ce supports the notion of modes for use by Ruby extensions. Modes are similar to major modes in Emacs. A mode consists of a name (which is arbitrary) and a set of key bindings, both of which are attached to a specific buffer. By default, buffers in ce do not have modes, but a Ruby extensions can define a mode for a buffer in order to create key bindings specific to that buffer. Modes can be used to implement features for particular types of code, or to provide special types of buffers that are not treated solely as plain text.

An Example

The file dired.rb, in the ruby directory of the MicroEAMCS repository, is an example of a mode. It implements a directory browser similar to the dired mode in Emacs. It provides a new command, dired, that is globally bound to the keystroke C-X D:

ruby_command "dired"
bind "dired", ctlx('d')

It also creates three new commands but does not bind them to keystrokes immediately:

ruby_command "visitfile"
ruby_command "openfile"
ruby_command "displayfile"

The initialization of the mode happens in the dired function when you enter the keystroke C-X D. This function prompts you for a directory name, then calls showdir to open a view on the directory.

The showdir function opens a new window with a buffer called *dired*, to avoid conflict with existing buffers. It clears any existing contents of the buffer. It then runs /bin/ls -laF to load the buffer with a directory listing. The first line in the buffer contains the directory name, and each subsequent line contains information about a file in that directory, as provided by ls. If the name of a file ends in a '/' character, that file is actually a subdirectory. Finally, showdir marks the buffer as read-only.

The showdir function then performs some string matching to determine that starting column for filenames in the directory listings. Finally, it creates a mode for the directory listing and attaches three key bindings to it:

setmode "dired"
bind "visitfile", ctrl('m'), true
bind "openfile", key('o'), true
bind "displayfile", ctrl('o'), true

The bind calls create key bindings for the three new commands that were defined earlier. These bindings perform three distinct actions on the file under the cursor (called the "selected" file):

• The Enter key opens the selected file in a new window, replacing the current dired window (which still exists).

• The o key splits the screen into two windows, one containing the dired buffer, and the other containing the selected file. It then moves the cursor to the selected file.

• The C-O key is similar to the o key, except that it does not move the cursor to the selected file.

You can load the dired mode support automatically by copying the file mode.rb to ~/pe.rb.