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Paul Evans
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IO::Async::MergePoint - resynchronise diverged control flow


 use IO::Async::MergePoint;

 my $merge = IO::Async::MergePoint->new(
    needs => [ "leaves", "water" ],

    on_finished => sub {
       my %items = @_;
       # Make tea using $items{leaves} and $items{water}

    on_boiled => sub { $merge->done( "water", $_[0] ) }

    on_fetched => sub { $merge->done( "leaves", $_[0] ) }


Often in program logic, multiple different steps need to be taken that are independent of each other, but their total result is needed before the next step can be taken. In synchonous code, the usual approach is to do them sequentially.

An IO::Async-based program could do this, but if each step involves some IO idle time, better overall performance can often be gained by running the steps in parallel. A IO::Async::MergePoint object can then be used to wait for all of the steps to complete, before passing the combined result of each step on to the next stage.

A merge point maintains a set of outstanding operations it is waiting on; these are arbitrary string values provided at the object's construction. Each time the done() method is called, the named item is marked as being complete. When all of the required items are so marked, the on_finished callback is invoked.

When an item is marked as complete, a value can also be provided, which would contain the results of that step. The on_finished callback is passed a hash (in list form, rather than by reference) of the collected item values.


$merge = IO::Async::MergePoint->new( %params )

This function returns a new instance of a IO::Async::MergePoint object. The %params hash takes the following keys:

needs => ARRAY

An array containing unique item names to wait on. The order of this array is not significant.

on_finished => CODE

CODE reference to the handler for when the merge point becomes ready.

The on_finished handler will be called when every key in the needs list has been notified by the done() method. It will be called as

 $on_finished->( %items )

where the %items hash will contain the item names that were waited on, and the values passed to the done() method for each one.


$merge->done( $item, $value )

This method informs the merge point that the $item is now ready, and passes it a value to store, to be passed into the on_finished callback. If this call gives the final remaining item being waited for, the on_finished callback is called within it, and the method will not return until it has completed.


Paul Evans <leonerd@leonerd.org.uk>