目标C-阻止问题？

dispatch_async

dispatch_sync

dispatch_main

+ (NSString*)getMeMyString
{
   __block NSString *result;
   dispatch_sync(dispatch_get_main_queue(), ^{
        result = [ClassNotThreadSafe getString];
    });
   return result;
}

dispatch_sync

dispatch_async

SEL theSelector;
NSMethodSignature *aSignature;
NSInvocation *anInvocation;

theSelector = @selector(getString);
aSignature = [ClassNotThreadSafe instanceMethodSignatureForSelector:theSelector];
anInvocation = [NSInvocation invocationWithMethodSignature:aSignature];
[anInvocation setSelector:theSelector];

NSString *result;

[anInvocation performSelectorOnMainThread:@selector(invoke) withObject:nil waitUntilDone:YES];
[anInvocation getReturnValue:result];