Flex Snippet - Using ChangeWatcher

One of the many features in Flex is Binding and it is a very important one at that. A feature that I feel is closely related (but that we have not yet covered here on the blog) is the ChangeWatcher class. The ChangeWatcher class does pretty much what the name says - it watches for changes on objects. Note that the properties that can be watched by the ChangeWatcher must be bindable - hence the connection to binding. Today I am going to give a quick rundown on how it works.

The first thing we are going to take a look at it is how to check if a property is actually watchable. This is done with the canWatch static function and returns a boolean (true if can be watched, false otherwise). The parameters we need to pass to the function are the host to watch (target object) and the property name as a string. So the code should look something like.

There is another static function on ChangeWatcher that is worth a mention, it is watch. This is the bread and butter function of the class. This function allows you to watch a property and then run a function each time the property changes. The first two function parameters are same as the canWatch parameters, but we add add one more argument - the function we want to be called when the watcher sees a change. This can be the name of a defined function or you can pass in an anonymous function:

The watch function also returns a ChangeWatcher instance which we can then use to do other things. The most important of these other tasks is stopping the ChangeWatcher. To stop watching we use a function named unwatch. We would end up doing something like:

That brings this little tutorial to an end. I am happy to field any questions anyone has on using the ChangeWatcher class or really anything else. Until next time, keep programming.