Add a form field.

def addWidget(self, widget):
        """Add a form field.
        """
        CheckParent(self)
        doc = self.parent
        if not doc.isPDF:
            raise ValueError("not a PDF")
        widget._validate()

    # Check if PDF already has our fonts.
    # If none insert all of them in a new object and store the xref.
    # Else only add any missing fonts.
    # To determine the situation, /DR object is checked.
        xref = 0
        ff = doc.FormFonts               # /DR object: existing fonts
        if not widget.text_font:         # ensure default
            widget.text_font = "Helv"
        if not widget.text_font in ff:   # if no existent font ...
            if not doc.isFormPDF or not ff:   # a fresh /AcroForm PDF!
                xref = doc._getNewXref()      # insert all our fonts
                doc._updateObject(xref, Widget_fontobjects)
            else:                        # add any missing fonts
                for k in Widget_fontdict.keys():
                    if not k in ff:      # add our font if missing
                        doc._addFormFont(k, Widget_fontdict[k])
            widget._adjust_font()        # ensure correct font spelling
        widget._dr_xref = xref           # non-zero causes /DR creation

    # now create the /DA string
        if   len(widget.text_color) == 3:
            fmt = "{:g} {:g} {:g} rg /{f:s} {s:g} Tf " + widget._text_da
        elif len(widget.text_color) == 1:
            fmt = "{:g} g /{f:s} {s:g} Tf " + widget._text_da
        elif len(widget.text_color) == 4:
            fmt = "{:g} {:g} {:g} {:g} k /{f:s} {s:g} Tf " + widget._text_da
        widget._text_da = fmt.format(*widget.text_color, f=widget.text_font,
                                     s=widget.text_fontsize)
    # create the widget at last
        annot = self._addWidget(widget)
        if annot:
            annot.thisown = True
            annot.parent = weakref.proxy(self) # owning page object
            self._annot_refs[id(annot)] = annot
        return annot