Most of the intricacies of the
Document interface are of interest only if you implement your own text editor. There is, however, one common use of the interface: tracking changes.
Sometimes, you may want to update a part of your user interface whenever a user edits text, without waiting for the user to click a button. Here is a simple example. We show three text fields for the red, blue, and green component of a color. Whenever the content of the text fields changes, the color should be updated. Figure 6.36 shows the running application of Listing 6.19.