NAME

App::Info - Information about software packages on a system

SYNOPSIS

use App::Info::Category::FooApp;

my $app = App::Info::Category::FooApp->new;

if ($app->installed) {
    print "App name: ", $app->name, "\n";
    print "Version:  ", $app->version, "\n";
    print "Bin dir:  ", $app->bin_dir, "\n";
} else {
    print "App not installed on your system. :-(\n";
}

DESCRIPTION

App::Info is an abstract base class designed to provide a generalized interface for subclasses that provide metadata about software packages installed on a system. The idea is that these classes can be used in Perl application installers in order to determine whether software dependencies have been fulfilled, and to get necessary metadata about those software packages.

App::Info provides an event model for handling events triggered by App::Info subclasses. The events are classified as "info", "error", "unknown", and "confirm" events, and multiple handlers may be specified to handle any or all of these event types. This allows App::Info clients to flexibly handle events in any way they deem necessary. Implementing new event handlers is straight-forward, and use the triggering of events by App::Info subclasses is likewise kept easy-to-use.

A few sample subclasses are provided with the distribution, but others are invited to write their own subclasses and contribute them to the CPAN. Contributors are welcome to extend their subclasses to provide more information relevant to the application for which data is to be provided (see App::Info::HTTPD::Apache for an example), but are encouraged to, at a minimum, implement the abstract methods defined here and in the category abstract base classes (e.g., App::Info::HTTPD and App::Info::Lib). See Subclassing for more information on implementing new subclasses.

INTERFACE

This section documents the public interface of App::Info.

Constructor

new

my $app = App::Info::Category::FooApp->new(@params);

Constructs an App::Info object and returns it. The @params arguments define how the App::Info object will respond to certain events, and correspond to their like-named methods. See the "Event Handler Object Methods" section for more information on App::Info events and how to handle them. The parameters to new() for the different types of App::Info events are:

on_info
on_error
on_unknown
on_confirm

When passing event handlers to new(), the list of handlers for each type should be an anonymous array, for example:

my $app = App::Info::Category::FooApp->new( on_info => \@handlers );

Metadata Object Methods

These are abstract methods in App::Info and must be provided by its subclasses. They provide the essential metadata of the software package supported by the App::Info subclass.

key_name

my $key_name = $app->key_name;

Returns a string that uniquely identifies the software for which the App::Info subclass provides data. This value should be unique across all App::Info classes. Typically, it's simply the name of the software.

installed

if ($app->installed) {
    print "App is installed.\n"
} else {
    print "App is not installed.\n"
}

Returns a true value if the application is installed, and a false value if it is not.

name

my $name = $app->name;

Returns the name of the application.

version

my $version = $app->version;

Returns the full version number of the application.

major_version

my $major_version = $app->major_version;

Returns the major version number of the application. For example, if version() returns "7.1.2", then this method returns "7".

minor_version

my $minor_version = $app->minor_version;

Returns the minor version number of the application. For example, if version() returns "7.1.2", then this method returns "1".

patch_version

my $patch_version = $app->patch_version;

Returns the patch version number of the application. For example, if version() returns "7.1.2", then this method returns "2".

bin_dir

my $bin_dir = $app->bin_dir;

Returns the full path the application's bin directory, if it exists.

inc_dir

my $inc_dir = $app->inc_dir;

Returns the full path the application's include directory, if it exists.

lib_dir

my $lib_dir = $app->lib_dir;

Returns the full path the application's lib directory, if it exists.

so_lib_dir

my $so_lib_dir = $app->so_lib_dir;

Returns the full path the application's shared library directory, if it exists.

home_url

my $home_url = $app->home_url;

The URL for the software's home page.

download_url

my $download_url = $app->download_url;

The URL for the software's download page.

Event Handler Object Methods

These methods provide control over App::Info event handling. Events can be handled by one or more objects of subclasses of App::Info::Handler. The first to return a true value will be the last to execute. This approach allows handlers to be stacked, and makes it relatively easy to create new handlers. App::Info::Handler for information on writing event handlers.

Each of the event handler methods takes a list of event handlers as its arguments. If none are passed, the existing list of handlers for the relevant event type will be returned. If new handlers are passed in, they will be returned.

The event handlers may be specified as one or more objects of the App::Info::Handler class or subclasses, as one or more strings that tell App::Info construct such handlers itself, or a combination of the two. The strings can only be used if the relevant App::Info::Handler subclasses have registered strings with App::Info. For example, the App::Info::Handler::Print class included in the App::Info distribution registers the strings "stderr" and "stdout" when it starts up. These strings may then be used to tell App::Info to construct App::Info::Handler::Print objects that print to STDERR or to STDOUT, respectively. See the App::Info::Handler subclasses for what strings they register with App::Info.

on_info

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

Info events are triggered when the App::Info subclass wants to send an informational status message. By default, these events are ignored, but a common need is for such messages to simply print to STDOUT. Use the App::Info::Handler::Print class included with the App::Info distribution to have info messages print to STDOUT:

use App::Info::Handler::Print;
$app->on_info('stdout');
# Or:
my $stdout_handler = App::Info::Handler::Print->new('stdout');
$app->on_info($stdout_handler);

on_error

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

Error events are triggered when the App::Info subclass runs into an unexpected but not fatal problem. (Note that fatal problems will likely throw an exception.) By default, these events are ignored. A common way of handling these events is to print them to STDERR, once again using the App::Info::Handler::Print class included with the App::Info distribution:

use App::Info::Handler::Print;
my $app->on_error('stderr');
# Or:
my $stderr_handler = App::Info::Handler::Print->new('stderr');
$app->on_error($stderr_handler);

Another approach might be to turn such events into fatal exceptions. Use the included App::Info::Handler::Carp class for this purpose:

use App::Info::Handler::Carp;
my $app->on_error('croak');
# Or:
my $croaker = App::Info::Handler::Carp->new('croak');
$app->on_error($croaker);

on_unknown

my @handlers = $app->on_unknown;
$app->on_uknown(@handlers);

Unknown events are trigged when the App::Info subclass cannot find the value to be returned by a method call. By default, these events are ignored. A common way of handling them is to have the application prompt the user for the relevant data. The App::Info::Handler::Prompt class included with the App::Info distribution can do just that:

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

See App::Info::Handler::Prompt for information on how it works.

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);

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 metadata 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.

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 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.

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 File::Spec in order to offer easy access to commonly-used methods from that class, e.g., 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 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.

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 caonnot 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 "Event Handler Object Methods" section.

info

$self->info(@message);

Use this method to display status messages for the user. You may wish to use it to inform users that you're searching for a particular file, or attempting to parse a file or some other resource for the data you need. For example, a common use might be in the object constructor: generally, when an App::Info object is created, some important initial piece of information is being sought, such as an executable file. That file may be in one of many locations, so it makes sense to let the user know that you're looking for it:

$self->info("Searching for executable");

Note that, due to the nature of App::Info event handlers, your informational message may be used or displayed any number of ways, or indeed not at all (as is the default behavior).

The @message will be joined into a single string and stored in the message attribute of the App::Info::Request object passed to info event handlers.

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 @erorr will be joined into a single string and stored in the 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." :-)

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 unknown(). The parameters are as follows:

key: The 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 unknown() as was returned by the first call, and no handlers will be activated. Typical values are "version" and "lib_dir".
prompt: The 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' key_name() method and the key parameter. The result would be something like "Enter a valid FooApp version". The prompt parameter value will be stored in the message attribute of the App::Info::Request object passed to event handlers.
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 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 $_ .This makes it easy to validate using functions or operators that, er, operate on $_ by default, but still allows you to get more information from @_ if necessary. For the directory example, a good callback might be sub { -d }. The callback parameter code reference will be stored in the callback attribute of the App::Info::Request object passed to event handlers.
error: The error parameter is the error message to display in the event that the 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 callback identified as invalid, a message to display might be something like "Invalid directory path". Note that if the error parameter is not provided, App::Info will supply the generic error message "Invalid value". This value will be stored in the error attribute of the App::Info::Request object passed to event handlers.

This may be the event method you use most, as it should be called in every metadata 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");

confirm

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

This method is very similar to unknown(), but serves a different purpose. Use this method for significant data points where you've found an appropriate value, but want to ensure it's really the correct value. A "significant data point" is usually a value essential for your class to collect metadata values. For example, you might need to locate an executable that you can then call to collect other data. In general, this will only happen once for an object -- during object construction -- but there may be cases in which it is needed more than that. But hopefully, once you've confirmed in the constructor that you've found what you need, you can use that information to collect the data needed by all of the metadata methods and can assume that they'll be right because that first, significant data point has been confirmed.

Other than where and how often to call confirm(), its use is quite similar to that of unknown(). Its parameters are as follows:

key: Same as for unknown(), a string that uniquely identifies the data point in your class, and ensures that the event is handled only once for a given key. The same value will be returned by subsequent calls to confirm() as was returned by the first call for a given key.
prompt: Same as for unknown(). Although confirm() is called to confirm a value, typically the prompt should request the relevant value, just as for unknown(). The difference is that the handler should use the value parameter as the default should the user not provide a value. The prompt parameter will be stored in the message attribute of the App::Info::Request object passed to event handlers.
value: The value to be confirmed. This is the value you've found, and it will be provided to the user as the default option when they're prompted for a new value. This value will be stored in the value attribute of the App::Info::Request object passed to event handlers.
callback: Same as for unknown(). Because the user can enter data to replace the default value provided via the value parameter, you might want to validate it. Use this code reference to do so. The callback will be stored in the callback attribute of the App::Info::Request object passed to event handlers.
error: Same as for unknown(): an error message to display in the event that a value entered by the user isn't validated by the callback code reference. This value will be stored in the error attribute of the App::Info::Request object passed to event handlers.

Here's an example usage demonstrating all of the above arguments:

my $exe = $self->confirm( key      => 'shell',
                          prompt   => 'Path to your shell?',
                          value    => '/bin/sh',
                          callback => sub { -x },
                          error    => 'Not an executable');

Event Examples

Below I provide some examples demonstrating the use of the event methods. These are meant to emphasize the contexts in which it's appropriate to use them.

Let's start with the simplest, first. Let's say that to find the version number for an application, you need to search a file for the relevant data. Your App::Info concrete subclass might have a private method that handles this work, and this method is the appropriate place to use the info() and, if necessary, error() methods.

sub _find_version {
    my $self = shift;

    # Try to find the revelant file. We cover this method below.
    # Just return if we cant' find it.
    my $file = $self->_find_file('version.conf') or return;

    # 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 info() method to display a status message to let the user know what we're doing. Then we used the 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 _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 info() method call. But rather than call the 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 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 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 _find_version() method above simply returns if _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 error() and when to use 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 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 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 metadata object methods. And of course, 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 _find_version() would be called by the version() method, which is a metadata 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 metadata methods should make use of the unknown() method. The 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 version() will return the same value that was returned the first time it was called. Of course, thanks to the key parameter in the call to unknown(), we could have have tried to enumerate the version number every time, as unknown() will return the same value every time it is called (as, indeed, should _find_version(). But by checking for the version key in $self ourselves, we save some of the overhead.

But as I said before, every metadata method should make use of the unknown() method. Thus, the 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};
}

Finally, the confirm() method should be used to verify core pieces of data that significant numbers of other methods rely on. Typically such data are executables or configuration files from which will be drawn other metadata. Most often, such major data points will be sought in the object constructor. Here's an example:

sub new {
    # Construct the object so that handlers will work properly.
    my $self = shift->SUPER::new(@_);

    # Try to find the executable.
    $self->info("Searching for executable");
    if (my $exe = $util->first_exe('/bin/myapp', '/usr/bin/myapp')) {
        # Confirm it.
        $self->{exe} =
          $self->confirm( key      => 'binary',
                          prompt   => 'Path to your executable?',
                          value    => $exe,
                          callback => sub { -x },
                          error    => 'Not an executable');
    } else {
        # Handle an unknown value.
        $self->{exe} =
          $self->unknown( key      => 'binary',
                          prompt   => 'Path to your executable?',
                          callback => sub { -x },
                          error    => 'Not an executable');
    }

    # We're done.
    return $self;
}

By now, most of what's going on here should be quite familiar. The use of the confirm() method is quite similar to that of unknown(). Really the only difference is that the value is known, but we need verification or a new value supplied if the value we found isn't correct. Such may be the case when multiple copies of the executable have been installed on the system, we found /bin/myapp, but the user may really be interested in /usr/bin/myapp. Thus the confirm() event gives the user the chance to change the value if the confirm event is handled.

The final thing to note about this constructor is the first line:

my $self = shift->SUPER::new(@_);

The first thing an App::Info subclass should do is execute this line to allow the super class to construct the object first. Doing so allows any event handling arguments to set up the event handlers, so that when we call confirm() or unknown() the event will be handled as the client expects.

If we needed our subclass constructor to take its own parameter argumente, the approach is to specify the same key = $arg> syntax as is used by App::Info's new() method. Say we wanted to allow clients of our App::Info subclass to pass in a list of alternate executable locations for us to search. Such an argument would most make sense as an array reference. So we specify that the key be alt_paths and allow the user to construct an object like this:

my $app = App::Info::Category::FooApp->new( alt_paths => \@paths );

This approach allows the super class constructor arguments to pass unmolested (as long as we use unique keys!):

my $app = App::Info::Category::FooApp->new( on_error  => \@handlers,
                                            alt_paths => \@paths );

Then, to retrieve these paths inside our new() constructor, all we need do is access them directly from the object:

my $self = shift->SUPER::new(@_);
my $alt_paths = $self->{alt_paths};

Subclassing Guidelines

To summarize, here are some guidelines for subclassing App::Info.

Always subclass an App::Info category subclass. This will help to keep the App::Info namespace well-organized. New categories can be added as needed.
When you create the new() constructor, always call SUPER::new(@_). This ensures that the event handling methods methods defined by the App::Info base classes (e.g., error()) will work properly.
Use a package-scoped lexical App::Info::Util object 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, and you think that others might find your solution useful, consider submitting a patch to App::Info::Util to add the functionality you need. See App::Info::Util for complete documentation of its interface.
Use the info() event triggering method to send messages to users of your subclass.
Use the error() event triggering method to alert users of unexpected conditions. Fatal errors should still be fatal; use Carp::croak() to throw exceptions for fatal errors.
Use the unknown() event triggering method when a metadata or other important value is unknown and you want to give any event handlers the chance to provide the data.
Use the confirm() event triggering method when a core piece of data is known (such as the location of an executable in the new() constructor) and you need to make sure that you have the correct information.
Be sure to implement all of the abstract methods defined by App::Info and by your category abstract base class -- even if they don't do anything. Doing so ensures that all App::Info subclasses share a common interface, and can, if necessary, be used without regard to subclass. Any method not implemented but called on an object will generate a fatal exception.

Otherwise, have fun! There are a lot of software packages for which relevant information might be collected and aggregated into an App::Info concrete subclass (witness all of the Automake macros in the world!), and folks who are knowledgeable about particular software packages or categories of software are warmly invited to contribute. As more subclasses are implemented, it will make sense, I think, to create separate distributions based on category -- or even, when necessary, on a single software package. Broader categories can then be aggregated in Bundle distributions.

But I get ahead of myself...

BUGS

Report all bugs via the CPAN Request Tracker at http://rt.cpan.org/NoAuth/Bugs.html?Dist=App-Info.

AUTHOR

David Wheeler <david@wheeler.net>

COPYRIGHT AND LICENSE

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

To install App::Info, copy and paste the appropriate command in to your terminal.

cpanm

cpanm App::Info

CPAN shell

perl -MCPAN -e shell
install App::Info

For more information on module installation, please visit the detailed CPAN module installation guide.

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