ios - 从错误线程访问的 Realm

Question

我写了下面的方法来处理文件的下载。

- (void)downloadFileFromURL:(NSURL *)url {
    if (url != nil) {
        if ([url pathExtension] != nil) {
            RLMRealm *realm = [Utilities getRealm];

            RLMResults<Download *> *results = [[Download allObjectsInRealm:realm] sortedResultsUsingKeyPath:@"id" ascending:NO];
            Download *oldestDownload = [results firstObject];

            Download *download = [[Download alloc] init];
            download.id = oldestDownload.id+1;
            download.name = [url path];
            download.type = @"html";
            download.state = DownloadStateNew;
            download.token = [Utilities generateToken];
            download.url = [url absoluteString];

            [realm beginWriteTransaction];
            [realm addObject:download];
            self.current = download;
            [realm commitWriteTransaction];

            NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
            NSString *path = [[paths objectAtIndex:0] stringByAppendingPathComponent:download.name];

            AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
            manager.responseSerializer = [AFHTTPResponseSerializer serializer];
            manager.responseSerializer.acceptableContentTypes = [NSSet setWithObjects:@"text/plain", @"text/html", @"application/json", @"image/png", nil];
            [manager GET:[NSString stringWithFormat:@"%@", url]
              parameters:nil
                 success:^(AFHTTPRequestOperation *operation, id responseObject) {
                     NSData *data = [[NSData alloc] initWithData:responseObject];
                     [data writeToFile:path atomically:YES];
                     NSLog(@"successful download to %@", path);

                     RLMRealm *realm = [Utilities getRealm];
                     [realm beginWriteTransaction];
                     self.current.state = DownloadStateDone;
                     [realm commitWriteTransaction];
                 } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
                     NSLog(@"Error: %@", error);


                     RLMRealm *realm = [Utilities getRealm];
                     [realm beginWriteTransaction];
                     self.current.state = DownloadStateCancelled;
                     [realm commitWriteTransaction];
                 }];
        }
    }
}

但是，这会不断抛出一个错误，表明从不正确的线程访问了 Realm。我不知道是什么原因造成的。有什么想法吗？

Answer 1

从 AFNetworking 代码来看，success 和 failure block 都将在与最初用于启动请求。

为了保证 ACID 合规性，Realm 对象是线程限制的，如果您尝试将一个对象传递给另一个线程，然后访问它的任何属性，这将触发您看到的异常。

我们最近在 Realm Objective-C 和 Realm Swift 中添加了一项功能，可以传递表示 Realm 对象的线程安全引用，然后您可以使用这些引用在新线程上查询相同的对象:

// Do work with `self.current`

RLMThreadSafeReference *currentReference = [RLMThreadSafeReference referenceWithThreadConfined:self.current];

AFHTTPRequestOperationManager *manager  =[AFHTTPRequestOperationManager manager];

//Configure `manager`

[manager GET:[NSString stringWithFormat:@"%@", url]
  parameters:nil
     success:^(AFHTTPRequestOperation *operation, id responseObject) {
         MyCurrentClass *threadCurrent = [RLMRealm resolveThreadSafeReference:currentReference];
         [threadCurrent.realm beginWriteTransaction];
         threadCurrent.state = DownloadStateDone;
         [threadCurrent.realm commitWriteTransaction];
     }
     failure:^(AFHTTPRequestOperation *operation, NSError *error) {
         // Handle `threadReference` here too if needed
     }];