Why resolveInstanceMethod: called twice

Question

Recently, I have been studying the runtime in Objective-C.

I created a class with the name TO:

@interface TO : NSObject
@end

#import "TO.h"

@implementation TO

- (id)forwardingTargetForSelector:(SEL)aSelector {
    NSLog(@"%@ sel: %@", NSStringFromSelector(_cmd), NSStringFromSelector(aSelector));
    return nil;
}

- (BOOL)respondsToSelector:(SEL)aSelector {
    NSLog(@"%@ sel: %@", NSStringFromSelector(_cmd), NSStringFromSelector(aSelector));
    return NO;
}

+ (BOOL)resolveClassMethod:(SEL)sel {
    NSLog(@"%@ sel: %@", NSStringFromSelector(_cmd), NSStringFromSelector(sel));
    return NO;
}

+ (BOOL)resolveInstanceMethod:(SEL)sel {
    NSLog(@"%@ sel: %@", NSStringFromSelector(_cmd), NSStringFromSelector(sel));
    return NO;
}

+ (IMP)instanceMethodForSelector:(SEL)aSelector {
    NSLog(@"%@ sel: %@", NSStringFromSelector(_cmd), NSStringFromSelector(aSelector));
    return nil;
}

- (void)forwardInvocation:(NSInvocation *)anInvocation {
    NSLog(@"%@", NSStringFromSelector(_cmd));
}

@end

Then I call the irrevocable selector somewhere:

TO *to = [TO new];
id res = [(NSString *)to uppercaseString];

Subsequently, I got the following output:

2015-12-22 22:27:04.319 OCDemo[81920:7728539] resolveInstanceMethod: sel: uppercaseString
2015-12-22 22:27:04.320 OCDemo[81920:7728539] forwardingTargetForSelector: sel: uppercaseString
2015-12-22 22:27:04.320 OCDemo[81920:7728539] resolveInstanceMethod: sel: uppercaseString
2015-12-22 22:27:04.320 OCDemo[81920:7728539] -[TO uppercaseString]: unrecognized selector sent to instance 0x7fdd3ad120a0
2015-12-22 22:27:04.322 OCDemo[81920:7728539] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[TO uppercaseString]: unrecognized selector sent to instance 0x7fdd3ad120a0'

As we see, it resolveInstanceMethod:is called twice.

However, if I first call -[description]:

TO *to = [TO new];
[to description];
id res = [(NSString *)to uppercaseString];

Then the output will be:

2015-12-22 22:58:50.458 OCDemo[82137:7813436] resolveInstanceMethod: sel: uppercaseString
2015-12-22 22:58:50.459 OCDemo[82137:7813436] forwardingTargetForSelector: sel: uppercaseString
2015-12-22 22:58:50.459 OCDemo[82137:7813436] -[TO uppercaseString]: unrecognized selector sent to instance 0x7f9bcad59960
2015-12-22 22:58:50.461 OCDemo[82137:7813436] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[TO uppercaseString]: unrecognized selector sent to instance 0x7f9bcad59960'

This time it resolveInstanceMethod:is called only once.

Can someone explain this?

Answer 1

, ( [to description] ) . resolveInstanceMethod: .

,

#import <objc/runtime.h>
void _objc_flush_caches(Class cls)

:

TO *to = [TO new];
[to description];
_objc_flush_caches([to class]);
id res = [(NSString *)to uppercaseString];

:

2017-04-15 21:48:21.575527+0200 pro objective c games[29820:1116842] resolveInstanceMethod: sel: uppercaseString
2017-04-15 21:48:21.575760+0200 pro objective c games[29820:1116842] forwardingTargetForSelector: sel: uppercaseString
2017-04-15 21:48:21.575808+0200 pro objective c games[29820:1116842] resolveInstanceMethod: sel: uppercaseString
2017-04-15 21:48:21.575833+0200 pro objective c games[29820:1116842] -[TO uppercaseString]: unrecognized selector sent to instance 0x100303600
2017-04-15 21:48:21.579989+0200 pro objective c games[29820:1116842] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[TO uppercaseString]: unrecognized selector sent to instance 0x100303600'

, resolveInstanceMethod: , [to description] . \\_objc_flush_caches([to class]);, resolveInstanceMethod: . . , , , .