objective c - Can't get custom @protocol working on iOS -

note: below using ios automatic reference counting (arc) enabled. think arc may have lot why isn't working set per examples i've found via google.

i trying create protocol notify delegate of filename user selects 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 error @

@synthesize delegate; 

line "filelistviewcontroller.m: error: automatic reference counting issue: existing ivar 'delegate' unsafe_unretained property 'delegate' must __unsafe_unretained"

if change filelistviewcontroller.h putting __weak , (weak) 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 try set delegate app crashes. view called 'importviewcontroller' creating view 'filelistviewcontroller' , setting delegate (importviewcontroller) can implement custom protocol of 'didselectfilename'. error is;

* terminating app due uncaught exception 'nsinvalidargumentexception', reason: '-[importviewcontroller setdelegate:]: unrecognized selector sent instance 0x6c7d430'

the code 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 putting (weak) , __weak in make work? don't understand why works found googling , there wasn't explanation.
• why can't set delegate using '[filelistviewcontroller setdelegate:self];' ? seems compiler doesn't know 'delegate' exists.

it seems :

filelistviewcontroller *filelistviewcontroller =     [self.storyboard instantiateviewcontrollerwithidentifier:@"filelist"]; 

you didn't filelistviewcontroller object. @ message says :

-[importviewcontroller setdelegate:]: unrecognized selector sent instance 0x6c7d430

and why app crashes. try define retain property, instead of assign, in case delegate deallocated elsewhere, app won't crash.