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-# 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
-		}
-	}
-}