Tiles will not load after usage without a network

[ristiisa]

When the connectivity was lost and is restored the tiles that failed to load when the connectivity was lost will continue to fail to load:
SocketException: Failed host lookup: 'a.tile.openstreetmap.org' (OS Error: No address associated with hostname, errno = 7)

Video of the issue:
https://streamable.com/9kaguw

Some time after the connectivity is restored an exception is caught:

Tested with ac52f28
Flutter version 1.15.17
Framework revision 2294d75bfa (5 weeks ago), 2020-03-07 00:28:38 +0900
Dart version 2.8.0 (build 2.8.0-dev.12.0 9983424a3c)

[maRci002]

This seems serious bug however I tested this against 0.8.2 release and it behaves same way.
git checkout 031fa41544f34a91acf69d0c3b758514f1ea2f5c or if you are using it as dependency flutter_map: 0.8.2 (not ^0.8.2) then flutter pub upgrade.

Here is a fast 0.8.2 preview:

I think I know how to fix this issue, give me some time and I will make a new PR, the main problem is within flutter because it caches the failed Image, see:

/// Fetches the given URL from the network, associating it with the given scale.
///
/// The image will be cached regardless of cache headers from the server.
///
/// When a network image is used on the Web platform, the [cacheWidth] and
/// [cacheHeight] parameters of the [DecoderCallback] are ignored as the Web
/// engine delegates image decoding of network images to the Web, which does
/// not support custom decode sizes.
///
/// See also:
///
///  * [Image.network] for a shorthand of an [Image] widget backed by [NetworkImage].
// TODO(ianh): Find some way to honor cache headers to the extent that when the
// last reference to an image is released, we proactively evict the image from
// our cache if the headers describe the image as having expired at that point.
abstract class NetworkImage extends ImageProvider<NetworkImage> {

[maRci002]

@ristiisa take a test ride with #577 don't forget to provide evictErrorTiles flag to TileLayerOptions since error tiles will become active they will be retained too, however _pruneTiles will have chance to evict error tiles if they are not retained anymore.

[maRci002]

Some time after the connectivity is restored an exception is caught: it was because they were not retained anymore and phone wanted to refetch the Image, forinstance this happens if an error happened on zoom lvl 5 and you changed zoom to lvl 7 or if overpanning the default 2 tiles margin.

[ristiisa]

@ristiisa take a test ride with #577 don't forget to provide evictErrorTiles flag to TileLayerOptions since error tiles will become active they will be retained too, however _pruneTiles will have chance to evict error tiles if they are not retained anymore.

I'm not sure if I'm seeing any changes in the behavior.

[maRci002]

It's because _pruneTiles / _abortLoading wasn't called, I have seen your video and map was moved by a MapController which took small steps and error tiles was still retained. I think I can make evictErrorTiles even stricter forinstance if error Tile is not in viewport / not on same zoom lvl as mapState.zoom then evict an error tile may seem fair enough. So I think evictErrorTiles will be better evictErrorTilesStrategy enum which will support/allow this behaviour.

  // If a Tile was loaded with error and if this flag is true, then TileProvider
  // will be asked to evict Image during _pruneTiles / _abortLoading calls
  // (see #576 - even Error Images are cached in flutter)
  final bool evictErrorTiles;

Anyway if you are moving map by a controller then #579 contains some fix.

[maRci002]

@ristiisa I updated the PR with more strategy like EvictErrorTileStrategy.notVisible.

[eis6aer]

Is there any workaround that you can think of for the moment? Until the real fix is implemented ?