从0到1实现小说阅读器（二、实现一个简单的排版引擎）

上篇展示了CoreText排版的基础能力，但是制作一个排版引擎需要进一步封装，根据设计模式的“单一功能”原则应该把不同功能分给不同类中处理。如下：

1.一个显示用的类，仅负责内容显示，不负责排版；
2.一个模型类，用于承载显示做需要的所有数据；
3.一个排版类，用于实现文字内容的排版；
4.一个配置类，用于实现排版是的可配置项。

根据以上原则，我们拆分出了4个类：
1.CTFrameParserConfig，用于配置绘制参数，比如：字体大小、字体颜色、行间距等等；
2.CTFrameParser，用于生成最后需要绘制的CTFrameRef实例；
3.CoreTextData，用于CTFrameParser生成的CTFrameRef实例，以及CTFrameRef实际绘制需要的高度。
4.CTDisplayView，持有CoreTextData类的实例，负责讲CTFrameRef绘制到界面上。具体代码实现如下：

CTFrameParserConfig 类：

#import <Foundation/Foundation.h>

@interface CTFrameParserConfig : NSObject

@property (nonatomic, assign) CGFloat width;
@property (nonatomic, assign) CGFloat fontSize;
@property (nonatomic, assign) CGFloat lineSpace;
@property (nonatomic, strong) UIColor *textColor;

@end

#import "CTFrameParserConfig.h"

@implementation CTFrameParserConfig

- (instancetype)init {
    self = [super init];
    if (self) {
        _width = 200.0f;
        _fontSize = 16.0f;
        _lineSpace = 8.0f;
        _textColor = RGB(108, 108, 108);
    }
    return self;
}

@end

CTFrameParser 类：

#import <Foundation/Foundation.h>
#import "CoreTextData.h"
#import "CTFrameParserConfig.h"

@interface CTFrameParser : NSObject

+ (CoreTextData *)parserContent:(NSString *)content config:(CTFrameParserConfig *)config;

@end

+ (CoreTextData *)parserContent:(NSString *)content config:(CTFrameParserConfig *)config {
    NSDictionary *attributes = [self attributesWithConfig:config];
    NSAttributedString *contentString = [[NSAttributedString alloc] initWithString:content attributes:attributes];
    
    // 创建CTFramesetterRef实例
    CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((CFAttributedStringRef)contentString);
    
    // 获得绘制区域高度
    CGSize restrictSize = CGSizeMake(config.width, CGFLOAT_MAX);
    CGSize coreTextSize = CTFramesetterSuggestFrameSizeWithConstraints(framesetter, CFRangeMake(0, 0), nil, restrictSize, nil);
    CGFloat textHeight = coreTextSize.height;
    
    // 生成CTFrameRef实例
    CTFrameRef frame = [self createFrameWithFramesetter:framesetter config:config height:textHeight];
    
    // 讲生成好的CTFrameRef实例和计算好的绘制高度保存到CoreTextData实例中，最后返回CoreTextData实例
    CoreTextData *data = [[CoreTextData alloc] init];
    data.ctFrame = frame;
    data.height = textHeight;
    
    // 释放内存
    CFRelease(frame);
    CFRelease(framesetter);
    return data;
}

+ (NSDictionary *)attributesWithConfig:(CTFrameParserConfig *)config {
    CGFloat fontSize = config.fontSize;
    CTFontRef fontRef = CTFontCreateWithName((CFStringRef)@"ArialMT", fontSize, NULL);
    
    CGFloat lineSpacing = config.lineSpace;
    const CFIndex kNumberOfSettings = 3;
    CTParagraphStyleSetting theSettings[kNumberOfSettings] = {
        {kCTParagraphStyleSpecifierLineSpacingAdjustment, sizeof(CGFloat), &lineSpacing},
        {kCTParagraphStyleSpecifierMaximumLineSpacing, sizeof(CGFloat), &lineSpacing},
        {kCTParagraphStyleSpecifierMinimumLineSpacing, sizeof(CGFloat), &lineSpacing}
    };
    
    CTParagraphStyleRef theParagraphRef = CTParagraphStyleCreate(theSettings, kNumberOfSettings);
    
    UIColor *textColor = config.textColor;
    
    NSMutableDictionary *dict = [NSMutableDictionary dictionary];
    dict[(id)kCTForegroundColorAttributeName] = (id)textColor.CGColor;
    dict[(id)kCTFontAttributeName] = (__bridge id)fontRef;
    dict[(id)kCTParagraphStyleAttributeName] = (__bridge id)theParagraphRef;
    
    CFRelease(theParagraphRef);
    CFRelease(fontRef);
    return dict;
}

+ (CTFrameRef)createFrameWithFramesetter:(CTFramesetterRef)framesetter config:(CTFrameParserConfig *)config height:(CGFloat)height {
    CGMutablePathRef path = CGPathCreateMutable();
    CGPathAddRect(path, NULL, CGRectMake(0, 0, config.width, height));
    
    CTFrameRef frame = CTFramesetterCreateFrame(framesetter, CFRangeMake(0, 0), path, NULL);
    CFRelease(path);
    return frame;
}

@end

CoreTextData 类：

#import <Foundation/Foundation.h>

@interface CoreTextData : NSObject

@property (nonatomic, assign) CTFrameRef ctFrame;
@property (nonatomic, assign) CGFloat height;

@end

#import "CoreTextData.h"

@implementation CoreTextData

- (void)setCtFrame:(CTFrameRef)ctFrame {
    if (_ctFrame != ctFrame) {
        if (_ctFrame != nil) {
            CFRelease(_ctFrame);
        }
        CFRetain(ctFrame);
        _ctFrame = ctFrame;
    }
}

- (void)dealloc {
    if (_ctFrame != nil) {
        CFRelease(_ctFrame);
        _ctFrame = nil;
    }
}

@end

CTDisplayView 类：

#import <UIKit/UIKit.h>
#import "CoreTextData.h"

@interface CTDisplayView : UIView

@property (nonatomic, strong) CoreTextData *data;

@end

#import "CTDisplayView.h"

@implementation CTDisplayView

- (void)drawRect:(CGRect)rect {
    [super drawRect:rect];
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetTextMatrix(context, CGAffineTransformIdentity);
    CGContextTranslateCTM(context, 0, self.bounds.size.height);
    CGContextScaleCTM(context, 1.0, -1.0);
    if (self.data) {
        CTFrameDraw(self.data.ctFrame, context);
    }
}

最终在 ViewController 类中的实现：

#import "ViewController.h"
#import "CTDisplayView.h"
#import "CTFrameParser.h"

@interface ViewController ()

@property (weak, nonatomic) IBOutlet CTDisplayView *displayView;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    CTFrameParserConfig *config = [[CTFrameParserConfig alloc] init];
    config.textColor = [UIColor redColor];
    config.width = self.displayView.width;
    
    CoreTextData *data = [CTFrameParser parserContent:@"小说阅读器，开始...." config:config];
    self.displayView.data = data;
    self.displayView.height = data.height;
    self.displayView.backgroundColor = [UIColor yellowColor];
}

@end

我们通过UML来看一下各个类之间的关系：

总结：(关键类：CTFrameParser)

如此可以看出来CTFrameParser通过CTFrameParserConfig和CoreTextData来完成排版工作，使得排版的工作从CTDisplayView类中完全解放出来，让其只负责最后的绘制和显示。