Cannot access Objective-C singleton's array from Swift code

Question

I made an array in a singleton to write objects into it from multiple parts of my code. Here's how:

// in singleton.h
#import <UIKit/UIKit.h>

// make globally accessible array
@interface MyManager : NSObject {
    NSMutableArray *imgArray;
}
@property (nonatomic, retain) NSMutableArray *imgArray;
+ (id)sharedManager;
@end

// in singleton.m
#import "singleton.h"

For my .m file :

@implementation MyManager

@synthesize imgArray;

#pragma mark Singleton Methods

+ (id)sharedManager {
    static MyManager *sharedMyManager = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedMyManager = [[self alloc] init];
    });
    return sharedMyManager;
}

- (id) init {
    if (self = [super init]) {
        self.imgArray = [NSMutableArray new];
        }
    NSLog(@"initialized");
    return self;
}

@end

I can access my array called imgArray it from my objective C code. However, In swift I get an error when I do this:

let array  = MyManager.sharedManager()

array.imgArray.add("hello world") .    (!!!) Value of type 'Any?' has no member 'imgArray'

I can access MyManager.sharedManager(), but Why can't I access imgArray the same way as in objective C?

score 3 · Accepted Answer · answered Mar 05 '19 at 01:10

You should declare it as instancetype or MyManager *. E.g.

+ (MyManager *)sharedManager;

or

+ (instancetype)sharedManager;

A couple of suggestions:

The Swift convention for singleton’s is to use a class property name of shared, not a class method of sharedManager(). When you declare it in Objective-C, you might want to explicitly say that it’s a class property:
```
@property (class, readonly, strong) MyManager *sharedManager NS_SWIFT_NAME(shared);
```
This won’t change any of the Objective-C behavior, but in Swift, you can just do:
```
let manager = MyManager.shared
manager.images.add(image)
```
This results in more concise and idiomatic Swift code.
I’d suggest that you audit your Objective-C for nullability. I.e., confirm what can be nil and what can’t. Since both imgArray (which I might just call images) and sharedManager can never be nil, I would just use the NS_ASSUME_NONNULL_BEGIN/END macros which tells the compiler “unless I tell you otherwise, assuming this property cannot be nil”:
```
//  MyManager.h

@import UIKit;

NS_ASSUME_NONNULL_BEGIN

@interface MyManager : NSObject

@property (nonatomic, strong) NSMutableArray <UIImage *> *images;
@property (class, readonly, strong) MyManager *sharedManager NS_SWIFT_NAME(shared);

@end

NS_ASSUME_NONNULL_END
```
By telling the compiler that these two cannot be nil, that means that you’ll have to do less unnecessary unwrapping of optionals in your Swift code.

As an aside, notice that I didn't declare an instance variable. (And if you did need one, I wouldn’t advise declaring it in the public interface.) Objective-C will now synthesize the ivars backing our properties automatically for us. (So my property images will have an ivar called _images that will be synthesized for me.) And you don’t need/want the @synthesize line, either:

//  MyManager.m

#import "MyManager.h"

@implementation MyManager

+ (instancetype)sharedManager {
    static MyManager *sharedMyManager = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedMyManager = [[self alloc] init];
    });
    return sharedMyManager;
}

- (instancetype)init {
    if (self = [super init]) {
        self.images = [NSMutableArray new];
    }
    NSLog(@"initialized");
    return self;
}

@end

score 2 · Answer 2 · answered Mar 04 '19 at 23:30

2

Change + (id)sharedManager; to + (MyManager *)sharedManager;. Otherwise Swift doesn't know what kind of object sharedManager is and it will assume it's Any.

answered Mar 04 '19 at 23:30

EmilioPelaez

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3

Or `instancetype` instead of `id`, too. – Rob Mar 04 '19 at 23:36

Cannot access Objective-C singleton's array from Swift code

2 Answers2