feat: Added closeToVector in flame_test (#1245)

Added matcher closeToVector to help test equality of vectors. This matcher

    follows standard Flutter convention for writing tests as expect(value, matcher);
    provides meaningful description of the mismatch in case the test fails.

Also: fixed the name of the fails_assert_test, which wasn't running automatically because the file name did not end with _test.

packages/flame_test/.min_coverage

@@ -0,0 +1 @@
+46.0

packages/flame_test/lib/src/close_to_vector.dart

@@ -0,0 +1,43 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:vector_math/vector_math_64.dart';
+
+/// Returns a matcher which matches if the argument vector is within distance
+/// [epsilon] of point ([x], [y]). For example:
+///
+/// ```dart
+/// expect(scale, closeToVector(2, -2));
+/// expect(position, closeToVector(120, 150, epsilon: 1e-10));
+/// ```
+Matcher closeToVector(double x, double y, {double epsilon = 1e-15}) {
+  return _IsCloseTo(Vector2(x, y), epsilon);
+}
+
+class _IsCloseTo extends Matcher {
+  const _IsCloseTo(this._value, this._epsilon);
+
+  final Vector2 _value;
+  final double _epsilon;
+
+  @override
+  bool matches(dynamic item, Map matchState) {
+    return (item is Vector2) && (item - _value).length <= _epsilon;
+  }
+
+  @override
+  Description describe(Description description) => description
+      .add('a Vector2 object within $_epsilon of (${_value.x}, ${_value.y})');
+
+  @override
+  Description describeMismatch(
+    dynamic item,
+    Description mismatchDescription,
+    Map matchState,
+    bool verbose,
+  ) {
+    if (item is! Vector2) {
+      return mismatchDescription.add('is not an instance of Vector2');
+    }
+    final distance = (item - _value).length;
+    return mismatchDescription.add('is at distance $distance');
+  }
+}

packages/flame_test/test/close_to_vector_test.dart

@@ -0,0 +1,42 @@
+import 'package:flame_test/src/close_to_vector.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:vector_math/vector_math_64.dart';
+
+void main() {
+  group('closeToVector', () {
+    test('matches normally', () {
+      expect(Vector2.zero(), closeToVector(0, 0));
+      expect(Vector2(-14, 99), closeToVector(-14, 99));
+      expect(Vector2(1e-20, -1e-16), closeToVector(0, 0));
+
+      expect(Vector2(1.0001, 2.0), closeToVector(1, 2, epsilon: 0.01));
+      expect(Vector2(13, 14), closeToVector(10, 10, epsilon: 5));
+    });
+
+    test('fails on type mismatch', () {
+      try {
+        expect(3.14, closeToVector(0, 0));
+      } on TestFailure catch (e) {
+        expect(
+          e.message,
+          contains('Expected: a Vector2 object within 1e-15 of (0.0, 0.0)'),
+        );
+        expect(e.message, contains('Actual: <3.14>'));
+        expect(e.message, contains('Which: is not an instance of Vector2'));
+      }
+    });
+
+    test('fails on value mismatch', () {
+      try {
+        expect(Vector2(101, 217), closeToVector(100, 220));
+      } on TestFailure catch (e) {
+        expect(
+          e.message,
+          contains('Expected: a Vector2 object within 1e-15 of (100.0, 220.0)'),
+        );
+        expect(e.message, contains('Actual: Vector2:<[101.0,217.0]>'));
+        expect(e.message, contains('Which: is at distance 3.16227766'));
+      }
+    });
+  });
+}