BApplication Overview¶

BApplication¶

The BApplication class defines an object that represents your application, creates a connection to the App Server, and runs your app’s main message loop. An app can only create one BApplication object; the system automatically sets the global be_app object to point to the BApplication object you create.

A BApplication object’s most pervasive task is to handle messages that are sent to your app, a subject that’s described in detail below. But message handling aside, you can also use your BApplication object to…

Control the cursor.: BApplication defines functions that hide and show the cursor, and set the cursor’s image. See SetCursor().
Access the window list.: You can iterate through the windows that your application has created with WindowAt().
Get information about your application.: Your app’s signature, executable location, and launch flags can be retrieved through GetAppInfo(). Additional information icons, version strings, recognized file types can be retrieved by creating an BAppFileInfo object based on your app’s executable file. BAppFileInfo is defined in The Storage Kit.

`be_app` and Subclassing BApplication¶

Because of its importance, the BApplication object that you create is automatically assigned to the global be_app variable. Anytime you need to refer to your BApplication object from anywhere in your code you can use be_app instead.

Unless you’re creating a very simple application, you should subclass BApplication. But be aware that the be_app variable is typed as BApplication *. You’ll have to cast be_app when you call a function that’s declared by your subclass:

((MyApp *)be_app)->MyAppFunction();

Constructing the Object and Running the Message Loop¶

As with all BLooper``s, to use a ``BApplication you construct the object and then tell it to start its message loop by calling the Run() function. However, unlike other loopers, BApplication``s ``Run() doesn’t return until the application is told to quit. And after Run() returns, you delete the object, it isn’t deleted for you.

Typically, you create your BApplication object in your main() function–it’s usually the first object you create. The barest outline of a typical main() function looks something like this:

#include <Application.h>

main()
{
        new BApplication("application/x-vnd.your-app-sig");
        /* Further initialization goes here -- read settings,
           set globals, etc. */
        be_app->Run();
        /* Clean up -- write settings, etc. */
        delete be_app;
}

The main() function doesn’t declare argc and argv parameters
(used for passing along command line arguments). If the user passes command line arguments to your app, they’ll show up in the ArgsReceived() hook function.
Why no pointer assignment? The constructor automatically assigns the object
to be_app, so you don’t have to assign it yourself.
The string passed to the constructor sets the application’s signature. This
is a precautionary measure; it’s better to add the signature as a resource than to define it here (a resource signature overrides the constructor signature). Use the FileTypes app to set the signature as a resource.

Application Messages¶

After you tell your BApplication to run, its message loop begins to receive messages. In general, the messages are handled in the expected fashion: They show up in BApplication’s MessageReceived() function (or that of a designated BHandler; for more on how messages are dispatched to handlers, see “From Looper to Handler”).

But BApplication also recognizes a set of application messages that it handles by invoking corresponding hook functions. The hook functions are invoked by DispatchMessage() so the application messages never show up in MessageRecieved().

Overriding the hook functions that correspond to the application messages is an important part of the implementation of a BApplication subclass.

In the table below, the application messages (identified by their command constants) are listed in roughly the order your BApplication can expect to receive them.

— table —

The protocols for the application messages are described in the “Message Constants” section of this chapter.

For more information on the details of when and why the hook functions are invoked, see the individual function descriptions.

A BApplication can also receive the B_QUIT_REQUESTED looper message. As explained in BLooper, B_QUIT_REQUESTED causes Quit() to be called, contigent on the value returned by the QuitRequested() hook function. However, BApplication’s implementation of Quit() is different from BLooper’s version. Don’t miss it.

API Documentation