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Python Programming On Win32 by Mark Hammond, Andy Robinson

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Implementing the Frontend

Once the PIDDLE API was stable, it was fairly straightforward to implement a PDFCanvas object to provide a simple API. Let’s take a quick look at some of the methods of the PDFCanvas class and how they hook up with the backend:

def __init__(self, filename):
        Canvas.__init__(self)
        self.filename = filename
        self.code = []  # list of strings to join later
        self.doc = pdfgen.PDFDocument()
        self.pageNumber = 1   # keep a count
        # various settings omitted

When it starts up, the PDFCanvas instance creates a PDFDocument instance. This is the class that manages the overall document structure. It also creates an empty list, self.code, to hold strings of page-marking operators. The various drawing methods add the right operators to the list in the same way the PostScript snippets did earlier. If you compare the methods and output with the PostScript PSCanvas, it’s easy to see the correspondence.

When you ask for a new page with the showPage() method, this happens:

def showPage(self):
        page = pdfgen.PDFPage()
        stream = string.join([self.preamble] + self.code, '\n')
        #print stream
        page.setStream(stream)
        self.doc.addPage(page)
        self.pageNumber = self.pageNumber + 1
        self.code = []    # ready for more...

First, create a PDFgen object called PDFPage , which is responsible for generating the output later. Then make a big string of page-marking operators by joining a standard preamble (which does some work to set up coordinate systems, default fonts, and so forth) and the list of accumulated ...

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