尝试同步 SyncTable 时 Xamarin 应用程序崩溃

Question

我使用 xamarin 和 azure 移动服务制作应用程序。我正在尝试添加离线同步功能，但我被卡住了。我有一个看起来像这样的服务

class AzureService
    {

        public MobileServiceClient Client;

        AuthHandler authHandler;
        IMobileServiceTable<Subscription> subscriptionTable;
        IMobileServiceSyncTable<ShopItem> shopItemTable;
        IMobileServiceSyncTable<ContraceptionCenter> contraceptionCenterTable;
        IMobileServiceTable<Member> memberTable;
        const string offlineDbPath = @"localstore.db";


        static AzureService defaultInstance = new AzureService();
        private AzureService()
        {
            this.authHandler = new AuthHandler();
            this.Client = new MobileServiceClient(Constants.ApplicationURL, authHandler);

            if (!string.IsNullOrWhiteSpace(Settings.AuthToken) && !string.IsNullOrWhiteSpace(Settings.UserId))
            {
                Client.CurrentUser = new MobileServiceUser(Settings.UserId);
                Client.CurrentUser.MobileServiceAuthenticationToken = Settings.AuthToken;
            }

            authHandler.Client = Client;

            //local sync table definitions
            //var path = "syncstore.db";
            //path = Path.Combine(MobileServiceClient.DefaultDatabasePath, path);

            //setup our local sqlite store and intialize our table
            var store = new MobileServiceSQLiteStore(offlineDbPath);

            //Define sync table
            store.DefineTable<ShopItem>();
            store.DefineTable<ContraceptionCenter>();

            //Initialize file sync context
            //Client.InitializeFileSyncContext(new ShopItemFileSyncHandler(this), store);

            //Initialize SyncContext
            this.Client.SyncContext.InitializeAsync(store);

            //Tables
            contraceptionCenterTable = Client.GetSyncTable<ContraceptionCenter>();
            subscriptionTable = Client.GetTable<Subscription>();
            shopItemTable = Client.GetSyncTable<ShopItem>();
            memberTable = Client.GetTable<Member>();

        }

        public static AzureService defaultManager
        {
            get { return defaultInstance; }
            set { defaultInstance = value; }
        }

        public MobileServiceClient CurrentClient
        {
            get { return Client; }
        }
 public async Task<IEnumerable<ContraceptionCenter>> GetContraceptionCenters()
        {
            try
            {
                await this.SyncContraceptionCenters();
                return await contraceptionCenterTable.ToEnumerableAsync();
            }
            catch (MobileServiceInvalidOperationException msioe)
            {
                Debug.WriteLine(@"Invalid sync operation: {0}", msioe.Message);
            }
            catch (Exception e)
            {
                Debug.WriteLine(@"Sync error: {0}", e.Message);
            }
            return null;



        }
public async Task SyncContraceptionCenters()
        {

            ReadOnlyCollection<MobileServiceTableOperationError> syncErrors = null;

            try
            {
                //await this.Client.SyncContext.PushAsync();

                await this.contraceptionCenterTable.PullAsync(
                    //The first parameter is a query name that is used internally by the client SDK to implement incremental sync.
                    //Use a different query name for each unique query in your program
                    "allContraceptionCenters",
                    this.contraceptionCenterTable.CreateQuery());
            }
            catch (MobileServicePushFailedException exc)
            {
                if (exc.PushResult != null)
                {
                    syncErrors = exc.PushResult.Errors;
                }
            }

            // Simple error/conflict handling. A real application would handle the various errors like network conditions,
            // server conflicts and others via the IMobileServiceSyncHandler.
            if (syncErrors != null)
            {
                foreach (var error in syncErrors)
                {
                    if (error.OperationKind == MobileServiceTableOperationKind.Update && error.Result != null)
                    {
                        //Update failed, reverting to server's copy.
                        await error.CancelAndUpdateItemAsync(error.Result);
                    }
                    else
                    {
                        // Discard local change.
                        await error.CancelAndDiscardItemAsync();
                    }

                    Debug.WriteLine(@"Error executing sync operation. Item: {0} ({1}). Operation discarded.", error.TableName, error.Item["id"]);
                }
            }
        }

我收到此错误： System.NullReferenceException: Object reference not set to an instance of an object. 当SyncContraceptionCenters() 运行时。据我所知，我在我的服务中复制了 coffeeItems 示例但我被卡住了。

Answer 1

我想我找到了解决方案。问题是表同步的方式。

通过同时调用SyncContraceptionCenters() 和SyncShop() shopItemtable.PullAsync 和contraceptionTable.PullAsync 同时发生。这显然很糟糕。因此，但是将它们放在相同的方法中并等待它们单独运行并且它们按预期工作。