1Async::MergePoint(3)  User Contributed Perl Documentation Async::MergePoint(3)
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NAME

6       "Async::MergePoint" - resynchronise diverged control flow
7

SYNOPSIS

9        use Async::MergePoint;
10
11        my $merge = Async::MergePoint->new(
12           needs => [ "leaves", "water" ],
13        );
14
15        my $water;
16        Kettle->boil(
17           on_boiled => sub { $water = shift; $merge->done( "water" ); }
18        );
19
20        my $tea_leaves;
21        Cupboard->get_tea_leaves(
22           on_fetched => sub { $tea_leaves = shift; $merge->done( "leaves" ); }
23        );
24
25        $merge->close(
26           on_finished => sub {
27              # Make tea using $water and $tea_leaves
28           }
29        );
30

DESCRIPTION

32       Often in program logic, multiple different steps need to be taken that
33       are independent of each other, but their total result is needed before
34       the next step can be taken. In synchonous code, the usual approach is
35       to do them sequentially.
36
37       An asynchronous or event-based program could do this, but if each step
38       involves some IO idle time, better overall performance can often be
39       gained by running the steps in parallel. A "Async::MergePoint" object
40       can then be used to wait for all of the steps to complete, before
41       passing the combined result of each step on to the next stage.
42
43       A merge point maintains a set of outstanding operations it is waiting
44       on; these are arbitrary string values provided at the object's
45       construction. Each time the "done()" method is called, the named item
46       is marked as being complete. When all of the required items are so
47       marked, the "on_finished" continuation is invoked.
48
49       For use cases where code may be split across several different lexical
50       scopes, it may not be convenient or possible to share a lexical
51       variable, to pass on the result of some asynchronous operation. In
52       these cases, when an item is marked as complete a value can also be
53       provided which contains the results of that step. The "on_finished"
54       callback is passed a hash (in list form, rather than by reference) of
55       the collected item values.
56
57       This module was originally part of the IO::Async distribution, but was
58       removed under the inspiration of Pedro Melo's Async::Hooks
59       distribution, because it doesn't itself contain anything IO-specific.
60

CONSTRUCTOR

62   $merge = Async::MergePoint->new( %params )
63       This function returns a new instance of a "Async::MergePoint" object.
64       The %params hash takes the following keys:
65
66       needs => ARRAY
67               Optional. An array containing unique item names to wait on. The
68               order of this array is not significant.
69
70       on_finished => CODE
71               Optional. CODE reference to the continuation for when the merge
72               point becomes ready. If provided, will be passed to the "close"
73               method.
74

METHODS

76   $merge->close( %params )
77       Allows an "on_finished" continuation to be set if one was not provided
78       to the constructor.
79
80       on_finished => CODE
81               CODE reference to the continuation for when the merge point
82               becomes ready.
83
84       The "on_finished" continuation will be called when every key in the
85       "needs" list has been notified by the "done()" method. It will be
86       called as
87
88        $on_finished->( %items )
89
90       where the %items hash will contain the item names that were waited on,
91       and the values passed to the "done()" method for each one. Note that
92       this is passed as a list, not as a HASH reference.
93
94       While this feature can be used to pass data from the component parts
95       back up into the continuation, it may be more direct to use normal
96       lexical variables instead. This method allows the continuation to be
97       placed after the blocks of code that execute the component parts, so it
98       reads downwards, and may make it more readable.
99
100   $merge->needs( @keys )
101       When called on an open MergePoint (i.e. one that does not yet have an
102       "on_finished" continuation), this method adds extra key names to the
103       set of outstanding names. The order of this list is not significant.
104
105       This method throws an exception if the MergePoint is already closed.
106
107   $merge->done( $item, $value )
108       This method informs the merge point that the $item is now ready, and
109       passes it a value to store, to be passed into the "on_finished"
110       continuation.  If this call gives the final remaining item being waited
111       for, the "on_finished" continuation is called within it, and the method
112       will not return until it has completed.
113

EXAMPLES

115   Asynchronous Plugins
116       Consider a program using "Module::Pluggable" to provide a plugin
117       architecture to respond to events, where sometimes the response to an
118       event may require asynchronous work. A "MergePoint" object can be used
119       to coordinate the responses from the plugins to this event.
120
121        my $merge = Async::MergePoint->new();
122
123        foreach my $plugin ( $self->plugins ) {
124           $plugin->handle_event( "event", $merge, @args );
125        }
126
127        $merge->close( on_finished => sub {
128           my %results = @_;
129           print "All plugins have recognised $event\n";
130        } );
131
132       Each plugin that wishes to handle the event can use its own package
133       name, for example, as its unique key name for the MergePoint. A plugin
134       handling the event synchonously could perform something such as:
135
136        sub handle_event
137        {
138           my ( $event, $merge, @args ) = @_;
139           ....
140           $merge->needs( __PACKAGE__ );
141           $merge->done( __PACKAGE__ => $result );
142        }
143
144       Whereas, to handle the event asynchronously the plugin can instead
145       perform:
146
147        sub handle_event
148        {
149           my ( $event, $merge, @args ) = @_;
150           ....
151           $merge->needs( __PACKAGE__ );
152
153           sometime_later( sub {
154              $merge->done( __PACKAGE__ => $result );
155           } );
156        }
157

AUTHOR

159       Paul Evans <leonerd@leonerd.org.uk>
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163perl v5.34.0                      2022-01-20              Async::MergePoint(3)
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