CAShapeLayer animation (arc from 0 to final size)

Question

I have a CAShapeLayer in which an arc is added using UIBezierPath. I saw a couple of posts (one actually) here on stackoverflow but none seemed to give me the answer. As said in the title I would like to animate an arc (yes it will be for a pie chart).

How can one accomplish an animation from an "empty" arc to the fully extended one? Kinda like a curved progress bar. Is it really impossible to do it via CoreAnimation?

Here's how I "do" the arc. Oh and ignore the comments and calculation since I'm used to counter clockwise unit circle and apple goes clockwise. The arcs appear just fine, I only need animation:

    //Apple's unit circle goes in clockwise rotation while im used to counter clockwise that's why my angles are mirrored along x-axis
    workAngle = 6.283185307 - DEGREES_TO_RADIANS(360 * item.value / total);

    //My unit circle, that's why negative currentAngle
    [path moveToPoint:CGPointMake(center.x + cosf(outerPaddingAngle - currentAngle) * (bounds.size.width / 2), center.y - (sinf(outerPaddingAngle - currentAngle) * (bounds.size.height / 2)))];

    //Apple's unit circle, clockwise rotation, we add the angles
    [path addArcWithCenter:center radius:120 startAngle:currentAngle endAngle:currentAngle + workAngle clockwise:NO];

    //My unit circle, counter clockwise rotation, we subtract the angles
    [path addLineToPoint:CGPointMake(center.x + cosf(-currentAngle - workAngle) * ((bounds.size.width / 2) - pieWidth), center.y - sinf(-currentAngle - workAngle) * ((bounds.size.height / 2) - pieWidth))];

    //Apple's unit circle, clockwise rotation, addition of angles
    [path addArcWithCenter:center radius:120 - pieWidth startAngle:currentAngle + workAngle endAngle:currentAngle - innerPaddingAngle clockwise:YES];

    //No need for my custom calculations since I can simply close the path
    [path closePath];

    shape = [CAShapeLayer layer];
    shape.frame = self.bounds;
    shape.path = path.CGPath;
    shape.fillColor = kRGBAColor(255, 255, 255, 0.2f + 0.1f * (i + 1)).CGColor; //kRGBAColor is a #defined

    [self.layer addSublayer:shape];


    //THIS IS THE PART IM INTERESTED IN
    if (_shouldAnimate)
    {
        CABasicAnimation *pieAnimation = [CABasicAnimation animationWithKeyPath:@"path"];
        pieAnimation.duration = 1.0;
        pieAnimation.removedOnCompletion = NO;
        pieAnimation.fillMode = kCAFillModeForwards;
        //from and to are just dummies so I dont get errors
        pieAnimation.fromValue = [NSNumber numberWithInt:0]; //from what
        pieAnimation.toValue = [NSNumber numberWithFloat:1.0f]; //to what

        [shape addAnimation:pieAnimation forKey:@"animatePath"];
    }

Answer 1

Here is an implementation of a CALayer based on CAShapeLayer:

ProgressLayer.h/m

#import <QuartzCore/QuartzCore.h>

@interface ProgressLayer : CAShapeLayer

-(void) computePath:(CGRect)r;
-(void) showProgress;
@end

#import "ProgressLayer.h"

@implementation ProgressLayer
-(id)init {
    self=[super init];
    if (self) {
        self.path = CGPathCreateWithEllipseInRect(CGRectMake(0, 0, 50, 50), 0);
        self.strokeColor = [UIColor greenColor].CGColor;
        self.lineWidth=40;
        self.lineCap = kCALineCapRound;
        self.strokeEnd=0.001;
    }
    return self;
}

-(void) computePath:(CGRect)r {
    self.path = CGPathCreateWithEllipseInRect(r, 0);
}


-(void)showProgress {
    float advance=0.1;
    if (self.strokeEnd>1) self.strokeEnd=0;

    CABasicAnimation * swipe = [CABasicAnimation animationWithKeyPath:@"strokeEnd"];
    swipe.duration=0.25;
    swipe.fromValue=[NSNumber numberWithDouble:self.strokeEnd];
    swipe.toValue=  [NSNumber numberWithDouble:self.strokeEnd + advance];

    swipe.fillMode = kCAFillModeForwards;
    swipe.timingFunction= [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut];
    swipe.removedOnCompletion=NO;
    self.strokeEnd = self.strokeEnd + advance;
    [self addAnimation:swipe forKey:@"strokeEnd animation"];
}

@end

I used this layer as a backing layer of an UIView:

#import "ProgressView.h"
#import "ProgressLayer.h"

@implementation ProgressView


-(id)initWithCoder:(NSCoder *)aDecoder {
    self=[super initWithCoder:aDecoder];
    if (self) {
        [(ProgressLayer *)self.layer computePath:self.bounds];
    }
    return self;
}

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    ProgressLayer * l =  self.layer;
    [l showProgress];
}


- (void)drawRect:(CGRect)rect {
    [[UIColor redColor] setStroke];
    UIRectFrame(self.bounds);
}


+(Class)layerClass {
    return [ProgressLayer class];
}

@end