A few weeks ago I started reviewing the PHoneyC DOM emulation code and realized it was turning to be hard to maintain and debug due to a huge amount of undocumented (and sometimes awful) hacks. For this reason I decided it was time to patch (and sometimes rewrite from scratch) such code. These posts will describe how the new DOM emulation code will work. The patch is not available right now since I’m testing the code but plans exists to commit it in the PHoneyC SVN in the next days.

In this first post we will take a look at the Window object in DOM/Window.py. During object inizialization, the following code is executed.

156     def __init_context(self):
157         “”"
158             Spidermonkey Context initialization.
159         “”"
160         document = Document(self)
161         self.__dict__['__cx'] = self.__dict__['__rt'].new_context(alertlist = [])
162         self.__dict__['__sl'] = []
163         self.__dict__['__fl'] = [document]
164
165         self.__init_properties(document)
166         self.__init_methods()
167         self.__finalize_context()

Let’s go into further details. First of all Window object properties are initialized through the __init_properties method.

181     def __init_properties(self, document):
182         self.__dict__['__cx'].add_global(‘window’, self)
183         self.__dict__['__cx'].add_global(‘self’  , self)
184         self.__dict__['__cx'].execute(“window.window = window;”)
185
186         self.__dict__['__cx'].add_global(‘document’, document)
187         self.__dict__['__cx'].execute(“window.document = document;”)
188
189         self.__dict__['__cx'].add_global(‘location’, document.location)
190         self.__dict__['__cx'].execute(“window.location = location;”)
191
192         self.__dict__['__cx'].add_global(“ActiveXObject”, ActiveXObject)
193
194         self.__dict__['__cx'].add_global(“navigator”, Navigator())
195         self.__dict__['__cx'].execute(“window.navigator = navigator;”)
196
197         self.__dict__['__cx'].add_global(“screen”, unknown())
198         self.__dict__['__cx'].execute(“window.screen = screen;”)
199
200         if ‘top_window’ in self.__dict__['__root'].__dict__:
201             if self.__dict__['__referrer']:
202                 top = self.__dict__['__referrer']
203             else:
204                 top = self.__dict__['__root'].top_window
205         else:
206             top = self
207
208         self.__dict__['__cx'].add_global(“top”, top)
209         self.__dict__['__cx'].execute(“window.top = top;”)
210
211         self.__dict__['__cx'].add_global(“parent”, top)
212         self.__dict__['__cx'].execute(“window.parent = parent;”)
213
214         self.__dict__['__cx'].add_global(“history”, History(document))
215         self.__dict__['__cx'].execute(“window.history = history;”)
216
217         self.__dict__['__cx'].execute(“window.innerWidth = 400;”)
218         self.__dict__['__cx'].execute(“window.innerHeight = 200;”)
219
220         self.__init_undefined_properties()
221
222     def __init_undefined_properties(self):
223         properties = (‘external’, ‘opera’, )
224
225         for prop in properties:
226             self.__dict__['__cx'].execute(“window.%s = undefined;” % (prop, ))

The code should be straightforward to understand. The idea beyond it is really simple. Simply stated this code allows Python objects’ variables and methods to be accessible from JS. Let’s move to most interesting stuff. Following the __init_methods method is called.

228     def __init_methods(self):
229         for attr in dir(self):
230             if attr.startswith(‘_Window__window’):
231                 p = attr.split(‘_Window__window_’)[1]
232                 self.__dict__['__cx'].add_global(p, getattr(self, attr))
233                 self.__dict__['__cx'].execute(“window.%s = %s;” % (p, p, ))

Not so easy to understand? Let’s take a look to the definition of a method.

322     def __window_back(self):
323         “”"
324         Returns the window to the previous item in the history.
325         Syntax
326
327         window.back()
328
329         Parameters
330
331         None.
332         “”"
333         pass

This is a private class method since its name starts with __. “If you try to call a private method, Python will raise a slightly misleading exception, saying that the method does not exist. Of course it does exist, but it’s private, so it’s not accessible outside the class. Strictly speaking, private methods are accessible outside their class, just not easily accessible. Nothing in Python is truly private; internally, the names of private methods and attributes are mangled and unmangled on the fly to make them seem inaccessible by their given names.” (taken from Dive Into Python). We can access the __window_back method of the Window class by the name _Window__window_back. This is the black magic __init_methods use for initializing methods. It’s quite easy to realize that adding a new method is really easy. All you need is to simply define a method named __window_<window_method_name> and match the signature of such method. How to emulate such method it’s up to you but a simple pass could do the trick.

The last step happens in __finalize_context method.

169     def __finalize_context(self):
170         self.__dict__['__cx'].execute(“Event = function(){}”)
171         self.__dict__['__cx'].execute(“function CollectGarbage() {};”)
172         self.__dict__['__cx'].execute(“function quit() {};”)
173         self.__dict__['__cx'].execute(“function prompt() {};”)
174
175         for clsname in dataetc.classlist:
176             inits = {‘window’ : self,
177                      ‘tagName’: dataetc.classtotag(clsname),
178                      ‘parser’ : None}
179             self.__dict__['__cx'].add_global(clsname, DOMObjectFactory(clsname, inits))

The most interesting code is in lines 175-179. First of all let’s take a look at the DOMObjectFactory code (DOM/ClassFactory.py) which is a genuine Python hack.

3 class DynamicDOMObject(DOMObject):
4     def __init__(self):
5         self.__dict__.update(self.inits)
6         DOMObject.__init__(self, self.window, self.tagName, self.parser)
7
8 def DOMObjectFactory(name, initializers):
9     return type(name, (DynamicDOMObject,), {‘inits’ : initializers})

After reading Python documentation it should be easy to understand how this code works and how it’s able to dynamically add new DOM objects to the context.

type(name, bases, dict)

Return a new type object. This is essentially a dynamic form of the class statement. The name string is the class name and becomes the __name__ attribute; the bases tuple itemizes the base classes and becomes the __bases__ attribute; and the dict dictionary is the namespace containing definitions for class body and becomes the __dict__ attribute. For example, the following two statements create identical type objects:

>>> class X(object):
… a = 1

>>> X = type(‘X’, (object,), dict(a=1))

What about the Window event handlers? They are handled with a different mechanism which can be fully understood just by analyzing how the new DOM emulation code preparses the pages deferring code execution until the last possible moment. I’ll analyze such feature in a future post in greater detail. Right now what you have to know is that if the handler for the event <event> is set, the Window attribute on<event> is set and contains the handler code. Once you understand it, the following code in DOM/DOM.py used for event handling should be easy to understand.

171     def get_event_func(self, name, f):
172         begin = str(f).index(‘{‘) + 1
173         s = str(f)[begin:].split(‘}’)
174         script = ‘}’.join(s[:-1]) + s[-1]
175         return script
176
177     def event_handler(self, window, name, f):
178         if name in window.__dict__:
179             try:
180                 script = self.get_event_func(name, f)
181                 window.__dict__['__cx'].execute(script)
182             except:
183                 #print str(f)
184                 traceback.print_exc()
185                 pass
186
187     def handle_events(self, window):
188         window.__dict__['__warning'] = False
189         self.event_handler(window, ‘onabort’         , window.onabort)
190         self.event_handler(window, ‘onbeforeunload’  , window.onbeforeunload)
191         self.event_handler(window, ‘onblur’          , window.onblur)
192         self.event_handler(window, ‘onchange’        , window.onchange)
193         self.event_handler(window, ‘onclick’         , window.onclick)
194         self.event_handler(window, ‘onclose’         , window.onclose)
195         self.event_handler(window, ‘oncontextmenu’   , window.oncontextmenu)
196         self.event_handler(window, ‘ondragdrop’      , window.ondragdrop)
197         self.event_handler(window, ‘onerror’         , window.onerror)
198         self.event_handler(window, ‘onfocus’         , window.onfocus)
199         self.event_handler(window, ‘onhashchange’    , window.hashchange)
200         self.event_handler(window, ‘onkeydown’       , window.onkeydown)
201         self.event_handler(window, ‘onkeypress’      , window.onkeypress)
202         self.event_handler(window, ‘onkeyup’         , window.onkeyup)
203         self.event_handler(window, ‘onload’          , window.onload)
204         self.event_handler(window, ‘onmousedown’     , window.onmousedown)
205         self.event_handler(window, ‘onmousemove’     , window.onmousemove)
206         self.event_handler(window, ‘onmouseout’      , window.onmouseout)
207         self.event_handler(window, ‘onmouseover’     , window.onmouseover)
208         self.event_handler(window, ‘onmouseup’       , window.onmouseup)
209         self.event_handler(window, ‘onmozorientation’, window.onmozorientation)
210         self.event_handler(window, ‘onpaint’         , window.onpaint)
211         self.event_handler(window, ‘onpopstate’      , window.onpopstate)
212         self.event_handler(window, ‘onreset’         , window.onreset)
213         self.event_handler(window, ‘onresize’        , window.onresize)
214         self.event_handler(window, ‘onscroll’        , window.onscroll)
215         self.event_handler(window, ‘onselect’        , window.onselect)
216         self.event_handler(window, ‘onsubmit’        , window.onsubmit)
217         self.event_handler(window, ‘onunload’        , window.onunload)
218         self.event_handler(window, ‘onpageshow’      , window.onpageshow)
219         self.event_handler(window, ‘onpagehide’      , window.onpagehide)
220         window.__dict__['__warning'] = True

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