This document describes the platform used to develop the Remote Ballot Marker (RBM), especially to those new to dynamic PDF forms.
The RBM is built using the XML Forms Architecture (XFA) technology. As its name implies, it defines an application (form), in terms of XML. XFA relies on a series of document object models (DOMs), each representing a separate concern:
| DOM | Purpose |
|---|---|
| XML Data DOM | Representing raw XML data |
| XFA Data DOM | Abstraction of the XML Data DOM |
| Template DOM | Representing boilerplate and bindable objects |
| Form DOM | Representing the binding of the XFA Data DOM to the Template DOM |
| Config DOM | Declarative configuration of the form |
To provide a modern analogy, consider an web application using a Model–view–viewmodel (MVVM) framework. The data source for various template parts is the XML Data DOM, the internal representation of the data (the ViewModel), is the XFA Data DOM, and the templates themselves are the Template DOM. As with modern web frameworks, applications can be composed (stitched) of multiple templates.
When a web page is rendered, the HTML DOM is constituted. Likewise, when an XFA form is rendered, the Form DOM is constituted.
Unlike the HTML DOM, which is always in memory representation, the Form DOM can be saved to disk as part of a
XML Data Package(XDP). End users rarely interact with XDPs directly, instead relying on a PDF's support for embedding (see below).
The Config DOM is used to configure the application, somewhat similar to how a web.config functions in a .NET application.
Binding between DOMs is accomplished using Script Object Model (SOM) expressions, which behaves somewhat similarly to XPath.
As much as possible, RBM leverages the declarative binding capabilities of XFA. Where a declarative solution is not feasible, additional scripting is offered through the use of JavaScript.
For those with familiarity with the PDF specification, the above section about XML DOMs might seem out of place. Indeed, XFA forms should be seen as being packaged as PDFs rather than being PDFs. XFA forms rely on the renderer being aware of their capabilities to be properly displayed.
XFA specification is referenced as an external specification necessary for full application of the ISO 32000-1 specification (PDF 1.7). Most PDF viewers on the other hand, only implement a subset of the PDF specification. Because of this reality, it is strongly recommended to use the Adobe desktop line of PDF readers, such as Adobe Acrobat.
Mitigations for end-users of the RBM using an incompatible PDF viewer are described here.