1# Simple Chord for Tcl
2#
3# A "chord" is a method with more than one entrypoint and only one body, such
4# that the body runs only once all the entrypoints have been called by
5# different asynchronous tasks. In this implementation, the chord is defined
6# dynamically for each invocation. A SimpleChord object is created, supplying
7# body script to be run when the chord is completed, and then one or more notes
8# are added to the chord. Each note can be called like a proc, and returns
9# immediately if the chord isn't yet complete. When the last remaining note is
10# called, the body runs before the note returns.
11#
12# The SimpleChord class has a constructor that takes the body script, and a
13# method add_note that returns a note object. Since the body script does not
14# run in the context of the procedure that defined it, a mechanism is provided
15# for injecting variables into the chord for use by the body script. The
16# activation of a note is idempotent; multiple calls have the same effect as
17# a simple call.
18#
19# If you are invoking asynchronous operations with chord notes as completion
20# callbacks, and there is a possibility that earlier operations could complete
21# before later ones are started, it is a good practice to create a "common"
22# note on the chord that prevents it from being complete until you're certain
23# you've added all the notes you need.
24#
25# Example:
26#
27#   # Turn off the UI while running a couple of async operations.
28#   lock_ui
29#
30#   set chord [SimpleChord::new {
31#     unlock_ui
32#     # Note: $notice here is not referenced in the calling scope
33#     if {$notice} { info_popup $notice }
34#   }
35#
36#   # Configure a note to keep the chord from completing until
37#   # all operations have been initiated.
38#   set common_note [$chord add_note]
39#
40#   # Activate notes in 'after' callbacks to other operations
41#   set newnote [$chord add_note]
42#   async_operation $args [list $newnote activate]
43#
44#   # Communicate with the chord body
45#   if {$condition} {
46#     # This sets $notice in the same context that the chord body runs in.
47#     $chord eval { set notice "Something interesting" }
48#   }
49#
50#   # Activate the common note, making the chord eligible to complete
51#   $common_note activate
52#
53# At this point, the chord will complete at some unknown point in the future.
54# The common note might have been the first note activated, or the async
55# operations might have completed synchronously and the common note is the
56# last one, completing the chord before this code finishes, or anything in
57# between. The purpose of the chord is to not have to worry about the order.
58
59# SimpleChord class:
60#   Represents a procedure that conceptually has multiple entrypoints that must
61#   all be called before the procedure executes. Each entrypoint is called a
62#   "note". The chord is only "completed" when all the notes are "activated".
63class SimpleChord {
64	field notes
65	field body
66	field is_completed
67	field eval_ns
68
69	# Constructor:
70	#   set chord [SimpleChord::new {body}]
71	#     Creates a new chord object with the specified body script. The
72	#     body script is evaluated at most once, when a note is activated
73	#     and the chord has no other non-activated notes.
74	constructor new {i_body} {
75		set notes [list]
76		set body $i_body
77		set is_completed 0
78		set eval_ns "[namespace qualifiers $this]::eval"
79		return $this
80	}
81
82	# Method:
83	#   $chord eval {script}
84	#     Runs the specified script in the same context (namespace) in which
85	#     the chord body will be evaluated. This can be used to set variable
86	#     values for the chord body to use.
87	method eval {script} {
88		namespace eval $eval_ns $script
89	}
90
91	# Method:
92	#   set note [$chord add_note]
93	#     Adds a new note to the chord, an instance of ChordNote. Raises an
94	#     error if the chord is already completed, otherwise the chord is
95	#     updated so that the new note must also be activated before the
96	#     body is evaluated.
97	method add_note {} {
98		if {$is_completed} { error "Cannot add a note to a completed chord" }
99
100		set note [ChordNote::new $this]
101
102		lappend notes $note
103
104		return $note
105	}
106
107	# This method is for internal use only and is intentionally undocumented.
108	method notify_note_activation {} {
109		if {!$is_completed} {
110			foreach note $notes {
111				if {![$note is_activated]} { return }
112			}
113
114			set is_completed 1
115
116			namespace eval $eval_ns $body
117			delete_this
118		}
119	}
120}
121
122# ChordNote class:
123#   Represents a note within a chord, providing a way to activate it. When the
124#   final note of the chord is activated (this can be any note in the chord,
125#   with all other notes already previously activated in any order), the chord's
126#   body is evaluated.
127class ChordNote {
128	field chord
129	field is_activated
130
131	# Constructor:
132	#   Instances of ChordNote are created internally by calling add_note on
133	#   SimpleChord objects.
134	constructor new {c} {
135		set chord $c
136		set is_activated 0
137		return $this
138	}
139
140	# Method:
141	#   [$note is_activated]
142	#     Returns true if this note has already been activated.
143	method is_activated {} {
144		return $is_activated
145	}
146
147	# Method:
148	#   $note activate
149	#     Activates the note, if it has not already been activated, and
150	#     completes the chord if there are no other notes awaiting
151	#     activation. Subsequent calls will have no further effect.
152	method activate {} {
153		if {!$is_activated} {
154			set is_activated 1
155			$chord notify_note_activation
156		}
157	}
158}
159