Re: Next/Prev navigation on model

Question

question, please bear with me). As you know already, I have a model which
gets data from Mainframe. I have retrieve method in that model (extends
defaultModel, implements RetrievingDataSetModel).In that method after
populating the base class's arrayList . I am setting the row index to
position specified in the context. The problem, I am having is I have a
repeated view , which is bound to this model. The view is gettingpopulated
correctly for the first time. On this response page, if I hit next button,
it is again brining the records from
0-25 five only(I am expecting 26 to 50). Auto retrieving is enabled. Am I
missing anything here ?I think I may see what the issue is. The view fetches data from the model
using the starting boundaries established by the first() (or beforeFirst())
method. The view acts like a viewport, but the model is the one that
determines the data that is actually visible.
What this means is that the view is stupid--it doesn't really understand
that it is looking at only some portion of the model's data. It uses the
model's beforeFirst() and the next() methods to show the data from the
model. Therefore, the model must define these operations so that they do
the correct thing and only show the data relevant to the latest execution.
For example, on the first hit of your page, the model should contain only
the data from rows 0-25. Where the Next button is pressed, the model should
contain the data only for rows 26-50. If the model contains rows 0-50, then
the beforeFirst() method on the model will move to the very beginning of the
data (row 0), and thus the view will only show the first 25 rows. Even
setting the current row index to begin at row 25 won't help, because as you
noticed, the view will reset the current row index to the beginning of the
model's data after retrieval. (I don't advise trying to override this
behavior, since such a solution would be order dependent and I can't
guarantee that the order won't change in the future.)
There are two possible solutions to your issue that I'd recommend. The
first is to do what I've done in the QueryModels, which is to only store in
the model the rows that are indicated in the execution context. For
example, when the user presses the Next button, the execution context should
indicate an offset of 26, and a fetch size of 25, indicating that the caller
wants to retrieve only rows 26-50. After executing the the query, I discard
rows 0-25 and cache only rows 26-50 in the model. When the model's first()
method is called, it will position the current row to the first available
row of data in the current model instance, which will actually be the 26th
row of the full result set. The next time the user presses the Next button,
I will only store rows 51-75 in the model, discarding rows 0-50.
The second possible solution is to store all the data starting from row 0,
but override the first(), next(), previous(), and other related methods to
position the current row to the data row you are interested in. In the case
of the Next button being pressed, you would have override the first() method
to position the current row at the 26th data row instead of at the 0th row.
In general, I would recommend the first solution above simply because it
tends to be more efficient. If the model is passed an execution context
that specifies rows 26-50, then those are the only rows of data you should
need to be concerned with. Therefore, storing rows 0-25 is extra overhead.
The exception to this may be if you are incrementally building up the result
set because you are storing your model in session. If this is the case,
then I would recommend the second solution above.
I realize I haven't done a very good job of explaining this, so don't
hesitate to ask questions. This is inherently a difficult topic to explain
through just email. A whiteboard would be of terrific help here. <g>
Todd
Todd Fast
Senior Engineer
Sun/Netscape Alliance
todd.fast@e...
512.656.5068 - Cell
303.733.6829 - Fax

Answer 1

Hi everybody,
this is something I would like to know too. It should be possible, since in the APEX developer GUI it seems to be working: for example when I open an item on a page and I click on a region selector, it remains on the same region even if I navigate to another item.
This feature is very interesting also because it could be a good alternative of the region customization by end user.
So if anybody has an idea how to make it work, please share some hints with us.
Thanks a lot,
Ladislav