SYNOPSIS

     use Monkey::Patch::Action qw(patch_package);
    
     package Foo;
     sub sub1  { say "Foo's sub1" }
     sub sub2  { say "Foo's sub2, args=", join(",", @_) }
     sub meth1 { my $self = shift; say "Foo's meth1" }
    
     package Bar;
     our @ISA = qw(Foo);
    
     package main;
     my $h; # handle object
     my $foo = Foo->new;
     my $bar = Bar->new;
    
     # replacing a subroutine
     $h = patch_package('Foo', 'sub1', 'replace', sub { "qux" });
     Foo::sub1(); # says "qux"
     undef $h;
     Foo::sub1(); # says "Foo's sub1"
    
     # adding a subroutine
     $h = patch_package('Foo', 'sub3', 'add', sub { "qux" });
     Foo::sub3(); # says "qux"
     undef $h;
     Foo::sub3(); # dies
    
     # deleting a subroutine
     $h = patch_package('Foo', 'sub2', 'delete');
     Foo::sub2(); # dies
     undef $h;
     Foo::sub2(); # says "Foo's sub2, args="
    
     # wrapping a subroutine
     $h = patch_package('Foo', 'sub2', 'wrap',
         sub {
             my $ctx = shift;
             say "wrapping $ctx->{package}::$ctx->{subname}";
             $ctx->{orig}->(@_);
         }
     );
     Foo::sub2(1,2,3); # says "wrapping Foo::sub2" then "Foo's sub2, args=1,2,3"
     undef $h;
     Foo::sub2(1,2,3); # says "Foo's sub2, args=1,2,3"
    
     # stacking patches (note: can actually be unapplied in random order)
     my ($h2, $h3);
     $h  = patch_package('Foo', 'sub1', 'replace', sub { "qux" });
     Foo::sub1(); # says "qux"
     $h2 = patch_package('Foo', 'sub1', 'delete');
     Foo::sub1(); # dies
     $h3 = patch_package('Foo', 'sub1', 'replace', sub { "quux" });
     Foo::sub1(); # says "quux"
     undef $h3;
     Foo::sub1(); # dies
     undef $h2;
     Foo::sub1(); # says "qux"
     undef $h;
     Foo::sub1(); # says "Foo's sub1"

DESCRIPTION

    Monkey-patching is the act of modifying a package at runtime: adding a
    subroutine/method, replacing/deleting/wrapping another, etc. Perl makes
    it easy to do that, for example:

     # add a subroutine
     *{"Target::sub1"} = sub { ... };
    
     # another way, can be done from any file
     package Target;
     sub sub2 { ... }
    
     # delete a subroutine
     undef *{"Target::sub3"};

    This module makes things even easier by helping you apply a stack of
    patches and unapply them later in flexible order.

FUNCTIONS

 patch_package($package, $subname, $action, $code, @extra) => HANDLE

    Patch $package's subroutine named $subname. $action is either:

      * wrap

      $subname must already exist. code is required.

      Your code receives a context hash as its first argument, followed by
      any arguments the subroutine would have normally gotten. Context hash
      contains: orig (the original subroutine that is being wrapped),
      subname, package, extra.

      * add

      subname must not already exist. code is required.

      * replace

      subname must already exist. code is required.

      * add_or_replace

      code is required.

      * delete

      code is not needed.

    Die on error.

    Function returns a handle object. As soon as you lose the value of the
    handle (by calling in void context, assigning over the variable,
    undeffing the variable, letting it go out of scope, etc), the patch is
    unapplied.

    Patches can be unapplied in random order, but unapplying a patch where
    the next patch is a wrapper can lead to an error. Example: first patch
    (P1) adds a subroutine and second patch (P2) wraps it. If P1 is
    unapplied before P2, the subroutine is now no longer there, and P2 no
    longer works. Unapplying P1 after P2 works, of course.

FAQ

 Differences with Monkey::Patch?

    This module is based on the wonderful Monkey::Patch by Paul Driver. The
    differences are:

      * This module adds the ability to add/replace/delete subroutines
      instead of just wrapping them.

      * Interface to patch_package() is slightly different (see previous
      item for the cause).

      * Using this module, the wrapper receives a context hash instead of
      just the original subroutine.

      * Monkey::Patch adds convenience for patching classes and objects. To
      keep things simple, no such convenience is currently provided by this
      module. patch_package() *can* patch classes and objects as well (see
      the next FAQ entry).

 How to patch classes and objects?

    Patching a class is basically the same as patching any other package,
    since Perl implements a class with a package. One thing to note is that
    to call a parent's method inside your wrapper code, instead of:

     $self->SUPER::methname(...)

    you need to do something like:

     use SUPER;
     SUPER::find_parent(ref($self), 'methname')->methname(...)

    Patching an object is also basically patching a class/package, because
    Perl does not have per-object method like Ruby. But if you just want to
    provide a modified behavior for a certain object only, you can do
    something like:

     patch_package($package, $methname, 'wrap',
     sub {
         my $ctx = shift;
         my $self = shift;
    
         my $obj = $ctx->{extra}[0];
         no warnings 'numeric';
         if ($obj == $self) {
             # do stuff
         }
         $ctx->{orig}->(@_);
     }, $obj);

SEE ALSO

    Monkey::Patch