ActionScript: The Definitive Guide

Event Handler Scope

As with any function, the statements in an event handler execute within a predefined scope. Scope dictates where the interpreter looks to resolve the variables, subfunctions, objects, or properties referenced in an event handler’s body. We’ll consider event handler scope in relation to movie clip events, button events, and other object events.

Movie Clip Event Handler Scope

Unlike regular functions, movie clip event handlers do not define a local scope ! When we attach a handler to a clip, the scope of the handler is the clip, not just the event handler itself. This means that all variables are retrieved from the clip’s timeline. For example, if we attach an enterFrame event handler to a clip named navigation and write trace(x); inside the handler, the interpreter looks for the value of x on navigation’s timeline:

onClipEvent (enterFrame) {
  trace(x);  // Displays the value of navigation.x
}

The interpreter does not consult a local scope first because there is no local scope to consult. If we write var y = 10; in our handler, y is defined on navigation’s timeline, even though the var keyword ordinarily declares a local variable when used in a function.

The easiest way to remember the scope rules of a clip event handler is to treat the handler’s statements as though they were attached to a frame of the handler’s clip. For example, suppose we have a clip named ball that has a variable called xVelocity in it. To access xVelocity from inside a ball event handler, we simply ...