Channels In IE And Navigator
Another area where Netscape and Microsoft
browsers differ is in their support for channels. While this is still a
developing technology for both companies, it's nice to be able to support them
both now. The problem is that the techniques are completely different. In
Navigator, we use the Netcaster accessory that comes with the full
Netscape Communicator
suite, while in IE4 and 5 we use a Channel Definition Format (CDF) file
written in a version of XML. We aren't going to be describing the way you
actually create channel content for either of these in detail here. Our aim is
simply to show how you can support both of them.
The Wrox Web-Developer Home Page Channels
The Home
page of our Web-Developer site offers a Wrox channel to both Navigator 4+ and
IE4+ users. To achieve this we first detect the browser type, using some simple
ASP script in the page. If we find a suitable browser, we set one of the two
variables, blnIE4
or blnNC4, to True:
...
<%
blnIE4 = False
blnNC4 = False
strUA =
Request.ServerVariables("HTTP_USER_AGENT")
intMSIE = Instr(strUA, "MSIE ") +
5
If intMSIE > 5 Then
If
CInt(Mid(strUA, intMSIE, Instr(intMSIE,strUA,".") - intMSIE)) >= 4
Then
blnIE4 = True
End
If
Else
If
(Instr(strUA,"Mozilla/4.") Or Instr(strUA,"Mozilla/5.")) _
And
Instr(strUA, "compatible;") = 0 Then
blnNC4 = True
End
If
End If
%>
...
Of course, you could use the Browser
Capabilities or BrowserHawk components instead. We chose to do it directly in
script purely to minimize server load.
Displaying The Channel Button
Now we can display the appropriate button image, or, in
older browsers, some text to indicate that we do support channels:
...
<% If blnIE4 Then %>
<a HREF="/webdev/channels/add_chan.cdf">
<img HSPACE="5" SRC="/images/add_chan.gif"
BORDER="0" WIDTH="136"
HEIGHT="20" ALT="Add to Internet Explorer 4
Channels"></a>
<% End If %>
<% If blnNC4 Then %>
<a HREF onClick="addChannel(); return false">
<img HSPACE="5" ONCLICK="return
addChannel()" SRC="/images/add_netc.gif"
BORDER="0" WIDTH="136" HEIGHT="20"
ALT="Add to Netcaster Channels"></a>
<% End If %>
<% If blnIE4 = False And blnNC4 = False
Then %>
Channels are available<br>for Internet
Explorer 4 and<br>Netscape Navigator 4 users.
<% End If %>
...
The result in IE 4 is that we get an Add to Active Channels button in
the page. When clicked this simply loads a CDF file that defines the channel;
this is all that's required to start the process in IE 4/5:
Creating A Netcaster Channel
Netcaster works
in a different way. To create a channel here, we have to use some client-side
JavaScript to load Netcaster,
and then define and add the channel to it. The following script does just this.
Notice that it's only included in the page we send to the client when we are
serving to Navigator 4 or better:
<% If blnNC4 Then %>
<script
LANGUAGE="JavaScript1.2">
function addChannel() {
var
nc = components["netcaster"];
if
(nc.active == false){
alert ('If you do not see the Add Channel dialog after Netcaster has
loaded, ' +
'come back to this page and click our "Add Netcaster Channel" button ' +
'again to install your new channel');
nc.activate();
}
if
(nc.active == true) {
import nc.getChannelObject;
import nc.addChannel;
channel
= getChannelObject();
channel.url = 'http://rapid.wrox.co.uk/Default.htm';
channel.name = 'The Wrox Web-Developer Community';
channel.desc = 'The center for Web Developers, with information and ...
etc.';
channel.intervalTime = -6;
channel.absoluteTime = 4320;
channel.estCacheSize = -1;
channel.depth = 1;
channel.active = 1;
channel.mode = 'window';
channel.type = 1;
channel.cardURL = 'http://webdev.wrox.co.uk/webdev/channels/chlogo.gif';
addChannel(channel)
}
}
</script>
<% End If %>
You may be tempted just to include it in the
page anyway, knowing that it won't get executed in IE4+. The problem is that
the browser parses the script when it loads the page, even if it is never going
to be executed. This means that the window object of the browser must
support the components
property, seen in the first line of the function code. If it doesn't (and only
Navigator 4/5 does) you'll get a script error when it loads the page.
When you load this page into Navigator 4 or Communicator 4,
you get the Add Netcaster Channel
button image, which, when clicked, starts to load Netcaster if it's not already running:
Note that, for this to work, you must have installed
Netcaster when you installed Navigator or the Netscape Communicator package.
Once Netcaster loads,
the Channel Properties dialog is
displayed. However, sometimes you'll find that the channel isn't added
automatically if Netcaster has to
be started up by our code, so we display a message to this effect. If Netcaster is already running, the process works a lot
more smoothly:
For more information about creating channels for
Netcaster,
check out http://developer.netscape.com/docs/manuals/netcaster.html.
