10.3-old A simple text formatter system

Here 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 our 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. The text formatter we define, 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.

We start by defining class Document:

class Document(temp: ref Template, lineMax: var integer):
    class Paragraph(theContent: var String, theStyle: ref Style):
        format -> theStyledPar: var String:
            theStyledPar :=
                   theStyle.format(theContent.asString,lineMax)
    content: obj OrderedList(Paragraph)    
    addPar(P: ref Paragraph):
       content.insert(P)
    format -> formattedContent: var String:
       theStyledPar := ""
       content.scan 
          formattedContent := formattedContent + current.format
    print:
       print(format)
  • Class Document has two parameters temp, and lineMax where temp is the Template defining the format of the Document, and lineMax which defines the maximum length of a line in a paragraph. Temp and lineMax are used when adding paragraphs to a given document.
  • It has a local class Paragraph, which has two parameters: theContent which is a String, and theStyle which is a reference to a Style object defining the style of the paragraph.
  • A Paragraph has a format method that invokes the format method of the Style with the String of the Paragraph and the lineMax as an argument. Class Paragraph is an example of a nested class.
  • The data-item content holds the paragraphs of the Document. The object content is of type OrderedList(Paragraph). OrderedList is similar to class Set as described in Section 4. The difference is that there is no ordering of the elements in a Set object whereas the elements of an OrderedList are ordered in the order they are inserted into the list. For details about OrderedList, see Section xxx.
  • The method addPar adds a Paragraph to the Document by inserting it into OrderedList.
  • The print method of Document invokes the format method and prints the String returned by format.

Next, we define class Template:

class Template:
   styles:  obj OrderedList(Style) -- not used
   heading: ref Style
   bodyText: ref Style 
   inner(Template)
  • The Styles defined for a given Template are held in a styles object of type OrderedList.
  • A Template object has two styles heading and bodyText which must be defined in subclasses of Template

Next we define class Style:

class Style -> S: ref Style:
   format(content: var String, lineMax: var integer) 
                                           -> formattedContent: var String:<
      inner(format)
   inner(Style)
   S := this(Style)

In this simple example, a Style-object only has a virtual format method. Note that a generation of a Style object returns a reference to the generated object as expressed by assigning this(Style) to the return variable S.

We may now define DefaultHeading and DefaultBodyText as subclasses of Style:

class DefaultHeading: Style
   secNo: var integer
   format::<
      secNo := secNo + 1
      formattedContent := "\n" + secNo + ". " + content 
class DefaultBodyText: Style
   format::<
      pos: var integer
      pos := 3
      formattedContent := "\n   " 
      for (1):to(content.length):repeat
          pos := pos + 1
          if (pos > lineMax):then
             formattedContent := formattedContent + "\n   "
             pos := 3
          formattedContent := formattedContent + content[inx]

A DefaultHeading has a local variable secNo keeping track of the heading numbers. The format method increments secNo by one and returns a String consisting of a newline ("\n"), followed by secNo followed by a dot and a blank (". ") and the string hold in the content parameter of format.

The format method of class DefaultBodyText appends the characters in content to formattedContent. A newline and there blanks ("\n ") are inserted so each line has a maximum of lineMax characters.

We may define a standard template as a subclass of Template:

class StandardTemplate: Template
   heading := DefaultHeading
   bodyText:= DefaultBodyText

A StandardTemplate object initializes the predefined styles heading and bodyText to defaultHeading and defaultBodyText respectively.

We may enclose the above classes in a class SimpleTextFormatter:

class SimpleTextFormatter:
   class Document:
      ...
   class Template:
      ...
   class Style:
      ...
   class StandardTemplate: Template
     ...

We may now show a simple example of using the text formatter:

myFormatter: obj SimpleTextFormatter
   doc: obj Document(StandardTemplate,10)
      addCont(T: ref Text, S: ref Style):
         addPar(Paragraph(T,S))
   doc.addCont("Introduction",temp.heading)
   doc.addCont("Once upon a time, ...",temp.bodyText)
   doc.addCont("The problem",temp.heading)
   doc.addCont("There was a programmer, ...",temp.bodyText)
   doc.print
  • The singular object myFormatter is subclassed from SimpleTextFormatter.
  • It defines a singular object doc subclassed from Document with arguments StandardTemplate and 10 – the latter setting lineMax to 10 characters. Note that StandardTemplate generates an object and a reference to this object is passed as the first argument to Document.
  • The doc-object has a method addCont, which inserts a Paragraph of a given Style in the Document.
  • Then doc.addCont is invoked four times putting two headings and two body texts into the document. For the headings, the theStyle.bodyText is used and for the body texts theStyle.bodyText is used.
  • Finally it prints the document which gives the following output:
1. Introduction
   Once up
   on a tim
   e, ...
2. The problem
   There w
   as a pro
   grammer,
    ...

As can be seen, the format method may insert newlines inside a word – a real text formatting system should of course not do that.

Discussion

In the example above, we have only defined a single subclass, StandardDocument of Document and a single StandardTemplate. We may of course defined several subclasses of Document and Template. Most publishers, professional societies, etc., have their own document and template formats. For MS Word and LaTex there are numerous possibilities depending on the use of the document.

The above text formatter is very simple. If you consider at systems like MS Word, it contains a large amount of options for formating a given document. This includes fonts and font families – class Style may be extended to include a font, a font size, etc. A Paragraph may include how it is to be adjusted. A Document may have a size (A4, US Letter, …) and it may have a page orientation (landscape or portrait).

In MS Word its is possible to change the template of a given document. This implies that there must be a procedure/algorithm for converting the style of each paragraph to a similar style in the new tempalte. In MS Word this is mainly done by selecting a style of the same name.

It is a major task to develop a text system that resembles MS Word. The above classes may be extended to some extent, but it is unlikely that the structure of these classes may expand into something like MS Word. We leave it is a challenge!

Exercise

Add methods that generates a HTML-document from the content of a Document object.

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