Kalen Gibbons

In a previous post I explained how a rendering bug in Flex was causing items to disappear in an itemRenderer. In the example below, you can see that when you scroll, the items in the list disappear.  This is caused by the varying heights of the elements.  If you explicitly set the height for each element or set variableRowHeights to false, then the problem goes away.

What was causing the problem was apparent but the solution was not.  Thankfully, Olivier Lalonde was nice enough to shed some light on an easy fix.  The itemRenderer needs to be validated as the user scrolls.  So, as Olivier suggests, you simply need to listen for the dataChange event and execute the validateNow() function.

You can see in the example below that the list now has varying row heights… and the data no longer disappears. NICE!  The key point to focus on here is the following line: <mx:VBox dataChange=”validateNow()” verticalGap=”0″>

Note: An update to this post has been posted here. It provides a better solution than the one offered below, so I recommend you check it out.

Although I love Flex, it can still be a bit buggy at times. I was working with an itemRenderer the other day and came across some very odd behavior. I had a list using an itemRenderer and when the data originally loaded everything was fine, but whenever I scrolled some of the data started disappearing. You can see this demonstrated in the sample below.

Apparently, Flex likes to have all the items in a list the same height; so when an item such as the Text field in the example causes variation, Flex simply doesn’t display it. I found that the easiest way to remedy the issue was to set explicit heights for the items in the renderer. Here is an example of how it works with the height explicitly set:

Now, this isn’t a perfect solution. What we are doing here is effectively the same thing as setting variableRowHeight to false; thus making all the items in the List the same height. So if anybody has a better solution for this issue please let me know.

There are occasions when you need to programmatically determine if a List or DataGrid has enough rows to require ScrollBars. If your DataGrid, for example, does not have an explicit rowCount or height specified, then it’s not as easy as looking at its data provider.

The easiest way I’ve found it to use the “maxVerticalScrollPosition” property, which the Flex 3 Language Reference describes as:

“The maximum value for the verticalScrollPosition property. Note that this is not the height of the content because the maxVerticalScrollPosition property contains the height of the content minus the height of the displayable area.”

So if the the maxVerticalScrollPosition is greater than zero, then scrollbars must be available.

If anybody know of an easier or more efficient way of determine if a List has ScrollBars, please let me know.