predicateWithFormat在比對NSNumber的坑

環境: iOS SDK9
在你想比對NSNumber時，從JSON轉出來的數值出現了NSTaggedPointerString，你會比對不到，於是得到nil.
- (Visitor *)visitorByID:(NSNumber *)uid {
        NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
        [fetchRequest setEntity:[NSEntityDescription entityForName:@"Visitor" inManagedObjectContext:self.managedObjectContext]];
   
    if ([uid isKindOfClass:[NSNumber class]]) {
        [fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"self.visitor_id == %@", uid]];
    } else {
        NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
        NSNumber *numUid = [numberFormatter numberFromString:(NSString *)uid];
        [fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"self.visitor_id == %@", numUid]];
    }
       
        NSError * error = nil;
        NSArray * result = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
       
        if (error!=nil) {
            SOLogError(@"%s[%d:%s]: %@", __FILE__, __LINE__, __FUNCTION__,error.localizedDescription);
        }
}

解法：
把NSTaggedPointerString再轉回NSNumber一次
NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
NSNumber *numUid = [numberFormatter numberFromString:(NSString *)uid];

CoreData的performBlock和performBlockAndWait的差異

performBlock

會異步執行在context的thread之中（不會阻塞當前的thread）

performBlockAndWait

會同步執行在context的thread之中（會阻塞當前的thread，直到執行完block中的code再返回）

NSManagedObjectContext save()

https://developer.apple.com/documentation/coredata/nsmanagedobjectcontext/1506866-save
https://developer.apple.com/documentation/coredata/nsmanagedobjectcontext/1506954-haschanges