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DOM Implementation and Scriptability

From MDC

Contents

[edit] What is the DOM?

  • API for access and manipulation of a markup document
  • Started as DOM0, NS3 & IE3
  • Evolved into W3C DOM Level 1 Core + HTML
  • Kept going to Level 2 and Level 3...

[edit] What's standardized?

  • The document object
  • Elements, Text nodes, Comment, Style objects...
  • HTML DOM
  • Range, Traversal
  • ...

[edit] What's not?

  • The window object
  • Proprietary properties
document.all
document.location
element.offset*
element.innerHTML
element.scroll*
...
  • JS-isms
    • Variable number and types of arguments
    • Dynamic element lookup on the document object and form objects
    • Scoping and lookup order

[edit] The Mozilla DOM

  • Tries to be as W3C DOM compliant as possible
  • Tries to be as backwards compatible as possible (except for document.layers)
  • Mozilla DOM objects are XPCOM objects
  • XPIDL interfaces, "nsIDOM" prefixed, some already frozen
  • DOM objects are refcounted, DOM object wrapper lifetime is managed by the JS GC.

[edit] The Mozilla document tree

  • Document classes
    • nsDocument, nsHTMLDocument, and nsXULDocument
    • Internal interface is nsIDocument
  • Element classes
    • nsGenericElement (nsGenericContainerElement) nsHTML*Element, nsXMLElement, nsXULElement
    • Internal interface is nsIContent n' friends
  • Data node classes
    • nsGenericDOMDataNode
    • nsTextNode, nsCommentNode, nsCDATASection, nsProcessingInstruction, nsDocumentType, ...
    • Internal interface is nsITextContent (nsIContent)
  • "Tearoff" classes
    • 3Node and EventTarget on elements and data nodes

[edit] The Mozilla "window" object

[edit] Scriptability in Mozilla

  • Used to be done with static generated C/C++ code (IDLC) that hooked the DOM objects into the JS environment.
  • Now done using XPConnect and additional JS specific support code.

[edit] XPConnect and the DOM

  • XPConnect - the XPCOM/JS glue
  • Deals with the refcount/GC memory management models

[edit] nsIClassInfo

Exposes:

  • Class name
  • Implemented interfaces
  • Language specific helpers
  • Class flags (threading, singleton, ...)

[edit] Scriptable helpers

  • nsIXPCScriptable (XPConnect hooks, JS specific)
  • Dynamic name resolution
  • Property get/set/enum/... hooks

[edit] Scriptable helpers, wrapper parenting

  • JS object parent chain to match DOM object parent chain
  • DOM objects w/o a parent are parented at the global object

[edit] Scriptable helpers, wrapper prototype setup

  • Initial DOM wrapper prototype chain (done by XPConnect) 
    XPC wrapper -> XPC wrapper proto -> Object
  • DOM wrapper prototype chain done by the helper 
    XPC wrapper -> XPC wrapper proto -> [class proto]* -> Object

[edit] Security and the DOM

  • Default security model is same-origin
  • Security checks done on all objects
  • Special security code for access to script globals

[edit] The Mozilla DOM vs. the IE DOM

  • No document.all before Gecko 1.7.5 (undetectable document.all was implemented for quirks mode documents - bug 248549)
  • No window.event, and different interface for event objects
  • IE exposes elements by id in the global namespace (we do that too since Gecko 1.7.5 for quirks mode documents - bug 256932)
  • Different property lookup order (i.e. form.submit() and <input name="submit">)

[edit] Original Document Information

  • Author: Johnny Stenback
Retrieved from "http://developer.mozilla.org/en/docs/DOM_Implementation_and_Scriptability"