Question

Note: the below is using iOS with Automatic Reference Counting (ARC) enabled. I think ARC may have a lot to do with why it isn’t working as this is set up as per examples i’ve found via google.

I am trying to create a protocol to notify a delegate of the filename the user selects from a UITableView.

FileListViewController.h

@protocol FileListDelegate <NSObject>
- (void)didSelectFileName:(NSString *)fileName;

@end

@interface FileListViewController : UITableViewController
{
    @private
        NSArray *fileList;
        id <FileListDelegate> delegate;
}
@property (nonatomic, retain) NSArray *fileList;
@property (nonatomic, assign) id <FileListDelegate> delegate;

@end

FileListViewController.m

#import "FileListViewController.h"

@implementation FileListViewController

@synthesize fileList;
@synthesize delegate;

This gives an error at the

@synthesize delegate;

line which is “FileListViewController.m: error: Automatic Reference Counting Issue: Existing ivar ‘delegate’ for unsafe_unretained property ‘delegate’ must be __unsafe_unretained”

If i change FileListViewController.h putting __weak and (weak) then it will run.

@protocol FileListDelegate <NSObject>
- (void)didSelectFileName:(NSString *)fileName;

@end

@interface FileListViewController : UITableViewController
{
    @private
        NSArray *fileList;
        __weak id <FileListDelegate> delegate;
}
@property (nonatomic, retain) NSArray *fileList;
@property (weak) id <FileListDelegate> delegate;

@end

But when I try to set the delegate the app crashes. A view called ‘ImportViewController’ is creating a view from ‘FileListViewController’ and setting the delegate to itself (ImportViewController) so I can implement my custom protocol of ‘didSelectFileName’. The error I get is;

* Terminating app due to uncaught exception ‘NSInvalidArgumentException’, reason: ‘-[ImportViewController setDelegate:]: unrecognized selector sent to instance 0x6c7d430’

The code I am running is;

ImportViewController.m

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    FileListViewController *fileListViewController = [self.storyboard instantiateViewControllerWithIdentifier:@"filelist"];

    [fileListViewController setDelegate:self];
    [self.navigationController pushViewController:fileListViewController animated:YES];

}

My Questions are:

• Why does putting (weak) and __weak in make it work? I don’t
  understand why this works as I found it googling and there wasn’t an
  explanation.
• Why can’t I set my delegate using this
  ‘[fileListViewController setDelegate:self];’ ? It seems like the
  compiler doesn’t know ‘delegate’ exists.

Answer 1

It seems that with :

FileListViewController *fileListViewController =
    [self.storyboard instantiateViewControllerWithIdentifier:@"filelist"];

you didn’t get an FileListViewController object. Look at the message it says :

-[ImportViewController setDelegate:]: unrecognized selector sent to instance 0x6c7d430

and that why your app crashes. Also try to define a retain property, instead of just assign, in case the delegate is deallocated elsewhere, your app won’t crash.