David Stiller’s blog

You may have seen this one in the forums; it comes up from time to time.  Everyone knows that gotoAndPlay() supports Scene names as an optional parameter, right?  You can either supply a frame number or frame label …

gotoAndPlay(15);
gotoAndPlay("roll-out animation");

… or a Scene name and a frame number …

gotoAndPlay("game over", 1);

… and that works.  So what’s the deal? — why does it break if you specify _root (or some other path) in front of that gotoAndPlay() function? 

Doppelgänger

The answer involves what may come as a surprise if you haven’t studied the MovieClip class entry of the ActionScript Language Reference.  There are, in fact, two versions of gotoAndPlay():  the global function (the very function we’re discussing, here) and the MovieClip.gotoAndPlay() method.  Methods are like functions, but are specific to a given class.  (For more information on classes in general, see Objects: Your ActionScript Building Blocks, elsewhere in this blog.)

When you supply a prefix of any kind to a function, such as _root.gotoAndPlay(), you’re no longer referring to a function, but to a method of the object in question.  _root refers to the main timeline, and believe it or not, the main timeline is a movie clip (in fact, the whole published SWF is one big movie clip).  So, to understand what’s going on here, we have to investigate the MovieClip.gotoAndPlay() entry of the ActionScript Language Reference.  I quote from the Parameters section …

frame:Object - A number representing the frame number, or a string representing the label of the frame, to which the playhead is sent.

Notice, no mention of Scenes.  In a nutshell, then, that’s it:  _root.gotoAndPlay() — or this.gotoAndPlay(), or this._parent.gotoAndPlay(), etc. — doesn’t accept Scenes because the MovieClip.gotoAndPlay() method doesn’t.

Working around it

What to do?  You’re scoped to a button, perhaps, and need to refer to the main timeline in order to specify a Scene.  Here’s the best approach.

Put simply, forego Scene names altogether when dealing with ActionScript.  I’m not saying you need to forego Scenes — they’re a good, organizational design convenience during the authoring process — but all you really need is a unique frame label at each of your points of interest.  It doesn’t matter what Scene they belong to, this.gotoAndPlay("frame label here"); (or _root.gotoAndPlay();) will work just fine.  Problem solved.

If, for some reason, you’re absolutely stuck on using the function version, you could write a function of your own — let’s call it gotoSceneAndPlay() — that accepts the expected two parameters and passes them along, but there’s a catch:  the optional Scene parameter for gotoAndPlay() must be a quoted string.  So you couldn’t simply accept a scene:String parameter and use that.  (Try it, and watch the compiler complain!)  You would have to use an if or switch statement to evaluate the string and invoke gotoAndPlay() with something quoted …

function gotoSceneAndPlay(scene:String, frame:Number):Void {
switch(scene) {
  case "uno":
    gotoAndPlay("uno", frame);
    break;
  case "dos":
    gotoAndPlay("dos", frame);
    break;
  case "tres":
    gotoAndPlay("tres", frame);
    break;
  }
}

Tedious, to say the least.  Stick with frame labels!

A bit of history

I should point out that in general, gotoAndPlay() the function, with or without Scenes, is something to avoid.  Just chalk it up as one of those gotchas that occurs in every IDE, platform, or language developed by any software company.  If your code is in the main timeline, simply use this.gotoAndPlay() to throw yourself into method mode.  Why?

The macromedia.com ActionScript forum archives spell it out, so you’ll have to search a bit.  The most helpful thread I can think of is one named “Scene Bliss” from mid-May, 2005.  Use something like Google Groups to search the phrase “Jeckyl Stiller Cadillacula gotoAndPlay” and you’ll find it.  It’s a looooong thread and gets somewhat emotional at times, so perform an in-page search for Jeckyl’s input if you want to cut to the chase.

Note, there are actually a handful of such “Doppelgängers” in ActionScript — duplicateMovieClip(), gotoAndStop(), loadMovie(), play(), and stop(), to name an additional few.  Most, if not all, have to do with movie clips.  Since Flash 5, and especially since Flash MX (aka Flash 6), ActionScript has taken more and more of an OOP approach.  The newer classes are, well, newer, so it’s reasonable to conclude they’re better built (less prone to the occasional bug from previous incarnations).  For backwards compatibility, the function versions are still around.

When you see one of these functions, take the initiative to check if a method version is available.  The version to use is, of course, up to your own good judgment, but make sure your judgment is informed.  There are subtle differences between the two versions of many of these Doppelgängers, so choose the one that best suits your needs.

This entry was posted on Monday, March 27th, 2006 at 8:39 pm and is filed under ActionScript 2.0. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.