TDataDriverEh component

TDataDriverEh is universal data provider for TMemTableEh and a handler of the changes in TMemTableEh. TDataDriverEh is a simplest supplier of datas, it is a agent between TMemTableEh and other DataSet, that can to work with the database. Although many DataSet’s can be connected to DBGrid directly, avoiding TMemTableEh, ligament DBGridEh-DataSource-MemTableEh-DataDriverEh-DataSet is possible to use if you want to use advantages of DBGridEh functioning in ligaments with MemTableEh such as ViewScroll mode, tree-type viewing of data, using same data by the several MemTableEh’s.

In the mode of the supplier, TDataDriverEh takes data from DataSet that assigned to ProviderDataSet property, and copies them in TMemTableEh. Before getting first record, MemTableEh requests structure of fields from TDataDriverEh and create internal array of records. TDataDriverEh build structure of fields on basis of fields list of ProviderDataSet. As required, TDataDriverEh takes values of the current record, writes their to a new record of TMemTableEh and goes over to the next record in ProviderDataSet.

Furthermore, TDataDriverEh can refresh values of the existing MemTable record from ProviderDataSet (TMemTableEh.RefreshRecord method). TDataDriverEh find record in ProviderDataSet using fields value from KeyFields property and reread values of record in MemTable record. When TDataDriverEh is used as handler of change (TMemTableEh.ApplyUpdates method), it takes a changed record from TMemTableEh, using characteristic KeyFields, finds its position in ProviderDataSet, updates a record in ProviderDataSet, queries it again from ProviderDataSet and returns a record back in TMemTableEh.

Events of TDataDriverEh allows to control data which are transferred between TDataDriverEh and TMemTableEh , as well as transfer and process changes from any/to other types source of data.

TDataDriverEh have next events:

OnBuildDataStruct

write this event to change a structure of fields created when TMemTableEh requests structure of fields. You can call DafaultBuildDataStruct method to build a structure by default.

OnProduceDataReader

write this event to return dataset-cursor to read data. You can call DefaultProduceDataReader method to define dataset-cursor by default. By default dataset-cursor will be a ProviderDataSet.

OnReadRecord

write this event to assign values of record of new record that delivered to TMemTableEh, or indicate that no more data. You can call DefaultReadRecord method to assign values by default. By default values of record will be assigned from dataset-cursor. If Eof function of dataset-cursor is True then TDataDriverEh indicate that no more data.

OnAssignFieldValue

write this event to assign a value of each field of new record that is delivered to TMemTableEh, or when record is refreshing. Use DataValueVersion to determine the mode of assigning:

dvvValueEh – when need to assign value for a new record,

dvvRefreshValue - when need to assign value for existing record.

You can call DefaultAssignFieldValue to assign a value of the field by default.

OnRefreshRecord

write this event to assign fresh value of fields at the refreshment of record from the server. You an call DefaultRefreshRecord to assign fresh values by default.

OnUpdateRecord

write this event to process updated records in TMemTableEh. Use MemRec.UpdateStatus property to determine the type of changes: Updating, Deleting or Inserting. You can call DefaultUpdateRecord to process updated record by default. By default DataDriver conducts changes in ProviderDataSet.

OnUpdateError

write this event to respond certain actions when error is arising in time of processing updates. Following reaction is possible when error is arising:

ueaBreakAbortEh Break this and all following operations of changes, exception is not raising.

ueaBreakRaiseEh Break this and all following operations of changes, rollback transactions, exception is raising.

ueaCountinueEh Ignore an error, does not change a status of a record and continue a performing the rest operations.

ueaRetryEh Repeat an operation (You should undertake actions to prevent this error on next time).

ueaUpdated CountinueSkip Ignore an error, set record status to Unchanged, continue a performing the rest operations.

You can call DefaultUpdateError to execute actions by default.