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tut-treeview-renderer-pango Diff - Ruby-GNOME2 Project Website

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= Gtk::CellRendererText, UTF8, and pango markup

All text used in Gtk+-2.0 widgets needs to be in UTF8 encoding, and
Gtk::CellRendererText is no exception. Text in plain ASCII is
automatically valid UTF8, but as soon as you have special characters
that do not exist in plain ASCII (usually characters that are not used
in the English language alphabet), they need to be in UTF8
encoding. There are many different character encodings that all
specify different ways to tell the computer which character is
meant. Gtk+-2.0 uses UTF8, and whenever you have text that is in a
different encoding, you need to convert it to UTF8 encoding first,
using the GLib.convert method.
# [since this doesn't set the #encoding of the resulting String
# to UTF-8, you should probably use String#encode instead.]  
If you only use text input from other Gtk+ widgets, you are on the
safe side, as they will return all text in UTF8 as well.

However, if you use 'external' sources of text input, then you must
convert that text from the text's encoding (or the user's locale) to
UTF8, or it will not be rendered correctly (either not at all, or it
will be cut off after the first invalid character). Filenames are
especially hard, because there is no indication whatsoever what
character encoding a filename is in (it might have been created when
the user was using a different locale, so filename encoding is
basically unreliable and broken).
# You may want to convert to UTF8 with fallback characters in that case.
# [g_convert_with_fallback() is not accessible from Ruby.]
You can check whether a string is
valid UTF8 with GLib.utf8_validate. You should, in this author's opinion
at least, put these checks into your code at crucial places wherever
it is not affecting performance, especially if you are an
English-speaking programmer that has little experience with
non-English locales. It will make it easier for others and yourself to
spot problems with non-English locales later on.

In addition to the "text" property, GtkCellRendererText also has a
"markup" property that takes text with pango markup as input. Pango
markup allows you to place special tags into a text string that affect
the style the text is rendered (see the pango
documentation). Basically you can achieve everything you can achieve
with the other properties also with pango markup (only that using
properties is more efficient and less messy). Pango markup has one
distinct advantage though that you cannot achieve with text cell
renderer properties: with pango markup, you can change the text style
in the middle of the text, so you could, for example, render one part
of a text string in bold print, and the rest of the text in
normal. Here is an example of a string with pango markup:

"You can have text in <b>bold</b> or in a <span color='Orange'>different color</span>"

When using the "markup" property, you need to take into account that
the "markup" and "text" properties do not seem to be mutually
exclusive (I suppose this could be called a bug). In other words:
whenever you set "markup" (and have used the "text" property before),
set the "text" property to nil, and vice versa. Example:


  ...
  column.set_cell_data_func(renderer) { | ... |
    if (foo.important?)
      renderer.set_property("markup", "<b>important</b>")
      renderer.set_property("text", nil)
      # or, you can simply use Gtk::CellRendererText#markup= and #text= .
    else
      renderer.set_property("markup", nil)
      renderer.set_property("text", "not important")
    end
  }
  ...

Another thing to keep in mind when using pango markup text is that you
might need to escape text if you construct strings with pango markup
on the fly using random input data. For example:

  ...
  column.set_cell_data_func(renderer) do | ... |
    ...
    # This might be problematic if artist_string or title_string
    # contain markup characters/entities:
    markuptxt = sprintf("<b>%s</b> - <i>%s</i>",
                        artist_string, title_string)
    ...
    renderer.markup = markuptxt
    renderer.text = nil
    ...
  end
  ...


The above example will not work if artist_string is "Simon &
Garfunkel" for example, because the & character is one of the
characters that is special. They need to be escaped, so that pango
knows that they do not refer to any pango markup, but are just
characters. In this case the string would need to be "Simon &amp;
Garfunkel" in order to make sense in between the pango markup in which
it is going to be pasted.
# You can escape a string with g_markup_escape
# [which is again not accessible. CGI.escapeHTML should achieve similar results, though].
# (and you will need to free the resulting newly-allocated string again
# with g_free).

It is possible to combine both pango markup and text cell renderer
properties. Both will be 'added' together to render the string in
question, only that the text cell renderer properties will be applied
to the whole string. If you set the "markup" property to normal text
without any pango markup, it will render as normal text just as if you
had used the "text" property. However, as opposed to the "text"
property, special characters in the "markup" property text would still
need to be escaped, even if you do not use pango markup in the text.