Obj-C, How do I use a category to supply methods which I will use in delegate methods?

I want to provide methods used in several view controllers called in my delegate methods.

For example, I have some CloudKit functionality (I've added this to my own framework, but I don't think thats important), where I want to provide some crash logging. Previosuly I had a crashLog function in each of my view controllers, which worked fine, but I have a lot of duplicate code.

Therefore I'd like to produce a category with these methods instead.

However I'm having difficulty getting my delegate methods to see these category methods.

Here's my code..

UIViewController+CloudKitDelegates.h

@interface UIViewController (CloudKitDelegates) <iCloudDBDelegate>

@property (weak,nonatomic) id<iCloudDBDelegate>iCloudDBDelegate;

-(void)crashLog:(NSString*)message, ...;

@end

UIViewController+CloudKitDelegates.m

#import "UIViewController+CloudKitDelegates.h"

@implementation UIViewController (CloudKitDelegates)
@dynamic iCloudDBDelegate;

-(void)crashLog:(NSString*)message, ...
{
    va_list args;
    va_start(args, message);

    NSLog(@"%@", [[NSString alloc] initWithFormat:message arguments:args]);

    va_end(args);
}

@end

h file - my calling view controller (e.g. My View Controller)

#import "UIViewController+CloudKitDelegates.h"

m file - delegate method

-(NSString*)getDBPath
{
    [self.iCloudDBDelegate crashLog: @"testing"];

From this call I'm getting an error ...

'NSInvalidArgumentException', reason: '-[MyViewController crashLog:]: 
    unrecognized selector sent to instance

The error is showing that my calling view controller called MyViewController doesn't have the crashLog method, which I have in my category.

Any ideas where I'm going wrong ?

The problem: identical method crashLog: in multiple classes, for example

@interface ViewController : UIViewController
@end

@implementation ViewController

- (void)someMethod {
    [self crashLog:@"error"];
}

-(void)crashLog:(NSString *)message {
    NSLog(@"%@", message);
}

@end

Solution A: move crashLog: to a common superclass (or a category on superclass UIViewController)

@interface CommonViewController : UIViewController

-(void)crashLog:(NSString *)message;

@end

@implementation CommonViewController

-(void)crashLog:(NSString *)message {
    NSLog(@"%@", message);
}

@end


@interface ViewController : CommonViewController
@end

@implementation ViewController

- (void)someMethod {
    [self crashLog:@"error"];
}

@end

Solution B: move crashLog: to a delegate and protocol

@protocol ICloudDBDelegate

-(void)crashLog:(NSString *)message;

@end


@interface DelegateClass : AnyClass <ICloudDBDelegate>
@end

@implementation DelegateClass

-(void)crashLog:(NSString *)message {
    NSLog(@"%@", message);
}

@end


@interface ViewController : UIViewController
@end

@implementation ViewController

@property (weak, nonatomic) id <ICloudDBDelegate> iCloudDBDelegate;

- (void)viewDidLoad
{
    [super viewDidLoad];
    AppDelegate *appDel = (AppDelegate *)[[UIApplication sharedApplication] delegate];
    self.iCloudDBDelegate = appDel.iCloudDBDelegate;
}

- (void)someMethod {
    [self.iCloudDBDelegate crashLog:@"error"];
}

@end


@interface AppDelegate : UIResponder <UIApplicationDelegate, AppDelProtocolDelegate, iCloudDBDelegate>

@property (strong, nonatomic) id<iCloudDBDelegate>iCloudDBDelegate;

@end

@implementation AppDelegate

- (id<iCloudDBDelegate>)iCloudDBDelegate {
    if (!_iCloudDBDelegate) {
        _iCloudDBDelegate = [[DelegateClass alloc] init];
    }
    return _iCloudDBDelegate;
}

@end

Now we have new problem: property iCloudDBDelegate in multiple classes

Solution B + A: move crashLog to a delegate, move iCloudDBDelegate property to a superclass

@protocol ICloudDBDelegate

-(void)crashLog:(NSString *)message;

@end


@interface DelegateClass : AnyClass <ICloudDBDelegate>
@end

@implementation DelegateClass

-(void)crashLog:(NSString *)message {
    NSLog(@"%@", message);
}

@end


@interface CommonViewController : UIViewController

@property (weak, nonatomic) id <ICloudDBDelegate> iCloudDBDelegate;

@end

@implementation CommonViewController
@end


@interface ViewController : CommonViewController
@end

@implementation ViewController

- (void)someMethod {
    [self.iCloudDBDelegate crashLog:@"error"];
}

@end

Solution C: Another approach is a singleton object like NSUserDefaults.standardUserDefaults or NSFontManager.sharedFontManager: CloudDBManager.sharedCloudDBManager. No category or protocol required, just include CloudDBManager.h and use CloudDBManager.sharedCloudDBManager from everywhere.

@interface CloudDBManager : NSObject

@property(class, readonly, strong) CloudDBManager *sharedCloudDBManager;

-(void)crashLog:(NSString *)message;

@end

@implementation CloudDBManager

+ (CloudDBManager *)sharedCloudDBManager {
    static CloudDBManager *sharedInstance = nil;
    static dispatch_once_t onceToken = 0;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[CloudDBManager alloc] init];
        // Do any other initialisation stuff here
    });
    return sharedInstance;
}

-(void)crashLog:(NSString *)message {
    NSLog(@"%@", message);
}

@end


@interface ViewController : CommonViewController
@end

@implementation ViewController

- (void)someMethod {
    [CloudDBManager.sharedCloudDBManager crashLog:@"error"];
}

@end