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Bug 1768

Summary: Lazy scrollbars
Product: TeeGrid for Delphi Reporter: david berneda <david>
Component: GridAssignee: david berneda <david>
Status: IN_PROGRESS ---    
Severity: enhancement    
Priority: ---    
Version: unspecified   
Target Milestone: ---   
Hardware: All   
OS: All   
Chart Series: --- Delphi / C++ Builder RAD IDE Version:

Description david berneda 2017-02-07 09:56:51 EST 
Scrollbars currently operate in "perfect" mode, that is, the scrollbar needs to know the maximum value, the current value and the size units the scrollbar thumb represents.

A new "lazy" mode is needed to support for example buffered TDataSet in the near future.

"Lazy" mode is used in VCL DBGrid vertical scrollbar. In this mode, the maximum number of rows (RecordCount) is not necessarily known. 

Only the current value (current selected row) and the thumb size (the number of rows that can be displayed) are known.

The scrollbar thumb is just set to an arbritrary size (eg: a third of the scrollbar size), and clicking the scrollbar goes up and down only when they are relatively close to 0 (dataset BOF) or max (dataset EOF).

A proposal to implement lazy mode is:

1) A new TVirtualData.ScrollMode enum (for example: Automatic, Lazy) so TVirtualDBData can return Lazy by default.

2) In lazy mode, xxxTee.Control.pas should never call GetMaxBottom (or GetMaxRight for horizontal scrollbars)

3) Scrolling down to the end of the scrollbar should just keep advancing "next" number of rows while possible, as we don't know the "max" bottom value.

Note: Horizontal "lazy" mode is not yet necessary (column widths are always calculated so the GetMaxRight function is always correct)
Comment 1 david berneda 2017-02-21 11:44:21 EST 
Partial work done. 

Scrollbars can now be set to "Lazy" mode, but still handling scrolling from / to grid <--> vertical scrollbar is not yet finished.

>>2) In lazy mode, xxxTee.Control.pas should never call GetMaxBottom

Done.

>>TVirtualDBData can return Lazy by default

Still returning "perfect" mode, until scrollbar work is finished. 

Some datasets (eg: BDE Paradox tables) can work in perfect mode.
Also FireDAC datasets with FetchOptions.RecordCountMode = Total can also work in perfect mode.