An Extension on NSAttributedString to allow for easier creation.

NSAttributedString+RZExtensions.h

//
//  NSAttributedString+RZExtensions.h
//  Raizlabs
//
//  Created by Alex Rouse on 12/13/13.

// Copyright 2014 Raizlabs and other contributors
// http://raizlabs.com/
// 
// Permission is hereby granted, free of charge, to any person obtaining
// a copy of this software and associated documentation files (the
// "Software"), to deal in the Software without restriction, including
// without limitation the rights to use, copy, modify, merge, publish,
// distribute, sublicense, and/or sell copies of the Software, and to
// permit persons to whom the Software is furnished to do so, subject to
// the following conditions:
// 
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
//

#import <Foundation/Foundation.h>

@interface NSAttributedString (RZExtensions)

// Provide a list of strings followed by attributes and returns a NSAttributedString for their combination.
//
//  Example:
//  [NSAttributedString attributedStringWithStringsAndAttributes:@"test",attributes,@"test2",attributes2,nil];
// 

+ (instancetype)rz_attributedStringWithStringsAndAttributes:(id)firstString, ... NS_REQUIRES_NIL_TERMINATION;

@end

NSAttributedString+RZExtensions.m

//
//  NSAttributedString+RZExtensions.m
//  Raizlabs
//
//  Created by Alex Rouse on 12/13/13.

// Copyright 2014 Raizlabs and other contributors
// http://raizlabs.com/
// 
// Permission is hereby granted, free of charge, to any person obtaining
// a copy of this software and associated documentation files (the
// "Software"), to deal in the Software without restriction, including
// without limitation the rights to use, copy, modify, merge, publish,
// distribute, sublicense, and/or sell copies of the Software, and to
// permit persons to whom the Software is furnished to do so, subject to
// the following conditions:
// 
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
//

#import "NSAttributedString+RZExtensions.h"

@implementation NSAttributedString (RZExtensions)

+ (instancetype)rz_attributedStringWithStringsAndAttributes:(id)firstString, ... NS_REQUIRES_NIL_TERMINATION
{
    NSMutableAttributedString* string = [[NSMutableAttributedString alloc] init];
    id param;
    NSDictionary* attributes;
    NSString* content = firstString;
    va_list argumentList;
    if (content)
    {
        va_start(argumentList, firstString);
        while ((param = va_arg(argumentList, id)) != nil)
        {
            if ([param isKindOfClass:[NSString class]])
            {
                content = param;
            }
            else if ([param isKindOfClass:[NSDictionary class]])
            {
                attributes = param;
            }
            
            if ([content isKindOfClass:[NSString class]] && [attributes isKindOfClass:[NSDictionary class]] )
            {
                NSAttributedString* str = [[NSAttributedString alloc] initWithString:content attributes:attributes];
                [string appendAttributedString:str];
                content = nil;
                attributes = nil;
            }
        }
        va_end(argumentList);
    }
    return [[NSAttributedString alloc] initWithAttributedString:string];
}

@end

Hi, could you just do "return string", NSMutableAttributedString is a subclass of NSAttributedString so it'll get auto casted to NSAttributedString at runtime when it returns to the caller. And you'll be able to keep the mutability aspect if the caller needs that in the future.