Xamarin中的Azure移动服务412失败

Question

每当尝试通过MobileServices更新项目时,我都会遇到412错误(PreconditionFailed).我相信这已经开始了,但不幸的是错误给了我很少的信息.

我设置了一个冲突解决方案,根据https://azure.microsoft.com/en-us/documentation/articles/mobile-services-windows,将客户端版本作为赢家的想法发生冲突.-store-dotnet-handling-conflicts-offline-data /.为此我正在重复推动呼叫.但是我在BOTH推送时失败了412,而不仅仅是第一次失败.

这是我的处理程序代码...

    public class AzureConflictHandler : IMobileServiceSyncHandler
{
    readonly ILogger _logger;
    public AzureConflictHandler(ILogger logger){
        _logger = logger;
    }
    #region IMobileServiceSyncHandler implementation

    public Task OnPushCompleteAsync (MobileServicePushCompletionResult result)
    {
        return Task.FromResult (false);
    }

    public async Task<JObject> ExecuteTableOperationAsync (IMobileServiceTableOperation operation)
    {
        try{
            await operation.ExecuteAsync ();
            return null;
        }                 
        catch (MobileServiceConflictException ex)
        {
            _logger.HandleException (ex);
        }
        catch (MobileServicePreconditionFailedException ex)
        {
            _logger.HandleException (ex);

            //https://codemilltech.com/why-cant-we-be-friends-conflict-resolution-in-azure-mobile-services/
        }
        catch(Exception e){
            _logger.HandleException (e);
            throw;
        }

        try
        {
            //retry so we'll take the client value
            await operation.ExecuteAsync();
            return null;
        }
        catch (MobileServicePreconditionFailedException e)
        {
            _logger.HandleException(e, LogLevel.Fatal);
            return e.Value;
        }
    }

    #endregion


}

有没有其他人看到这个错误？我的数据对象中有一个Version字段,如下所示...

        public string id { get; set; }
    public string EntityType { get; set; }
    public Guid EntityID { get; set; }
    public string EntityJSON { get; set; }
    public DateTimeOffset LastUpdatedDate { get; set; }

    [Microsoft.WindowsAzure.MobileServices.Version]
    public string Version { set; get; }

Answer 1

我不确定为什么该教程说明了这一点,但它目前是不正确的.

第二次调用执行不会更改结果,永远不会自动修改本地项.(从技术上讲,如果你只想要最后一次写入胜利(客户端),你可以从数据模型中删除版本,而你永远不会得到412.)

假设您需要一个更长期的策略,那么您需要使用服务器的版本副本更新本地项目的版本.

catch (MobileServicePreconditionFailedException ex) {
  var serverValue = ex.Value;

  // Resolve in favor of our client by just using the server's version
  var item = operation.Item;
  item[MobileServiceSystemColumns.Version] = serverValue[MobileServiceSystemColumns.Version];

  // this will save the item changes to the local store so next Push()
  // will resolve this
  operation.UpdateOperationAsync(item)
  throw ex;

}

在你上面的代码中,虽然你马上重试,但要注意它应该在循环中,因为另一个客户端也可以更新它,等等.

在这种情况下,你可以做更多的事情

while (some condition) {
  try {
     return await operation.ExecuteAsync();
  } catch (obileServicePreconditionFailedException ex) {
    var serverItem = ex.Value;
    operation.item[MobileServiceSystemColumns.Version] = serverValue[MobileServiceSystemColumns.Version];
  } catch ...
}

https://github.com/Azure/mobile-services-samples/blob/master/TodoOffline/WindowsUniversal/TodoOffline/TodoOffline.Shared/SyncHandler.cs#L27