ios - 湾曲したテキストが中央に配置されないのはなぜですか?

Question

というUIViewサブクラスがありdpCurvedLabelます。CATextLayers円弧の周りにテキストを曲げるのに使用します。レイヤーの親ビューで完全に中央に配置できないことを除いて、正常に機能します。すべてのテキスト文字が中心から同じ距離になるように、円弧の中心点をビューの中心に配置します (ビューが小さくても)。

近づけることはできますが、常に少なくとも数ピクセルはずれています。それが「オフ」である量は、私がそれぞれに与えるフレームサイズによって影響を受けるようCATextLayerです. 数学に問題がありますが、何が原因かわかりません。私のコード:

---

// instantiate a dpCurvedLabel in a super view
dpCurvedLabel *curvedLabel = [[dpCurvedLabel alloc] 
initWithFrame:CGRectMake(0, 0, 200, 200) arcSize:360 radius:50 
text:@"curve curve curve " font:[UIFont fontWithName:@"HelveticaNeue" size:18] 
textColor:[UIColor whiteColor]];

// You can animate a rotation to see a more pronounced effect
// [curvedLabel rotateContinously];

[self addSubview:curvedLabel];

---

dpCurvedLabel.h

#import <UIKit/UIKit.h>

@interface dpCurvedLabel : UIView

@property CGFloat arcSize;
@property CGFloat radius;
@property (strong) NSString *text;
@property (strong) UIFont *font;
@property (strong) UIColor *textColor;

- (id)initWithFrame:(CGRect)frame arcSize:(CGFloat)arcSize radius:(CGFloat)radius
text:(NSString *)text font:(UIFont *)font 
textColor:(UIColor *)textColor;
- (void)rotateContinously;

+ (void)makeCurvedText:(CALayer *)layer arcSize:(CGFloat)arcSize radius:(CGFloat)radius 
text:(NSString *)text font:(UIFont *)font textColor:(UIColor *)textColor;

@end

dpCurvedLabel.m

#import "dpCurvedLabel.h"

@implementation dpCurvedLabel

- (id)initWithFrame:(CGRect)frame arcSize:(CGFloat)arcSize radius:(CGFloat)radius text:(NSString *)text font:(UIFont *)font textColor:(UIColor *)textColor
{
    self = [super initWithFrame:frame];
    if (self) {
        // Initialization code

        self.opaque = NO;
        self.clipsToBounds = NO;

        self.arcSize = arcSize;
        self.radius = radius;
        self.text = text;
        self.font = font;
        self.textColor = textColor;
    }
    return self;
}



- (void)layoutSublayersOfLayer:(CALayer *)layer
{
    [super layoutSublayersOfLayer:layer];

    NSLog(@"laying out sublayers!");
    [dpCurvedLabel makeCurvedText:layer arcSize:self.arcSize radius:self.radius text:self.text font:self.font textColor:self.textColor];
}

+ (void)makeCurvedText:(CALayer *)layer arcSize:(CGFloat)arcSize radius:(CGFloat)radius text:(NSString *)text font:(UIFont *)font textColor:(UIColor *)textColor
{

    layer.sublayers = nil;
    layer.masksToBounds = NO;

    CGFloat arcStart = 0;
    CGFloat shiftH = 0;
    CGFloat shiftV = 0;


    BOOL clockwise = YES;
    BOOL debugMode = YES;



    CGFloat xcenter = CGRectGetMidX(layer.bounds);
    CGFloat ycenter = CGRectGetMidY(layer.bounds);

    CGFloat angle = arcStart;
    CGFloat angleStep = arcSize / text.length;

    for ( NSUInteger i = 0; i < text.length; ++i )
    {
        NSRange range = { .location = i, .length = 1 };
        NSString *c = [text substringWithRange:range];

        CGFloat yoffset = sin( degreesToRadians(angle) ) * radius;
        CGFloat xoffset = cos( degreesToRadians(angle) ) * radius;

        CGFloat rotAngle = 90 - angle;

        if ( clockwise )
        {
            yoffset = -yoffset;
            rotAngle = -90 + angle;
        }

        CATextLayer* tl = [[CATextLayer alloc] init];
        tl.masksToBounds = NO;
        tl.wrapped = NO;
        tl.truncationMode = kCATruncationNone;

        if ( debugMode )
        {
            tl.borderWidth = 1;
            tl.cornerRadius = 3;
            tl.borderColor = [UIColor whiteColor].CGColor;
        }

        // Text layer frame determined here. Effects how arc is centered. 
        CGSize charSize = CGSizeMake(20, 20);
        tl.frame = CGRectMake( shiftH + xcenter - xoffset, shiftV + ycenter + yoffset, charSize.width, charSize.height );
        // *******

        tl.font = (__bridge CFTypeRef)(font.fontName);
        tl.fontSize = font.pointSize;
        tl.foregroundColor = textColor.CGColor;
        tl.string = c;
        tl.alignmentMode = @"center";

        tl.transform = CATransform3DMakeAffineTransform( CGAffineTransformMakeRotation( degreesToRadians(rotAngle) ) );
        [layer addSublayer:tl];
        angle += angleStep;
    }

    if ( debugMode )
    {
        layer.backgroundColor = RGBA(0x00, 0x00, 0x00, .6).CGColor;
    }
}

- (void)rotateContinously
{
    CABasicAnimation *rotationAnimation;
    rotationAnimation = [CABasicAnimation animationWithKeyPath:@"transform.rotation.z"];
    rotationAnimation.toValue = [NSNumber numberWithFloat: M_PI * 2.0 /* full rotation*/ * 1 * 1 ];
    rotationAnimation.duration = 5;
    rotationAnimation.cumulative = YES;
    rotationAnimation.repeatCount = INT_MAX;

    [self.layer addAnimation:rotationAnimation forKey:@"rotationAnimation"];
}
@end

ここで数学の何が問題になっていますか? このテキスト自体が中心にならないのはなぜですか?