Wednesday, December 12, 2012

Changed rendering (and evaluation) behaviour in APEX 4.2

Today we ran into a problem in our recently upgraded APEX environment. In one of the pages we had a report defined on a pipelined table function. This function also changed a package global variable. The value of that global variable is used as a source for a page item that shows up in the same region just below the report. This ran fine in 4.1, it runs fine when we set the "Compatibility Mode" to 4.1 as well. But if we set that mode to 4.2...the page item shows...nothing !!!

What happened? According to the help on that Compatibility Mode property, items (and regions) in a display point are evaluated before rendering that display point. This is done to find out whether items / regions are displayed or not - to support the grid layout. To dig a little deeper (thanks to Patrick Wolf, who was helping us with this issue): As soon as the region grid is calculated, the same is done for the page item grid in the region which gets painted next. This calculation also has to determine the page item values, because they could be used in a condition of one of the following page items. This is all done before the region source is actually executed.

So, in our case, the value of the page item - what was based on the result of the pipelined table function - was evaluated before that pipelined table function was actually executed...and therefore returned null. We could easily spot that in the debug output: When we compared the order of evaluations of the region and the items we saw a (huge) difference in 4.1 and 4.2 mode.

So now we know what caused the issue. But how to solve it?
We created a PL/SQL sub region to the reports region and moved the page item to that subregion. That was not enough to evaluate the page item value after running the report. So we removed the PL/SQL source of that page item and moved it to the source of it's PL/SQL region. Even that didn't work out. But when we set the Item Display Position to "Below Content", we - finally - saw that the item was evaluated and rendered in the right position of the complete page flow....

So when you notice - similar -  strange behaviour when running your upgraded pages in 4.2, it is worthwhile to check the difference in evaluation and rendering order is the cause of that!

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