Bindings to E4X don’t work as expected
Our current application uses a large XML structure to send data back and forth between the frontend and backend. This is a legacy interface which was created 2-3 years ago by the developers of a Flash 6 version of the same application and has stuck with the system since then. If I had my druthers I’d use Flex Data Services to retool this into asynchronous micro-transfers to speed up the whole process but we haven’t been able to convince anyone to buy a license for FDS yet.
To get the data into and out of the XML structure I decided to try Flex’s Bindings. This would mean we’d just have to specify the relationships and whenever the values changed the data/UI would be automatically upated. This worked fine for each individual page I created but once I started running through the entire application each additional page created caused the application to slow down noticeably. By the 20th page or so the UI was nearly unresponsive when touching any element attached to a binding. Typing text into a TextInput was a laborious character-by-character process.
It was obvious that adding the bindings has caused this but at first I couldn’t understand why. Each element was tied to its specific place in the XML and so should have been firing a single binding. Looking closer I noticed that all of the bindings from the XML to the UI were getting fired *every time* a bound UI element changed.
It appears that the reason for this is that an E4X XML object fires a change event on the top-level XML object whenever anything inside it is changed and that bindings which have an E4X source fire when the top level changes. This means that every time I updated a UI element and the binding into the XML changed the value that all of the bindings *out* of the XML to the UI were fired, causing a huge slowdown.
I ended up writing the “bindings” as simple “=” statements and calling them when a page is navigated to or away from. This introduced extra work to make sure these happen at the right time, but it’s far faster and will be simpler to maintain given the structure of the code.