Implement FlipEffect test

lib/src/effects/flip_effect.dart

@@ -64,26 +64,33 @@ class FlipEffect extends Effect<double> {
     Animation<double> animation = buildAnimation(controller, entry);
     return getOptimizedBuilder<double>(
       animation: animation,
-      builder: (_, __) {
-        double value = animation.value * pi;
-
-        final Matrix4 mtx = Matrix4(
-          1.0, 0.0, 0.0, 0.0, //
-          0.0, 1.0, 0.0, 0.0, //
-          0.0, 0.0, 1.0, 0.002 * perspective, //
-          0.0, 0.0, 0.0, 1.0,
-        );
-        if (value != 0) {
-          if (direction == Axis.vertical) {
-            mtx.rotateX(value);
-          } else {
-            mtx.rotateY(value);
-          }
-        }
+      builder: (_, __) => Transform(
+        alignment: alignment,
+        transform: getTransformMatrix(animation.value, direction, perspective),
+        child: child,
+      ),
+    );
+  }
 
-        return Transform(alignment: alignment, transform: mtx, child: child);
-      },
+  static Matrix4 getTransformMatrix(
+    double value,
+    Axis direction,
+    double perspective,
+  ) {
+    final Matrix4 mtx = Matrix4(
+      1.0, 0.0, 0.0, 0.0, //
+      0.0, 1.0, 0.0, 0.0, //
+      0.0, 0.0, 1.0, 0.002 * perspective, //
+      0.0, 0.0, 0.0, 1.0,
     );
+    if (value != 0) {
+      if (direction == Axis.vertical) {
+        mtx.rotateX(value * pi);
+      } else {
+        mtx.rotateY(value * pi);
+      }
+    }
+    return mtx;
   }
 }
 

test/effects/flip_test.dart

@@ -1,3 +1,26 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter_animate/flutter_animate.dart';
+
+import '../tester_extensions.dart';
+
 void main() {
-  // TODO: expose FlipEffect.getTransformMatrix (name TBD) and use in test
+  testWidgets('FlipEffect: core', (tester) async {
+    Axis direction = Axis.vertical;
+    double perspective = 2;
+    final animation = const FlutterLogo().animate().flip(
+          duration: 1000.ms,
+          direction: direction,
+          perspective: perspective,
+          begin: 0,
+          end: 1,
+        );
+
+    // Check halfway,
+    await tester.pumpAnimation(animation, initialDelay: 500.ms);
+    // Create a matrix and compare to the one in the widget tree
+    var mtx = FlipEffect.getTransformMatrix(0.5, direction, perspective);
+    tester.expectWidgetWithBool<Transform>(
+        (o) => o.transform == mtx, true, 'transform @ 500ms');
+  });
 }