Plug-in is the core element of an extensible application. The extensions and also the main application logic are implemented as plug-ins. Plug-ins can be developed, distributed and deployed independently, subject to inter-plugin dependencies. Deploying a new plug-in does not require recompilation or relinking if the operating system platform supports required dynamic linking facilities.
A plug-in includes the following structural elements.
A plug-in descriptor is an XML document describing a plug-in. It includes information about the contents of the plug-in, the features provided by the plug-in, plug-in version information and static dependencies of the plug-in. Most of the elements are optional. Most of the descriptor information described here is available to software via cp_plugin_info_t structure. The plug-in descriptor must be located in the plug-in directory as plugin.xml.
The formal declaration of plug-in descriptor is available as XML Schema Definition in plugin.xsd located in the top level source directory. Currently there is no namespace associated with the plug-in descriptor. Here is an example of a plug-in descriptor. Click element name to jump into documentation for that element.
<plugin id="org.c-pluff.example" name="Example Plug-in" version="0.3.2" provider-name="Johannes Lehtinen">
<backwards-compatibility abi="0.3" api="0.2.8"/>
<requires>
<c-pluff version="0.1"/>
<import plugin="org.c-pluff.util" version="0.2"/>
<import plugin="org.c-pluff.extra" optional="true"/>
</requires>
<runtime library="libruntime" funcs="org_cpluff_example_funcs"/>
<extension-point id="editors" name="Text Editors" schema="editors_schema.xsd"/>
<extension-point id="url-families"/>
<extension point="org.c-pluff.util.archivers" id="tar" name="Tar Archiver Support">
<type random-access="false"/>
<exec bin="tar"/>
</extension>
<extension point="org.c-pluff.example.editors>
<editor name="Text Editor" runtime="org_cpluff_example_txteditor_runtime">
<file-types>
<file-type mime-type="text/plain"/>
</file-types>
</editor>
</extension>
</plugin>
A descriptor can also be much simpler, depending on the plug-in. Here is an example of a minimal descriptor (of a useless plug-in).
<plugin id="org.c-pluff.useless"/>
This is the top level element of the plug-in descriptor. It can have following attributes.
- id: A mandatory unique identifier for the plug-in. Plug-in identifiers should preferably be generated using a reversed DNS domain name as prefix to prevent identifier conflicts.
- name: An optional human-readable name for the plug-in.
- version: An optional version number for the plug-in. Version numbers are used for checking compatibility when resolving versioned plug-in dependencies. See also information about plug-in versions.
- provider-name: The name of the plug-in provider or author. Optional.
This element can contain following elements.
- backwards-compatibility: Optional information about backwards compatibility of this plug-in version.
- requires: Information about static plug-in dependencies. Can be omitted if the plug-in does not have static dependencies.
- runtime: Information about the plug-in runtime library. Can be omitted if the plug-in does not have a runtime library but only data.
- extension-point: Information about extension points provided by the plug-in. This element is repeated if there are multiple extension points and omitted if there are none.
- extension: Information about extensions provided by the plug-in. This element is repeated if there are multiple extensions and omitted if there are none.
This element includes optional information about the backwards compatibility of this plug-in version. It can have following attributes.
- abi: Backwards compatibility of the application binary interface (ABI) of the plug-in. ABI includes any public symbols exported by the plug-in, data structures associated with exported symbols and any extension points provided by the plug-in. The ABI of the current plug-in version is backwards compatible with any plug-in version from the version specified here to the current version. This information is used when resolving versioned plug-in dependencies. See also information about plug-in versions.
- api: Backwards compatibility of the application programming interface (API) of the plug-in. API compatibility means that source code developed against one version of the plug-in also compiles against another version of the plug-in. This information is not used by framework but it can be used by a developer developing dependent plug-ins.
These apply to plug-ins that provide header files and runtime libraries. For example, a plug-in might export global functions to other plug-ins or it might provide an extension point where an extension installed by other plug-in must conform to data structures defined by the plug-in. Both attributes are optional.
This element includes information about static plug-in dependencies. It can be omitted if there are no dependencies. It can contain following elements.
- c-pluff: An optional version dependency on the C-Pluff implementation.
- import: Declares a static dependency on other plug-in. This element is repeated if there are multiple dependencies and omitted if there are none.
This element declares a version dependency on the C-Pluff implementation. It can be used to ensure that the plug-in is not loaded by incompatible C-Pluff version. It has the following attribute.
- version: The required version of the C-Pluff implementation. This is used when resolving the plug-in. It is checked that the used C-Pluff implementation is backwards compatible with the version specified here when it comes to the application binary interface (ABI) of C-Pluff.
This element declares a static dependency on other plug-in. It must be used when a plug-in uses global symbols or data from other plug-in or when a plug-in uses an extension point defined by other plug-in or whenever some other plug-in needs to be there for the plug-in to work. The framework takes care of resolving and starting the dependencies whenever the plug-in is resolved or started.
This element can have following attributes.
- plugin: The identifier of the imported plug-in.
- version: An optional version dependency on the imported plug-in. The plug-in can be resolved only if the version of the imported plug-in is backwards compatible with the version specified here when it comes to the application binary interface (ABI) of the imported plug-in.
- optional: Is the import optional or not ("true" or "false"). Default is false, a mandatory import. An optional import behaves just like the mandatory import as long as the imported plug-in is present. However, if it is not present then the import is ignored. Optional import can be used if the plug-in works in limited capacity even without the specified plug-in.
This element contains information about the plug-in runtime library. It is omitted if the plug-in does not have a runtime library but contains only data. It can have following attributes.
- library: The name of the plug-in runtime library in the plug-in directory. A platform specific extension (for example, ".so" or ".dll") is added to the value specified here when loading the library.
- funcs: The functions to be used to create an instance of the plug-in runtime. This attribute is optional. It is needed if the plug-in has a start or stop function. The value specified here is a name of an exported symbol which contains a pointer to cp_plugin_runtime_t structure.
This element defines an extension point provided by the plug-in. It can have following attributes.
- id: The local identifier of the extension point. The value specified here is prefixed with the identifier of the plug-in and dot to construct the global identifier of the extension point.
- name: An optional human-readable name describing the use of the extension point.
- schema: An optional path to the extension point XML schema in the plug-in directory. This information is not currently used by the framework. But it can be used by a developer to determine what information should be provided by extensions attached to this extension point.
This element defines an extension installed into a specified extension point provided by the defining plug-in or some other plug-in. It can have following attributes.
- point: The global identifier of the associated extension point.
- id: An optional local identifier for the extension. The value specified here is prefixed with the identifier of the plug-in and dot to construct the global identifier for the extension.
- name: An optional human-readable name describing the extension.
The extension element can contain XML elements specific to the associated extension point (conforming to the schema defined by the extension point).
A plug-in runtime library is an optional plug-in element. Plug-ins only supplying static data in form of XML data and files do not need a runtime library. However, a typical plug-in does provide program logic as well.
The plug-in runtime library includes all program logic and program data provided by the plug-in. It is simply a shared library, or a dynamically linked library, which is linked in to the application when the plug-in is started. When plug-in is unloaded, the runtime library is unloaded as well. The framework has been designed to manage dependencies so that unloading of the runtime library does not cause problems, provided that plug-ins behave properly.
A plug-in can expose functionality to other plug-ins either as exported global symbols that are directly resolved by other plug-ins or by supplying extensions. When other plug-ins are directly using exported symbols the plug-in acts just like any standard shared library. Nothing special there. The more interesting case is exposing functionality as extensions. Because the extension is registered at a specific extension point, the logic in other plug-ins can use the extension and the associated program logic even if they are not aware of the existence of the extension supplying plug-in.
The extension points accepting program logic as extensions define a way to specify the name of the symbol pointing to the supplied logic. This is typically an attribute of an XML element contained in the extension definition. The plug-in supplying the extension can then export the program logic as a global symbol with arbitrary name and then place the name of the symbol in extension data. Alternatively, the plug-in can define a virtual symbol at runtime using cp_define_symbol. Other plug-ins that are using extensions registered at the extension point can then resolve the named symbol using cp_resolve_symbol at runtime. The framework automatically creates a dependency from the symbol using plug-in to the symbol supplying plug-in to prevent failures in case the symbol supplying plug-in is stopped or unloaded.
Plug-in can supply static data to other plug-ins using at least two different mechanisms. A plug-in can easily provide static XML data as part of extension elements. Additionally, a plug-in directory can contain files that may be accessed by other plug-ins. Currently the platform does not provide assistance in accessing data files provided by other plug-ins. However, a plug-in can locate the plug-in directory and thus any included data files by using plug-in path available in cp_plugin_info_t structure of the data providing plug-in.
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