问题
I am using Firefox Scratchpad to understand the scaling behavior that
results when the user types the CTRL-PLUS and CTRL-MINUS keys.
In Javascript Scratchpad, I'm evaluating a snippet of Javascript code that accesses the
Components.interfaces.nsIDOMWindowUtils.screenPixelsPerCSSPixel
property like this:
var util = window.QueryInterface(Components.interfaces.nsIInterfaceRequestor).getInterface(Components.interfaces.nsIDOMWindowUtils);
util.screenPixelsPerCSSPixel
In Scratchpad typing CTRL-L after the last l in screenPixelsPerCSSPixel above results in an Exception:
Exception: The operation is insecure.
(Note that the current active Firefox tab is pointing to http://www.google.com).
The devtools.chrome.enabled flag is set to true in this
context, but that does not seem to matter as an Exception is still being thrown.
What do I need to do in order to get Javascript Scratchpad to display that value?
This is intended only to be temporary to gain understanding during a
debug ritual. Set aside for now the obvious question of "Why is it a
security risk to show a scaling factor?". I'm only after getting at
the value from the confines of standard, and safe, Firefox controls
such as enabling/disabling about:config values that I can easily
undo later when I'm done.
回答1:
Here is the answer I found:
Accessing the window variable from the Scratchpad scope whose
Environment (see its Environment menu) is set to Content yeilds one
type of object, while if Environment was set to Browser, a different
type of object is in play (CTRL-L on it in Scratch pad reports it is
of type ChromeWindow). The window.content object is the content
object relative to the ChromeWindow object.
The value of
....QueryInterface(Components.interfaces.nsIInterfaceRequestor).getInterface(Components.interfaces.nsIDOMWindowUtils).screenPixelsPerCSSPixel
relative to the window object (ChromeWindow) object returns
1, while calling it on the window.content shows something other
than 1 and show that scaling is involved.
(EDIT: I relocated the Javascript code that used to be here; see below.)
To demonstrate using the value, I've provided some sample Javascript in an answer to my other question at Finding top-level X Window x/y coordinates of CTRL-F text in Firefox
来源：https://stackoverflow.com/questions/14176792/inspecting-firefox-components-interfaces-nsidomwindowutils-screenpixelspercsspix