🌔 Drawing an accurate moon with UIBezierPath

Moon.h

//
//  Moon.h
//  Blackbox
//
//  Created by Ryan McLeod on 12/2/14.
//  Copyright (c) 2014 Ryan McLeod. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface Moon : UIView

// 0.0 = new moon
// 0.1 = sliver forming/waxing from right
// 0.5 = full mooon! fading/wanning from right
// 0.9 = almost new moon
// 1.0 = new moon
@property float moonPhase;
@property UIColor *moonColor;

@end

moon.m

//
//  Moon.m
//  Blackbox
//
//  Created by Ryan McLeod on 12/2/14.
//  Copyright (c) 2014 Ryan McLeod. All rights reserved.
//

#import "Moon.h"

@implementation Moon

static float moonInset = 1;

- (id) initWithFrame:(CGRect)frame {
    self = [super initWithFrame:frame];
    if (self) {
        self.moonPhase = 0.0f;
        self.moonColor = [UIColor fallbackPrimary];
        [self setBackgroundColor:[UIColor clearColor]];
        
        self.layer.backgroundColor = [UIColor backgroundColor].CGColor;
        self.layer.cornerRadius = (frame.size.width / 2.f) - moonInset;
        self.layer.shadowColor = [UIColor colorWithRed:0.75 green:0.32 blue:0.56 alpha:1].CGColor;
        
        self.accessibilityPath = [UIBezierPath bezierPathWithOvalInRect:self.frame];
    }
    return self;
}

- (void) drawRect:(CGRect)rect {
    [super drawRect:rect];
    
    // Outline when close to new moons (inset helps avoid stroke clipping
    UIBezierPath *outlineCircle = [[UIBezierPath bezierPathWithOvalInRect:CGRectInset(rect, moonInset, moonInset)] bezierPathByReversingPath];
    outlineCircle.usesEvenOddFillRule = NO;
    float alpha = CLAMP(5*fabsf(self.moonPhase-0.5f)/0.5f - 4.f, 0.0, 1);
    [[self.moonColor colorWithAlphaComponent:alpha] setStroke];
    [outlineCircle stroke];

    CGRect bounds = self.bounds;
    NSAssert(bounds.size.width == bounds.size.height, @"Moon view must be a square!");

    CGFloat moonPhase = self.moonPhase;
    CGFloat diameter = CGRectGetWidth(bounds);
    CGFloat radius = diameter/2.f;
    UIBezierPath *circlePath = [UIBezierPath bezierPathWithArcCenter:CGPointMake(radius, radius)
                                                              radius:radius-moonInset
                                                          startAngle:-M_PI_2
                                                            endAngle:M_PI_2
                                                           clockwise:(moonPhase < 0.5f)];
    


    // 0.0 -> Full, Subtractive
    // 0.125 -> Half, Subtractive
    // 0.25 -> Zero, Additive
    // 0.375 -> Half, Additive
    // 0.5 -> Full, Additive
    // 0.625 -> Half, Additive
    // 0.75 -> Zero, Additive
    // 0.875 -> Half, Subtractive
    // 1.0 -> Full, Subtractive

    // visible area of the moon is based on the area not the radius so we sqrt our multiplier
    CGFloat ovalWidth = 2 * radius * sqrtf(fabs(4.0f * (fmod(moonPhase, 0.5) - 0.25)));
    UIBezierPath *ovalPath = [UIBezierPath bezierPathWithOvalInRect:CGRectInset(CGRectMake(((diameter/2.f) - (ovalWidth/2.f)), 0, ovalWidth, diameter), moonInset, moonInset)];
    [self.moonColor setFill];

    if (moonPhase < 0.25 || moonPhase > 0.75) {
        CGFloat nudgeAmount = 0.5; // Avoid small anti-aliasing lines on some devices
        CGFloat originX = (moonPhase < 0.25f ? (bounds.size.width / 2.f + nudgeAmount) : -nudgeAmount);
        CGRect otherHalf = CGRectMake(originX, 0, (bounds.size.width / 2.f), bounds.size.height);
        [[UIBezierPath bezierPathWithRect:otherHalf] addClip];

        UIBezierPath *moonPath = [UIBezierPath bezierPath];
        moonPath.usesEvenOddFillRule = YES;
        [moonPath appendPath:circlePath];
        [moonPath appendPath:ovalPath];
        [moonPath fill];
    } else {
        [circlePath fill];
        [ovalPath fill];
    }
    
    
    // Moon shadow movement
    float newToFull = -2 * (self.moonPhase - 0.5); // [–1, 0, +1]
    float xOffset = newToFull * 20;
    float yOffset = newToFull * 20;
    self.layer.shadowOffset = CGSizeMake(xOffset, yOffset);
    self.layer.shadowOpacity = (fabs(newToFull) * 0.20) + 0.15;
    self.layer.shadowRadius = (fabs(newToFull) * 35) + 15;
    self.layer.shadowPath = circlePath.CGPath;
}

@end