Thursday, October 23, 2014

TabLayoutPanel can get into desynchronized state, eventually causing an IndexOutOfBounds error (on remove)

I have a TabLayoutPanel that is constantly having tabs added and removed. Most of the time, I remove the last tab or two, then add their replacements. 

Sometimes, I run into the issue where the deckPanel and tabBar components of the TabLayoutPanel get out of sync with each other. This happens if a Widget being added to the deckPanel errors out during its onLoad, for example, causing the TabLayoutPanel.insert method to drop execution before it can add the corresponding tab to tabBar. While exceptions can be caught and handled, this state (where deckPanel's size is one larger than tabBar's) is essentially unrecoverable and undetectable outside the class. 

Next time I try to remove the last tab, using TabLayoutPanel.getWidgetCount() - 1 as the index, I get an IndexOutOfBounds exception. I can't check for this condition (since deckPanel and tabBar are private and the latter doesn't offer any accessors), and even if I catch the IndexOutOfBounds exception and let it drop on the floor, I'm still left with a rogue Widget inside the deckPanel (which I still can't remove).

What would really help mitigate the issue is if the order of additions (in TabLayoutPanel.insert:736) was the same as order of removal (in TabLayoutPanel.remove:550), e.g. either:

deckPanel.insertProtected(child, beforeIndex);
tabs.add(beforeIndex, tab);
...
deckPanel.removeProtected(child);
tabBar.remove(index);

or 

tabs.add(beforeIndex, tab);
deckPanel.insertProtected(child, beforeIndex);
...
tabBar.remove(index);
deckPanel.removeProtected(child); 

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