Demonstrate how to load an OTF file, change some if its internal values in-memory (Ascender, Descender, LineGap), register it and then create a UIFont object

otfHorizontalTableModify.m

#import <CoreText/CoreText.h>

#define kOTFhorizontalHeaderTableRecord_tag htonl((int32_t)('hhea'))

typedef struct otfTableRecordStruct
{
    int32_t tag;
    int32_t checksum;
    int32_t offset;
    int32_t length;
}otfTableRecord, *otfTableRecordPTR;

typedef struct otfHorizontalHeaderTableStruct
{
    int32_t version;
    int16_t ascender;
    int16_t descender;
    int16_t lineGap;
    int16_t advancedWidthMax;
}otfHorizontalHeaderTable, *otfHorizontalHeaderTablePTR;

@interface OTFModifier ()

@property(nonatomic, strong) NSString* fontPostScriptName;
@property(nonatomic, strong) UIFont* uiFont;

@end

@implementation OTFModifier

CGFontRef cgFont;

- (void)dealloc
{
    [self unregisterOTF];
}

#pragma mark - OTFun

- (BOOL)loadOTF:(NSURL*)otfFileURL withModifyAscender:(int)modifiedAscender modifiedDescender:(int)modifiedDescender modifiedLineGap:(int)modifiedLineGap
{
    [self unregisterOTF];
    
    NSData* otfData = [NSData dataWithContentsOfURL:otfFileURL];
    const char* pRawDataBuffer = (const char*)[otfData bytes];
    char* pCurOffset = ((char*)pRawDataBuffer) + 0x04;  //Offset where the value that specifies the number of "Table Records" reside
    
    int16_t numTables = htons(*((int16_t*)(pCurOffset)));
    NSLog(@"numTables = %d", (int)numTables);
    
    otfHorizontalHeaderTablePTR pHorizontalHeaderTable = NULL;
    
    pCurOffset = ((char*)pRawDataBuffer) + 0x0C;    //Offset where the list of "Table Records" begin
    
    for (int16_t curTableIndex = 0; curTableIndex < numTables; ++curTableIndex)
    {
        otfTableRecordPTR pTableRecord = (otfTableRecordPTR)pCurOffset;
        
        if (pTableRecord->tag == kOTFhorizontalHeaderTableRecord_tag)
        {
            NSLog(@"\nFound the Horizontal Header Table Record (tag=0x%x -> 'hhea')", (pTableRecord->tag));
            NSLog(@"Loading Horizontal Header Table (offset=0x%x)\n", htonl(pTableRecord->offset));
            
            pHorizontalHeaderTable = ((otfHorizontalHeaderTablePTR)(((char*)pRawDataBuffer) + htonl(pTableRecord->offset)));
        }
        else
        {
            NSLog(@"Skipping Header Table Record (tag=0x%x -> '%c%c%c%c')",
                  (pTableRecord->tag),
                  ((char*)(&(pTableRecord->tag)))[0],
                  ((char*)(&(pTableRecord->tag)))[1],
                  ((char*)(&(pTableRecord->tag)))[2],
                  ((char*)(&(pTableRecord->tag)))[3]
                  );
        }
        
        pCurOffset+= sizeof(otfTableRecord);    //+=16
    }
    
    if (pHorizontalHeaderTable == NULL)
    {
        NSLog(@"Sorry, I looked and I looked but I could not find the Horizontal Header Table Record :(");
        return NO;
    }
    
    NSLog(@"\nHorizontal Header Table Record:");
    NSLog(@"version\t=\t0x%x",(int32_t)htonl(pHorizontalHeaderTable->version & 0xFFFFFFFF));
    NSLog(@"ascender\t=\t%hi",(int16_t)htons(pHorizontalHeaderTable->ascender & 0x0000FFFF));
    NSLog(@"descender\t=\t%hi",(int16_t)htons(pHorizontalHeaderTable->descender & 0x0000FFFF));
    NSLog(@"lineGap\t=\t%hi",(int16_t)htons(pHorizontalHeaderTable->lineGap & 0x0000FFFF));
    NSLog(@"advancedWidthMax\t=\t%hi",(int16_t)htons(pHorizontalHeaderTable->advancedWidthMax & 0x0000FFFF));
    NSLog(@"\n");
    
    NSLog(@"Patching \"ascender\" value to \"%d\"", (int)modifiedAscender);
    pHorizontalHeaderTable->ascender = (int16_t)htons(modifiedAscender);
    
    NSLog(@"Patching \"descender\" value to \"%d\"", (int)modifiedDescender);
    pHorizontalHeaderTable->descender = (int16_t)htons(modifiedDescender);
    
    NSLog(@"Patching \"lineGap\" value to \"%d\"", (int)modifiedLineGap);
    pHorizontalHeaderTable->lineGap = (int16_t)htons(modifiedLineGap);
    
    NSLog(@"Done patching\n");
    
    NSLog(@"Creating Core-Graphics Font object");
    CFErrorRef error;
    CGDataProviderRef provider = CGDataProviderCreateWithCFData((__bridge CFDataRef)otfData);
    cgFont = CGFontCreateWithDataProvider(provider);
    
    if (cgFont == NULL)
    {
        NSLog(@"Failed :(");
        CFRelease(provider);
        return NO;
    }
    
    NSLog(@"Done\n");
    
    CFStringRef cfFontPostScriptName = CGFontCopyPostScriptName(cgFont);
    self.fontPostScriptName = CFBridgingRelease(cfFontPostScriptName);
    
    if ([self.fontPostScriptName length] > 0)
    {
        self.title = self.fontPostScriptName;
    }
    
    
    NSLog(@"Registering the font \"%@\"", self.fontPostScriptName);
    
    if (! CTFontManagerRegisterGraphicsFont(cgFont, &error)) {
        CFStringRef errorDescription = CFErrorCopyDescription(error);
        NSLog(@"Failed to register font: %@", errorDescription);
        CFRelease(errorDescription);
    }

    CFRelease(provider);
    
    NSLog(@"Done\n");
    
    NSLog(@"Creating a UIFont object");
    self.uiFont = [UIFont fontWithName:self.fontPostScriptName size:14.0f];
    
    if (self.uiFont == nil)
    {
        NSLog(@"Failed :(");
        return NO;
    }
    
    NSLog(@"Done\n");
    
    //Test :)
    self.uiLabel.font = self.uiFont;
    self.uiTextField.font = self.uiFont;
    self.uiButton.titleLabel.font = self.uiFont;
    self.uiButton.titleLabel.text = [NSString stringWithFormat:@"%@", self.uiButton.titleLabel.text];
    self.uiTextView.font = self.uiFont;
    
    return YES;
}

- (BOOL)unregisterOTF
{
    if (cgFont)
    {
        NSLog(@"Unregistering the font");
        
        CFErrorRef error;
        if (!CTFontManagerUnregisterGraphicsFont(cgFont, &error)) {
            CFStringRef errorDescription = CFErrorCopyDescription(error);
            NSLog(@"Failed to unregister font: %@", errorDescription);
            CFRelease(errorDescription);
            return NO;
        }
        else
        {
            NSLog(@"Done\n");
        }
        
        CGFontRelease(cgFont);
        cgFont = NULL;
    }
    
    return YES;
}

@end