The magazine of the Melbourne PC User Group A Site On The World Wide Reb Trevor Gosbell |
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Trevor Gosbell gives us the third in this series of articles on REBOL — this month we learn about the REBOL Distributed Desktop |
The World Wide Reb is the tongue-in-cheek name given to the network of distributed REBOL applications accessible from the REBOL/View desktop - more formally it is known as the REBOL Distributed Desktop. Once you've set up a rebsite you can add it to Distributed Desktop and make it available to all REBOL/View users.
The REBOL folks reckon that setting up a rebsite is easier than setting up an ordinary HTML Web page. Let's put that claim to the test.
A Rebol Mini-Application For the Internet
Because of the fact that users browse the World Wide Reb using their REBOL/View desktop, we'll need some type of Internet-enabled application. The file renamer we developed in the previous articles is not really suitable as it is intended for use only on the local machine. Also REBOL has built-in security that stops its own programs from accessing the user's computer without permission. I don't want my rebsite asking visitors for access to files on their computers - I certainly wouldn't like it!
No, it must be a network-oriented application that doesn't need access to files on the user's computer.
The "Melbourne Weather" Application
Recently I had the dubious privilege of working in a basement level office that lacked any natural light, let alone a window. Before leaving the office we always wanted to know what conditions were like up on the surface. Rather than climbing the stairs to the nearest window we would hit the Bureau of Meteorology Web site and check the latest observations at http://www.bom.gov.au/products/IDV65119.shtml.
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The trouble with this Web page is that it shows data from a number of
observation stations (Figure 1) but all I wanted was Melbourne. Also it includes
more observations than I needed - two measures of wind speed? And what is a dew
point anyway? Information overload! What I need is a little program that will
access this page and tell me the temperature, whether it's windy out and if it
has been raining.
There are countless approaches to this task, and a REBOL solution appears in
Figure 2. This will be the mini-application to include on the rebsite.
The program (Listing 1 - below, or download here) addresses the two parts to the problem - extracting the
required data and presenting the information in an easy-to-read form.
Parsing The Web Page
First we read the Web page into the variable page, then use the REBOL parse
command to extract the data. Just as the layout command uses a dialect (the
Visual Interface Dialect), the parse command also uses a dialect but this one is
for text-processing rather than visual display. The format of the parse command
is:
| parse some_text [using these rules] |
So the parse dialect allows us to build up a set of text-processing rules, as follows:
parse page [ |
Or rephrased in English:
Now the variable
data_list holds a list that includes a date stamp and all of
the current observational data for Melbourne (time of observation, temperature,
dew point, relative humidity, wind direction, wind speed (km/h and knots), wind
gust (km/h and knots), air pressure, rainfall since 9.00 am) - a total of 12
fields. We now have our observations - it's time to present them in a graphical
user interface (GUI).
But First...
A quick word about the REBOL path syntax. There are two ways to access each data
element in a block such as
data_list. One is to use the ordinals first data_list,
second data_list, third data_list, and so on. But the method used here is the
REBOL path syntax:
| data_list/1, data_list/2, data_list/3 - a bit like arrays in other languages. |
A GUI Display
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Looking at the second part of Listing 1, we are familiar with the view layout
command combination from the previous article. Next we introduce a backdrop
effect - in this case a colour gradient starting as black (0.0.0) in the top
left corner to a dark green (0.128.128) at the bottom right. The banner command
gives the window a nice big title. The next line is a doozy - and all it does is make a subheading in the format: "Monday 12 May 2003 at 18:00"! Let's take it apart: data_list/1 returns the first item in data_list, which is text in the format "18:12 AEST Monday 12 May 2003" but all I need is the "Monday 12 May 2003" portion. parse data_list/1 none Using parse without any text rules (none) breaks the string at each whitespace to make the block: ["18:12" "AEST" "Monday" "12" "May" "2003"] |
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skip...2 takes the result of the parse function and skips the first two
elements, resulting in the block: ["Monday" "12" "May" "2003"]
parse data_list/2 none
parses the second element of data_list (the observation time stamp), a string of
the format "12 18:00" into the block ["12" "18:00"]
second selects the second string out of ["12" "18:00"], and returns "18:00"
reform after the above processing -
reform gets passed a block like this [["Monday" "12" "May" "2003"] "at"
"18:00"], which it reduces and forms into the string "Monday 12 May 2003 at
18:00"
vh1 is a level 1 heading for video display
Phew! Sit down and have a headache pill if you need to. It gets easier from
here.
The pad command adds some space (60 pixels at the left and 20 pixels at the top)
for the widgets that follow.
The next line uses the rejoin command to link up everything in the block and
pass it as a string to
vtext, which is a text field for video display. (Note:
#"^(B0)" is the hexadecimal character code for the degree symbol.) And
return
moves the insertion point back to the left margin set by guide. This pattern
repeats until we set a new margin and put in the Done button.
Save the code in Listing 1 to file
melb_weather.r and we have an application
that we can run from our own machine (run rebol melb_weather.r or double-click
the melb_weather.r) or via the Internet as outlined on later.
Set Up a Rebsite
Remember, this is the easy bit! It should not come as a surprise that you set up
a rebsite using a REBOL script (Listing 2 - see below or
download here).
The key points to note are:
And that's all there is to it - other files and folders are added in a similar
way. The code in Listing 2 is saved as
index.r and uploaded to a public Web
server along with
melb_weather.r. To access the rebsite choose Goto on the REBOL/View
desktop and enter the URL eg.
http://member.melbpc.org.au/~tgosbell/rebsite/index.r
into the text field (Figure 3) and click Goto.
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Maybe the REBOL folks are right - it does seem easy.
Rebol Resources On the Web
More information about the REBOL/View Distributed Desktop
http://www.rebol.com/docs/desktop.html
More information on parse can be found at:
http://www.codeconscious.com/rebol/parse-tutorial.html
http://www.rebolforces.com/zine/rzine-1-06.html#sect4
and in the REBOL manual
http://www.rebol.com/docs/core23/rebolcore-15.html.
| REBOL [ Title: "Melbourne Weather Update" File: %melb_weather.r Date: "May 2003" Purpose: "Retrieve current weather observations from Bureau of Meteorology and display." ] ; first process the web page page: read http://www.bom.gov.au/products/IDV65119.shtml data_list: [] parse page [ thru "Last updated: " copy data_item to "^/" (append data_list data_item) thru "Melbourne</a></td>" thru <td nowrap align="center"> copy data_item to </td> (append data_list data_item) 10 [thru <td align="center"> copy data_item to </td> (append data_list data_item)] ] ; second display the information view layout [ backdrop effect [gradient 0x0 0.0.0 0.128.128] banner "Melbourne Weather Update" vh1 reform [skip parse data_list/1 none 2 "at" second parse data_list/2 none ] pad 60x20 vtext rejoin ["Temperature: " data_list/3 #"^(B0)" "C"] vtext rejoin ["Relative Humidity: " data_list/5 "%"] vtext rejoin ["Wind:"data_list/6 " at " data_list/8 "Knots"] vtext rejoin ["Rain since 9am: " data_list/12 "mm"] pad 180x20 button " Done " [quit] ] |
| Listing 1. |
| REBOL [type: 'index] summary { text-color 255.255.255 128.128.128 |
| Listing 2 |
About The Author
Readers will be relieved to learn that Trevor now occupies an office closer to
ground level, where the ankles of passing pedestrians are dimly visible through
a small air vent.
Reprinted from the July 2003 issue of PC Update, the magazine of Melbourne PC User Group, Australia
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