Creating a path to the documentsDirectory results in a NSInvalidArgumentException and unrecognized selector issue

Question

I am learning Obj-C, and I cannot for the life of me figure out why I am getting this crash. Here is the error message:

2018-01-22 16:10:00.009334-0500 ParkSmartObjC[1997:189530] Finished picking media
2018-01-22 16:10:00.163798-0500 ParkSmartObjC[1997:189530] B4A78674-6164-44CE-8EED-153AA8C0BDA2
2018-01-22 16:10:00.164109-0500 ParkSmartObjC[1997:189530] -[__NSSingleObjectArrayI stringByAppendingPathComponent:]: unrecognized selector sent to instance 0x60000020bd00
2018-01-22 16:10:00.167466-0500 ParkSmartObjC[1997:189530] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSSingleObjectArrayI stringByAppendingPathComponent:]: unrecognized selector sent to instance 0x60000020bd00'
*** First throw call stack:
(
0   CoreFoundation                      0x0000000109d731cb __exceptionPreprocess + 171
1   libobjc.A.dylib                     0x000000010928ff41 objc_exception_throw + 48
2   CoreFoundation                      0x0000000109df3914 -[NSObject(NSObject) doesNotRecognizeSelector:] + 132
3   CoreFoundation                      0x0000000109cf6178 ___forwarding___ + 1432
4   CoreFoundation                      0x0000000109cf5b58 _CF_forwarding_prep_0 + 120
5   ParkSmartObjC                       0x0000000107f4d9fa -[PhotoVC imagePickerController:didFinishPickingMediaWithInfo:] + 426
6   UIKit                               0x000000010a64507f -[UIImagePickerController _imagePickerDidCompleteWithInfo:] + 127
7   UIKit                               0x000000010a644999 __60-[UIImagePickerController didSelectMediaWithInfoDictionary:]_block_invoke + 42
8   libdispatch.dylib                   0x000000010c5193f7 _dispatch_call_block_and_release + 12
9   libdispatch.dylib                   0x000000010c51a43c _dispatch_client_callout + 8
10  libdispatch.dylib                   0x000000010c5256f0 _dispatch_main_queue_callback_4CF + 628
11  CoreFoundation                      0x0000000109d35ef9 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 9
12  CoreFoundation                      0x0000000109cfa662 __CFRunLoopRun + 2402
13  CoreFoundation                      0x0000000109cf9a89 CFRunLoopRunSpecific + 409
14  GraphicsServices                    0x000000010eeaf9c6 GSEventRunModal + 62
15  UIKit                               0x000000010a1ee23c UIApplicationMain + 159
16  ParkSmartObjC                       0x0000000107f4ca9f main + 111
17  libdyld.dylib                       0x000000010c596d81 start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException

Here is the code where it crashes:

- (void) imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary<NSString *,id> *)info {

if(info[UIImagePickerControllerOriginalImage]) {

    UIImage *image = info[UIImagePickerControllerOriginalImage];
    NSData *data = UIImageJPEGRepresentation(image, 0.6);
    NSString *uuid = [[NSUUID UUID] UUIDString];

    NSString *path = [ImageSaveHelper getDocumentsDirectory];
    NSLog(@"%@", uuid);
    //The UUID prints fine...
    NSString *filePath = [[NSString alloc] init];
   ->>> filePath = [path stringByAppendingPathComponent:uuid]; //SEEMS TO CRASH HERE. WHAT HAVE I DONE WRONG?
    [data writeToFile:filePath atomically:YES];

    //Save photo to current CarLocation object
    RLMRealm *realm = [RLMRealm defaultRealm];
    [realm beginWriteTransaction];
    self.carLoc.photoUID = uuid;
    self.carLoc.isPhotoTaken = YES;
    [realm commitWriteTransaction];

    self.carImage.image = image;
    [picker dismissViewControllerAnimated:YES completion:nil];
}
}

I am trying to select an image from the ImagePickerController, then save the data to a location on the disk with a UUID. It seems to be crashing when I try to append the UUID to the filePath. I get the filePath from a helper function. What am I doing wrong?

EDIT: Here is the helper function...

+ (NSString*) getDocumentsDirectory {
NSArray *paths = [NSArray arrayWithObjects:NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES), nil];
NSString *filePath = [paths objectAtIndex:0];
return filePath;

}

Answer 1

Please read the error message:

[__NSSingleObjectArrayI stringByAppendingPathComponent:]: unrecognized selector

clearly states that the method getDocumentsDirectory returns an Array, not a single string. So you have to get the first object of the array:

NSArray *pathArray = [ImageSaveHelper getDocumentsDirectory];
NSString *path = pathArray[0];
NSString *filePath = [path stringByAppendingPathComponent:uuid];

Edit:

Your helper method returns an array because you are initializing an array with the result of NSSearchPathForDirectoriesInDomains which is already an array. The result is a nested array

@[@[@"path", @"path", @"path"]];

and getting the object at index 0 is still an array.

---

The solution is to change getDocumentsDirectory to

+ (NSString*) getDocumentsDirectory {
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    return paths[0];
}

then you can keep the syntax

NSString *path = [ImageSaveHelper getDocumentsDirectory];
NSString *filePath = [path stringByAppendingPathComponent:uuid];

PS: I recommend to learn Swift rather than ObjC. The Swift compiler doesn't let you do those type mismatch mess 😉