Perl Catalyst - Obtaining JSON data from AJAX POST request

Question 1

My other answer is (hopefully) useful information for anyone debugging Catalyst/JSON issues. But subsequent updates to the question have shown the problem here is actually something entirely different.

There are not enough actions to support the required functionality here. If the index action forwards to the homescreen action which renders the homescreen.html template, then the $.json() call has to be to some other action which is responsible for handling the data save request and nothing else.

package MyProject::Controller::Homescreen;

use strict;
use warnings;
use parent 'Catalyst::Controller';
use JSON;

sub homescreen :Path('/homescreen') :Args(0)  {

    my ( $self, $c ) = @_;
    $c->stash({title => 'Home Screen',
               pagetype => 'html',
               template => 'homescreen.html'
    });
}

sub savecanvasstates :Local {
    my ($self, $c, @args) = @_;
    my $p = $c->req->param->{stateForUserNumber7} || die "wrong parameters!";
    my $result = {};
    foreach my $r (@{$p}) {
        # ... do something with $r->{x} etc
        $result->{output} = # construct whatever object the ajax caller expects
    }
    $c->res->body(to_json($result))
}

1;

And in some nearby JavaScript:

$("#saveCanvasStates").click(function () {
    // button to save canvas states to a database

    // Serialize the states array
    var JsonStringForTransport = JSON.stringify({stateForUserNumber7: states});

    // POST the JSON to the server
    var thePost = $.ajax({
        url: 'savecanvasstates',
        type: 'POST',
        dataType: 'json',
        data: JsonStringForTransport,
        contentType: 'application/json; charset=utf-8',
        success: function(data, textStatus){ ... }
        error: function(XMLHttpRequest, textStatus, errorThrown){ ... }
    });
});

I hope all that makes what you have to do clearer.

Question 2

Although the built in logging stuff is great when you are making logs to persist and to do analysts on, I find it not terribly using for basic debugging. I've lately been using Devel::Dwarn which is an easy install. This is like a warn dumper + rolled up very neatly.

If you start your app like so

perl -MDevel::Dwarn ...

You can stick Dwarn $ref and get a nice stderr output:

Dwarn $c->req->body_data

Should give you a neat review. Plus if you drop the -MDevel::Dwarn you'll get errors in all the places you accidentally left Dwarn out (something I should have done with the borked release of Runner 002 last month).

If that is confusing to you just try:

(after cpanm Devel::Dwarn)

use Devel::Dwarn;
Dwarn $c->req->body_data

You should get a deep structure output to your console. Best of luck!

Question 3

Your Catalyst app won't receive JSON. The dataType argument tells $.ajax() how to parse the data returned from the server. What's sent to the server is a bog-standard GET or POST request with HTML form parameters.

By the time Catalyst receives them, they're plain old keys inside $c->req->params, perfectly suitable for whatever database actions you want to whip up.

So if

/* JS */
JsonStringForTransport = { foo: "bar", baz: "quux" };

then at the Catalyst end, you'll receive:

# perl
my $foo = $c->req->params->{foo}; # or $c->req->body_params->{foo}
if ($c->req->params->{baz} eq "quux"){ ... }

... and so on.

UPDATE

So body_data contains a hash (unsurprisingly), which has a key stateForUserNumber7 that contains a reference to an array of hashes. That's what that Dumper output is telling you.

You should be able to access body_data using the following:

my $first_x = $c->req->body_data->{stateForUserNumber7}->[0]->{x};
print STDERR "first_x = $first_x\n"; # should print first_x = 11

However, it's more likely you'll want to do something like:

foreach my $data (@{$c->req->body_data->{stateForUserNumber7}}) {
    # do something with $data->{x}, $data->{y}, $data->{height} etc
}

UPDATE

jnap has published a very good article on how to use body_data in the 2013 Catalyst Advent Calendar. If you are using this feature in your app instead of the more traditional params, you should definitely take the time to read and absorb it.

UPDATE

Sorry, misspoke earlier regarding dataType, so I've corrected that. But it doesn't change the fundamentals of my answer.

I think you're going to have to introspect your body_data to figure out what's going wrong. Is it always 'stateForUserNumber7', or does that vary from request to request? What does

$c->log->debug("body_params keys: " _ join(", ", keys %{ $c->req->body_data }));

produce?

You have to use the development server (i.e. bin/myapp_server.pl) so that you get the full debug output to assist you. Debugging via Apache or FastCGI is not recommended at all.

Perl Catalyst - Obtaining JSON data from AJAX POST request

Update (relating to RET's answer)

Further update