Kontinuität - And transforming XML into a UI.

Das frohe neue Jahr hat fahrt aufgenommen - es beginnt fröhlich stürmisch mit frischem Wind.

During the holidays I had some time and looked into xsl transforms again.
This time I needed a nice way to build a simple UI for a specific XML document.
My initial intention was to use a visitor pattern to transform the XML document into a corresponding Gtk UI, but then my eyes catched GtkBuilder.
GtkBuilder builds a UI from an XML definition.
And there it is, an ideal case for xslt: XSL ransform allow you to transform an XML document into some other plain-text document, this includes into other XML or - quite common - XHTML documents.

The simple document looked something like this:

<document id="document" title="About something" author="Jane Doe">
  <text id="intro">Hello World.</text>
  <text id="more">This is something like <i>xml-to-gtk</i></text>
</document>

Using this xsl stylesheet and a bit of glue, resulted in a UI looking like this:

 

Not pretty - but - hey - there are also other techniques that take some markup to built UIs, just think of clutter ...

Another approach to modfiy structured files (like config or XML files)

Have you ever wondered if there is a more convenient way to modify xml documents, beside libxml2? Sure, there is the nice dom within Firefox, but outside of that?
Well, there is augeas. AFAIK initially a project to standardize the interface to modify configuration files, in other words a common syntax for different formats.
Files are parsed trough so called lenses. Those lenses transform the actual file (e.g. httpd.conf or passwd) into a tree, which than can be modified by a set of augeas commands.
As lenses are bi-directional the same lense can be used to read a file, and to dump the (internal augeas) tree back into a valid configuration file. A nice concept, ey?

A back to the topic, one of the lenses - Xml.lns - can be used to modify XML files.
The following example creates a dummy xml file and modifies it using augtool.

$ sudo yum install augeas tidyp

$ cat > abc.xml <<EOF
<world poo="bar">
<child>Hi there.</child>
</world>
EOF

$ augtool -LA -e <<EOF
# Load XML lense
set /augeas/load/xml/lens "Xml.lns"
set /augeas/load/xml/incl "$PWD/abc.xml"
load

# Dump internal tree
print /files

# Add a note
set /files/$PWD/abc.xml/world/note/#attribute/timestamp "now"
set /files/$PWD/abc.xml/world/note/#text "Wtf"

# Change the text of a node.
set /files/$PWD/abc.xml/world/child/#text "Hello World."

# Insert a new child after the present child
ins child after /files/$PWD/abc.xml/world/child
set /files/$PWD/abc.xml/world/child[2]/#text "Hello Mars."

# Insert a new child - anywhere ..
set /files/$PWD/abc.xml/world/child[3]/#text "Hello Jupiter Jones."

# More children
set /files/$PWD/abc.xml/world/child[4]/child/child/#text "Hello Mars."

print /files/$PWD/abc.xml

save
print /augeas//error
EOF

$ tidyp -xml -i -q < abc.xml

$ unlink abc.xml

With the help from the people at #augeas on freenode, I found out that (a) there is a small bug preventing the usage of relative paths and (b) You need a little knowledge about the format you are writin to. In the (xml) example above, you need to specify attributes before specifying a #text node, because they are written first. If they were specified the other way round, augeas would fail while saving.