From 32c78d4d93b6c82ff55693c54176b2826f718e5e Mon Sep 17 00:00:00 2001
From: featherless <jverkoey@gmail.com>
Date: Thu, 14 Dec 2017 15:48:57 -0500
Subject: [PATCH] Add support for additively animating bounds. (#93)

Closes https://github.com/material-motion/motion-animator-objc/issues/74
---
 src/MDMAnimatableKeyPaths.h                   |  4 +-
 src/private/CABasicAnimation+MotionAnimator.m | 42 +++++++++++++++++++
 2 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/src/MDMAnimatableKeyPaths.h b/src/MDMAnimatableKeyPaths.h
index 2a9bbc3..e02a037 100644
--- a/src/MDMAnimatableKeyPaths.h
+++ b/src/MDMAnimatableKeyPaths.h
@@ -58,9 +58,7 @@ FOUNDATION_EXPORT MDMAnimatableKeyPath MDMKeyPathBackgroundColor NS_SWIFT_NAME(b
  Equivalent UIView property: bounds
  Equivalent CALayer property: bounds
  Expected value type: CGRect or NSValue (containing a CGRect).
- Additive animation supported: No.
- TODO( https://github.com/material-motion/motion-animator-objc/issues/74 ):
-      Add support for additively animating CGRect types.
+ Additive animation supported: Yes.
  */
 FOUNDATION_EXPORT MDMAnimatableKeyPath MDMKeyPathBounds NS_SWIFT_NAME(bounds);
 
diff --git a/src/private/CABasicAnimation+MotionAnimator.m b/src/private/CABasicAnimation+MotionAnimator.m
index 087f97f..3f212b5 100644
--- a/src/private/CABasicAnimation+MotionAnimator.m
+++ b/src/private/CABasicAnimation+MotionAnimator.m
@@ -44,6 +44,15 @@ static BOOL IsCGSizeType(id someValue) {
   return NO;
 }
 
+static BOOL IsCGRectType(id someValue) {
+  if ([someValue isKindOfClass:[NSValue class]]) {
+    NSValue *asValue = (NSValue *)someValue;
+    const char *objCType = @encode(CGRect);
+    return strncmp(asValue.objCType, objCType, strlen(objCType)) == 0;
+  }
+  return NO;
+}
+
 static BOOL IsCATransform3DType(id someValue) {
   if ([someValue isKindOfClass:[NSValue class]]) {
     NSValue *asValue = (NSValue *)someValue;
@@ -270,6 +279,39 @@ void MDMConfigureAnimation(CABasicAnimation *animation, MDMAnimationTraits * tra
       }
     }
 
+  } else if (IsCGRectType(animation.toValue)) {
+    CGRect from = [animation.fromValue CGRectValue];
+    CGRect to = [animation.toValue CGRectValue];
+    CGRect additiveDisplacement = CGRectMake(from.origin.x - to.origin.x,
+                                             from.origin.y - to.origin.y,
+                                             from.size.width - to.size.width,
+                                             from.size.height - to.size.height);
+
+    if (animation.additive) {
+      animation.fromValue = [NSValue valueWithCGRect:additiveDisplacement];
+      animation.toValue = [NSValue valueWithCGRect:CGRectZero];
+    }
+
+    if (isSpringAnimation) {
+      // Core Animation's velocity system is single dimensional, so we pick the dominant direction
+      // of movement and normalize accordingly.
+      CGFloat biggestDelta = additiveDisplacement.origin.x;
+      if (fabs(additiveDisplacement.origin.y) > fabs(biggestDelta)) {
+        biggestDelta = additiveDisplacement.origin.y;
+      }
+      if (fabs(additiveDisplacement.size.width) > fabs(biggestDelta)) {
+        biggestDelta = additiveDisplacement.size.width;
+      }
+      if (fabs(additiveDisplacement.size.height) > fabs(biggestDelta)) {
+        biggestDelta = additiveDisplacement.size.height;
+      }
+      CGFloat displacement = -biggestDelta;
+      CGFloat absoluteInitialVelocity = springTimingCurve.initialVelocity;
+      if (fabs(displacement) > 0.00001) {
+        springAnimation.initialVelocity = absoluteInitialVelocity / displacement;
+      }
+    }
+
   } else if (IsCATransform3DType(animation.toValue)) {
     CATransform3D from = [animation.fromValue CATransform3DValue];
     CATransform3D to = [animation.toValue CATransform3DValue];