当表数据发生更改时,如何在前端触发NOTIFICATION事件

前端:

Public Sub test()

Dim conn = New NpgsqlConnection(" Server=server; port=portno;User Id=uid; pwd=pwd; DataBase=Db; ")
conn.Open()

Dim command = New NpgsqlCommand("listen notifytest;", conn)
command.ExecuteNonQuery()

AddHandler conn.Notification, AddressOf NotificationSupportHelper


command = New NpgsqlCommand("update testtable set field='test' where id=1", conn)
Dim x As NpgsqlDataReader = command.ExecuteReader
End Sub

Private Sub NotificationSupportHelper(ByVal sender As Object, ByVal e As NpgsqlNotificationEventArgs)
    MsgBox(e.Condition)
End Sub

触发:

CREATE OR REPLACE FUNCTION testingNotify()
  RETURNS trigger AS
$BODY$ 
    begin
       notify notifytest;
       return null;
    end;     
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;
ALTER FUNCTION testingNotify() OWNER TO testserver;

这个 trigger 如果 Insert 或 delete 或 update 发生在桌子上 testtable .所以在上面的代码中,在名为 test ,我写了一个查询 updating the table tabletest ,因此在执行查询时 NotificationSupportHelper 接到电话。