---
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page_name: conventions-and-patterns-for-multi-platform-development
title: Conventions and patterns for multi-platform development
---

Chromium is a large and complex cross-platform product. We try to share as much
code as possible between platforms, while implementing the UI and OS integration
in the most appropriate way for each. While this gives a better user experience,
it adds extra complexity to the code. This document describes the recommended
practices for keeping such cross-platform code clean.

We use a variety of different file naming suffixes to indicate when a file
should be used:

*   Mac files use the _mac suffix for lower-level files and Cocoa (Mac
            UI) files use the _cocoa suffix.
*   iOS files use the _ios suffix (although iOS also uses some specific
            _mac files).
*   Linux files use _linux suffix for lower-level files, _gtk for
            GTK-specific files, and _x for X Windows (with no GTK) specific
            files.
*   Chrome OS files use _chromeos suffix when needed but otherwise also
            include _linux files by default.
*   Windows files use the _win suffix.
*   Posix files shared between Mac, iOS, and Linux use the _posix
            suffix.
*   Files for Chrome's "Views" UI (on Windows and experimental GTK)
            layout system use the _views suffix.

Platform-specific suffixes are encoded using conditionals in their respective
BUILD.gn files:
https://chromium.googlesource.com/chromium/src/+/HEAD/docs/no_sources_assignment_filter.md

The separate front-ends of the browser are contained in their own directories:

*   Mac Cocoa: chrome/browser/ui/cocoa
*   Linux GTK: chrome/browser/ui/gtk
*   Windows Views (and the experimental GTK-views):
            chrome/browser/ui/views

The [Coding Style](/developers/coding-style) page lists some stylistic rules
affecting platform-specific defines.

### How to separate platform-specific code

#### Small platform differences: #ifdefs

When you have a class with many shared functions or data members, but a few
differences, use #ifdefs around the platform-specific parts. If there are no
significant differences, it's easier for everybody to keep everything in one
place.

#### Small platform differences in the header, larger ones in the implementation: split the implementation

There may be cases where there are few header file differences, but significant
implementation differences for parts of the implementation. For example,
base/waitable_event.h defines a common API with a couple of platform
differences.

With significant implementation differences, the implementation files can be
split. The prevents you from having to do a lot of #ifdefs for the includes
necessary for each platform and also makes it easier to follow (three versions
each of a set of functions in a file can get confusing). There can be different
.cc files for each platform, as in base/waitable_event_posix.cc that implements
posix-specific functions. If there were cross-platform functions in this class,
they would be put in a file called base/waitable_event.cc.

#### Complete platform implementations and callers: separate implementations

When virtually none of the implementation is shared, implement the class
separately for each platform in separate files.

If all implementations are in a cross-platform directory such as base, they
should be named with the platform name, such as FooBarWin in base/foo_bar_win.h.
This case will generally be rare since files in these cross-platform files are
normally designed to be used by cross-platform code, and separate header files
makes this impossible. In some places we've defined a commonly named class in
different files, so PlatformDevice is defined in skia/ext/platform_device_win.h,
skia/ext/platform_device_linux.h, and skia/ext/platform_device_mac.h. This is OK
if you really need to refer to this class in cross-platform code. But generally,
cases like this will fall into the following rule.

If the implementations live in platform-specific directories such as
chrome/browser/ui/cocoa or chrome/browser/ui/views, there is no chance that the
class will be used by cross-platform code. In this case, the classes and
filenames should omit the platform name since it would be redundant. So you
would have FooBar implemented in chrome/browser/ui/cocoa/foo_bar.h.

Don't create different classes with different names for each platform and
typedef it to a shared name. We used to have this for PlatformCanvas, where it
was a typedef of PlatformCanvasMac, PlatformCanvasLinux, or PlatformCanvasWin
depending on the platform. This makes it impossible to forward-declare the
class, which is an important tool for reducing dependencies.

### When to use virtual interfaces

In general, virtual interfaces and factories should not be used for the sole
purpose of separating platform differences. Instead, it should be be used to
separate interfaces from implementations to make the code better designed. This
comes up mostly when implementing the view as separate from the model, as in
TabContentsView or RenderWidgetHostView. In these cases, it's desirable for the
model not to depend on implementation details of the view. In many cases, there
will only be one implementation of the view for each platform, but gives cleaner
separation and more flexibility in the future.

In some places like TabContentsView, the virtual interface has non-virtual
functions that do things shared between platforms. Avoid this. If the code is
always the same regardless of the view, it probably shouldn't be in the view in
the first place.

### Implementing platform-specific UI

In general, construct platform specific user interface elements from other
platform-specific user interface elements. For instance, the views-specific
class BrowserView is responsible for constructing many of the browser dialog
boxes. The alternative is to wrap the UI element in a platform-independent
interface and construct it from a model via a factory. This is significantly
less desirable as it confuses ownership: in most cases of construction by
factory, the UI element returned ends up being owned by the model that created
it. However in many cases the UI element is most easily managed by the UI
framework to which it belongs. For example, a views::View is owned by its view
hierarchy and is automatically deleted when its containing window is destroyed.
If you have a dialog box views::View that implements a platform independent
interface that is then owned by another object, the views::View instance now
needs to explicitly tell its view hierarchy not to try and manage its lifetime.

e.g. prefer this:

> // browser.cc:
> Browser::ExecuteCommand(..) {
> ...
> case IDC_COMMAND_EDIT_FOO:
> window()-&gt;ShowFooDialog();
> break;
> ...
> }
> // browser_window.h:
> class BrowserWindow {
> ...
> virtual void ShowFooDialog() = 0;
> ...
> };
> // browser_view.cc:
> BrowserView::ShowFooDialog() {
> views::Widget::CreateWindow(new FooDialogView)-&gt;Show();
> }
> // foo_dialog_view.cc:
> // FooDialogView and FooDialogController are automatically cleaned up when the
> window is closed.
> class FooDialogView : public views::View {
> ...
> private:
> scoped_ptr&lt;FooDialogController&gt; controller_; // Cross-platform
> state/control logic
> ...
> }

to this:

> // browser.cc:
> Browser::ExecuteCommand(..) {
> ...
> case IDC_COMMAND_EDIT_FOO: {
> FooDialogController::instance()-&gt;ShowUI();
> break;
> }
> ...
> }
> // foo_dialog_controller.h:
> class FooDialog {
> public:
> static FooDialog\* CreateFooDialog(FooDialogController\* controller);
> virtual void Show() = 0;
> virtual void Bar() = 0;
> };
> class FooDialogController {
> public:
> ...
> static FooDialogController\* instance() {
> static FooDialogController\* instance = NULL;
> if (!instance)
> instance = Singleton&lt;FooDialogController&gt;::get();
> return instance;
> }
> ...
> private:
> ...
> void ShowUI() {
> if (!dialog_.get())
> dialog_.reset(FooDialog::CreateFooDialog(this));
> dialog_-&gt;Show();
> }
> // Why bother keeping FooDialog or even FooDialogController around?
> // Most dialogs are very seldom used.
> scoped_ptr&lt;FooDialog&gt; dialog_;
> };
> // foo_dialog_win.cc:
> class FooDialogView : public views::View,
> public FooDialogController {
> public:
> ...
> explicit FooDialogView(FooDialogController\* controller) {
> set_parent_owned(false); // Now necessary due to scoped_ptr in
> FooDialogController.
> }
> ...
> };
> FooDialog\* FooDialog::CreateFooDialog(FooDialogController\* controller) {
> return new FooDialogView(controller);
> }

Sometimes this latter pattern is necessary, but these occasions are rare, and
very well understood by the frontend team. When porting, consider converting
cases of the latter model to the former model if the UI element is something
simple like a dialog box.
