show old tiles until new ones are downloaded

lib/src/layer/image_tile.dart

@@ -0,0 +1,65 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_map/flutter_map.dart';
+import 'package:transparent_image/transparent_image.dart';
+
+typedef DownloadListener = void Function(Coords downloadedCoords);
+
+class ImageTile extends StatefulWidget {
+
+  final TileLayerOptions options;
+  final Coords coords;
+  final Size size;
+  final DownloadListener listener;
+
+  const ImageTile({Key key, this.options, this.coords, this.size, this.listener}) : super(key: key);
+
+  @override
+  _ImageTileState createState() => _ImageTileState();
+}
+
+class _ImageTileState extends State<ImageTile> {
+
+  bool _isDownloaded = false;
+  ImageInfo _imageInfo;
+  ImageProvider _provider;
+  ImageStream _stream;
+
+  @override
+  void initState() {
+    super.initState();
+    _provider = widget.options.tileProvider.getImage(widget.coords, widget.options);
+    _stream = _provider.resolve(ImageConfiguration(size: widget.size));
+    _stream.addListener(ImageStreamListener(_updateImage));
+  }
+
+  @override
+  void dispose() {
+    _stream.removeListener(ImageStreamListener(_updateImage));
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return AnimatedSwitcher(
+      duration: const Duration(milliseconds: 100),
+      child: _isDownloaded
+          ? ConstrainedBox(
+        constraints: BoxConstraints.expand(),
+        child: RawImage(
+          image: _imageInfo?.image,
+          fit: BoxFit.fill,
+        ),
+      )
+          : Image(image: MemoryImage(kTransparentImage)),
+    );
+  }
+
+  void _updateImage(ImageInfo imageInfo, bool synchronousCall) {
+    widget.listener(widget.coords);
+    setState(() {
+      _isDownloaded = true;
+      _imageInfo = imageInfo;
+    });
+  }
+}