Sketch: Working on the Timeline, with currentFrame, currentLabel, and FrameTracker August 25, 2007

Sketch is a way for letting designers work on the timeline with essentially no code, and coders in pure code no visuals, where they are happiest, typically in separate swfs. Which introduces the key questions, how does they talk to each other? and coordinate actions?

There are two methods one is lateViewBinding which turns MovieClips into components at runtime via composition, which I'll cover in a future post, the other is using the frameLabel as a contract. Current 'active' framelabels as events indicating when the playhead has changed.

Things to be aware of when using currentFrame and currentLabel
There are few things that one should be aware of when working in this fashion.

• currentFrame changes when hitting a new frameLabel, regardless of which way the playhead is going. It's essentially a rendered a frame named this event, not a box indicating the framelabel is valid from say it's start's Frame to some other frame as it's often intended to mean when looking at the timeline. This maps onto a finite state machine one labelled frame == one active State, only one active at a time.
• New frames with no frame labels don't change the currentLabel So if you have an empty keyframe after a labelled keyframe, it will keep the value until it's changed to something not empty. So if your looking to clear out a frame label, you should use something like " " (which is a valid if somewhat odd frame label)
• FrameLabels are intended to be used one at a time. While the FlashIDE allows for multiple layers, each potentially having overlapping frame labels, this isn't easily accessible in script. So if you hit a particular frame with multiple layers on it, each having a label, it will choose as the currentLable whatever the level load order is. So say you had frameA on layer 1 and frameB on layer 2. currentLabel only reflects frameB. Most the time this isn't an issue. But there are times when using changes as events.
• you'll have to roll your own change event to deal with frameLabel changes. This strikes me a bit wierd given all the other changeEvents. But so be it, it's only 2 paragraphs of code.

Creating and Using FrameChangedEvents

All Labels of a given are known via the currentLabels array property. which gives you a list of frame labels, each having a name and a frame number. As enterFrame events happen, changes to the currentFrame and currentLabel can be dectected by comparing them to what they were on the last onEnterFrame event. Querying this collection when the frameNumber changes is how we can get access to multiple frame labels being active or not, and then by keeping a history of when they are active or not, broadcast events to the controller/model.
Using a FrameTracker MovieClip

An alternate approach for frame Enter and frameLeave events is using a frameTracker, basically a special basically empty MovieClip that is put on the timeline and based on when it loads and when it's unloaded it generates the appropriate events. The adding and removal of this is watching the DisplayObject for add and removal, and using the instance name as the base of the event. e.g. myFrameTracker becomes "myFrameTracker_ENTER", "myFrameTracker_LEAVE"

The detection of adding and removal of the FrameTracker from the stage is the basis of LateViewBinding.