全局修改字体（Change font globally）

NSObject+SwizzelingUtils.h

//
//  NSObject+SwizzelingUtils.h
//  Font
//
//  Created by 宗仁 on 2017/1/23.
//  Copyright © 2017年 宗仁. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface NSObject(SwizzelingUtils)

+(void)swizzlingClassMethod:(SEL)originalSelector toMethod:(SEL)swizzledSelector;

@end

NSObject+SwizzelingUtils.m

//
//  NSObject+SwizzelingUtils.m
//  Font
//
//  Created by 宗仁 on 2017/1/23.
//  Copyright © 2017年 宗仁. All rights reserved.
//

#import "NSObject+SwizzelingUtils.h"
#import <objc/runtime.h>

@implementation NSObject(SwizzelingUtils)

+(void)swizzlingClassMethod:(SEL)originalSelector toMethod:(SEL)swizzledSelector{

    Class class = object_getClass((id)self);

    Method originalMethod = class_getClassMethod(class, originalSelector);
    Method swizzledMethod = class_getClassMethod(class, swizzledSelector);
    
    BOOL didAddMethod =
    class_addMethod(class,
                    originalSelector,
                    method_getImplementation(swizzledMethod),
                    method_getTypeEncoding(swizzledMethod));
    
    if (didAddMethod) {
        class_replaceMethod(class,
                            swizzledSelector,
                            method_getImplementation(originalMethod),
                            method_getTypeEncoding(originalMethod));
    } else {
        method_exchangeImplementations(originalMethod, swizzledMethod);
    }

}

@end

UIFont+Swizzeling.h

//
//  UIFont+Swizzeling.h
//  Font
//
//  Created by 宗仁 on 2017/1/23.
//  Copyright © 2017年 宗仁. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface UIFont(Swizzeling)

@end

UIFont+Swizzeling.m

//
//  UIFont+Swizzeling.m
//  Font
//
//  Created by 宗仁 on 2017/1/23.
//  Copyright © 2017年 宗仁. All rights reserved.
//

#import "UIFont+Swizzeling.h"
#import "UIFont+YSCCustom.h"
#import "NSObject+SwizzelingUtils.h"

@implementation UIFont(Swizzeling)

+ (void)load {
    
    [UIFont swizzlingClassMethod:@selector(zr_fontWithName:size:) toMethod:@selector(fontWithName:size:)];
    [UIFont swizzlingClassMethod:@selector(zr_systemFontOfSize:) toMethod:@selector(systemFontOfSize:)];
    
}

@end

UIFont+YSCCustom.h

//
//  UIFont+YSCCustom.h
//  Font
//
//  Created by 宗仁 on 2017/1/23.
//  Copyright © 2017年 宗仁. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface UIFont(YSCCustom)

+ (nullable UIFont *)customFontOfSize:(CGFloat)fontSize;
+ (nullable UIFont *)zr_systemFontOfSize:(CGFloat)fontSize;
+ (nullable UIFont *)zr_fontWithName:(NSString * _Nonnull)fontName size:(CGFloat)fontSize;

@end

UIFont+YSCCustom.m

//
//  UIFont+YSCCustom.m
//  Font
//
//  Created by 宗仁 on 2017/1/23.
//  Copyright © 2017年 宗仁. All rights reserved.
//

#import "UIFont+YSCCustom.h"

static NSString* const CUSTOM_FONT_NAME = @"HYm7gj";

@implementation UIFont(YSCCustom)

+ (nullable UIFont *)customFontOfSize:(CGFloat)fontSize{
    return [UIFont zr_fontWithName:CUSTOM_FONT_NAME size:fontSize];
}

+ (UIFont *)zr_systemFontOfSize:(CGFloat)fontSize{
    return [UIFont customFontOfSize:fontSize];
}

+ (nullable UIFont *)zr_fontWithName:(NSString *)fontName size:(CGFloat)fontSize{
    return [UIFont zr_fontWithName:CUSTOM_FONT_NAME size:fontSize];
}

@end

Usage.m

@implementation Usage
-(instancetype)init{
  UIFont *font = [UIFont systemFontOfSize:48];    
}