Archive for May, 2006

Cairngorm 2 Stateless Commands

You may have seen over on Steven Webster’s blog that Cairngorm 2 for Flex 2 Beta 3 has been released.

The main change in this release, compared with previous releases, is how commands are instantiated and used. I’d like to touch on the motivation behind this change.

In Cairngorm for Flex 1.x, a single instance of each command was instantiated by the controller and that one instance is used for each and every invocation of the command.

There are reasons why the original implementation was done in such a manner, mainly historical due to Cairngorm originally being developed for Flash applications, but since the advent of Flex, we’ve wanted to change it. With the API scrub taking place between Flex 1.5 and Flex 2, we see now is a good time to make this change.

Steven explains on his blog the single change you’ll have to make to your concrete front controller because of this API change.

So now, rather than the single instance of the command being used for each command invocation, the front controller now instantiates a new instance of the command on each invocation. There are a couple of implications here:

1) If, in your existing Cairngorm applications, you rely on the same command being used for each command invocation, (eg, if you store state in the command, and reuse that state on the next invocation), you will have to change your implementation. What we’d recommend here is that you store your state in your model, possibly via the ModelLocator pattern.

2) Because a new instance of the command is instantiated on each command invocation, you can now store local state in that command for the duration of its invocation. This is particularly useful for cases where you make server side calls (eg, via RemoteObject). Previously, you’d have to use a pattern such as the Asynchronous Completion Token, but now, you can store your state in a command’s instance variable in the execute() method (for example, you may store the event object), and then retrieve it again in your result or fault handler. We find this useful for cases where you want to update different parts of the model in the result handler, depending on the context of the initial command execution.

So, go get Cairngorm 2 for Flex 2 Beta 3 and let us know what you think.