# BMessageFilter Use Cases and Implementation Details:

This document describes the BMessageFilter interface and some basics of how it is implemented. The document has the following sections:

1. BMessageFilter Interface
2. BMessageFilter Use Cases
3. BMessageFilter Implementation

## BMessageFilter Interface:

The BMessageFilter class is a simple class for processing incoming BMessage's before they are dispatched to a BLooper. The best source of information for the BMessageFilter interface can be found here in the Be Book.

## BMessageFilter Use Cases:

The following use cases cover the BMessageFilter functionality:

1. Construction 1: A BMessageFilter can be created by specifying a message_delivery option, a message_source option, a command code and an optional filter function. These options are used to determine the messages on which the filter will act and the filter itself (see the "Filter" use cases).

2. Construction 2: A BMessageFilter can be created by specifying a message_delivery option, a message_source option and an optional filter function. These options are used to determine the messages on which the filter will act and the filter itself (see the "Filter" use cases).

3. Construction 3: A BMessageFilter can be created by specifying a command code and an optional filter function. These options are used to determine the messages on which the filter will act and the filter itself (see the "Filter" use cases).

4. Construction 4: A BMessageFilter can be constructed by use of a copy constructor which can take a reference to or a pointer to another BMessageFilter. The new BMessageFilter will have all the options of the source BMessageFilter when that source filter was constructed. Any other state of the source BMessageFilter that it has accumulated after construction is not copied to the new filter.

5. Assignment: A BMessageFilter can be assigned the attributes of a source BMessageFilter by the use of the assignment operator. The target BMessageFilter will have all the options of the source BMessageFilter when that source filter was constructed. Any other state of the source BMessageFilter that it has accumulated after construction is not copied to the new filter.

6. Destruction: When a BMessageFilter is deconstructed, the BMessageFilter releases any shared resources it may have allocated.

7. Command: When Command() is used on a BMessageFilter, the command code specified at construction time for this filter is returned. If a BMessageFilter was not constructed with a specific command code, this member function does not return a valid result.

8. FiltersAnyCommand: When FiltersAnyCommand() is called on a BMessageFilter, it returns true if a command code was not specified at construction time for this filter. It returns false if a command code was specified at construction time. The Command() member function (see above use case) is only valid when FiltersAnyCommand() returns false.

9. Looper: The Looper() member function returns a pointer to the BLooper (or BHandler) to which this filter has been added. If the filter has not been added to a BLooper, NULL is returned. The member functions BLooper::AddCommonFilter() or BHandler::AddFilter() can be used to add the filter to a looper (see use cases for BLooper and BHandler for details about adding a filter).

10. MessageDelivery: The MessageDelivery() member function returns the message delivery value that was specified at construction time for this filter. The possible values are B_DROPPED_DELIVERY, B_PROGRAMMED_DELIVERY and B_ANY_DELIVERY. If no message delivery value was specified at construction time, B_ANY_DELIVERY is returned.

11. MessageSource: The MessageSource() member function returns the message source value that was specified at construction time for this filter. The possible values are B_LOCAL_SOURCE, B_REMOTE_SOURCE and B_ANY_SOURCE. If no message source value was specified at construction time, B_ANY_SOURCE is returned.

12. Filter 1: The filter can be applied for a message by passing the message and the target BHandler which has received that message to the Filter() member function. This member function returns B_DISPATCH_MESSAGE or B_SKIP_MESSAGE to the caller. The actual impact of these filter results is dependent on the BLooper and BHandler behaviour (see the use cases for these classes for more details).

13. Filter 2: If a filter function was not supplied on construction of the BMessageFilter, then the Filter() member function determines the result of the filter. If the Filter() member has been overridden in a derived class, the result depends on the behaviour of this derived class. If the class has not been overridden and no filter function was provided on construction, the B_DISPATCH_MESSAGE is returned. If a filter function was provided on construction, then the filter function will be called and the Filter() member will return what this filter function returns.

## BMessageFilter Implementation:

The implementation of the BMessageFilter is pretty simple. It is mainly a container for properties of the filter itself and a few simple methods for getting and setting these properties.

The actual act of dispatching or skipping the message as dictated by the filter is implemented in BLooper or BHandler. The BMessageFilter just provides a mechanism for these classes to determine which to do. Also the BLooper or BHandler decide whether to pass the message through the filter on their own. The filter does not look at every message to see whether the source and delivery of this message is such that it should pass through the filter. Instead, BLooper or BHandler look at the filter, check the source and delivery options of the filter against the message itself and call the filter if appropriate. So, the filter is really just a container for these options and doesn't take action based on them.