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

Question

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 ?

Answer 1

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

Answer 2

(I've added this to my own framework, but I don't think thats important)
Yep, that's the typical problem. You've failed to include -ObjC in the link flags.
See Building Objective-C static libraries with categories. This applies to frameworks as well.
ObjC does not create linker symbols for methods. It can't, they're not resolved until runtime. So the category methods aren't seen by the linker as "missing" and it doesn't bother linking the relevant compile unit. This is an important optimization that keeps you from linking all of a massive C library just because you use one function in it, but Objective-C categories break some of the linker's assumptions. The compiler saw the definition (via the header), but the linker didn't care, so there's no error until runtime.
The -ObjC flag says "this C-looking compile unit is actually Objective-C; link all of it even if you don't think you need to."