Yes, I completely agree. But a single "undoable event" generates
multiple changes to the database (which might as well be done inside a
single transaction). Thus one click on Undo results in multiple changes
to undo the event (ie, undoing the entire event's transaction). This
implies that somehow the undo log either:
- Tags multiple rows in the undo log as comprising a single event, or
- Accumulates multiple queries into a single row in the undo log
I really liked the elegance of using triggers to automatically create
the undo log, but I don't see how to accomplish either of the above
strategies (tagging a group of undo rows, or concatenating multiple
undos into a single row) in that design without some manual legwork.
So my question was whether there is some "COMMIT_COUNT()" SQL function
or constant that represents "the number of committed transactions on
this database", as then I could just stick that in the trigger query and
insert it along with the undo query into the undo log as follows:
CREATE TEMP TRIGGER _ex1_it AFTER INSERT ON ex1 BEGIN
INSERT INTO undolog VALUES(NULL,COMMIT_COUNT(),'DELETE FROM ex1 WHERE
rowid='||new.rowid);
END;
Then multiple INSERTs in a single transaction would generate multiple
DELETEs in the undolog, all tagged with the same commit count. Later, I
could look up the set of queries necessary to undo that transaction with:
SELECT sql FROM undolog WHERE commitcount=XXX;
The upshot is I could roll back one transaction at a time, all the way
back to the beginning (or as far as the undo log goes).
But lacking a COMMIT_COUNT() function, I might instead just create a
simple table:
CREATE TABLE commitcount(num)
And then increment that at the start of each transaction:
UPDATE commitcount SET num=(SELECT num FROM commitcount)+1;
And then my trigger could be something like:
CREATE TEMP TRIGGER _ex1_it AFTER INSERT ON ex1 BEGIN
INSERT INTO undolog VALUES(NULL,(SELECT num FROM commitcount),'DELETE
FROM ex1 WHERE rowid='||new.rowid);
END;
This'll do the same thing, but requires a subquery, an extra table, and
manual incrementing of the commit count. Works, but is yucky.
Another way might be to create a table that just keeps track of which
range in the undolog corresponds to which distinct undoable event (aka
transaction)... Perhaps a bit cleaner, but still not great.
Anyway, I was just curious if the COMMIT_COUNT() function existed,
because then it'd be really clean and easy. But it sounds it doesn't,
and that's what I wanted to know. Thanks!
-david
D. Richard Hipp wrote:
> On May 13, 2008, at 6:21 PM, David Barrett wrote:
>
>> True, but even an application would need to "undo" in transactions,
>> I'd
>> think. Like, if a user drags a widget from column A to B, it
>> generates
>> an INSERT in one column and a DELETE in another. Pressing Undo once
>> would leave it in both columns, which is probably unexpected.
>>
>
> And undo/redo mechanism typically groups actions together by user
> input event. The application is typically event driven. It sits idle
> waiting for a user event, such as a mouse click. That event might
> trigger a cascade of related events, some of which involve database
> changes. All database changes associated with that one event are
> undone together. You know that you have reached the end of your event
> cascade when the event loop goes idle again.
>
> For processing gestures (drags, and other inputs that involve multiple
> events spread out over time) you acquire a lock or a "grab" at the
> beginning of the gesture, hold it throughout the gesture, then release
> it when the gesture completes. You do not finish the undo package
> until the end of the gesture. So the complete rule for when you stop
> one undoable entry and start a new one is: you are idle (no pending
> events) and you are not holding a grab.
>
> D. Richard Hipp
> [EMAIL PROTECTED]
>
>
>
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