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440 Day 12
NOTE
Don’t confuse TDatabase’s KeepConnections property with TSession’s
property of the same name. TSession’s property affects only temporary
TDatabase components, not those you create. Setting TSession’s
KeepConnections property will have no effect on whether your explicit
TDatabase connections are retained.
Changing Server Login Parameters
You can use TDatabase’s OnLogin event handler to keep the default password dialog box from
displaying when a connection is initiated. OnLogin gets passed two parameters: a TDatabase
component that points to the database the user is trying to log on to, and a TStrings object
for storing the required login parameters. Here’s the header definition for a typical OnLogin
event method handler:
void __fastcall TForm1::Database1Login(TDatabase *Database, TStrings
*LoginParams)
From inside the OnLogin method handler, you can use TStrings’s indexed Values property
to access individual parameters, like so:
LoginParams->Values[“SERVER NAME”] = “VH1”;
LoginParams->Values[“USER NAME”] = “dave”;
LoginParams->Values[“PASSWORD”] = “ureally”;
To prevent the default login dialog box from displaying, you’ll have to at least set the PASSWORD
parameter. You can gather the parameters you need from a dialog box of your own, retrieve
them from another Database component, or hard-code them—it doesn’t matter. If they leave
your OnLogin method handler with values, C++Builder will attempt to use them to establish
a connection.
Application-Controlled Transaction Processing
A transaction is a unit of database work—a set of data modifications that you want to treat
as a single unit. It may consist of a single data-modification command; it may consist of
thousands. When you group a series of data changes together as a single transaction, you
ensure that either all the changes occur or none of them do. If a transaction consists of 1,000
changes and the 999th one fails, they all fail, and the database behaves as though none of them
ever occurred. Normally, C++Builder handles transaction-related issues for you automatically
by starting and committing transactions when your application attempts to make
changes to a database. If this level of control isn’t sufficient, you can guide transaction
processing yourself using the TransIsolation property and the StartTransaction, Commit,
and Rollback methods.