# Simple Chord for Tcl
#
# A "chord" is a method with more than one entrypoint and only one body, such
# that the body runs only once all the entrypoints have been called by
# different asynchronous tasks. In this implementation, the chord is defined
# dynamically for each invocation. A SimpleChord object is created, supplying
# body script to be run when the chord is completed, and then one or more notes
# are added to the chord. Each note can be called like a proc, and returns
# immediately if the chord isn't yet complete. When the last remaining note is
# called, the body runs before the note returns.
#
# The SimpleChord class has a constructor that takes the body script, and a
# method add_note that returns a note object. Since the body script does not
# run in the context of the procedure that defined it, a mechanism is provided
# for injecting variables into the chord for use by the body script. The
# activation of a note is idempotent; multiple calls have the same effect as
# a simple call.
#
# If you are invoking asynchronous operations with chord notes as completion
# callbacks, and there is a possibility that earlier operations could complete
# before later ones are started, it is a good practice to create a "common"
# note on the chord that prevents it from being complete until you're certain
# you've added all the notes you need.
#
# Example:
#
# # Turn off the UI while running a couple of async operations.
# lock_ui
#
# set chord [SimpleChord::new {
# unlock_ui
# # Note: $notice here is not referenced in the calling scope
# if {$notice} { info_popup $notice }
# }
#
# # Configure a note to keep the chord from completing until
# # all operations have been initiated.
# set common_note [$chord add_note]
#
# # Activate notes in 'after' callbacks to other operations
# set newnote [$chord add_note]
# async_operation $args [list $newnote activate]
#
# # Communicate with the chord body
# if {$condition} {
# # This sets $notice in the same context that the chord body runs in.
# $chord eval { set notice "Something interesting" }
# }
#
# # Activate the common note, making the chord eligible to complete
# $common_note activate
#
# At this point, the chord will complete at some unknown point in the future.
# The common note might have been the first note activated, or the async
# operations might have completed synchronously and the common note is the
# last one, completing the chord before this code finishes, or anything in
# between. The purpose of the chord is to not have to worry about the order.
# SimpleChord class:
# Represents a procedure that conceptually has multiple entrypoints that must
# all be called before the procedure executes. Each entrypoint is called a
# "note". The chord is only "completed" when all the notes are "activated".
class SimpleChord {
field notes
field body
field is_completed
field eval_ns
# Constructor:
# set chord [SimpleChord::new {body}]
# Creates a new chord object with the specified body script. The
# body script is evaluated at most once, when a note is activated
# and the chord has no other non-activated notes.
constructor new {i_body} {
set notes [list]
set body $i_body
set is_completed 0
set eval_ns "[namespace qualifiers $this]::eval"
return $this
}
# Method:
# $chord eval {script}
# Runs the specified script in the same context (namespace) in which
# the chord body will be evaluated. This can be used to set variable
# values for the chord body to use.
method eval {script} {
namespace eval $eval_ns $script
}
# Method:
# set note [$chord add_note]
# Adds a new note to the chord, an instance of ChordNote. Raises an
# error if the chord is already completed, otherwise the chord is
# updated so that the new note must also be activated before the
# body is evaluated.
method add_note {} {
if {$is_completed} { error "Cannot add a note to a completed chord" }
set note [ChordNote::new $this]
lappend notes $note
return $note
}
# This method is for internal use only and is intentionally undocumented.
method notify_note_activation {} {
if {!$is_completed} {
foreach note $notes {
if {![$note is_activated]} { return }
}
set is_completed 1
namespace eval $eval_ns $body
delete_this
}
}
}
# ChordNote class:
# Represents a note within a chord, providing a way to activate it. When the
# final note of the chord is activated (this can be any note in the chord,
# with all other notes already previously activated in any order), the chord's
# body is evaluated.
class ChordNote {
field chord
field is_activated
# Constructor:
# Instances of ChordNote are created internally by calling add_note on
# SimpleChord objects.
constructor new {c} {
set chord $c
set is_activated 0
return $this
}
# Method:
# [$note is_activated]
# Returns true if this note has already been activated.
method is_activated {} {
return $is_activated
}
# Method:
# $note activate
# Activates the note, if it has not already been activated, and
# completes the chord if there are no other notes awaiting
# activation. Subsequent calls will have no further effect.
method activate {} {
if {!$is_activated} {
set is_activated 1
$chord notify_note_activation
}
}
}
