How to highlight changed cells when updating a DBgrid?

Question

Let's say I am showing stock prices, or sports scores, or movie attendance or something.

Periodically, I will refresh the grid by Close() and then Open() of a query linked to its associated datasource.

I know how to owner draw a cell with OnDrawCell() - what I can't figure out is how to know if the new value is the same as or different from the previous value for a given cell.

I suppose there are two use cases here, one where the number of rows is fixed and they remain in the same row order and one where rows can change (insert/delete or reorder).

For the former, I can take a snapshopshot before updating and compare after the update, but that might be a lot of data. I am not sure if I want to restrict the operation to the currently visible rows. I think that a user might want to scroll down and still be notified of any which have changed during the last update.

For the latter, I am stumped, unless, of course, each row has a unique key.

How can I do this (efficiently)? A solution for TDbGrid would help everyone, a solution with TMS Software's TAdvDbGrid would be fine by me (as would a (preferably free) 3rd party component).

Answer 1

TDBGrid reads the data currently contained in its assigned dataset. It has no capacity to remember prior values, perform calculations, or anything else. If you want to track changes, you have to do it yourself. You can do it by multiple means (a prior value column, a history table, or whatever), but it can't be done by the grid itself. TDBGrid is for presenting data, not analyzing or storing it.

One suggestion would be to track it in the dataset using the BeforePost event, where you can store the _oldvalue of a your into a LastValue column, and then use that to see if the value has changed in your TDBGrid.OnDrawColumnCell event and alter the drawing/coloring as needed. Something like if LastValue <> CurrValue then... should work.