Dynamic GCD accessors with resolveInstanceMethod: and dispatch_barrier_async.

gistfile1.m

#include <objc/runtime.h>
#import <Foundation/Foundation.h>

@interface Hi : NSObject
@end


@implementation Hi {
    NSString *something;
}

dispatch_queue_t _queue;

-(instancetype)init {
    self = [super init];
    if (self){
        _queue = dispatch_queue_create("com.example", DISPATCH_QUEUE_CONCURRENT);
    }
    return self;
}

static id ivarIMP(id self, SEL _cmd) {
    __block NSString *localSomething;
    dispatch_sync(_queue, ^{
        Ivar ivar = class_getInstanceVariable([self class], [NSStringFromSelector(_cmd) UTF8String]);
        localSomething = object_getIvar(self, ivar);
    });
    return localSomething;
}

static void setIvarIMP(id self, SEL _cmd, id value) {
    dispatch_barrier_async(_queue, ^{
        
        // ouch, slow block here
        const char* sName = sel_getName(_cmd);
        size_t len = strlen(sName);
        char *name = malloc(len-4);
        strncpy(name,sName+3,len-4);
        name[0] = tolower(name[0]);
        
        Ivar ivar = class_getInstanceVariable([self class], name);
        object_setIvar(self, ivar, value);
    });
}

+ (BOOL)resolveInstanceMethod:(SEL)aSEL {
    NSMutableString *name = [NSStringFromSelector(aSEL) mutableCopy];
    if ([name hasPrefix:@"set"]) {
        class_addMethod([self class], aSEL, (IMP)setIvarIMP, "v@:@");
    } else {
        class_addMethod([self class], aSEL,(IMP)ivarIMP, "@@:");
    }
    return YES;
}

@end


int main(int argc, char *argv[]) {
    @autoreleasepool {

        id hi = [Hi new];
        [hi performSelector:@selector(setSomething:) withObject:@"X"];
        NSLog(@"%@",[hi performSelector:@selector(something)]);
        
        // to avoid “No known instance method” use performSelector, @dynamic,
        // objc_msgSend, a protocol, a compiler flag or pragma (if there is any).
    }
}