refactor: Use a temp vector for delta calculations of FollowBehavior (

#3230)

This PR add a temp vector in `FollowBehavior` to avoid creating a new
vector every frame for the delta calculations.

packages/flame/lib/src/camera/behaviors/follow_behavior.dart

@@ -3,6 +3,7 @@ import 'package:flame/src/camera/viewport.dart';
 import 'package:flame/src/components/core/component.dart';
 import 'package:flame/src/components/position_component.dart';
 import 'package:flame/src/effects/provider_interfaces.dart';
+import 'package:vector_math/vector_math_64.dart';
 
 /// This behavior will make the [owner] follow the [target].
 ///
@@ -46,6 +47,8 @@ class FollowBehavior extends Component {
  final bool horizontalOnly;
  final bool verticalOnly;
 
+ final _tempDelta = Vector2.zero();
+
  @override
  void onMount() {
  if (_owner == null) {
@@ -59,19 +62,18 @@ class FollowBehavior extends Component {
 
  @override
  void update(double dt) {
- final delta = target.position - owner.position;
- if (horizontalOnly) {
- delta.y = 0;
- }
- if (verticalOnly) {
- delta.x = 0;
- }
- final distance = delta.length;
- if (distance > _speed * dt) {
- delta.scale(_speed * dt / distance);
+ _tempDelta.setValues(
+ verticalOnly ? 0 : target.position.x - owner.position.x,
+ horizontalOnly ? 0 : target.position.y - owner.position.y,
+ );
+
+ final distance = _tempDelta.length;
+ final deltaOffset = _speed * dt;
+ if (distance > deltaOffset) {
+ _tempDelta.scale(deltaOffset / distance);
  }
- if (delta.x != 0 || delta.y != 0) {
- owner.position = delta..add(owner.position);
+ if (_tempDelta.x != 0 || _tempDelta.y != 0) {
+ owner.position = _tempDelta..add(owner.position);
  }
  }
 }