Animate a UIView along an arc

Question

I wish to animate a view along an arc as shown in the image from position 1 to position 2.

What actually happens is that the view animation describes a full circle instead of an arc. My questions are:

1. Should I be using CGPathAddArc or CGPathAddArcToPoint ?
2. Do I need to use CGPathMoveToPoint to describe the initial location of the view or not?

When I step through the code the CGPoints values are as I expect them to be, as are the angles.

I use the following function to get CGPoints on a circle

CGPoint pointOnCircle(CGPoint centre, float radius, float angle)
{
    float x = centre.x + radius * cosf(angle);
    float y = centre.y + radius * sinf(angle);

    return CGPointMake(x, y);
}

I am using the following code and variants thereof but as yet I have not managed to crack it.

CAKeyframeAnimation *pathAnimation = [CAKeyframeAnimation animationWithKeyPath:@"position"];
pathAnimation.calculationMode = kCAAnimationPaced;
pathAnimation.fillMode = kCAFillModeForwards;
pathAnimation.removedOnCompletion = NO;
pathAnimation.duration = duration;
pathAnimation.repeatCount = 1;

CGPoint viewCenter = dynamicView.objectCenter;
CGMutablePathRef arcPath = CGPathCreateMutable();
//  CGPathMoveToPoint(arcPath, NULL, viewCenter.x, viewCenter.y);

//work out the half way point
float halfwayAngle = dynamicView.angle + (0.5f * self.angleIncrement);
float fullwayAngle = dynamicView.angle + self.angleIncrement;
CGPoint halfwayPoint = pointOnCircle(self.center, self.radius, halfwayAngle);
CGPoint fullwayPoint = pointOnCircle(self.center, self.radius, fullwayAngle);

CGPathAddArc(arcPath, NULL, halfwayPoint.x, halfwayPoint.y, self.radius, dynamicView.angle, fullwayAngle, clockwise);
//    CGPathAddArcToPoint(arcPath, NULL, viewCenter.x, viewCenter.y, fullwayPoint.x, fullwayPoint.y, self.radius);

pathAnimation.path = arcPath;
CGPathRelease(arcPath);

[dynamicView.layer addAnimation:pathAnimation forKey:@"arc"];

Answer 1

I coded up a quick example that hopefully puts it all together. I started from the "single view" application template and added this action to the view controller along with a button to trigger the action.

- (IBAction)animateArc:(id)sender
{
    // Create the arc
    CGPoint arcStart = CGPointMake(0.2 * self.view.bounds.size.width, 0.5 * self.view.bounds.size.height);
    CGPoint arcCenter = CGPointMake(0.5 * self.view.bounds.size.width, 0.5 * self.view.bounds.size.height);
    CGFloat arcRadius = 0.3 * MIN(self.view.bounds.size.width, self.view.bounds.size.height);

    CGMutablePathRef arcPath = CGPathCreateMutable();
    CGPathMoveToPoint(arcPath, NULL, arcStart.x, arcStart.y);
    CGPathAddArc(arcPath, NULL, arcCenter.x, arcCenter.y, arcRadius, M_PI, 0, NO);

    // The layer we're going to animate (a 50x50pt red box)
    UIView* dynamicView = [[UIView alloc] initWithFrame: CGRectMake(0, 0, 50, 50)];
    dynamicView.layer.backgroundColor = [[UIColor redColor] CGColor];
    [self.view addSubview: dynamicView];
    dynamicView.center = arcStart;

    // An additional view that shows the arc.
    BOOL showArc = NO;
    UIView* drawArcView = nil;
    if (showArc)
    {
        drawArcView = [[UIView alloc] initWithFrame: self.view.bounds];
        CAShapeLayer* showArcLayer = [[CAShapeLayer alloc] init];
        showArcLayer.frame = drawArcView.layer.bounds;
        showArcLayer.path = arcPath;
        showArcLayer.strokeColor = [[UIColor blackColor] CGColor];
        showArcLayer.fillColor = nil;
        showArcLayer.lineWidth = 3.0;
        [drawArcView.layer addSublayer: showArcLayer];
        [self.view insertSubview: drawArcView belowSubview: dynamicView];
    }

    // The animation
    CAKeyframeAnimation *pathAnimation = [CAKeyframeAnimation animationWithKeyPath:@"position"];
    pathAnimation.calculationMode = kCAAnimationPaced;
    pathAnimation.duration = 5.0;
    pathAnimation.path = arcPath;
    CGPathRelease(arcPath);

    // Add the animation and reset the state so we can run again.
    [CATransaction begin];
    [CATransaction setCompletionBlock:^{
        [drawArcView removeFromSuperview];
        [dynamicView removeFromSuperview];
    }];
    [dynamicView.layer addAnimation:pathAnimation forKey:@"arc"];
    [CATransaction commit];
}

Answer 2

Ultimately I used CGPathAddRelativeArc to get the transition I wanted.

CGPoint viewCenter = dynamicView.objectCenter;
CGMutablePathRef arcPath = CGPathCreateMutable();
CGPathMoveToPoint(arcPath, NULL, viewCenter.x, viewCenter.y);

float angleOfRotation = self.angleIncrement;
if (clockwise == NO)
{
    angleOfRotation *= -1.0f;
}

float fullwayAngle = dynamicView.angle + angleOfRotation;
CGPoint fullwayPoint = pointOnCircle(self.center, self.radius, fullwayAngle);

CGPathAddRelativeArc(arcPath, NULL, self.center.x, self.center.y, self.radius, dynamicView.angle, angleOfRotation);
pathAnimation.path = arcPath;
CGPathRelease(arcPath);

[dynamicView.layer addAnimation:pathAnimation forKey:@"arc"];

dynamicView.angle = fullwayAngle;
dynamicView.objectCenter = fullwayPoint;