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The word "parameter," in terms of the Media Kit, refers to a point at which the data manipulation performed by a BControllable node can be configured, and doesn't actually refer to a user interface object. BParameter-derived objects don't implement a user interface. See the BMediaTheme class for information on how to create a user interface for a BControllable node.
The BParameter class is an abstract class; you'll normally use the MakeNullParameter(), MakeDiscreteParameter(), and MakeContinuousParameter() functions in the BParameterGroup class to actually create parameters and add them toa BParameterWeb. For more detailed information on parameters and how to create a BParameterWeb, see the BParameterGroup and BParameterWeb classes.
Hook Function
Constructor and DestructorBParameter objects may only be created by using the appropriate functions in the BParameterGroup class.
Member Functions
AddInput() , AddOutput()
AddInput() adds the BParameter specified by input to the set of BParameters that transmit data into the BParameter. The specified input parameter's AddOutput() function is called to let it know that it's sending data to this parameter.AddOutput() adds the BParameter specified by output to the set of BParameters to which the BParameter outputs data. The specified output parameter's AddInput() function is called to let it know that it's receiving data from this parameter. RETURN CODES B_OK. The item was successfully added to the list.
CountChannels() , SetChannelCount()
Some BParameters have more than one channel of the same type. For example, a stereo volume parameter with independent left and right gain would have two channels, while a single parameter value that affects both the left and right by the same amount would have only one channel. CountChannels() returns the number of channels manipulated by the BParameter. The default is one channel; if your node provides more channels, call SetChannelCount() after creating the BParameter. SetChannelCount() specifies the number of channels the parameter manipulates.
CountInputs() , CountOutputs()
These two functions return the number of inputs or outputs currently connected to the BParameter. The number of inputs or outputs may be different from the number of channels, since an input or output might be comprised of multiple channels (a single stereo sound input or output might consist of a left channel and a right channel, for example).
GetValue() , SetValue()
GetValue() returns the current value of the BParameter. On entry, buffer points to ioSize bytes of memory, indicating where the current value of the BParameter should be stored. On return, ioSize indicates the actual number of bytes returned in the buffer, and lastChange indicates the time at which the BParameter's value last changed. SetValue() changes the value of the BParameter. The new value, which is size bytes long, is obtained from the specified buffer. The change occurs at the performance time specified by changeWhen. In either case, if the parameter is a multichannel parameter, the buffer should be large enough to contain the values for all the channels, and ioSize or size should indicate the total size of the buffer. These calls simply dispatch to the BControllable::GetControlValue() and BControllable::SetControlValue() functions. RETURN CODES B_OK. No errors.
Group()
Returns the BParameterGroup that most directly contains the BParameter.
ID()
Returns the BParameter's ID number. This ID is used by the node to route value change requests to the right place. This value should be unique within the BParameterWeb containing the BParameter.
InputAt() , OutputAt()
InputAt() returns the BParameter that feeds data into the input number specified by index, which can range from zero to CountInputs()-1. OutputAt() returns the BParameter that receives data from the output number specified by index, which can range from zero to CountOutputs()-1. Both functions return NULL if an invalid index is specified.
Kind()
Returns the BParameter's kind. The kind isn't necessarily user-readable, but it can be used to figure out what kind of BControl to create to visually represent the configuration option provided by the BParameter. See the BParameterGroup::MakeNullParameter(), BParameterGroup::MakeDiscreteParameter(), and BParameterGroup::MakeContinuousParameter() functions for more detailed information about the available kinds.
MediaType() , SetMediaType()
MediaType() returns the type of media data that flows through the BParameter. SetMediaType() changes the recorded type of media data. The default, if it hasn't been changed, is B_MEDIA_NO_TYPE. The media_type of a BParameter is used to inform interested parties as to the format of the data; it's an informational setting, and doesn't alter the data flowing through the BParameter in any way.
Name()
Returns the BParameter's name, which is suitable for display to the user (for example, it can be used as the label in a user interface object).
Channels()
SetMediaType() see MediaType()
SetValue() see GetValue()
Type()
Returns the kind of parameter the object represents.
Unit()
Returns the unit of measurement used by the BParameter. This might be, for example, "dB," "kHz," or "fps." It should be human-readable, as it may be displayed in a user interface.
ValueType()
Returns a type code indicating the type of data type of the BParameter's value. This is usually B_INT32_TYPE for selectors or B_FLOAT_TYPE for sliders.
Web()
Returns the BParameterWeb that contains the BParameter.
Constants
Parameter KindsDeclared in: <be/media/ParameterWeb.h>
Indicates the kind of parameter represented by a BParameter object. The B_VIDEO_FORMAT kind has specific values associated with it:
media_parameter_typeDeclared in: <be/media/ParameterWeb.h>
These are the possible parameter types, which are returned by the Type() function. They indicate what subclass of BParameter the object is.
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