Flash documents are fundamentally structured around the concept of animation. Every Flash document comprises a linear sequence of
frames
, or slices of visual and audio content. A great deal of ActionScript code is tied to this
frame-based
structure. By placing a code block on a frame, we stipulate the timing of that block's execution relative to the playback of the movie. Whenever we add code to a frame, we must always consider not only what we want the code to do, but also
when
we want the code to execute.
For example, we may want to write some code that places a new movie clip on stage when the 20
th
frame of a movie is displayed:
To attach code to a keyframe, we select the keyframe in the timeline,
open
the Actions panel, and then add the desired code to the Script pane. During playback, code on a keyframe is executed before the content of the frame is displayed.
Code on keyframes is used to perform
tasks
synchronized to the movie's playback and to establish program elements such as
variables
, functions, and objects for use throughout a movie clip or movie. A timeline loop is a simple example of a synchronized task. Suppose we attach the following code to frame 15 of a movie:
gotoAndPlay(10);
When the playhead of the movie
reaches
frame 15, the code executes, and the movie begins playing back at frame 10 again. When frame 15 is reached for the second time, the code is executed again, and the movie again starts playing at frame 10. This causes the movie to cycle endlessly between frames 10, 11, 12, 13, 14, and 15.
However, code on keyframes does not always control or synchronize with a movie's playhead. We can also use keyframes to represent application states (e.g., as a stopping point in a movie, such as for each quiz question in Chapter 1), or simply as storage devices for functions, variables, and objects. For example, we can attach the function
moveClipTo( )
to frame 1 of a movie clip and invoke
moveClipTo( )
from a button later in the movie:
function moveClipTo (x, y) {
_x = x;
_y = y;
}
One caution: because the execution of code on frames is dictated by the playback of a movie, we must always ensure that variables, functions, and other program elements are available before they are accessed. For example, we can't invoke a function at frame 3 if that function isn't defined until frame 10. Code used globally throughout a movie should therefore be placed on the first frame of the main timeline or on the first frame after the movie's preloader. See the
_global
object in the
Language Reference
for more information on global code placement. See also Chapter 2 and Chapter 9 for a discussion of variable and function scope.
We must also ensure that no code attempts to access portions of a movie that have not yet loaded. To check whether a specific portion of a movie has loaded, use either the movie clip
_framesloaded
property or the
getBytesLoaded( )
method. For sample code, see the entry for
Movieclip
._framesloaded
in the ActionScript Language Reference.
Keyframe or Insert
