Implement Matrix4Tween.lerp() (#10829)

packages/flutter/lib/src/widgets/implicit_animations.dart

@@ -111,12 +111,21 @@ class Matrix4Tween extends Tween<Matrix4> {
   Matrix4 lerp(double t) {
     assert(begin != null);
     assert(end != null);
-    // TODO(abarth): We should use [Matrix4.decompose] and animate the
-    // decomposed parameters instead of just animating the translation.
-    final Vector3 beginT = begin.getTranslation();
-    final Vector3 endT = end.getTranslation();
-    final Vector3 lerpT = beginT*(1.0-t) + endT*t;
-    return new Matrix4.identity()..translate(lerpT);
+    final Vector3 beginTranslation = new Vector3.zero();
+    final Vector3 endTranslation = new Vector3.zero();
+    final Quaternion beginRotation = new Quaternion.identity();
+    final Quaternion endRotation = new Quaternion.identity();
+    final Vector3 beginScale = new Vector3.zero();
+    final Vector3 endScale = new Vector3.zero();
+    begin.decompose(beginTranslation, beginRotation, beginScale);
+    end.decompose(endTranslation, endRotation, endScale);
+    final Vector3 lerpTranslation =
+        beginTranslation * (1.0 - t) + endTranslation * t;
+    // TODO(alangardner): Implement slerp for constant rotation
+    final Quaternion lerpRotation =
+        (beginRotation.scaled(1.0 - t) + endRotation.scaled(t)).normalized();
+    final Vector3 lerpScale = beginScale * (1.0 - t) + endScale * t;
+    return new Matrix4.compose(lerpTranslation, lerpRotation, lerpScale);
   }
 }
 

packages/flutter/test/animation/tween_test.dart

@@ -5,6 +5,7 @@
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/animation.dart';
 import 'package:flutter/widgets.dart';
+import 'package:vector_math/vector_math_64.dart';
 
 void main() {
   test('Can chain tweens', () {
@@ -49,4 +50,26 @@ void main() {
     expect(tween.lerp(0.5), equals(Rect.lerp(a, b, 0.5)));
     expect(tween, hasOneLineDescription);
   });
+
+  test('Matrix4Tween', () {
+    final Matrix4 a = new Matrix4.identity();
+    final Matrix4 b = a.clone()..translate(6.0, -8.0, 0.0)..scale(0.5, 1.0, 5.0);
+    final Matrix4Tween tween = new Matrix4Tween(begin: a, end: b);
+    expect(tween.lerp(0.0), equals(a));
+    expect(tween.lerp(1.0), equals(b));
+    expect(
+      tween.lerp(0.5),
+      equals(a.clone()..translate(3.0, -4.0, 0.0)..scale(0.75, 1.0, 3.0))
+    );
+    final Matrix4 c = a.clone()..rotateZ(1.0);
+    final Matrix4Tween rotationTween = new Matrix4Tween(begin: a, end: c);
+    expect(rotationTween.lerp(0.0), equals(a));
+    expect(rotationTween.lerp(1.0), equals(c));
+    expect(
+      rotationTween.lerp(0.5).absoluteError(
+        a.clone()..rotateZ(0.5)
+      ),
+      moreOrLessEquals(0.0)
+    );
+  });
 }