10.3 A simple text formatter system

In this section, we present another example of using nested classes. The domain is text formatting as known from e.g. Microsoft Word, and LaTex. The complexity of most of these systems is huge. Here we will thus stick to a very simple text formatting system.

The phenomena of the domain are documents that consists of a number of paragraphs. The paragraphs of a document are formatted according to a given template, which defines a number of styles for formating the paragraphs in the document. A style may e.g. define the font being used, the size of the font, the form of the font, etc. A paragraph may be left-aligned, centred, right-aligned, og with flushed margins.

In addition to styles, there are also properties associated with a document like page orientation (portrait or landscape), paper size (like A4, A3, and US letter), and number of columns.

To avoid ending up with a large example, we will keep the possible template and style properties to a minimum. Our text formatter can handle documents with two types of paragraphs: headings and body text. Headings are automatically numbered sequentially and body texts are indented by three spaces. A document has only one property which is the maximum length of a line in a paragraph.

Given a simple text formatter, represented by the class SimpleTextFormatter, it can be use as shown here.

myFormatter: obj SimpleTextFormatter
   doc: obj StandardDocument(StandardTemplate, 10)
   doc.addHeading("Introduction")
   doc.addBodyText("Once upon a time, ...")
   doc.addHeading("The problem")
   doc.addBodyText("There was a programmer, ...")
   doc.print 

• The objects myFormatter is derived from SimpleTextFormatter.
• It defines an object doc which is an instance of class StandardDocument.
  • StandardDocument has two arguments: StandardTemplate and 10.
  • StardardTemplate is an attribute of SimpleTextFormatter and defines the template to be used for doc.
  • The argument 10 sets the maximum line length to 10.
• Then 4 paragraphs are added to doc by invoking doc.addHeading and doc.addBodyText.
  • addHeading uses a heading style and addBodyText uses a body style defined in StandardTemplate.
  • Each invocation has a String.
• Finally doc is printed giving the output:

1. Introduction
   Once up
   on a tim
   e, ...
2. The problem
   There w
   as a pro
   grammer,
    ...

A real text formatter should of course not insert newlines inside a word, but as mentioned, we keep it simple here.

Class SimpleTextFormatter has the following structure:

class SimpleTextFormatter:
   class Document:
      :::
   class Template:
      :::
   class Style:
      :::
   class StandardTemplate: Template
      :::
   class StandardDocument: Document
      :::

• It defines classes Document, Template, Style, StandardTemplate, and StandardDocument. StandardTemplate is a subclass of Template, and StandardDocument is a subclass of Document.

Class Document has the following structure

class Document(theTemplate: ref Template, lineMax: var integer):
    class Paragraph(theContent: var String, theStyle: ref Style):
        format -> formattedPar: var String:<
            :::  
    addParagraph(theContent: var String, theStyle: ref Style):
       :::
    format -> formattedDoc: var String:
       ...
    print:
       :::

Class Document has the following attributes:

• Two parameters theTemplate and lineMax.
• A local class Paragraph representing a paragraph of the document.
• A method addParagraph for adding a Paragraph to the Document.
• A method format that returns a formatted version of the Document.
• A print method.

Class Paragraph has the following attributes:

• Two parameters theContent and theStyle.
• A virtual method format that returns a formatted version of the paragraph.

Class Template has the structure:

class Template:
   heading: ref Style
   bodyText: ref Style 
   ...

It has two attributes heading and bodyText holding references to the styles for paragraphs being headings and paragraphs being body text.

Class Style has the structure:

class Style -> s: ref Style:
   format(content: var String, lineMax: var integer) 
                                           -> formattedString: var String:<
      ...
   ... 

It has a virtual method format for formatting the String being hold in content given that lineMax defines the maximum line length.

In the next section we show the details of the above classes and methods. The reader may skip this part during a first reading.