Pre-calculate Marker's Anchor

This allows Marker to accept the same anchor argument as MarkerLayer,
anchorPos, whilst still precalculating the Anchor so it isn't calculated
on every build.

Co-authored-by: Luka S <github@jaffaketchup.dev>

example/lib/pages/markers.dart

@@ -129,8 +129,7 @@ class MarkerPageState extends State<MarkerPage> {
  47.18664724067855, -1.5436768515939427),
  width: 64,
  height: 64,
- anchor: Anchor.fromPos(
- AnchorPos.align(AnchorAlign.left), 64, 64),
+ anchorPos: AnchorPos.align(AnchorAlign.left),
  builder: (context) => const ColoredBox(
  color: Colors.lightBlue,
  child: Align(
@@ -144,8 +143,7 @@ class MarkerPageState extends State<MarkerPage> {
  47.18664724067855, -1.5436768515939427),
  width: 64,
  height: 64,
- anchor: Anchor.fromPos(
- AnchorPos.align(AnchorAlign.right), 64, 64),
+ anchorPos: AnchorPos.align(AnchorAlign.right),
  builder: (context) => const ColoredBox(
  color: Colors.pink,
  child: Align(
@@ -158,8 +156,7 @@ class MarkerPageState extends State<MarkerPage> {
  point: const LatLng(
  47.18664724067855, -1.5436768515939427),
  rotate: false,
- anchor: Anchor.fromPos(
- AnchorPos.align(AnchorAlign.center), 30, 30),
+ anchorPos: AnchorPos.align(AnchorAlign.center),
  builder: (context) =>
  const ColoredBox(color: Colors.black),
  ),

lib/src/layer/marker_layer.dart

@@ -10,11 +10,13 @@ import 'package:latlong2/latlong.dart';
 /// Can be defined exactly (using [AnchorPos.exactly] with an [Anchor]) or in
 /// a relative alignment (using [AnchorPos.align] with an [AnchorAlign]).
 class AnchorPos {
+ static const defaultAnchorPos = AnchorPos.align(AnchorAlign.center);
+
  final Anchor? anchor;
  final AnchorAlign? alignment;
 
- AnchorPos.exactly(Anchor this.anchor) : alignment = null;
- AnchorPos.align(AnchorAlign this.alignment) : anchor = null;
+ const AnchorPos.exactly(Anchor this.anchor) : alignment = null;
+ const AnchorPos.align(AnchorAlign this.alignment) : anchor = null;
 }
 
 /// Exact alignment for a [Marker.builder] widget relative to the center
@@ -26,7 +28,7 @@ class Anchor {
  final double left;
  final double top;
 
- Anchor(this.left, this.top);
+ const Anchor(this.left, this.top);
 
  factory Anchor.fromPos(AnchorPos pos, double width, double height) {
  if (pos.anchor case final anchor?) return anchor;
@@ -131,11 +133,12 @@ class Marker {
  required this.builder,
  this.width = 30.0,
  this.height = 30.0,
- this.anchor,
+ AnchorPos? anchorPos,
  this.rotate,
  this.rotateOrigin,
  this.rotateAlignment,
- });
+ }) : anchor =
+ anchorPos == null ? null : Anchor.fromPos(anchorPos, width, height);
 }
 
 class MarkerLayer extends StatelessWidget {
@@ -202,7 +205,7 @@ class MarkerLayer extends StatelessWidget {
  // unlike the map coordinates.
  final anchor = marker.anchor ??
  Anchor.fromPos(
- anchorPos ?? AnchorPos.align(AnchorAlign.center),
+ anchorPos ?? AnchorPos.defaultAnchorPos,
  marker.width,
  marker.height,
  );