How to set and change layer positions of visible objects

C4WorkSpace.m

#import "C4WorkSpace.h"

@implementation C4WorkSpace {
    C4Shape *zPositionShape;
    CGFloat zPosOfLastShape, offset;
    NSInteger circleCount;
}

-(void)setup {
    offset = 30;
    circleCount = (NSInteger)(self.canvas.height/offset)+10;
    for(int i = 0; i < circleCount; i++) {
        CGFloat x, y, w, h;
        w = 10 + (circleCount - i)*offset;
        h = w;
        x = 384 - w/2.0f;
        y = 512 - h/2.0f;
        
        if(i == 0) {
            zPositionShape = [C4Shape ellipse:CGRectMake(x,y,w,h)];
            zPositionShape.userInteractionEnabled = NO;
            zPositionShape.zPosition = i;
            zPositionShape.lineWidth = 0.0f;
            zPositionShape.fillColor = [[UIColor whiteColor] colorWithAlphaComponent:0.8];
            [self.canvas addShape:zPositionShape];
        } else {
            C4Shape *s = [C4Shape ellipse:CGRectMake(x,y,w,h)];
            s.zPosition = i;
            s.fillColor = [C4GREY colorWithAlphaComponent:1.0/offset];
            s.lineWidth = 1.0f;
            s.userInteractionEnabled = NO;
            [self.canvas addShape:s];
        }
    }
}

-(void)touchesBegan {
    zPositionShape.animationDuration = 5.0f;
    zPositionShape.animationOptions = AUTOREVERSE | REPEAT | LINEAR;
    zPositionShape.zPosition = circleCount;
}

@end