Question

I have designed an html document editor and I wanted some help in resolving a design problem in it. The problem is as follows –

The document editor consists of a surface (JFrame) and a menu bar. Inside the surface I have three panels – a tool panel, text panel and status panel (which are all extensions of JPanel). The text panel has the customized text pane (extension of JTextPane). Now the problem is that – there are lots of scenarios where the menus and the widgets items on the tool panel need access to the underlying document model of the JTextPane. E.g. to implement list/paragraph dragging functionality, the ruler needs to know the position of the caret inside the document model, so that I can mark the paragraph whose left inset I need to increase.

The way I have organized my design right now is that – surface is a singleton, so to access the html document model inside the JTextPane you need to code your way through the following maze –

Give me surface -> go to text panel -> go to text pane -> go to document of the text pane

Another alternative to make the reference of the document inside the text pane static, so that it can be accessed directly.

TextPane.getTextDocument ()

Also the html document inside the Text Pane I am using is not customized right now. So I am just using the default HTMLDocument returned by the text pane. Though in future I may have to replace it with a more customized extension of HTMLDocument (e.g. to implement cusom tags)

I am somewhat of a design novice. Can some design guru throw some light/insight into this?

Answer 1

I think if you get 10 responses you’ll get 11 answers. 🙂

First, there is no need for a Singleton. In this scenario, it’s just a way of cheating with a global variable. And what happens when you want to have two or three text documents open?

Do your menus have ready access to the tool panel? Since toolpanel is a sibling of the textpanel, and in a 1-1 relationship (I think?) it is somewhat “reasonable” for toolpanel to know about textdocument (have a link it) and a convenience method, getTextDocument(). And it seems reasonable for your widgets to know about their immediate parent, the tool panel.

That would be my way of approaching the problem – what links between objects “make sense”. You want as little coupling as possible, but there are places where you do need coupling. Maybe I have misunderstood your problem, or maybe you think a lot differently than me, in which case you should do it another way. Also, you are making a best guess as to how your code might evolve in the future. Good luck with that! 🙂

One big question for any solution – does the text document change? (e.g., is there a File->New menu or a File->Open menu?). If so, that needs to be considered. In my proposal, the JFrame, on a File->Open, would create the text document and then change the link to it in the ToolBar.

p.s. MVC purists – please add appropriate models and controllers! I just talked about the graphical components to keep things simpler…

p.p.s. Examples of “thinking ahead” for reasonable enhancements

1. Would this handle multiple JFrames open at once? Yes.
2. Would this handle a single JFrame, but with multiple documents (say, a tabbed interface for the text document)? Almost. The method has to be a little smarter to know which of the many documents is active, but you can easily imagine doing that.
3. How about when I need to hold a lot more info about the dicument, like the name (so Save gives a good default), last time saved, is it “dirty”, is it HTML or XHTML or whatever… At this point, you’ll probably want to add one more model layer. But you can sortof imagine, with one more layer of indirection (one more link) this being o.k. So it depends on how close you are to implementing #3 how you want to design.