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This document explains what makes an application "native" on Mac OS X. It shows the areas where Qt/Mac is compliant, and the grey areas where compliance is more questionable. (See also the document Qt/Mac Issues.)
Normally when referring to a native application, one really means an application that talks directly to the underlying window system and operating system, rather than one that uses some intermediary (for example the X11 server, or a web browser). Qt/Mac applications run as first class citizens, just like Cocoa, Java, and Carbon applications.
When an application is running as a first class citizen it means that it can interact with specific components of the Mac OS X experience:
Qt/Mac does this via the QMenuBar abstraction. Mac users expect to have a menubar at the top of the screen and Qt/Mac honors this.
Additionally, users expect certain conventions to be respected, for example the application menu should contain About, Preferences, Quit, etc. Qt/Mac handles this automatically, although it does not provide a means of interacting directly with the application menu. (By doing this automatically, Qt/Mac makes it easier to port Qt/Mac applications to other platforms.)
This is a critical piece of Mac OS X (documentation can be found at http://developer.apple.com/documentation/UserExperience/Conceptual/OSXHIGuidelines/index.html). It is a huge topic, but the most important guidelines for GUI design are probably these:
As with Cocoa/Carbon Qt/Mac provides widgets that look like those described in the Human Interface Descriptions. Qt/Mac's widgets use the Appearance Manager to implement the look, so Apple's own API's are doing the rendering (Qt/Mac <3.1 used an emulation style with pixmaps, however this quickly proved to be cumbersome, and unable to keep up with style changes at Apple).
This is a bit more subjective, but certainly Qt/Mac strives to provide the same feel as any Mac OS X application (and we consider situations where it doesn't achieve this to be bugs). Of course Qt has other concerns to bear in mind, especially remaining multiplatform. Some "baggage" that Qt carries is in an effort to provide a widget on a platform for which an equivelant doesn't exist, or so that a single API can be used to do something, even if the API doesn't make entire sense for a specific widget (for example pushbuttons with a popup menu are really bevel buttons in Mac OS X, but Qt/Mac cannot guess that this bevel button is right next to other real pushbuttons).
This is the most subjective, but there are many suggestions and guidelines in the Aqua style guidelines. This is the area where Qt/Mac is of least assistance. The decisions that must be made to conform (widget sizes, widget layouts with respect to other widgets, window margins, etc) must be made based on the user experience demanded by your application. If your user base is small or mostly comes from the Windows or Unix worlds, these are minor issues much less important than trying to make a mass market product. Qt/Mac is fully API compatible with Qt/Windows and Qt/X11, but Mac OS X is a significantly different platform to Windows and some special considerations must be made based on your audience.
Interaction with the dock is limited, but at the very least the icon should be able to be interacted with. This can be achieved with QWidget::setIcon(). The setIcon() call can be made as often as necessary, so can be used to provide a constantly updating pixmap that works as expected.
Although many users never use this, some users will only interact with your applications via assistive devices. With Qt the aim is to make this automatic in your application so that it conforms to accepted practice on its platform (X11 accessiblity support is still in the works due to the developing nature of its accessibility design). With Qt 3.3 Qt/Mac will support accessiblity, and hopefully a host of assistive devices.
Mac OS X developers expect a certain level of interopability between their development toolkit and the platform's developer tools (for example MSVC, gmake, etc). Qt/Mac supports both Unix style Makefiles, and ProjectBuilder/Xcode project files by using the qmake tool. For example:
qmake -spec macx-pbuilder project.pro
will generate an Xcode project file from project.pro. With qmake you do not have to worry about rules for Qt's preprocessors (moc and uic) since qmake automatically handles them and ensures that everything necessary is linked into your application.
Qt does not entirely interact with the development environment (for example plugins to set a file to 'mocable' from within the Xcode user interface). Trolltech is actively working on improving Qt's interoperability with various IDEs, so hopefully this will be supported soon.