Tile Providers
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The tileProvider parameter in TileLayer takes a TileProvider object specifying a tile provider to use for that layer.
This has a default of NetworkTileProvider which gets tiles from the internet through a dedicated image provider.
There's two situations in which you'll need to change the tile provider:
Sourcing tiles from the filesystem or asset store: Local Tile Providers
Using a plugin that instructs you to do so (Creating New Tile Providers)
These tile providers use the urlTemplate to get the appropriate tile from the a network, usually the World Wide Web.
The underlying custom ImageProviders will cache tiles in memory, so that they do not require another request to the tile server if they are pruned then re-loaded. This should result in them being loaded quicker, as well as enabling already loaded tiles to appear even without Internet connection (at least in the same session).
Specifying any fallbackUrl (even if it is not used) in the TileLayer will prevent loaded tiles from being cached in memory.
This is to avoid issues where the urlTemplate is flaky (sometimes works, sometimes doesn't), to prevent potentially different tilesets being displayed at the same time.
NetworkTileProviderThis is the default tile provider, and does nothing particularly special. It takes two arguments, but you'll usually never need to specify them:
httpClient: BaseClient
By default, a RetryClient backed by a standard Client is used
headers: Map<String, String>
By default, only headers sent by the platform are included with each request, plus an overridden (where possible) 'User-Agent' header based on the property
This requires the 'flutter_map_cancellable_tile_provider' plugin to be installed.
This plugin is part of the official 'flutter_map' organisation, and maintained by the same maintainers.
Tiles that are removed/pruned before they are fully loaded do not need to complete (down)loading, and therefore do not need to complete the HTTP interaction. Cancelling these unnecessary tile requests early could:
Reduce tile loading durations (particularly on the web)
Reduce users' (cellular) data and cache space consumption
Reduce costly tile requests to tile servers*
Improve performance by reducing CPU and IO work
This provider uses 'dio', which supports aborting unnecessary HTTP requests in-flight, after they have already been sent.
Although HTTP request abortion is supported on all platforms, it is especially useful on the web - and therefore recommended for web apps. This is because the web platform has a limited number of simulatous HTTP requests, and so closing the requests allows new requests to be made for new tiles. On other platforms, the other benefits may still occur, but may not be as visible as on the web.
Once HTTP request abortion is added to Dart's 'native' 'http' package (which already has a PR opened), NetworkTileProvider will be updated to take advantage of it, replacing and deprecating this provider. This tile provider is currently a separate package and not the default due to the reliance on the additional Dio dependency.
These tile providers use the urlTemplate to get the appropriate tile from the asset store of the application, or from a file on the users device, respectively.
Specifying any fallbackUrl (even if it is not used) in the TileLayer will reduce the performance of these providers.
It will cause 23% slower asset tile requests with AssetTileProvider, and will cause main thread blocking when requesting tiles from FileTileProvider.
AssetTileProviderThis tile providers uses the templateUrl to get the appropriate tile from the asset store of the application.
Asset management in Flutter leaves a lot to be desired! Unfortunately, every single sub-directory (to the level of tiles) must be listed.
FileTileProviderThis tile providers uses the templateUrl to get the appropriate tile from the a path/directory/file on the user's device - either internal application storage or external storage.
On the web, FileTileProvider() will throw an UnsupportedError when a tile request is attempted, due to the lack of the web platform's access to the local filesystem.
If you know you are running on the web platform, use a NetworkTileProvider or a custom tile provider.