Question

I’ve got a file named ColourUtils.h which contains the following code, I found this which converts hex strings to color.

#import <Foundation/Foundation.h>

@interface ColourUtils : NSObject

+ (UIColor *) colorWithHexString: (NSString *) hex;

@end

@implementation ColourUtils

+ (UIColor *) colorWithHexString: (NSString *) hex  
{  
    NSString *cString = [[hex stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] uppercaseString];  

    // String should be 6 or 8 characters  
    if ([cString length] < 6) return [UIColor grayColor];  

    // strip 0X if it appears  
    if ([cString hasPrefix:@"0X"]) cString = [cString substringFromIndex:2];  

    if ([cString length] != 6) return  [UIColor grayColor];  

    // Separate into r, g, b substrings  
    NSRange range;  
    range.location = 0;  
    range.length = 2;  
    NSString *rString = [cString substringWithRange:range];  

    range.location = 2;  
    NSString *gString = [cString substringWithRange:range];  

    range.location = 4;  
    NSString *bString = [cString substringWithRange:range];  

    // Scan values  
    unsigned int r, g, b;  
    [[NSScanner scannerWithString:rString] scanHexInt:&r];  
    [[NSScanner scannerWithString:gString] scanHexInt:&g];  
    [[NSScanner scannerWithString:bString] scanHexInt:&b];  

    return [UIColor colorWithRed:((float) r / 255.0f)  
                           green:((float) g / 255.0f)  
                            blue:((float) b / 255.0f)  
                           alpha:1.0f];  
} 

@end

I’ve got 2 ViewControllers that both need to use this class (for decorating the UI in viewDidLoad). In both .h files, I’ve got this imported : #import "ColourUtils.h".

However, I do get the following error at compile time :

ld: duplicate symbol _OBJC_METACLASS_$_ColourUtils in ...

What is the best way of being able to include this common method into multiple ViewControllers? I’m coming from a Java background whereby a static method and some imports would be fine, but this doesn’t appear to work the same in Objective-C

Answer 1

Don’t put @implementation parts of a class into a .h file. The .h should only be for the declarations of the class interface that other classes need in order to know what public methods and properties they can use. Create a matching .m file for the part of the class that others don’t need to know about (implementation and private declarations).