Firefox Hacks [Electronic resources] نسخه متنی

Hack 91. Handle Cross-Platform Differences

Manage the differences among standard Firefox
installs on different platforms.

Firefox works hard to provide the same user interface and web page displays on all
platforms: Windows, Unix/Linux, and Macintosh. A great deal of effort
has gone into this, especially with respect to the Gecko layout
engine. Where the underlying operating systems are different, or
where specific installations are different, differences can sometimes
show through. This hack points out the most obvious cases and shows
how to manage them.

At first, such differences might be irritating or awkward for users,
but the consequences are felt mostly in XUL-based user-interface
development, where a little care in constructing XUL documents is
required.

Fundamentally, a single, compiled Firefox program cannot run on all
platforms. Therefore, the standard Firefox install is created and
customized separately for each platform. Although there is only one
source code base, some small parts of it are
operating-system-specific.

9.2.1. Handling Obvious Big-Ticket Differences

Human Interface Guidelines (HIGs) are
standards that encourage all applications in a given desktop
environment to provide similar idioms of use. Since HIGs for Apple,
Microsoft, and Linux (e.g., GNOME) all differ, a tool that hopes to
get a standards tick on every platform must look different on each
platform. These are the big-ticket differences and how Firefox
handles them:

The hidden window menu bar

The menu bar on the Macintosh is lifted to the
desktop's system menu bar and
doesn't appear on each window.

The Firefox Macintosh menu bar is automatically boosted to the system
menu bar. This is the source of the hidden
window feature sometimes remarked upon in
Mozilla's Bugzilla. No action is required, unless
your XUL application has no menu bar at all, in which case the system
menu bar will be blank.

File and Print dialog boxes are different on each platform

Firefox wraps up the operating-system-supplied File and Print dialog
boxes in XPCOM objects that present one interface on all platforms;
use that. The XUL <dialog> tag contains
buttons designed to support HIG dialogs on every platform, but in
general, it must be carefully coded if it is to work portably and
look right.

Accelerator keys sometimes differ

Macintosh has a Command key, Windows a Windows key, and Unix/Linux
might make use of a Meta key. In general, keyboard shortcuts differ.

The XUL and XBL <key> tag accepts the
generic term accel. This refers to the accelerator
key native to the current platform: Alt on Windows, Command on
Macintosh, and Alt or Meta on Unix/Linux. Firefox is also accompanied
by a different set of keyboard shortcuts on each platform, with
common keystrokes for most but not all popular commands. See the
resource:/builtin/platformHTMLBindings.xml
file for more details.

Different locations for user configuration system

The main user configuration system is under EditPreferences
on Unix/Linux, under ToolsOptions on Windows, and under
FirefoxPreferences on the Macintosh.

At the time of writing, there's no extension yet to
make the menu location of the Options dialog box uniform across
operating systems.

Unix/Linux Window managers tend to lack support for modal dialog boxes

Although it might be possible to make an application modal,
it's usually not possible to force the focus to stay
on the dialog window.

In addition to window.open(), secure web pages
and chrome content can also call window.openDialog(). Both support the special keyword
modal in their feature string argument; that
keyword locks the parent window of the dialog box on all operating
systems, so that it (at least) cannot get the focus back while the
dialog box is open.

Command-line options differ

Differing command-line options is just part of life [Hack #10], unfortunately.

Desktop integration differs

Windows has a Windows Registry, Unix/Linux has a
.gtkrc file, and desktop application icons
(shortcuts/launchers) work differently.

The XPInstall system, which runs the install.js
file bundled with the standard install and
install.js scripts bundled with extensions,
includes operating-system-specific features that such scripts can
exploit. In particular it allows reading and writing to the Windows
Registry, and knows the operating-system-specific locations of many
important folders. Any secure script can exploit that information
nonportably if it wishes. Examine these scripts for examples of
nsIDirectory-style names at work.

Firefox obeys all window-management hints or instructions that it
receives. Under X11/Gtk, a window cannot be 0 0 pixels in size (it
can on Windows). There are no perfectly portable solutions for
desktop theme integration [Hack #79], unfortunately, but with a
lot of work you can come close.

9.2.2. Handling Widget Differences

Each operating system provides or supports a GUI library that can be
used to draw interactive widgets. Firefox does
not draw widgets using these libraries. Firefox
uses its own widget-drawing code to draw widgets on a blank canvas
supplied by those libraries. So, for example, Firefox for Windows
does not depend on COMCTL32.DLL (or
.OCX), and replacing that library is therefore
of no use.

There are two areas of the desktop GUI that Firefox widgets depend
upon. One is the theme engine for natively styled widgets [Hack #79] . Not all platforms support
all -moz-appearance values, so applying such a
style property can do nothing in some instances, but at least
that's harmless.

The second area is the operating-system accessibility API. This is
wrapped up in a portable XPCOM object with the
nsIAccessible interface. It is used in the XBL
definitions for XUL widgets [Hack #90] . You can't
see the equivalent use for HTML tags unless you turn on XHTML tag
support.

Where there is accessibility support, follow the guidelines at
http://www.mozilla.org/access/xul-guidelines
for best results in XUL.

In general, don't try to lay out XUL or HTML widgets
in pixel-perfect style. What looks good on one operating system will
be up for debate on another. It is better to pack widgets
loosely, using the naturally flowing layout of
the page: line boxes and blocks for HTML, boxes for XUL.

9.2.3. Handling Font Differences

Firefox must source, choose, analyze, resize and
lay out font information [Hack #30] before it can ultimately be
displayed. On Windows, Firefox uses some low-level features of GDI
such as GetTextExtentPoint32 or
ExtTextOutW, but ultimately it uses its own layout
and font selection algorithms.