ActionScript for Flash MX: The Definitive Guide, Second Edition

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Chapter 13: Movie Clips

Self-references with the this keyword

When we want to refer explicitly to the current instance from a frame in its timeline

or from one of its event handlers, we can use the

this keyword. For example, the fol-

lowing statements are synonymous when attached to a frame in the timeline of our

cloud_mc instance:

_alpha = 60; // Implicit reference to the current timeline

this._alpha = 60; // Explicit reference to the current timeline

There are three reasons to use this to refer to a clip even when we could legitimately

refer to the clip’s properties and methods directly.

• First, explicit references are easier for other developers to read, because they

make the intention of a statement unambiguous.

• Second, when used without an explicit instance reference, certain movie clip

methods are mistaken for global functions by the interpreter. If we omit the

this

reference, the interpreter thinks we’re trying to invoke the analogous global

function and complains that we’re missing the target movie clip parameter. To

work around the problem, we use

this, as follows:

if (this._name != "newClouds_mc") {

this.duplicateMovieClip("newClouds_mc", 0); // Invoke a method on an instance

}

// If we omit the this reference, we get an error

duplicateMovieClip("newClouds_mc", 0); // Oops! ...