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Last modified: July 26, 1996
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How can I determine when a tabular DataWindow in an MDI sheet needs to be refreshed? Other sheets, and even other users may have added, changed or deleted rows that are listed in the tabular DataWindow.
With all due respect to IBM, Oracle and even Powersoft and Sybase, Watcom does things the way they should be done.
The implementation of Default Timestamp in SQL Anywhere 5 is a case in point: "Not only can the new timestamp default be used to increase the level of consistency checking, it can also be used for application purposes. That's because the column contains real date and time values instead of the strange counter that SQL Server calls 'timestamp'."
The activate event is a good place to check to see if a DataWindow needs refreshing since that event is fired when the user switches back to that window after doing other work. The question is "How?" and that's where database columns defined with default timestamp come in to play.
Default timestamp columns are automatically updated whenever a row is inserted or updated, as opposed to default current timestamp which is set only on insert. By selecting max ( default_timestamp_column ) you can determine if the value has changed since the previous DataWindow retrieve. Selecting count(*) will tell you if any rows have been deleted.
Of course, count(*) won't change if one row was deleted and another one was inserted, but in that case max ( default_timestamp_column ) will be different.
The code in Listings 1, 2, 3 and 4 show the code to do the checking. The critical logic isn't placed in the activate event because that's too dangerous. It is placed in a "ue_post_activate" user defined event that is fired from the activate script via PostEvent(). Code is also included to prevent endless loops caused by activate firing again when the user clicks Yes or No to close the message box.
Listing 1: Window Instance Variables
public: long il_row_count = -1 // "uninitialized" value datetime idt_last_change = DateTime ( today(), now() ) boolean ib_activate_running = false boolean ib_skipping_refresh = false
Listing 2: Activate Event
this.ib_activate_running = true this.PostEvent ( "ue_post_activate" )
Listing 3: UE_Post_Activate Event
long ll_row_count // User has already been asked. if this.ib_skipping_refresh then this.ib_skipping_refresh = false return end if // This is a fresh activate. ll_row_count = this.of_refresh_if_necessary() if ll_row_count < 0 then return end if
Listing 4: OF_Refresh_If_Necessary() Function
integer li_RC DateTime ldt_last_change long ll_row_count // Check for adds, changes, deletes. select max ( last_change ), count (*) into :ldt_last_change, :ll_row_count from province using SQLCA; if SQLCA.SQLCode <> 0 then MessageBox ( "Error", "Select" ) this.ib_activate_running = false return -1 end if // Is it the first call, or has anything changed? if ( ldt_last_change <> this.idt_last_change ) & or ( ll_row_count <> this.il_row_count ) then if this.il_row_count = -1 then // The first call requires unconditional retrieval. li_RC = 1 else // Suppress activate after MessageBox closes. this.ib_skipping_refresh = true // Ask the user, which will cause a 2nd activate // after the MessageBox closes. li_RC = MessageBox ( "Confirm", & "The database has changed since the list of~r~n" & + "provinces was last displayed.~r~n~r~n" & + "Do you want to refresh the list of provinces?", & question!, & YesNo!, & 1 ) end if if li_RC = 1 then this.idt_last_change = ldt_last_change this.il_row_count = this.of_retrieve() end if end if this.ib_activate_running = false return this.il_row_count
Listings 5 shows the supporting code that does the actual retrieval. It is not necessary to put anything in the window open event because the activate script will take care of filling the DataWindow when the window first opens.
Listing 5: OF_Retrieve() Window Function
integer li_RC long ll_row_count li_RC = this.dw_province.SetTransObject ( SQLCA ) if li_RC <> 1 then MessageBox ( "Error", "SetTransObject" ) return -1 end if ll_row_count = this.dw_province.retrieve() if ll_row_count < 0 then MessageBox ( "Error", "Retrieve" ) return -1 end if return ll_row_count
Here's what you see when the table has changed and the you switch to the
window containing the DataWindow that is now out of date:
Breck Carter can be reached by phone at (416) 763-5200 or via email at email@example.com.