Researching XPIDL and IDL Technologies

Introduction

XPIDL stands for XP Interface Description Language. It is a specification for XPCOM which is a cross platform adapter description language. An adapter description language is used to describe an adapter which unrelated with its machine language. The description of the adapter can use specialized tools to handle automatoc generation of language that is unrelated to the adapter's specifications. Typically XPIDL is frequently used to generate C++ header files and typelib information.

Language

Interfaces

  An interface is declared with the interface keyword, and the simplest possible interface is  
 interface nsToolbar {
 };

To specify an interface's parent, follow the interface name with a colon and the parent name

interface nsIToolbar : nsIParent {
};

Methods

An interface can have both methods and attributes. Attributes are properties of interface objects that can be read and optionally set. The following example - Attributes, properties of interface objects, can be read and optionally set. - This is an example of an interface with one method (no return value, no parameters) named fun and an integer-valued attribute named attr

interface nsIFoo {
  attribute long attr;
  void fun();
};

Methods can have any number of in, out or inout parameters, with a variety of types.

In this example many parameters of different types and in an out specifications are seen

interface nsIStringStuff {
  void findStringLength(in string str, out long l);
  void concatenateStrings(in string str1, in string str2,
                         out string result);
  void replaceChar(inout string str, in char from, in char to,
                  in boolean foldCase);
};

Attributes can be made read-only, by placing the read-only keyword infront of the the definition

interface nsIThing {
  readonly attribute string lookButDontTouch;
};
C++ get and set methods for attributes are automatically declared for attributes in the   
interface. For example attribute name foo generates accessor methods GetFoo and SetFoo.

Compiler

The compiler capitalizes method names when generating C++ headers. It supports existing C++ convention in Mozilla, used the InterCaps method naming style. The method name used in XPIDL will appear with the same capitalization to JavaScript, so the best practice is to match JavaScript convention by declaring attributes and method names in InterCaps.

Built In Types

Type                    C++ 

void 	                void
boolean 	        PRBool
octet 	                PRUint8
short 	                PRInt16
long 	                PRInt32
long long 	        PRInt64
unsigned short 	PRUint16
unsigned long 	        PRUint32
unsigned long long 	PRUint64
float 	                float
double 	        double
char           	char
wchar 	                PRUnichar
string 	        char*
wstring 	        PRUnichar*

Native Types

     It is possible to use native types in XPIDL by declaring then using the native declaration syntax which works similar to a typedef in C++.
A native type can be given a name using the following syntax:
     native name(native_type);

Reference and pointer types can be defined by combining [ref] or [ptr] with native languages

     XPIDL code	Generated C++

     [ref] native        nsNativeFileRef(nsFileSpec);
     [ptr] native        nsNativeFilePtr(nsFileSpec);

     interface foo {
       void openByRef(in nsNativeFileRef aFileSpecRef);
       void openByPtr(in nsNativeFilePtr aFileSpecPtr);
     };      

     /* starting interface:    foo */
     class foo {
      public:

       /* void openByRef (in nsNativeFileRef aFileSpecRef); */
       NS_IMETHOD OpenByRef(nsFileSpec & aFileSpecRef) = 0;

       /* void openByPtr (in nsNativeFilePtr aFileSpecPtr); */
       NS_IMETHOD OpenByPtr(nsFileSpec * aFileSpecPtr) = 0;
     };                                  

Native types in your code can be a good way to migrate existing interfaces. However, using native types in a method will mean that method is not scriptable.

Include Terminology

1. include