Thursday 17 July 2008

My first video: creating a Hello World class in Squeak

Given the recent push to get Squeak video tutorials available, I decided to have a go myself. I took as my starting point my post from a few months back, intended to act as a quick introduction to developers coming to Squeak for the first time.
My first problem was to find a good screen capture utility. Unfortunately, Wink isn't available for OS X, but a bit of searching uncovered Snapz Pro X. Despite the terrible name, it's a very nice piece of software that makes recording video and screenshots very easy. It comes with a 14-day fully-functional trial, so if I get a taste for this, I may end up having to cough up the $69 soon. If anyone knows of other software I could use, please let me know.
You can view the video at, or find it in the new Squeak Smalltalk group that Randal Schwartz has set up. Have a look at the video and let me know what you think: too fast, too slow, too much like Ricky Gervais, whatever.

Monday 14 July 2008

Using Apache as a front-end for Seaside

I'll admit it, configuring Apache scares the bejeezus out of me. The documentation seems to be so focused on the trees, that the wood becomes an impenetrable, gloomy forest. I guess I'm not alone in this, which makes Ramon Leon's posts on configuring Apache with Seaside(1, 2, 3) so useful.
Despite this, I've still steered clear of going near Apache, until Ramon posted a sample extract of configuration text. Now, cut-and-paste is something I can do, so I decided to give it a go.
I'm on Mac OS X, so Apache is installed and running by default. Despite my earlier protestations, I have played with Apache before, so I knew that httpd.conf was the key file to control how Apache runs. A bit of poking about in man files uncovered the location of the file I needed: /private/etc/apache2/http.conf.
I opened a Terminal and cd'ed to /private/etc/apache2/ where I could execute sudo vi httpd.conf which prompted me to enter my password to edit this administrator file, and then allowed me to start hacking. I wanted to leave my existing configuration as untouched as possible, so below the line:
Listen 80
I added a new line:
Listen 81
which would start Apache listening on port 81 as well as port 80. I could then use port 81 for my experimentation.
Typing sudo /usr/sbin/apachectl restart caused Apache to restart, hopefully loading my change.
I then tried browsing http://localhost:81/

Success (so far) - Apache is trying to deal with my request, and using its default handler.
Now, I'd noticed an interesting line at the bottom of httpd.conf:
Include /private/etc/apache2/other/*.conf
which meant that I could make any other changes in a separate file. So sudo vi other/seaside.conf opened a new .conf file, into which I typed:
    #DocumentRoot /var/www/myExamplePath
    RewriteEngine On
    ProxyRequests Off
    ProxyPreserveHost On
    UseCanonicalName Off
    # if the path doesn't exist, rewrite it to be a Seaside file ref
    RewriteRule ^/seaside/files(.*)$ http://localhost:8080/seaside/files$1 [P,L]

    # redirect all requests to seaside - configure this as required
    RewriteRule ^/(.*)$ http://localhost:8080/seaside/$1 [P,L]


Which (when you put back the angle brackets that Blogger eats on the first and last lines) is taken directly from Ramon's post. It does two useful things. It takes any requests coming in to port 81, and first checks to see if you're requesting a physical file, and if so, serves it up (useful for the static parts of your site). If not, it passes them on to Seaside on the appropriate port, adding the standard /seaside prefix (so this is hidden from the outside world, which is a nice touch). Restarting Apache again and this time browsing http://localhost:81/examples/counter got me:

Again, good news. However, when I tried to navigate around, or use applications that made use of built-in #style and #script methods, things started to go wrong. The reason here is that Seaside prefixes its generated relative URLs with /seaside. So I simply added the following:
# re-write all requests starting with seaside - these links are exposed
# by the application internally. Alternatively, you can
# re-configure Seaside to change the urls it generates.
RewriteRule ^/seaside/(.*)$ http://localhost:8080/seaside/$1 [P,L]

into the virtualhost directive, and re-started Apache once more. This time it all worked like clockwork.
Twenty minutes of not very challenging experimentation, and I now have Apache working as my front-end webserver. That means, all the handling of static files can be done outside of Squeak, I can configure my responses if the Squeak images crashes, I get access to all of Apache's features such as logging, SSL etc. Not a bad half-hour's work really.

Tuesday 8 July 2008

Digging into the functionality behind morphs

Someone on the Seaside mailing list asked how to find out the meanings of all the icons against each method in the browser. The answer is quite interesting, as it helps you understand the importance of the "live environment" that Squeaks gives you.

Looking at a typical class, you'll see that many method definitions have a little icon by the name:

(nb alt- and cmd- prefixes used below may need to be changed depending on your image preferences).

In order to work out why things happen in morphs, you can easily access the underlying code. In this case, alt-click on the offending item repeatedly to bring up halo menus for more targeted morphs. The closest in you get is OBLazyList - if you go too far, you'll notice that you cycle round to the original morph again. Notice in this case that this morph is much bigger than its containing morph, and so much of its content is hidden from the user:

When you're on the morph you're after, you can then find out more about it by clicking on the "spanner" (=debug) icon, and selecting "browse morph class", which will open a browser on OBLazyListMorph.

Look in the "drawing" methods of this class, and you'll get your first clue in #display:atRow:on: which selects and draws the icons with the help of OBMorphicIcons class.

Selecting the name OBMorphicIcons and pressing Cmd-B will bring up a browser on that class, and you'll see that each icon is defined by an instance method. Click on one of these (I chose #arrowUpAndDown because that looks pretty likely to be unique to this usage), and press Cmd-n to find its senders.

Bingo! OBInheritanceFilter method #icon:forNode: decides which icon to display based on a number of tests, including a test for #halt: messages that puts a red flag against a method.

You'll also see that this code accidentally red-flags itself because it fails to distinguish between #halt: used as a symbol rather than as a message.