This post goes over the basics of writing a very simple custom layout that supports virtualization in spark. It assumes that you have already built a custom layout that handles the real (non-virtual) case and are comfortable doing so.
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The default spark List will add a horizontal scroll bar when the contents of the List are wider than its width. This post shows how to change that behavior using a custom item renderer so that a truncation mark like an ellipsis is used instead.
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The ItemRenderer class in Flex 4 has an itemIndex property that was added after beta2. You can use this property within your item renderer to display the row number or index of an item in a spark List.
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ItemRenderer has logic built in that changes between some standard states like normal, hovered, selected, etc. You can supplement this logic by overriding ItemRenderer.getCurrentRendererState(), or do away with the built in states altogether by subclassing DataRenderer instead of ItemRenderer.
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The spark List component in Flex 4 treats every item equally with regards to selection. Sometimes I find it useful to customize this behavior so certain items cannot be selected. You can simulate this behavior by customizing the item renderer to make the selected state visually identical to the normal state, but to get more accurate functionality you will need to subclass List.
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The spark List component in Flex 4 allows you to select multiple items when you set allowMultipleSelection to true. With this flag set you can select multiple items by using the control/command key.
If you want to have a List that allows multiple selection, but does not require the user to hold down the control key when selecting items you can subclass List and override the item_mouseDownHandler() method.
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If you have built a custom item renderer for a DataGroup or List you may have come across warnings in your Flash Builder console output similar to this:
warning: unable to bind to property 'firstName' on class 'Object' (class is not an IEventDispatcher)
The default spark List skin wraps the the underlying DataGroup with a Scroller component to easily provide scrolling functionality. Custom skinning the Scroller component isn’t very flexible; you can custom skin each scrollbar to change the visuals on them, but you can’t easily reposition them. In these cases it usually makes more sense to get rid of the Scroller and hook up the scrollbars directly.
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It’s easy to delete an item in a spark List from within an item renderer. This is handy if you have a List with a custom renderer that provides a button to delete the item that is associated with that renderer.
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The spark DataGroup component is designed to take data items and pass them through an item renderer to create a visual representation of that data. In Flex 4 it is very easy to build custom item renderers.
This sample application demonstrates how you can create an item renderer that only shows some information by default and allows the user to drill down to see more information by clicking on an item or it’s associated “twisty” icon.