App-Info

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lib/App/Info.pm  view on Meta::CPAN

=head3 on_confirm

  my @handlers = $app->on_confirm;
  $app->on_confirm(@handlers);

Confirm events are triggered when the App::Info subclass has found an
important piece of information (such as the location of the executable it'll
use to collect information for the rest of its methods) and wants to confirm
that the information is correct. These events will most often be triggered
during the App::Info subclass object construction. Here, too, the
App::Info::Handler::Prompt class included with the App::Info distribution can
help out:

  use App::Info::Handler::Prompt;
  my $app->on_confirm('prompt');
  # Or:
  my $prompter = App::Info::Handler::Prompt;
  $app->on_confirm($prompter);

=cut

sub on_confirm {
    my $self = shift;
    @{ $self->{on_confirm} } = $set_handlers->(\@_) if @_;
    return @{ $self->{on_confirm} };
}

##############################################################################
##############################################################################

=head1 SUBCLASSING

As an abstract base class, App::Info is not intended to be used directly.
Instead, you'll use concrete subclasses that implement the interface it
defines. These subclasses each provide the meta data necessary for a given
software package, via the interface outlined above (plus any additional
methods the class author deems sensible for a given application).

This section describes the facilities App::Info provides for subclassing. The
goal of the App::Info design has been to make subclassing straight-forward, so
that developers can focus on gathering the data they need for their
application and minimize the work necessary to handle unknown values or to
confirm values. As a result, there are essentially three concepts that
developers need to understand when subclassing App::Info: organization,
utility methods, and events.

=head2 Organization

The organizational idea behind App::Info is to name subclasses by broad
software categories. This approach allows the categories themselves to
function as abstract base classes that extend App::Info, so that they can
specify more methods for all of their base classes to implement. For example,
App::Info::HTTPD has specified the C<httpd_root()> abstract method that its
subclasses must implement. So as you get ready to implement your own subclass,
think about what category of software you're gathering information about.
New categories can be added as necessary.

=head2 Utility Methods

Once you've decided on the proper category, you can start implementing your
App::Info concrete subclass. As you do so, take advantage of App::Info::Util,
wherein I've tried to encapsulate common functionality to make subclassing
easier. I found that most of what I was doing repetitively was looking for
files and directories, and searching through files. Thus, App::Info::Util
subclasses L<File::Spec|File::Spec> in order to offer easy access to
commonly-used methods from that class, e.g., C<path()>. Plus, it has several
of its own methods to assist you in finding files and directories in lists of
files and directories, as well as methods for searching through files and
returning the values found in those files. See
L<App::Info::Util|App::Info::Util> for more information, and the App::Info
subclasses in this distribution for usage examples.

I recommend the use of a package-scoped lexical App::Info::Util object. That
way it's nice and handy when you need to carry out common tasks. If you find
you're doing something over and over that's not already addressed by an
App::Info::Util method, consider submitting a patch to App::Info::Util to add
the functionality you need.

=head2 Events

Use the methods described below to trigger events. Events are designed to
provide a simple way for App::Info subclass developers to send status messages
and errors, to confirm data values, and to request a value when the class
cannot determine a value itself. Events may optionally be handled by module
users who assign App::Info::Handler subclass objects to your App::Info
subclass object using the event handling methods described in the L<"Event
Handler Object Methods"> section.

=cut

##############################################################################
# This code reference is used by the event methods to manage the stack of
# event handlers that may be available to handle each of the events.
my $handler = sub {
    my ($self, $meth, $params) = @_;

    # Sanity check. We really want to keep control over this.
    Carp::croak("Cannot call protected method $meth()")
      unless UNIVERSAL::isa($self, scalar caller(1));

    # Create the request object.
    $params->{type} ||= $meth;
    my $req = App::Info::Request->new(%$params);

    # Do the deed. The ultimate handling handler may die.
    foreach my $eh (@{$self->{"on_$meth"}}) {
        last if $eh->handler($req);
    }

    # Return the request.
    return $req;
};

##############################################################################

=head3 info

  $self->info(@message);

Use this method to display status messages for the user. You may wish to use

lib/App/Info.pm  view on Meta::CPAN

=cut

sub info {
    my $self = shift;
    # Execute the handler sequence.
    my $req = $handler->($self, 'info', { message => join '', @_ });
}

##############################################################################

=head3 error

  $self->error(@error);

Use this method to inform the user that something unexpected has happened. An
example might be when you invoke another program to parse its output, but it's
output isn't what you expected:

  $self->error("Unable to parse version from `/bin/myapp -c`");

As with all events, keep in mind that error events may be handled in any
number of ways, or not at all.

The C<@erorr> will be joined into a single string and stored in the C<message>
attribute of the App::Info::Request object passed to error event handlers. If
that seems confusing, think of it as an "error message" rather than an "error
error." :-)

=cut

sub error {
    my $self = shift;
    # Execute the handler sequence.
    my $req = $handler->($self, 'error', { message => join '', @_ });
}

##############################################################################

=head3 unknown

  my $val = $self->unknown(@params);

Use this method when a value is unknown. This will give the user the option --
assuming the appropriate handler handles the event -- to provide the needed
data. The value entered will be returned by C<unknown()>. The parameters are
as follows:

=over 4

=item key

The C<key> parameter uniquely identifies the data point in your class, and is
used by App::Info to ensure that an unknown event is handled only once, no
matter how many times the method is called. The same value will be returned by
subsequent calls to C<unknown()> as was returned by the first call, and no
handlers will be activated. Typical values are "version" and "lib_dir".

=item prompt

The C<prompt> parameter is the prompt to be displayed should an event handler
decide to prompt for the appropriate value. Such a prompt might be something
like "Path to your httpd executable?". If this parameter is not provided,
App::Info will construct one for you using your class' C<key_name()> method
and the C<key> parameter. The result would be something like "Enter a valid
FooApp version". The C<prompt> parameter value will be stored in the
C<message> attribute of the App::Info::Request object passed to event
handlers.

=item callback

Assuming a handler has collected a value for your unknown data point, it might
make sense to validate the value. For example, if you prompt the user for a
directory location, and the user enters one, it makes sense to ensure that the
directory actually exists. The C<callback> parameter allows you to do this. It
is a code reference that takes the new value or values as its arguments, and
returns true if the value is valid, and false if it is not. For the sake of
convenience, the first argument to the callback code reference is also stored
in C<$_> .This makes it easy to validate using functions or operators that,
er, operate on C<$_> by default, but still allows you to get more information
from C<@_> if necessary. For the directory example, a good callback might be
C<sub { -d }>. The C<callback> parameter code reference will be stored in the
C<callback> attribute of the App::Info::Request object passed to event
handlers.

=item error

The error parameter is the error message to display in the event that the
C<callback> code reference returns false. This message may then be used by the
event handler to let the user know what went wrong with the data she entered.
For example, if the unknown value was a directory, and the user entered a
value that the C<callback> identified as invalid, a message to display might
be something like "Invalid directory path". Note that if the C<error>
parameter is not provided, App::Info will supply the generic error message
"Invalid value". This value will be stored in the C<error> attribute of the
App::Info::Request object passed to event handlers.

=back

This may be the event method you use most, as it should be called in every
meta data method if you cannot provide the data needed by that method. It will
typically be the last part of the method. Here's an example demonstrating each
of the above arguments:

  my $dir = $self->unknown( key      => 'lib_dir',
                            prompt   => "Enter lib directory path",
                            callback => sub { -d },
                            error    => "Not a directory");

=cut

sub unknown {
    my ($self, %params) = @_;
    my $key = $params{key}
      or Carp::croak("No key parameter passed to unknown()");
    # Just return the value if we've already handled this value. Ideally this
    # shouldn't happen.
    return $self->{__unknown__}{$key} if exists $self->{__unknown__}{$key};

    # Create a prompt and error message, if necessary.
    $params{message} = delete $params{prompt} ||
      "Enter a valid " . $self->key_name . " $key";

lib/App/Info.pm  view on Meta::CPAN

      # Send a status message.
      $self->info("Searching '$file' file for version");

      # Search the file. $util is an App::Info::Util object.
      my $ver = $util->search_file($file, qr/^Version\s+(.*)$/);

      # Trigger an error message, if necessary. We really think we'll have the
      # value, but we have to cover our butts in the unlikely event that we're
      # wrong.
      $self->error("Unable to find version in file '$file'") unless $ver;

      # Return the version number.
      return $ver;
  }

Here we've used the C<info()> method to display a status message to let the
user know what we're doing. Then we used the C<error()> method when something
unexpected happened, which in this case was that we weren't able to find the
version number in the file.

Note the C<_find_file()> method we've thrown in. This might be a method that
we call whenever we need to find a file that might be in one of a list of
directories. This method, too, will be an appropriate place for an C<info()>
method call. But rather than call the C<error()> method when the file can't be
found, you might want to give an event handler a chance to supply that value
for you. Use the C<unknown()> method for a case such as this:

  sub _find_file {
      my ($self, $file) = @_;

      # Send a status message.
      $self->info("Searching for '$file' file");

      # Look for the file. See App::Info:Utility for its interface.
      my @paths = qw(/usr/conf /etc/conf /foo/conf);
      my $found = $util->first_cat_path($file, @paths);

      # If we didn't find it, trigger an unknown event to
      # give a handler a chance to get the value.
      $found ||= $self->unknown( key      => "file_$file",
                                 prompt   => "Location of '$file' file?",
                                 callback => sub { -f },
                                 error    => "Not a file");

      # Now return the file name, regardless of whether we found it or not.
      return $found;
  }

Note how in this method, we've tried to locate the file ourselves, but if we
can't find it, we trigger an unknown event. This allows clients of our
App::Info subclass to try to establish the value themselves by having an
App::Info::Handler subclass handle the event. If a value is found by an
App::Info::Handler subclass, it will be returned by C<unknown()> and we can
continue. But we can't assume that the unknown event will even be handled, and
thus must expect that an unknown value may remain unknown. This is why the
C<_find_version()> method above simply returns if C<_find_file()> doesn't
return a file name; there's no point in searching through a file that doesn't
exist.

Attentive readers may be left to wonder how to decide when to use C<error()>
and when to use C<unknown()>. To a large extent, this decision must be based
on one's own understanding of what's most appropriate. Nevertheless, I offer
the following simple guidelines: Use C<error()> when you expect something to
work and then it just doesn't (as when a file exists and should contain the
information you seek, but then doesn't). Use C<unknown()> when you're less
sure of your processes for finding the value, and also for any of the values
that should be returned by any of the L<meta data object methods|"Metadata
Object Methods">. And of course, C<error()> would be more appropriate when you
encounter an unexpected condition and don't think that it could be handled in
any other way.

Now, more than likely, a method such C<_find_version()> would be called by the
C<version()> method, which is a meta data method mandated by the App::Info
abstract base class. This is an appropriate place to handle an unknown version
value. Indeed, every one of your meta data methods should make use of the
C<unknown()> method. The C<version()> method then should look something like
this:

  sub version {
      my $self = shift;

      unless (exists $self->{version}) {
          # Try to find the version number.
          $self->{version} = $self->_find_version ||
            $self->unknown( key    => 'version',
                            prompt => "Enter the version number");
      }

      # Now return the version number.
      return $self->{version};
  }

Note how this method only tries to find the version number once. Any
subsequent calls to C<version()> will return the same value that was returned
the first time it was called. Of course, thanks to the C<key> parameter in the
call to C<unknown()>, we could have have tried to enumerate the version number
every time, as C<unknown()> will return the same value every time it is called
(as, indeed, should C<_find_version()>. But by checking for the C<version> key
in C<$self> ourselves, we save some of the overhead.

But as I said before, every meta data method should make use of the
C<unknown()> method. Thus, the C<major()> method might looks something like
this:

  sub major {
      my $self = shift;

      unless (exists $self->{major}) {
          # Try to get the major version from the full version number.
          ($self->{major}) = $self->version =~ /^(\d+)\./;
          # Handle an unknown value.
          $self->{major} = $self->unknown( key      => 'major',
                                           prompt   => "Enter major version",
                                           callback => sub { /^\d+$/ },
                                           error    => "Not a number")
            unless defined $self->{major};
      }

      return $self->{version};
  }