Merge branch 'feature/pdf-print' of git.dorfsvald.net:ewandor/cht-lawfirm into feature/pdf-print

back/app/contract/print/pdf_generator.py

@@ -0,0 +1,152 @@
+from weasyprint import HTML, CSS
+
+
+class PdfGenerator:
+    """
+    Generate a PDF out of a rendered template, with the possibility to integrate nicely
+    a header and a footer if provided.
+
+    Notes:
+    ------
+    - When Weasyprint renders an html into a PDF, it goes though several intermediate steps.
+      Here, in this class, we deal mostly with a box representation: 1 `Document` have 1 `Page`
+      or more, each `Page` 1 `Box` or more. Each box can contain other box. Hence the recursive
+      method `get_element` for example.
+      For more, see:
+      https://weasyprint.readthedocs.io/en/stable/hacking.html#dive-into-the-source
+      https://weasyprint.readthedocs.io/en/stable/hacking.html#formatting-structure
+    - Warning: the logic of this class relies heavily on the internal Weasyprint API. This
+      snippet was written at the time of the release 47, it might break in the future.
+    - This generator draws its inspiration and, also a bit of its implementation, from this
+      discussion in the library github issues: https://github.com/Kozea/WeasyPrint/issues/92
+    """
+    OVERLAY_LAYOUT = '@page {size: A4 portrait; margin: 0;}'
+
+    def __init__(self, main_html, header_html=None, footer_html=None,
+                 base_url=None, side_margin=2, extra_vertical_margin=30):
+        """
+        Parameters
+        ----------
+        main_html: str
+            An HTML file (most of the time a template rendered into a string) which represents
+            the core of the PDF to generate.
+        header_html: str
+            An optional header html.
+        footer_html: str
+            An optional footer html.
+        base_url: str
+            An absolute url to the page which serves as a reference to Weasyprint to fetch assets,
+            required to get our media.
+        side_margin: int, interpreted in cm, by default 2cm
+            The margin to apply on the core of the rendered PDF (i.e. main_html).
+        extra_vertical_margin: int, interpreted in pixel, by default 30 pixels
+            An extra margin to apply between the main content and header and the footer.
+            The goal is to avoid having the content of `main_html` touching the header or the
+            footer.
+        """
+        self.main_html = main_html
+        self.header_html = header_html
+        self.footer_html = footer_html
+        self.base_url = base_url
+        self.side_margin = side_margin
+        self.extra_vertical_margin = extra_vertical_margin
+
+    def _compute_overlay_element(self, element: str):
+        """
+        Parameters
+        ----------
+        element: str
+            Either 'header' or 'footer'
+
+        Returns
+        -------
+        element_body: BlockBox
+            A Weasyprint pre-rendered representation of an html element
+        element_height: float
+            The height of this element, which will be then translated in a html height
+        """
+        html = HTML(
+            string=getattr(self, f'{element}_html'),
+            base_url=self.base_url,
+        )
+        element_doc = html.render(stylesheets=[CSS(string=self.OVERLAY_LAYOUT)])
+        element_page = element_doc.pages[0]
+        element_body = PdfGenerator.get_element(element_page._page_box.all_children(), 'body')
+        element_body = element_body.copy_with_children(element_body.all_children())
+        element_html = PdfGenerator.get_element(element_page._page_box.all_children(), element)
+
+        if element == 'header':
+            element_height = element_html.height
+        if element == 'footer':
+            element_height = element_page.height - element_html.position_y
+
+        return element_body, element_height
+
+    def _apply_overlay_on_main(self, main_doc, header_body=None, footer_body=None):
+        """
+        Insert the header and the footer in the main document.
+
+        Parameters
+        ----------
+        main_doc: Document
+            The top level representation for a PDF page in Weasyprint.
+        header_body: BlockBox
+            A representation for an html element in Weasyprint.
+        footer_body: BlockBox
+            A representation for an html element in Weasyprint.
+        """
+        for page in main_doc.pages:
+            page_body = PdfGenerator.get_element(page._page_box.all_children(), 'body')
+
+            if header_body:
+                page_body.children += header_body.all_children()
+            if footer_body:
+                page_body.children += footer_body.all_children()
+
+    def render_pdf(self):
+        """
+        Returns
+        -------
+        pdf: a bytes sequence
+            The rendered PDF.
+        """
+        if self.header_html:
+            header_body, header_height = self._compute_overlay_element('header')
+        else:
+            header_body, header_height = None, 0
+        if self.footer_html:
+            footer_body, footer_height = self._compute_overlay_element('footer')
+        else:
+            footer_body, footer_height = None, 0
+
+        margins = '{header_size}px {side_margin} {footer_size}px {side_margin}'.format(
+            header_size=header_height + self.extra_vertical_margin,
+            footer_size=footer_height + self.extra_vertical_margin,
+            side_margin=f'{self.side_margin}cm',
+        )
+        content_print_layout = '@page {size: A4 portrait; margin: %s;}' % margins
+
+        html = HTML(
+            string=self.main_html,
+            base_url=self.base_url,
+        )
+        main_doc = html.render(stylesheets=[CSS(string=content_print_layout)])
+
+        if self.header_html or self.footer_html:
+            self._apply_overlay_on_main(main_doc, header_body, footer_body)
+        pdf = main_doc.write_pdf()
+
+        return pdf
+
+    @staticmethod
+    def get_element(boxes, element):
+        """
+        Given a set of boxes representing the elements of a PDF page in a DOM-like way, find the
+        box which is named `element`.
+
+        Look at the notes of the class for more details on Weasyprint insides.
+        """
+        for box in boxes:
+            if box.element_tag == element:
+                return box
+            return PdfGenerator.get_element(box.all_children(), element)

back/app/contract/print/templates/styles.css

@@ -93,7 +93,6 @@ h2 {
     text-align: right;
 }
 
-
 .content h2 {
     page-break-before: always;
 }