Pcom xpcom
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XPCOM is a cross-platform component object model similar to COM. It allows components to be implemented and used in multiple languages including JavaScript, Java, and Python in addition to C++. Interfaces are defined in XPIDL and XPCOM provides core classes for file, memory, and data management. Components can be globally used across applications and languages. Drawbacks include needing to access objects through interfaces and increased memory leak potential. Components are registered via modules to simplify retrieval. Factories instantiate components by their interfaces.
Pcom xpcom
- 1. XPCO M CROSS PLATFORM COMPONENT OBJECT MODELING A Brief Overview
- 2. Catalogue Introduction Benefits Drawbacks Concepts Example Code Working References
- 3. Introduction XPCOM is a cross platform component object model, similar to Microsoft COM. It has multiple language bindings, allowing XPCOM components to be used and implemented in JavaScript, Java, and Python in addition to C++. Interfaces in XPCOM are defined in a dialect of IDL called XPIDL. XPCOM itself provides a set of core components and classes, e.g. file and memory management, threads, basic data structures (strings, arrays, variants), etc.
- 4. Benefits XPCOM components can be used by any programming language Mozilla supports. Objects implemented as XPCOM can be global to an application, and are not dependent on the scope of any one window. You can use an XPCOM service to share data between windows. XPCOM encourages modular programming - programming logic can be encapsulated within a component.
- 5. Drawbacks Objects must be accessed from their defined interfaces. Javascript shortcuts such as the global window object cannot be accessed. The only exception to this rule, is if you set a magical property wrappedJSObject on your component implemented in JavaScript, the object this property refers to will be accessible from the calling side. You can set ”this.wrappedJSObject = this” in your component's constructor and avoid writing any interfaces altogether. It is easier to have memory leaks.
- 6. Concepts Components: A component is simply an object that implements interfaces, and is registered with the component manager. Interfaces: Interfaces are used to define what functions and attributes an XPCOM component provides. nsISupports is an interface that all classes must implement. It provides a function to request the interfaces on a class.
- 7. Concepts Factories: (Cont.) Factories are objects used to instantiate components. Each factory object implements the nsIFactory interface. Modules: Multiple classes can be defined in a file. A module is an object that is used to register, unregister and provide factories for classes. It implements the nsIModule interface.
- 8. Example Code
- 11. Working I. Startup & Registration During the startup process if the file compreg.dat is missing or the file .autoreg exists, the application knows that it needs to register components. It then loads files from each of the components folders, including the components folder in each extension directory. It then calls the function NSGetModule on each file to get the object that implements the nsIModule Interface. The module object is then used to register your module. This is used to simplify the process of retrieving the module later in class creation. The registerSelf function is called to do the actual registering. When removing a class, the unregisterSelf function is called.
- 12. Working II. Class Creation (Cont.) The code calls getClassObject on your module. This returns a class factory. The factory then has a method called createInstance which is used to create the actual class object. III. Accessing the component You can now access your component by the code written. Mozilla also provides a component construction process to easy creation with JavaScript.
- 14. Thank You Jay Patel (M.Sc. IT - Part 1)