blah.mm

- (void)_setValue:(CGFloat)value forAnimatedKey:(NSString *)animationKey
{
  POPSpringAnimation *ps = [self pop_animationForKey:animationKey];
  if (!ps) {
    ps = [POPSpringAnimation animation];
    ps.property = [POPAnimatableProperty propertyWithName:animationKey initializer:^(POPMutableAnimatableProperty *prop) {
      prop.readBlock = ^(DEViewerView *view, CGFloat *vs) {
        vs[0] = [[view valueForKey:animationKey] floatValue];
      };
      prop.writeBlock = ^(DEViewerView *view, const CGFloat *vs) {
        [view setValue:@(vs[0]) forKey:animationKey];
        [view setNeedsLayout];
      };
      prop.threshold = 0.0001;
    }];
    ps.dynamicsFriction = fromQCFriction(8);
    ps.dynamicsTension = fromQCTension(30);
    [self pop_addAnimation:ps forKey:animationKey];
  }
  ps.toValue = @(value);
}