XPConnect Sample

Ariel Blackeroth <arielb@netscape.com>
Michael Ang <mang@subcarrier.org>
Last modified July 9, 1999

In the spirit of "worse is better" this somewhat rough guide is being released to the world. It will be expanded upon and improved.

Overview
This sample is located in the Mozilla tree at mozilla/xpcom/sample and is a simple demonstration of XPConnect. XPConnect allows JavaScript to transparantly access and manipulate XPCOM objects; this communication between JavaScript and native code is done by having their interfaces defined in the XPIDL interface definition language. See the Roadmap for documentation on XPCOM, XPConnect, XPTCall and XPIDL for more information. The sample demonstrates how a JavaScript call can access and manipulate a XPCOM interface.

nsISample.idl
This is the interface declaration for the XPCOM object. It defines two functions, their parameters, and one attribute. It also defines the interface's id. The idl file is compiled by the xpidl compiler into a C++ header, nsISample.h and a .xpt file which is a binary representation of the interface used at runtime.
`attribute string Value;`
`void WriteValue(in string aPrefix);`
`void Poke(in string aValue);`

nsSample.cpp
This contains the implementation of nsISample.idl. SampleImpl inherits from nsISample.h, the header dynamically created by the xpidl compiler. The attribute Value has been expanded into a get and set and the return values have been modified to NS_IMETHOD, a success status for the method. The macro NS_DECL_ISUPPORTS, defined in mozilla/xpcom/public/nsISupportsUtils.h defines the inherited methods from nsISupports.h.
`NS_IMPL_ISUPPORTS(SampleImpl, nsISample::GetIID());`
In the constructor, the macro NS_INIT_REFCNT is called which sets the reference count to 0.

nsSampleFactory.cpp
This is the class which builds the instance of the nsSample class. The COM framework uses factories to create instance of implementations rather than having the implementations instatiate themselves in order to increase portability of code. This factory inherits from nsFactory, which is also an XPCOM object. To gain more knowledge of factories see the generic factory document or the Modularization techniques document.

xpconnect-sample.html
This contains the calls from JavaScript to the XPCOM object. In the first two lines JavaScript obtains a reference to the XPCOM object.
`var sample = Components.classes["component://netscape/sample/sample-world"].createInstance();`
`sample = sample.QueryInterface(Components.interfaces.nsISample);`
After these instructions, the sample object is available seemlessly to the script.

Compiling the idl
The XPIDL compiler (xpidl on Unix, xpidl.exe on Windows, and a CodeWarrior plugin on Mac) is compiled at build time (except on Mac) thus you will have to build mozilla in order to test this out. If you have already built mozilla then the compiler will be located at `mozilla\dist\WIN32_D.OBJ\bin\xpidl.exe`.
Once you have the XPIDL compiler enter the following command at your prompt
`D:\mozilla\xpcom\sample>d:\mozilla\dist\WIN32_D.OBJ\bin\xpidl -I d:\mozilla\dist\idl -m header nsISample.idl`
The `-I d:\mozilla\dist\idl` points the compiler to the folder containing the other idl files, needed because nsISample.idl inherits from nsISupports.idl. The `-m header` instruction tells the compiler to build the C++ header. To build the .xpt file substitute `-m typelib`. For more information on compilation see the xpidl compiler page.

Building the Sample
To build the Sample just enter
`d:\mozilla\xpcom\sample>nmake /f makefile.win`
In order to do this you need to have your environment variables set correctly. See the Build page for more information.

Running the sample
Using your mozilla browser, load the xpconnect-sample.html document. The mozilla binary is located at `\mozilla\dist\WIN32_D.OBJ\bin\apprunner.exe`. Set the url to `file:///D|/mozilla/xpcom/sample/xpconnect-sample.html` or wherever you have the samle html file located. Pay attention to the console when clicking write. Notice that the value printed is calculated in C++ code defined in nsSample.cpp.

Resources:

- mozilla/xpcom/sample source directory

Comments to: Michael Ang <mang@subcarrier.org>