BInputServerFilter

Derived from: none
Declared in:  be/add-ons/input_server/InputServerFilter.h
Library: libbe.so
Allocation: By the Input Server only
Summary:  more...

BInputServerFilter is a base class for input filters; these are instances of BInputServerFilter that modify, generate, or eat input events. An input filter add-on is privy to all the events that pass through the Input Server's event stream. A filter is similar to the Interface Kit's BMessageFilter, but at a much lower level. The BInputServerFilter also sees all events, while a BMessageFilter only sees the events targeted at its BLooper. BInputServerFilters can also generate additional events in place of, or in addition to, the original input event.

BInputServerFilter objects are created and deleted by the Input Server only—you never create or delete these objects in your code.

Creating

To create a new input filter, you must:

• create a subclass of BInputServerFilter

• implement the instantiate_input_filter() C function to create an instance of your BInputServerFilter subclass

• compile the class and function as an add-on

• install the add-on in one of the input filter directories

At boot time (or whenever the Input Server is restarted; see "Loading" in The Input Server), the Input Server loads the add-ons it finds in the input filter directories. For each add-on it finds, the Server invokes instantiate_input_filter() to get a pointer to the add-ons's BInputServerFilter object. After constructing the object, the Server calls InitCheck() to give the add-on a chance to bail out if the constructor failed.

Installing an Input Filter

The input server looks for input filters in the "input_server/filters" subdirectories of B_BEOS_ADDONS_DIRECTORY, B_COMMON_ADDONS_DIRECTORY, and B_USER_ADDONS_DIRECTORY.

• You can install your input devices in the latter two directories—i.e. those under B_COMMON_ADDONS_DIRECTORY, and B_USER_ADDONS_DIRECTORY.

• The B_BEOS_ADDONS_DIRECTORY is reserved for add-ons that are supplied with BeOS.

Hook Functions

• Filter()

• InitCheck()

Constructor and Destructor

BInputServerFilter()

BInputServerFilter(void)

Creates a new BInputServerFilter object. You can initialize the object—set initial values, spawn threads, etc.—either here or in the InitCheck() function, which is called immediately after the constructor.

~BInputServerFilter()

~BInputServerFilter()

Deletes the BInputServerFilter object. The destructor is invoked by the Input Server only—you never delete a BInputServerFilter object from your own code. If this object has spawned its own threads or allocated memory on the heap, it must clean up after itself here.

Member Functions

Filter()

virtual filter_result Filter(BMessage *message, BList *outList)

The Filter() hook function is invoked by the Input Server to filter the in-coming message. You can discard the message, allow it to pass downstream (as is or modified), or replace it with one or more other messages:

• To discard message, implement Filter() to return B_SKIP_MESSAGE (don't delete the BMessage yourself—it's owned by the Input Server).

• If you implement Filter() to return B_DISPATCH_MESSAGE (and if you leave outList unchanged), message is allowed to continue downstream. You're allowed to modify (but not destroy or replace) message.

• To replace message, place one or more BMessages in outList and return B_DISPATCH_MESSAGE. message is discarded; the messages in outList are passed downstream in the order that they appear in the list. The Input Server owns all of the messages you pass back in outList, so don't delete them yourself. Note that if you want to include message in outList, you must copy the object before adding it to the list.

The default implementation returns B_DISPATCH_MESSAGE without modifying the message.

GetScreenRegion()

status_t GetScreenRegion(BRegion *region) const

The GetScreenRegion() function returns the screen's region in region. This is the most efficient way for an input filter to get the screen's region. The system screen saver's input filter uses this for its "sleep now"/"sleep never" corners.

GetScreenRegion() returns B_OK.

InputCheck()

virtual status_t InitCheck(void)

Invoked by the Input Server immediately after the object is constructed to test the validity of the initialization. If the object is properly initialized (i.e. all required resources are located or allocated), this function should return B_OK. If the object returns non-B_OK, the object is deleted and the add-on is unloaded.

The default implementation returns B_OK.