CODEWITHSUNDEEP
delphi
JC Boggio
JC Boggio

Reputation: 407

Is it safe to share a TDataSource between several controls at the same time?

I have moved all my lookup tables/queries in the main DataModule of my application. Now I wonder if I could also move the associated TDataSource to the Datamodule.

For example, if I have two TLookupCombobox on two different forms (or even on the same form) using the same TDataSource, will this have an impact ? Like when I choose an item in combobox1 will it move to the same item on combobox2 ?

I only want to use these TDataSources read only.

Upvotes: 2

Views: 336

Answers (2)

MartynA
MartynA

Reputation: 30715

Yes, it is possible for a TDataSource to be placed in a TDataModule and used as the datasource for db-aware components on a number of forms. However, it is generally not a good idea and is one you are likely to come to regret. Doing it creates a maintenance problem for yourself, because it is rather too easy to make some change to its properties which affects how the db-aware components behave and overlook the knock-on consequenes for the various forms (same is true for TDataSet descendants, of course).

Sharing a datasource across forms can impact on performance if instances of the forms exist at the same time, and this is particularly so if you carry out some procedure which iterates the records in the dataset (unless you surround the operation with calls to TDataSet.DisableControls and .EnableControls).

So imo although there are benefits to placing datasets in a datamodule which is used by a number of forms, it is far better to place the datasource(s) on the forms which contain the connected db-aware components.

So far as db-aware compound controls such as TDBComboBox, TDBListBox, etc are concerned, these always display the value from the current record in the related dataset. No amount of coding will permit these components to simultneously display different field values on different forms: if they are fed by the same datasource from the same record field, they will display the same value (though of course, the contents of associated lists in the controls, as in the drop-down list of a DBComboBox, may differ between them). This arises from the way TDataSets are designed to operate: they all implement a dataset "cursor" which makes all data-reading and -writing operations operate on a single, "current" record of the dataset and the value of the dataset field which a db-aware component displays is the value in the record the cursor is on.

Upvotes: 3

Ian Branch
Ian Branch

Reputation: 69

You are asking two questions there. - No problem. On the usage of the Datasource, absolutely. I have all my TTables, TQuerys and DatSources in the one DataModule aand reference them from multiple Forms with multiple components on each. On the combobox1 v combobox2 interaction, it will depend on the code behind them. Nominally in the OnChange event. Ian

Upvotes: 1

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