Apply gradient color to arc created with UIBezierPath

Question

I want to create a progress bar that has the form of an arc. The color of the progress bar has to change according to the values.

I created an arc using UIBezierPath bezierPathWithArcCenter. I used the following code:

- (void)viewDidLoad
{
    [super viewDidLoad];

    int radius = 100;

    CAShapeLayer *arc = [CAShapeLayer layer];

    arc.path = [UIBezierPath bezierPathWithArcCenter:CGPointMake(100, 50) radius:radius startAngle:60.0 endAngle:0.0 clockwise:YES].CGPath;

    arc.position = CGPointMake(CGRectGetMidX(self.view.frame)-radius,
                                  CGRectGetMidY(self.view.frame)-radius);

    arc.fillColor = [UIColor clearColor].CGColor;
    arc.strokeColor = [UIColor purpleColor].CGColor;
    arc.lineWidth = 15;

    [self.view.layer addSublayer:arc];

    CABasicAnimation *drawAnimation = [CABasicAnimation animationWithKeyPath:@"strokeEnd"];
    drawAnimation.duration            = 5.0; // "animate over 10 seconds or so.."
    drawAnimation.repeatCount         = 1.0;  // Animate only once..
    drawAnimation.removedOnCompletion = NO;   // Remain stroked after the animation..
    drawAnimation.fromValue = [NSNumber numberWithFloat:0.0f];
    drawAnimation.toValue   = [NSNumber numberWithFloat:10.0f];
    drawAnimation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseIn];
    [arc addAnimation:drawAnimation forKey:@"drawCircleAnimation"];

}

The result looks like this:

My question is: How to apply a gradient to the color if i.e. the value is <= 50%? I also created an UIButton that generates random CGFloat numbers in order to hook it up with the progress bar. Does anyone have an idea how to tackle this?

The gradient would look something like this:

Any help would be appreciated!

Thank you very much.

Granit

Answer 1

You can use a CAGradientLayer to get the gradient effect, and use the CAShapeLayer as a mask.

e.g.:

- (void)viewDidLoad
{
    [super viewDidLoad];

    int radius = 100;

    CAShapeLayer *arc = [CAShapeLayer layer];    
    arc.path = [UIBezierPath bezierPathWithArcCenter:CGPointMake(100, 50) radius:radius startAngle:60.0 endAngle:0.0 clockwise:YES].CGPath;

    arc.position = CGPointMake(CGRectGetMidX(self.view.frame)-radius,
                               CGRectGetMidY(self.view.frame)-radius);

    arc.fillColor = [UIColor clearColor].CGColor;
    arc.strokeColor = [UIColor purpleColor].CGColor;
    arc.lineWidth = 15; 
    CABasicAnimation *drawAnimation = [CABasicAnimation animationWithKeyPath:@"strokeEnd"];
    drawAnimation.duration            = 5.0; // "animate over 10 seconds or so.."
    drawAnimation.repeatCount         = 1.0;  // Animate only once..
    drawAnimation.removedOnCompletion = NO;   // Remain stroked after the animation..
    drawAnimation.fromValue = [NSNumber numberWithFloat:0.0f];
    drawAnimation.toValue   = [NSNumber numberWithFloat:10.0f];
    drawAnimation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseIn];
    [arc addAnimation:drawAnimation forKey:@"drawCircleAnimation"];

    CAGradientLayer *gradientLayer = [CAGradientLayer layer];
    gradientLayer.frame = self.view.frame;
    gradientLayer.colors = @[(__bridge id)[UIColor redColor].CGColor,(__bridge id)[UIColor blueColor].CGColor ];
    gradientLayer.startPoint = CGPointMake(0,0.5);
    gradientLayer.endPoint = CGPointMake(1,0.5);

    [self.view.layer addSublayer:gradientLayer];
     //Using arc as a mask instead of adding it as a sublayer.
     //[self.view.layer addSublayer:arc]; 
     gradientLayer.mask = arc;


}

Answer 2

To draw a gradient along a stroke, see this implementation: https://stackoverflow.com/a/43668420/917802

EDIT: In short, create a custom UIView class, add a radial gradient to it by iterating between two colours at increasing angles, e.g. colour1 = 1 degree, colour2 = 2 degrees etc, all the way up to 360. Then apply a donut mask to that. As you change the strokeEnd value of the masking CAShapeLayer, you also rotate the underlying radial gradient. Because they move together it looks like the stroke itself has a gradient.