fix: allowed LatLngBounds.center to work across world boundary & added simpleCenter (#1860)

Author: monsieurtanuki

lib/src/geo/latlng_bounds.dart

@@ -193,9 +193,15 @@ class LatLngBounds {
     final lambda3 = lambda1 + atan2(by, cos(phi1) + bx);
 
     // phi3 and lambda3 are actually in radians and LatLng wants degrees
-    return LatLng(phi3 * radians2Degrees, lambda3 * radians2Degrees);
+    return LatLng(
+      phi3 * radians2Degrees,
+      (lambda3 * radians2Degrees + 540) % 360 - 180,
+    );
   }
 
+  /// Obtain simple coordinates of the bounds center
+  LatLng get simpleCenter => LatLng((south + north) / 2, (east + west) / 2);
+
   /// Checks whether [point] is inside bounds
   bool contains(LatLng point) =>
       point.longitude >= west &&

test/geo/latlng_bounds_test.dart

@@ -47,5 +47,52 @@ void main() {
         expect(bounds1.hashCode, bounds2.hashCode);
       });
     });
+
+    group('center', () {
+      // cf. https://github.com/fleaflet/flutter_map/issues/1689
+      test('should calculate center point #1', () async {
+        final bounds = LatLngBounds(
+          const LatLng(-77.45, -171.16),
+          const LatLng(46.64, 25.88),
+        );
+        final center = bounds.center;
+        expect(center.latitude, greaterThanOrEqualTo(-90));
+        expect(center.latitude, lessThanOrEqualTo(90));
+        expect(center.longitude, greaterThanOrEqualTo(-180));
+        expect(center.longitude, lessThanOrEqualTo(180));
+      });
+      test('should calculate center point #2', () async {
+        final bounds = LatLngBounds(
+          const LatLng(-0.87, -179.86),
+          const LatLng(84.92, 23.86),
+        );
+        final center = bounds.center;
+        expect(center.latitude, greaterThanOrEqualTo(-90));
+        expect(center.latitude, lessThanOrEqualTo(90));
+        expect(center.longitude, greaterThanOrEqualTo(-180));
+        expect(center.longitude, lessThanOrEqualTo(180));
+      });
+    });
+
+    group('simpleCenter', () {
+      test('should calculate center point #1', () async {
+        final bounds = LatLngBounds(
+          const LatLng(-77.45, -171.16),
+          const LatLng(46.64, 25.88),
+        );
+        final center = bounds.simpleCenter;
+        expect(center.latitude, (-77.45 + 46.64) / 2);
+        expect(center.longitude, (-171.16 + 25.88) / 2);
+      });
+      test('should calculate center point #2', () async {
+        final bounds = LatLngBounds(
+          const LatLng(-0.87, -179.86),
+          const LatLng(84.92, 23.86),
+        );
+        final center = bounds.simpleCenter;
+        expect(center.latitude, (-0.87 + 84.92) / 2);
+        expect(center.longitude, (-179.86 + 23.86) / 2);
+      });
+    });
   });
 }