objective-c - 更新时无效时 Realm 崩溃

Question

当我仍在其他线程中运行更新事件时尝试删除数据库时，我在 Realm 上不断崩溃。

崩溃是:

2017-08-14 18:07:56.289 App Staging[28264:7828070] *** Terminating app due to uncaught exception 'RLMException', reason: 'Can only add, remove, or create objects in a Realm in a write transaction - call beginWriteTransaction on an RLMRealm instance first.'
*** First throw call stack:
(
    0   CoreFoundation                      0x000000010c67fb0b __exceptionPreprocess + 171
    1   libobjc.A.dylib                     0x000000010c0e4141 objc_exception_throw + 48
    2   Realm                               0x0000000108095f96 _ZL27RLMVerifyInWriteTransactionP8RLMRealm + 86
    3   Realm                               0x000000010809710a RLMCreateObjectInRealmWithValue + 138
    4   Realm                               0x00000001080820af +[RLMObject createOrUpdateInRealm:withValue:] + 607
    5   App Staging                         0x0000000107497ae0 +[RealmRoundable createOrUpdateInRealm:withMemberResponse:] + 400
    6   App Staging                         0x0000000107497916 +[RealmRoundable createOrUpdateWithMemberResponse:] + 118
    7   App Staging                         0x000000010742a0a0 +[RealmStaff createOrUpdateInRealm:withResponse:inCareProvider:] + 352
    8   App Staging                         0x000000010742ab92 +[RealmStaff createOrUpdateInRealm:withStaff:inCareProvider:] + 514
    9   App Staging                         0x000000010742a94b +[RealmStaff createOrUpdateStaff:inCareProvider:] + 139
    10  App Staging                         0x000000010733b803 -[StaffRoundableTableViewController updateRoundables:fromDataLoader:inCareProvider:] + 131
    11  App Staging                         0x000000010747ae3a __54-[RoundableTableViewController dataLoaderDidLoadData:]_block_invoke.324 + 122
    12  Realm                               0x00000001081d01a6 -[RLMRealm transactionWithBlock:error:] + 86
    13  Realm                               0x00000001081d010e -[RLMRealm transactionWithBlock:] + 62
    14  App Staging                         0x000000010747ab0d __54-[RoundableTableViewController dataLoaderDidLoadData:]_block_invoke + 765
    15  libdispatch.dylib                   0x000000010e15c4a6 _dispatch_call_block_and_release + 12
    16  libdispatch.dylib                   0x000000010e18505c _dispatch_client_callout + 8
    17  libdispatch.dylib                   0x000000010e164dcd _dispatch_queue_override_invoke + 1321
    18  libdispatch.dylib                   0x000000010e166ec4 _dispatch_root_queue_drain + 634
    19  libdispatch.dylib                   0x000000010e166bef _dispatch_worker_thread3 + 123
    20  libsystem_pthread.dylib             0x000000010e51c5a2 _pthread_wqthread + 1299
    21  libsystem_pthread.dylib             0x000000010e51c07d start_wqthread + 13
)
libc++abi.dylib: terminating with uncaught exception of type NSException

当我打电话时会发生这种情况:

[RealmManager deleteRealm];

实现为:

+ (void)deleteRealm
{
    @autoreleasepool {
        [[RLMRealm defaultRealm] invalidate];
        // Hack to force Realm to clear cache because config is cached and crashes eventually because it detects encryption key has changed
        SUPPRESS_UNDECLARED_SELECTOR_WARNING([[RLMRealm class] performSelector:@selector(resetRealmState)]);
    }

    NSFileManager *manager = [NSFileManager defaultManager];
    RLMRealmConfiguration *config = [RLMRealmConfiguration defaultConfiguration];
    NSArray<NSURL *> *realmFileURLs = @[
                                        config.fileURL,
                                        [config.fileURL URLByAppendingPathExtension:@"lock"],
                                        [config.fileURL URLByAppendingPathExtension:@"log_a"],
                                        [config.fileURL URLByAppendingPathExtension:@"log_b"],
                                        [config.fileURL URLByAppendingPathExtension:@"note"],
                                        [[config.fileURL URLByDeletingLastPathComponent] URLByAppendingPathComponent:[NSString stringWithFormat:@"%@.realm.management", [[config.fileURL URLByDeletingPathExtension] lastPathComponent]]]
                                        ];
    for (NSURL *URL in realmFileURLs) {
        NSError *error = nil;
        [manager removeItemAtURL:URL error:&error];
        if (error) {
            // handle error
            DDLogError(@"Error deleting realm file - %@", error);
        }
    }
}

我的问题是:有没有办法在运行这段代码之前停止所有 Realm 操作。

Answer 1

来自 Realm 关于 Deleting Realm Files 的文档:

Because Realm avoids copying data into memory except when absolutely required, all objects managed by a Realm contain references to the file on disk, and must be deallocated before the file can be safely deleted. This includes all objects read from (or added to) the Realm, all RLMArray, RLMResults, and RLMThreadSafeReference objects, and the RLMRealm itself.

In practice, this means that deleting a Realm file should be done either on application startup before you have opened the Realm, or after only opening the Realm within an explicit autorelease pool, which ensures that all of the Realm objects will have been deallocated.

在另一个线程仍在访问文件时删除文件会导致各种问题。调用 Realm 的私有(private)方法，例如 +[RLMRealm resetRealmState]，也是如此。我强烈建议不要做这两件事。

根据从磁盘中删除 Realm 文件的动机，您可能会以略微不同的方式处理此问题。如果您可以分享有关您的用例的更多信息，我或许可以提供更具体的建议。

例如，您可以跟踪您的后台线程是否正在积极使用 Realm，并且仅在它们空闲时才将其删除。但是，您需要非常小心，以确保在删除文件时删除了对 Realm 的所有引用，否则您最终可能会继续通过已经打开的文件句柄访问现已删除的文件。

或者，您可以为新的 Realm 文件生成一个新的、唯一的路径，而不是立即删除 Realm 文件。然后，当您确定应用程序的其余部分未使用它们时，您将删除未使用的 Realm 文件(下次启动是实现此目的的一种非常可靠的方法，或者将其绑定(bind)到应用程序生命周期中的点您知道无法再访问旧状态)。这将是我的偏好，因为在使用文件时没有机会删除它。它也非常适合许多具有注销用户概念的应用程序，因为每个用户不同的 Realm 路径是一个相对容易掌握的概念。