iOS: basic 2-column text layout

TwoColumnTextArea.h

//
//  TwoColumnTextArea.h
//  TastingRoom
//
//  Created by Keith Lazuka on 4/26/13.
//  Copyright (c) 2013 Keith Lazuka. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface TwoColumnTextArea : UIView

// the width of each column is equal to 0.5*(self.width - (self.width*self.gutterRatio)),
// with some rounding to keep everything on whole pixel boundaries

@property (nonatomic, readwrite, strong) NSString *text;
@property (nonatomic, readwrite, strong) UIColor *textColor;
@property (nonatomic, readwrite, strong) UIFont *font;
@property (nonatomic, readwrite, assign) float minimumLineHeight; // controls the leading
@property (nonatomic, readwrite, assign) float gutterRatio; // percentage of the view's width devoted to the center text gutter.

@end

TwoColumnTextArea.m

//
//  TwoColumnTextArea.m
//  TastingRoom
//
//  Created by Keith Lazuka on 4/26/13.
//  Copyright (c) 2013 Keith Lazuka. All rights reserved.
//

#import <CoreText/CoreText.h>

#import "TwoColumnTextArea.h"


@implementation TwoColumnTextArea

- (id)initWithFrame:(CGRect)frame
{
  if ((self = [super initWithFrame:frame]))
  {
    self.userInteractionEnabled = NO;
    self.font = [UIFont systemFontOfSize:[UIFont labelFontSize]];
    self.textColor = [UIColor blackColor];
    self.minimumLineHeight = self.font.pointSize;
    self.backgroundColor = [UIColor whiteColor];
    self.gutterRatio = 0.05; // 5% gutter by default
  }
  return self;
}

- (void)drawRect:(CGRect)rect
{
  if (!_text || _text.length == 0)
    return;
  
  NSMutableAttributedString *string = [[NSMutableAttributedString alloc] initWithString:_text];
  
  // style the text
  CTFontRef appFont = CTFontCreateWithName((__bridge CFStringRef)[_font fontName], [_font pointSize], NULL);
  NSRange entireStringRange = NSMakeRange(0, string.length);
  [string addAttribute:(id)kCTFontAttributeName value:(__bridge id)appFont range:entireStringRange];
  [string addAttribute:(id)kCTForegroundColorAttributeName value:(__bridge id)self.textColor.CGColor range:entireStringRange];
  CTParagraphStyleSetting leading = {
    .spec = kCTParagraphStyleSpecifierMinimumLineHeight,
    .value = &_minimumLineHeight,
    .valueSize = sizeof(_minimumLineHeight),
  };
  CTParagraphStyleRef paraStyle = CTParagraphStyleCreate(&leading, 1);
  [string addAttribute:(id)kCTParagraphStyleAttributeName value:(__bridge id)paraStyle range:entireStringRange];
  
  // derived layout parameters
  float gutterWidth = roundf(self.gutterRatio * self.width);
  float contentWidth = self.width - gutterWidth;
  float leftColumnWidth = floorf(contentWidth/2.0);
  float rightColumnWidth = contentWidth - leftColumnWidth;
  
  // layout master
  CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((__bridge CFAttributedStringRef)string);
  
  // left column frame
  CGMutablePathRef leftColumnPath = CGPathCreateMutable();
  CGPathAddRect(leftColumnPath, NULL, CGRectMake(0, 0, leftColumnWidth, self.bounds.size.height));
  CTFrameRef leftFrame = CTFramesetterCreateFrame(framesetter, CFRangeMake(0, 0), leftColumnPath, NULL);
  
  // right column frame
  CGMutablePathRef rightColumnPath = CGPathCreateMutable();
  CGPathAddRect(rightColumnPath, NULL, CGRectMake(leftColumnWidth + gutterWidth, 0, rightColumnWidth, self.bounds.size.height));
  NSInteger rightColumStart = CTFrameGetVisibleStringRange(leftFrame).length;
  CTFrameRef rightFrame = CTFramesetterCreateFrame(framesetter, CFRangeMake(rightColumStart, 0), rightColumnPath, NULL);
  
  // flip the coordinate system
  CGContextRef context = UIGraphicsGetCurrentContext();
  CGContextSetTextMatrix(context, CGAffineTransformIdentity);
  CGContextTranslateCTM(context, 0, self.bounds.size.height);
  CGContextScaleCTM(context, 1.0, -1.0);
  
  // draw
  CTFrameDraw(leftFrame, context);
  CTFrameDraw(rightFrame, context);
  
  // cleanup
  CFRelease(leftFrame);
  CGPathRelease(leftColumnPath);
  CFRelease(rightFrame);
  CGPathRelease(rightColumnPath);
  CFRelease(framesetter);
  CFRelease(appFont);
  CFRelease(paraStyle);
}

- (void)setText:(NSString *)text
{
  if (_text == text)
    return;
  
  _text = text;
  [self setNeedsDisplay];
}


@end