Structurally, a .fla file is a hierarchy of one or more animations, each with its own timeline. A timeline is a linear sequence of frames, akin to a physical filmstrip. Each frame can contain audio, video, text, vector graphics, bitmap graphics, and programmatic content.
When a .swf file is exported from a .fla file, the Flash compiler converts the .fla file's timelines to Flash file format (SWF), suitable for playback in a Flash client runtime. When the .swf file plays, the frames from the .fla file's timelines are displayed in rapid succession, producing the illusion of animation. The speed at which the Flash runtime displays frames is governed by the frame rate, which is measured in number of frames per second. (For more information on the frame rate, see Chapter 23).
While some .fla files contain a single timeline, most contain multiple timelines, allowing content to be created in discrete parts and combined to form a larger animation. For example, a scene depicting a car driving down a mountainous road might include three timelines—one for the mountains passing slowly, one for the road passing quickly, and one for the car's wheels spinning.
The first timeline created in a new .fla file is known as the main timeline. The main timeline forms the foundation of all content subsequently added to the .fla file.
To produce each frame of content in a timeline, we can either import external assets or create new graphics using Flash's built-in design tools (e.g., Pencil, ...