Plug-in Registration

For the plug-in host to know what types are available, each plug-in must register with the host. Registration consists of making the host aware of the types a plug-in implements and their associated factory functions. This information can be declared statically using a few special keys in the plug-in’s information property list or it can be registered dynamically by code in the plug-in. See Implementing a Plug-in for an example.

When a plug-in is found by a host, plug-ins use the value of the CFPlugInDynamicRegistration key in the plug-in’s information property list to determine whether a plug-in requires static or dynamic registration. If a plug-in uses dynamic registration, the plug-in’s code must be loaded immediately so the dynamic registration can take place. If the plug-in uses static registration, its code need not be loaded until the application actually needs to instantiate a type. For this reason, static registration is preferred when there’s no overriding reason for using dynamic registration.

For static registration, the information property list contains the CFPlugInFactories key whose value is a dictionary whose keys are factory UUIDs (expressed in the standard string format) and whose values are function names. Each key-value pair in this dictionary registers a factory function when the plug-in’s CFBundle is created. The information property list also contains a CFPlugInTypes key. The value of this key is a dictionary whose keys are type UUIDs and whose values are arrays of factory UUIDs. For each type then, there is a list of the factories within the plug-in that can create that type. See Implementing a Plug-in for an example plug-in’s information property list.

For dynamic registration, plug-ins load the plug-in’s code and allows it to do its dynamic registration as soon as the associated CFBundle is created. For plug-ins that do dynamic registration, the optional CFPlugInDynamicRegisterFunction key can be set to the name of the function that should be called to do the dynamic registration. If this key is not provided, plug-ins attempt to call a function named CFPlugInDynamicRegister. When you implement the CFPlugInDynamicRegister function (or your custom version) you use dynamic registration functions of plug-ins to create the type/factory associations that would otherwise be declared in the plug-in’s information property list.

A code entity can also register built-in types and factories using the dynamic registration functions. The code with built-in types could then use the plug-in model internally, or allow plug-ins to query the host for the built-in types and interfaces. This might be necessary if plug-ins need to discover some information about the host’s state before being used.