19.7 How It All Works

The modularity of the Swing text components can be confusing. Fortunately, most of the time it doesn't matter how it works as long as it does work. However, some understanding of what's going on behind the scenes is necessary for what is to come in the next four chapters.

Let's take a look at what needs to happen for a text component to be displayed. These behaviors describe the responsibilities of a JTextArea, but they are similar for other JTextComponents:

• The text component retrieves its UI delegate from the L&F and installs it. For JTextArea, this might be javax.swing.plaf.basic.BasicTextAreaUI.

• The UI delegate may set properties such as font, foreground, and selection color. The UI delegate may also set the caret, highlighter, InputMaps and ActionMap. The maps allow text components to respond to L&F-specific keyboard commands for actions such as cut/copy/paste, select-all, caret-to-end-of-line, page-down, and so on.

• The UI delegate also instantiates an EditorKit. For JTextArea this might be javax.swing.text.DefaultEditorKit. Most of the Actions in the text component's array come from the EditorKit.

• If the text component's constructor didn't receive a Document, it creates one. JTextArea creates its Document (a PlainDocument) directly, but other text components delegate this to the EditorKit.

• The Document is responsible for storing the component's text content. It does this by breaking it into a hierarchy of one or more Elements. Each Element can hold part of the Document's content and some style information. JTextArea creates one Element for each line of text and ignores style information. Other text components can be more sophisticated.

• The text component registers itself as a listener for events it needs to track. This includes registering as a DocumentListener so it can update itself in response to any changes that occur in its Document.

• The text component may delegate or partially delegate preferredSize, minimumSize, and maximumSize to the UI delegate. JTextArea does this, but if its rows and columns properties are nonzero, it enforces a minimum on preferredSize.

• The UI delegate is responsible for painting the component, but it uses the EditorKit to paint the text content. The EditorKit does this by way of a ViewFactory. The ViewFactory creates a hierarchy of one or more View objects from the hierarchy of Element objects. The View hierarchy is then painted to the screen. A one-to-one correspondence from Element to View is typical but not required of the ViewFactory.

In this chapter, we've shown how easy it is to do simple things with the Swing text framework. However, if you want to do more than we've demonstrated in this chapter, Swing has a lot to offer. In the next four chapters, we'll examine the rest of the Swing text package, building many interesting and powerful sample programs as we go.