummy tile provider constructs properly, i.e. no IOException occurs, and mDummyTile is not null.

这是我在地图设置中设置图块提供程序的方式(mMap是我的GoogleMap对象，已正确初始化):

This is the way I am setting the tile provider in the map setup (mMap is my GoogleMap object, properly initialized):

    mMap.setMapType(GoogleMap.MAP_TYPE_NONE);
    DummyTileProvider tileProvider = new DummyTileProvider(this);
    mMap.addTileOverlay(new TileOverlayOptions().tileProvider(tileProvider));

不幸的是，地图根本没有显示. 永远不会调用getTile方法. 不过，我在地图上绘制的所有标记和其他内容均能正常工作. 如果我删除上面的三行代码，从而使用默认的图块提供程序，则所有显示效果都很好，显示了标准的Google地图(仅在在线模式下). 谁能给我有用的提示?

Unfortunately, the map doesn't show at all. The getTile method is never called. All markers and other stuff I am drawing on the map work correctly, though. If I remove the three lines of code above, thus using the default tile provider, all works perfectly, showing the standard google maps (only in online mode). Can anyone give me a useful hint?