=head1 Maypole Request Hacking Cookbook
+Hacks; design patterns; recipes: call it what you like, this chapter is a
+developing collection of techniques which can be slotted in to Maypole
+applications to solve common problems or make the development process easier.
+
+As Maypole developers, we don't necessarily know the "best practice" for
+developing Maypole applications ourselves, in the same way that Larry Wall
+didn't know all about the best Perl programming style as soon as he wrote
+Perl. These techniques are what we're using at the moment, but they may
+be refined, modularized, or rendered irrelevant over time. But they've
+certainly saved us a bunch of hours work.
+
=head2 Frontend hacks
+These hacks deal with changing the way Maypole relates to the outside world;
+alternate front-ends to the Apache and CGI interfaces, or subclassing chunks
+of the front-end modules to alter Maypole's behaviour in particular ways.
+
+=head3 Debugging with the command line
+
+You're seeing bizarre problems with Maypole output, and you want to test it in
+some place outside of the whole Apache/mod_perl/HTTP/Internet/browser circus.
+
+B<Solution>: Use the C<Maypole::CLI> module to go directly from a URL to
+standard output, bypassing Apache and the network altogether.
+
+C<Maypole::CLI> is not a standalone front-end, but to allow you to debug your
+applications without having to change the front-end they use, it temporarily
+"borgs" an application. If you run it from the command line, you're expected
+to use it like so:
+
+ perl -MMaypole::CLI=Application -e1 'http://your.server/path/table/action'
+
+For example:
+
+ perl -MMaypole::CLI=BeerDB -e1 'http://localhost/beerdb/beer/view/1?o2=desc'
+
+You can also use the C<Maypole::CLI> module programatically to create
+test suites for your application. See the Maypole tests themselves or
+the documentation to C<Maypole::CLI> for examples of this.
+
=head3 Changing how params are parsed
=head3 REST
-=head2 Authentication hacks
+=head2 Content display hacks
-=head2 Creating and editing hacks
+These hacks deal primarily with the presentation of data to the user,
+modifying the C<view> template or changing the way that the results of
+particular actions are displayed.
-=head3 Getting data from external sources
+=head3 Template Switcheroo
-You want to supplement the data received from a form with additional
-data from another source.
+An action doesn't have any data of its own to display, but needs to display
+B<something>.
-B<Solution>: Munge the contents of C< $r-E<gt>params > before jumping
-to the original C<do_edit> routine. For instance, in this method,
-we use a C<Net::Amazon> object to fill in some fields of a database row based
-on an ISBN:
+B<Solution>: This is an B<extremely> common hack. You've just issued an
+action like C<beer/do_edit>, which updates the database. You don't want
+to display a page that says "Record updated" or similar. Lesser
+application servers would issue a redirect to have the browser request
+C</beer/view/I<id>> instead, but we can actually modify the Maypole
+request on the fly and, after doing the update, pretend that we were
+going to C</beer/view/I<id>> all along. We do this by setting the
+objects in the C<objects> slot and changing the C<template> to the
+one we wanted to go to.
- sub create_from_isbn :Exported {
- my ($self, $r) = @_;
- my $response = $ua->search(asin => $r->{params}{isbn});
- my ($prop) = $response->properties;
- # Rewrite the CGI parameters with the ones from Amazon
- @{$r->{params}{qw(title publisher author year)} =
- ($prop->title,
- $prop->publisher,
- (join "/", $prop->authors()),
- $prop->year());
- # And jump to the usual edit/create routine
- $self->do_edit($r);
+In this example from L<Flox.pod>, we've just performed an C<accept>
+method on a C<Flox::Invitation> object and we want to go back to viewing
+a user's page.
+
+ sub accept :Exported {
+ my ($self, $r) = @_;
+ my $invitation = $r->objects->[0];
+ # [... do stuff to $invitation ...]
+ $r->{objects} = [$r->{user}];
+ $r->{model_class} = "Flox::User";
+ $r->{template} = "view";
}
-The request will carry on as though it were a normal C<do_edit> POST, but
-with the additional fields we have provided.
+This hack is so common that it's expected that there'll be a neater
+way of doing this in the future.
-=head2 Content display hacks
+=head3 Maypole for mobile devices
+
+XXX
=head3 XSLT
=head3 Displaying pictures
-You want to serve a picture, or something else which doesn't have a
-content type of C<text/html>, out of your database.
+You want to serve a picture, a Word document, or something else which
+doesn't have a content type of C<text/html>, out of your database.
B<Solution>: Fill the content and content-type yourself.
sub view_picture :Exported {
my ($self, $r) = @_;
- my $user = $r->{objects}->[0] || $r->{user};
- if ($r->{content_type} = $user->photo_type) {
- $r->{output} = $user->photo;
+ my $user = $r->{objects}->[0];
+ $r->{content_type} = $user->photo_type;
+ $r->{output} = $user->photo;
+ }
+
+Of course, the file doesn't necessarily need to be in the database
+itself; if your file is stored in the filesystem, but you have a file
+name or some other pointer in the database, you can still arrange for
+the data to be fetched and inserted into C<$r-E<gt>{output}>.
+
+=head3 Component-based Pages
+
+You're designing something like a portal site which has a number of
+components, all displaying different bits of information about different
+objects. You want to include the output of one Maypole request call while
+building up another.
+
+B<Solution>: Use C<Maypole::Component>. By inheriting from this, you can
+call the C<component> method on the Maypole request object to make a
+"sub-request". For instance, if you have a template
+
+ <DIV class="latestnews">
+ [% request.component("/news/latest_comp") %]
+ </DIV>
+
+ <DIV class="links">
+ [% request.component("/links/list_comp") %]
+ </DIV>
+
+then the results of calling the C</news/latest_comp> action and template
+will be inserted in the C<latestnews> DIV, and the results of calling
+C</links/list_comp> will be placed in the C<links> DIV. Naturally, you're
+responsible for exporting actions and creating templates which return
+fragments of HTML suitable for inserting into the appropriate locations.
+
+=head3 Bailing out with an error
+
+Maypole's error handling sucks. Something really bad has happened to the
+current request, and you want to stop processing now and tell the user about
+it.
+
+B<Solution>: Maypole's error handling sucks because you haven't written it
+yet. Maypole doesn't know what you want to do with an error, so it doesn't
+guess. One common thing to do is to display a template with an error message
+in it somewhere.
+
+Put this in your driver class:
+
+ sub error {
+ my ($r, $message) = @_;
+ $r->{template} = "error";
+ $r->{template_args}{error} = $message;
+ return OK;
+ }
+
+And then have a F<custom/error> template like so:
+
+ [% PROCESS header %]
+ <H2> There was some kind of error... </H2>
+ <P>
+ I'm sorry, something went so badly wrong, we couldn't recover. This
+ may help:
+ </P>
+ <DIV CLASS="messages"> [% error %] </DIV>
+
+Now in your actions you can say things like this:
+
+ if (1 == 0) { return $r->error("Sky fell!") }
+
+This essentially uses the template switcheroo hack to always display the
+error template, while populating the template with an C<error> parameter.
+Since you C<return $r-E<gt>error>, this will terminate the processing
+of the current action.
+
+The really, really neat thing about this hack is that since C<error>
+returns C<OK>, you can even use it in your C<authenticate> routine:
+
+ sub authenticate {
+ my ($self, $r) = @_;
+ $r->get_user;
+ return $r->error("You do not exist. Go away.")
+ if $r->{user} and $r->{user}->status ne "real";
+ ...
+ }
+
+This will bail out processing the authentication, the model class, and
+everything, and just skip to displaying the error message.
+
+Non-showstopper errors or other notifications are best handled by tacking a
+C<messages> template variable onto the request:
+
+ if ((localtime)[6] == 1) {
+ push @{$r->{template_args}{messages}}, "Warning: Today is Monday";
+ }
+
+Now F<custom/messages> can contain:
+
+ [% IF messages %]
+ <DIV class="messages">
+ <UL>
+ [% FOR message = messages %]
+ <LI> [% message %] </LI>
+ [% END %]
+ </UL>
+ </DIV>
+ [% END %]
+
+And you can display messages to your user by adding C<PROCESS messages> at an
+appropriate point in your template; you may also want to use a template
+switcheroo to ensure that you're displaying a page that has the messages box in
+it.
+
+=head2 Authentication hacks
+
+The next series of hacks deals with providing the concept of a "user" for
+a site, and what you do with one when you've got one.
+
+=head3 Logging In
+
+You need the concept of a "current user".
+
+B<Solution>: Use something like
+C<Maypole::Authentication::UserSessionCookie> to authenticate a user
+against a user class and store a current user object in the request
+object.
+
+C<UserSessionCookie> provides the C<get_user> method which tries to get
+a user object, either based on the cookie for an already authenticated
+session, or by comparing C<username> and C<password> form parameters
+against a C<user> table in the database. Its behaviour is highly
+customizable, so see the documentation, or the authentication paper at
+C<http://maypole.simon-cozens.org/docs/authentication.html> for examples.
+
+=head3 Pass-through login
+
+You want to intercept a request from a non-logged-in user and have
+them log in before sending them on their way to wherever they were
+originally going.
+
+B<Solution>:
+
+ sub authenticate {
+ my ($self, $r) = @_;
+ $r->get_user;
+ return OK if $r->{user};
+ # Force them to the login page.
+ $r->{template} = "login";
+ return OK;
+ }
+
+This will display the C<login> template, which should look something
+like this:
+
+ [% INCLUDE header %]
+
+ <h2> You need to log in </h2>
+
+ <DIV class="login">
+ [% IF login_error %]
+ <FONT COLOR="#FF0000"> [% login_error %] </FONT>
+ [% END %]
+ <FORM ACTION="/[% request.path%]" METHOD="post">
+ Username:
+ <INPUT TYPE="text" NAME="[% config.auth.user_field || "user" %]"> <BR>
+ Password: <INPUT TYPE="password" NAME="password"> <BR>
+ <INPUT TYPE="submit">
+ </FORM>
+ </DIV>
+
+Notice that this request gets C<POST>ed back to wherever it came from, using
+C<request.path>. This is because if the user submits correct credentials,
+C<get_user> will now return a valid user object, and the request will pass
+through unhindered to the original URL.
+
+=head3 Logging Out
+
+Now your users are logged in, you want a way of having them log out
+again and taking the authentication cookie away from them, sending
+them back to the front page as an unprivileged user.
+
+B<Solution>: This action, on the user class, is probably overkill, but
+it does the job:
+
+ sub logout :Exported {
+ my ($class, $r) = @_;
+ # Remove the user from the request object
+ my $user = delete $r->{user};
+ # Destroy the session
+ tied(%{$r->{session}})->delete;
+ # Send a new cookie which expires the previous one
+ my $cookie = Apache::Cookie->new($r->{ar},
+ -name => $r->config->{auth}{cookie_name},
+ -value => undef,
+ -path => "/"
+ -expires => "-10m"
+ );
+ $cookie->bake();
+ # Template switcheroo
+ $r->template("frontpage");
+ }
+
+=head3 Multi-level Authentication
+
+You have both a global site access policy (for instance, requiring a
+user to be logged in except for certain pages) and a policy for
+particular tables. (Only allowing an admin to delete records in some
+tables, say, or not wanting people to get at the default set of methods
+provided by the model class.)
+
+You don't know whether to override the global C<authenticate> method or
+provide one for each class.
+
+B<Solution>: Do both. Have a global C<authenticate> method which calls
+a C<sub_authenticate> method based on the class:
+
+ sub authenticate {
+ ...
+ if ($r->{user}) {
+ return $r->model_class->sub_authenticate($r)
+ if $r->model_class->can("sub_authenticate");
+ return OK;
}
+ ...
+ }
+
+And now your C<sub_authenticate> methods can specify the policy for
+each table:
+
+ sub sub_authenticate { # Ensure we can only create, reject or accept
+ my ($self, $r) = @_;
+ return OK if $r->{action} =~ /^(issue|accept|reject|do_edit)$/;
+ return;
+ }
+
+=head2 Creating and editing hacks
+
+These hacks particularly deal with issues related to the C<do_edit>
+built-in action.
+
+=head3 Limiting data for display
+
+You want the user to be able to type in some text that you're later
+going to display on the site, but you don't want them to stick images in
+it, launch cross-site scripting attacks or otherwise insert messy HTML.
+
+B<Solution>: Use the C<CGI::Untaint::html> module to sanitize the HTML
+on input. C<CGI::Untaint::html> uses C<HTML::Sanitizer> to ensure that
+tags are properly closed and can restrict the use of certain tags and
+attributes to a pre-defined list.
+
+Simply replace:
+
+ App::Table->untaint_columns(
+ text => [qw/name description/]
+ );
+
+with:
+
+ App::Table->untaint_columns(
+ html => [qw/name description/]
+ );
+
+And incoming HTML will be checked and cleaned before it is written to
+the database.
+
+=head3 Getting data from external sources
+
+You want to supplement the data received from a form with additional
+data from another source.
+
+B<Solution>: Munge the contents of C< $r-E<gt>params > before jumping
+to the original C<do_edit> routine. For instance, in this method,
+we use a C<Net::Amazon> object to fill in some fields of a database row based
+on an ISBN:
+
+ sub create_from_isbn :Exported {
+ my ($self, $r) = @_;
+ my $response = $ua->search(asin => $r->{params}{isbn});
+ my ($prop) = $response->properties;
+ # Rewrite the CGI parameters with the ones from Amazon
+ @{$r->{params}{qw(title publisher author year)} =
+ ($prop->title,
+ $prop->publisher,
+ (join "/", $prop->authors()),
+ $prop->year());
+ # And jump to the usual edit/create routine
+ $self->do_edit($r);
+ }
+
+The request will carry on as though it were a normal C<do_edit> POST, but
+with the additional fields we have provided.
+
+=head3 Uploading files and other data
+
+You want the user to be able to upload files to store in the database.
+
+B<Solution>: It's messy.
+
+First, we set up an upload form, in an ordinary dummy action. Here's
+the action:
+
+ sub upload_picture : Exported {}
+
+And here's the template:
+
+ <FORM action="/user/do_upload" enctype="multipart/form-data" method="POST">
+
+ <P> Please provide a picture in JPEG, PNG or GIF format:
+ </P>
+ <INPUT TYPE="file" NAME="picture">
+ <BR>
+ <INPUT TYPE="submit">
+ </FORM>
+
+(Although you'll probably want a bit more HTML around it than that.)
+
+Now we need to write the C<do_upload> action. At this point we have to get a
+little friendly with the front-end system. If we're using C<Apache::Request>,
+then the C<upload> method of the C<Apache::Request> object (which
+C<Apache::MVC> helpfully stores in C<$r-E<gt>{ar}>) will work for us:
+
+ sub do_upload :Exported {
+ my ($class, $r) = @_;
+ my $user = $r->{user};
+ my $upload = $r->{ar}->upload("picture");
+
+This returns a C<Apache::Upload> object, which we can query for its
+content type and a file handle from which we can read the data. It's
+also worth checking the image isn't going to be too massive before we
+try reading it and running out of memory, and that the content type is
+something we're prepared to deal with.
+
+ if ($upload) {
+ my $ct = $upload->info("Content-type");
+ return $r->error("Unknown image file type $ct")
+ if $ct !~ m{image/(jpeg|gif|png)};
+ return $r->error("File too big! Maximum size is ".MAX_IMAGE_SIZE)
+ if $upload->size > MAX_IMAGE_SIZE;
+
+ my $fh = $upload->fh;
+ my $image = do { local $/; <$fh> };
+
+Now we can store the content type and data into our database, store it
+into a file, or whatever:
+
+ $r->{user}->photo_type($ct);
+ $r->{user}->photo($image);
}
+
+And finally, we use our familiar template switcheroo hack to get back to
+a useful page:
+
+ $r->objects([ $user ]);
+ $r->{template} = "view";
+ }
+
+Now, as we've mentioned, this only works because we're getting familiar with
+C<Apache::Request> and its C<Apache::Upload> objects. If we're planning to use
+C<CGI::Maypole> instead, or want to write our application in a generic way so
+that it'll work regardless of front-end, then we need to replace the C<upload>
+call with an equivalent which uses the C<CGI> module to get the upload data.
+This is convoluted and horrific and we're not going to show it here, but it's
+possible.
+
+Combine with the "Displaying pictures" hack above for a happy time.
projects / maypole.git / blobdiff