PDF In-Depth

JavaScript Advice - Intending to create forms

July 01, 2001

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If you intend to create forms for your website, whether authored in HTML or in PDF, you're going to have to learn some JavaScript. Not a lot, but some. If you're new to the language, you'll need to read up.

For a good hit-the-ground-running introduction to JavaScript, I recommend Peachpit Press's Visual Quickstart Guide on JavaScript by Negrino and Smith, ISBN 0-201-69648-7. At $17.95 list, it's a bargain. When you graduate from that book to needing a for-real heavy-duty text covering the language in some depth, you have no option (in my opinion) but to go straight to David Flanagan's JAVASCRIPT: THE DEFINITIVE GUIDE, an O'Reilly book (with a rhino on the cover).

PDF JavaScript vs. Browser JavaScript

Traditional JavaScript is browser-executed. It runs inside the browser and gives you access to a great deal of browser functionality via methods that access the Document Object Model components. None of this, however, has anything to do with PDF-based JavaScript.

While it's true that Adobe has implemented core JavaScript per the 1.2 specification inside the PDF spec, that does not mean you can access browser functions or HTML document objects from within PDF-based JavaScript. The JavaScript you put in a PDF form or document gets executed inside Acrobat or Acrobat Reader. The browser knows nothing about this. Indeed, you can't reach the browser from within a PDF file. They live in separate worlds.

Adobe has not only implemented core 1.2 JavaScript but has extended it to cover new objects, with methods available only to PDF files. These objects and methods are covered in Adobe's documentation. Look for a document called AcroJS.pdf on Adobe's web site or on your Acrobat CD. This docfile describes in detail how PDF-based JavaScript is implemented. Early in the document it states:

Acrobat Forms defines an object model on top of JavaScript 1.2. These objects are only defined within the Adobe Acrobat realm and are specific to Acrobat Forms. They basically mirror the Acrobat Forms components and give the forms developer a way to access these components programmatically in order to query and change their properties. In addition to defining forms specific objects, there are additional generic objects that allows the developer to access the underlying document and perform certain actions on it.

An example of an additional object is the app object, which has methods like beep and alert. (These methods issue a system beep and an alert dialog, respectively.) The app object also has properties. One property it has is the fullscreen attribute, which is a Boolean value. If you call

app.fullscreen = true

from inside your PDF-based JavaScript, you will force the user's viewing application (i.e., Acrobat Reader) into fullscreen mode, making menus disappear and generally making it seem as if your form has taken over the user's machine. (This may be disconcerting to the user, of course!) If you set this value to False, you will make the viewing app go back to its default viewing mode.

How to Attach JavaScript to a Widget

In HTML, you insert JS code into your main HTML document, usually in various places throughout the document, and the document ends up being quite long and almost impossible for a normal human being to read and understand.

In PDF, you attach JS snippets to individual widgets visually, with the mouse,using Acrobat's form tool. Choose the form tool, then doubleclick on whichever field you want to attach JS to. Click the Actions tab and decide whether to attach your code to a mouse action, or make it a Validate script, or a Calculate script, or a Format script. (These latter options will mainly be available in Text fields.) For example, select a mouse action, hit the Add button, and choose JavaScript from the pulldown menu. Then type or paste your script.

An Actual JavaScript Example

Take a look at the following code, which shows how to change the Fill Color of all of a PDF form's fields at runtime:

 var rgb = new Array("RGB",.5,.5,.5);      
   for(var i =0;i<this.numfields; i++)
      = this.getNthFieldName(i);  
      = this.getField(s);       

      f.fillColor=rgb;}

We will walk through this code step by step, in great detail, later.

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