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NAME ^

Form::Sensible::Overview - Getting started with Form::Sensible

INTRODUCTION ^

Form::Sensible is a different way of handling form based user input. Many people have heard of MVC. The separation of programming concerns into that of the data model, the view, and the controller is by now familiar territory. Many implementations are available and some of them are really good. Unfortunately, more often than not the user input portion of the View is neglected. Handling user interaction is pushed into the controller where direct access to the methods of input occur.

Form::Sensible is an attempt to abstract user input in a way that is similar to the abstraction provided for the presentation portion of the view. As such, Form::Sensible takes a different approach to representing a form. Instead of defining a form in terms of what your user will see (checkboxes, dropdowns, text fields, etc.) you define your form in terms of the information you want to collect from the user.

If you want to have a user select an item from a set of options, you create a Select field. If you want a number between 1975 and 2010, then you create a Number field. Ultimately, your code just wants the correct information. It doesn't matter whether the number is from a text entry field, or from a dropdown select box. Likewise when selecting a preferred item from a number of options, your code doesn't care whether the selection is presented as a dropdown box or a set of checkboxes. (How many times have you been asked to change a select dropdown to a set of checkboxes or vice-versa?)

Form::Sensible provides a conduit for information to and from the user. It is a mechanism for prompting the user with questions, for collecting the information you need, for ensuring that the value collected fits your constraints, and for indicating errors and other important information to the user.

With Form::Sensible, however, the method by which the user is queried for the information is abstracted from the code that uses the information. This allows you to change the details of what the interface looks like without adjusting your code. It also allows you to completely change your interface mechanism without rewriting your code. You can, for example, swap out your in-browser HTML based forms with a desktop-GUI by simply replacing the Renderer* (or, more accurately, you will be able to in the not-too-distant future.)

OVERVIEW ^

As we mentioned, Form::Sensible abstracts the method of querying the user from the code that's actually using the data. Many people will use Form::Sensible primarily for HTML based forms only and those who do use it for other things are likely familiar with HTML based form handling so we will use them as our unfortunate victim, er... example.

When comparing Form::Sensible to HTML forms, there are a few big differences from what you are probably used to. First off, you are probably used to defining fields in terms of how they will eventually be shown. You create text fields, password fields, radio buttons, etc. in a template of some kind. Then you collect the information in its raw form and boil it down to something that you can use in your code. Then you write code to perform validation of your data and then set up a mechanism for reporting any errors to the user. A number of modules and libraries have been created to make portions of this easier to deal with HTML generators, Form Validators, etc. But when it comes down to it, the process is the same just slightly more succinct.

In Form::Sensible you approach it from a different perspective. Fields are defined in terms of the data they represent. For each field in a form, you define the constraints and other parameters important to the type of data it represents. Once this is done, you simply pass the form to a Form renderer and it takes care of turning it into something presentable for the user.

It is important to note that unlike HTML forms which bind to a particular action for form processing, forms in Form::Sensible simply provide a way to bundle a set of fields together. They are simply a handle for working with more than one field at once.

Once the user's response to the form is collected, you can easily ensure that it is valid by triggering form validation. The easiest method for doing so is to simply call $form->validate(). This will validate the values provided by the user against the constraints defined for each field (and the collection of fields together) and will set up messaging for any errors that are found. Then you can choose to represent the form to the user or if validation succeeded, you can proceed to do whatever is appropriate for your data.

It is important to note also that Form::Sensible forms are intended to be queried for their data. Meaning if you want the value of your year_of_birth field, you call $form->field('year_of_birth')->value(). This allows your code to be separate from the mechanism of retrieving the value in the first place. This makes things significantly easier when it comes time to port your application to a new user interface method.

USING IT ^

Until now we've spoken in very general terms about how you do things with Form::Sensible. Now we will actually demonstrate it, starting with creating a form.

CREATING AND RENDERING A FORM

You can create Form::Sensible forms using an object-oriented process of creating a form object, creating field objects, and then adding each field object to the form object. While useful in certain circumstances, most people tend to create a form in a static manner. As such, an easier and significantly more brief option is available. That option is the Form::Sensible->create_form() method. Let's jump right in:

    use Form::Sensible;
    
    my $form = Form::Sensible->create_form(
        {
            name => 'login_form',
            fields => [
                         {
                            field_class => 'Text',
                            name => 'username',
                            validation => {
                                    regex => '^[0-9a-z]*',
                                    required => 1,
                                }
                         },
                         {
                             field_class => 'Text',
                             name => 'password',
                             validation => {
                                    required => 1,
                                },
                             render_hints => {
                                    'HTML' => {
                                                render_as => 'password'
                                              }
                                    },
                         },
                         {
                             field_class => 'Trigger',
                             name => 'submit'
                         }
                      ],
        } );

The above code snippet creates the all-too-familiar login form we are used to. Each field has a field_class , which corresponds with a Form::Sensible::Field classname, and a name which is used to get a hold of this particular field. The name element for any given field should be unique within the form.

Something to notice here is that all validation information required for validating this form is present in this definition. Another thing to notice is that we use a standard Text field to store our password data. Remember that Form::Sensible field definitions are centered around the data you want to collect, not how they are presented. In this case, we simply ask the HTML renderer to render it as a password by providing render_hints. We'll come back to render_hints later.

Now that we have a form, though, what do we do with it? Well, in most cases we want to render it. That is astoundingly simple. First we ask for the renderer we want, then we use the renderer to render the form:

    my $renderer = Form::Sensible->get_renderer('HTML');
    my $renderedform = $renderer->render($form);

At this point, we have a $renderedform. This is not, however, a string. The output of a Form::Sensible renderer is a 'rendered form handle.' What that means is specific to the type of Renderer being used. What they have in common, though, is that they are an object that can be used to actually initiate user interaction. They are also connected directly to the $form object which was used to create them. This means that changes to the $form object will be reflected in the User Interface and that values will be set in the $form in response to user action on the renderedform.

For HTML, this connection is limited by the transactional nature of HTTP, but it is still there and any changes you make to $form prior to actually presenting the rendered form will be reflected in the HTML generated.

Ok... so we have a $renderedform. Now what do we do? We display it, obviously. For an HTML form, that looks like this:

    print $renderedform->complete('/login', 'post');

This would print out the form rendered as HTML. The first argument to complete() is the action to submit this HTML form to. This is passed directly through to the generated <form> element's action attribute. The second argument is the method, either 'get' or 'post', and is likewise passed directly through to the method attribute on the form element.

In terms of rendering, we are done. The HTML representation of the form will be generated and printed out.

SETTING VALUES AND VALIDATION

Once the form is submitted, we have to get the values into our form somehow. This requires a bit of glue in your application and will be somewhat specific to the rendering model you are using, but the most straightforward option is simply to call $form->set_values() with a hashref of name-value pairs.

    $form->set_values($c->req->params);

The form is smart enough to recognize the fields it cares about and to load the values as appropriate. Note, however, that validation is not done at this point. It is entirely possible to put bad data into the values for the various fields. To make sure that what we got is valid, we call $form->validate():

    my $result = $form->validate();

Calling $form->validate() runs through all field and form validation according to the form definition and produces a Form::Sensible::Validator::Result object. The result object can then be used to indicate what exactly went wrong, if anything:

    if ($result->is_valid()) {
        # do valid-form stuff here.
        authenticate_user($form->field('username')->value, $form->field('password')->value);
    } else {
        my $renderer = Form::Sensible->get_renderer('HTML');
        my $renderedform = $renderer->render($form);
        $renderedform->add_errors_from_validator_result($result);
        print $renderedform->complete('/login','post');
    }

If $result->is_valid() is true, we can proceed to doing what we need to do with the form data. An interesting thing to note here is that if the form is invalid, we simply redisplay the form.

You've now gone through the entire lifecycle of a Form::Sensible form. We have used the HTML renderer because it is most familiar (and it is the only publicly released renderer at this point.) That said, however, for the most part only the print line and set_values line are likely to change to present using a different renderer.

DETAILS ^

In the previous section we showed briefly how to create a form, render it, and validate the user's input. The validation and rendering mechanisms in Form::Sensible are complex and flexible and some detail is needed to use them effectively.

VALIDATION

Field validation

Form and Field validation in Form::Sensible is fairly straightforward. Each field has a validation hashref which defines three elements for field validation: required, regex, and code. The required element is simply a true/false flag indicating whether it is an error if the field is empty. The regex element is exactly what it sounds like. The value for the field is checked against the regex provided; if it does not match, the field is marked invalid. The last element, code, is the most interesting. Its value is a coderef for a subroutine that checks the field's value. The subroutine provided is called with two arguments, first the field's value, then the field object itself. The subroutine is expected to return either a zero (0) value - indicating the field is OK, or a message indicating why the field is invalid. This is somewhat non-intuitive as you return 0, or a false value, to indicate the field is ok if you are not familiar with exit codes on UNIX-like platforms. If that is the case, it may help to think about the code coderef as a question, 'Is this value invalid?' An example of a code validator is:

    validation => {
                ## remember this is answering the question 'is_this_invalid()'
        code => sub {
                        my ($value, $field) = @_;
                        if ($value ne $field->name) {
                            return "_FIELDNAME_ is a bizarre field that can only contain its own name.  Field is invalid.";
                        } else {
                            ## I contain my own name, that means this validation should pass.  return 0.
                            return 0;
                        }
                    }
    }

Note in the example above the string '_FIELDNAME_' is used in place of the actual field name. This works this way to allow for easy localization of strings. This will be translated to the field's name later.

Validation is not an either-or: you can have both a regex and a coderef for validation and if you do, both must be successful for the value to be considered valid. When you provide both, the regex is tested first and if the value passes the regex check, then the coderef is run. Note that if the regex failed, the coderef subroutine will not be called.

Note that some field types have their own validation. For example, Select fields will not accept any value that is not part of their option list. Numbers, likewise, perform their own validation to check against their upper and lower bounds and other related items. If a field type has a builtin validator routine, it will be run after the regex and coderef are run. Unlike the coderef, however, the field's builtin validation routine is run regardless of the results of regex or coderef validators.

Another thing to note about Field validation is that if the field is empty no validation will be run and the form itself will be marked 'invalid.' The field itself will be marked as 'invalid' if it is not required and 'missing' if it is required. See "Error Messaging".

Complete Form validation

If field validation is completed successfully, the final step in form validation is run. That step is 'complete form validation.' You can provide a validation hashref on the form itself which will be run after all other fields have been validated (if and only if all the fields individually are valid.) The subroutine for a form is slightly different than the coderef for a field. The arguments are the form object and the validator result object that was created for this validation. Rather than returning any particular value, a form validation coderef should manipulate the validator result object directly. For example:

    # ... rest of form definition
    validation => {
        code => sub {
                        my ($form, $result) = @_;
                        if ($form->field('password')->value() ne $form->field('password2')->value()) {
                            ## add an error on the 'password' field.
                            $result->add_error('password', "Your passwords do not match, please try again.");
                        }
                    }
    }

Error Messaging

Finally we need to cover error messaging. With code-based validation of either Fields or the entire form, you provide the error message to use. For other types of validation (including regex and field-specific validation) you may want to provide custom error messages. If you want to provide a custom error message for a field you can provide two additional elements to the validation hashref: invalid_message and missing_message. Both of these are strings that will be used in place of the generic 'FieldX is invalid' type messaging. As you might suspect, invalid_message is used when a field is found to be invalid. The missing_message is used if the field is required but is not set. An example of custom messaging:

    # ... rest of field definition
    validation => {
                    regex => '^\d\d\d-\d\d\d-\d\d\d\d$',
                    invalid_message => "Phone must be formatted as ###-###-####",
                    missing_message => "Please provide a Phone number."
    }

RENDERING

render_hints

Render hints are a way to help the renderer decide how to present the field or form you are working with. Render hints can be placed into the form definition itself or can be inserted at various points along the rendering path. Generally speaking, the hints that are closest to final rendering take precedence over those at a higher distance.

That is to say that if you have render_hints provided for the field in the render_field() call, they will override any hints set in the complete() call which in turn would override any hints set in the field's original definition. Note that the all of the render_hints are merged before processing occurs with those closer to the actual rendering taking precedence on conflict.

Render hints are most often applied to fields, though they can be applied to Forms as well. How render hints are applied are specific to the renderer being used. An example of render hints might be:

    {
        field_class => 'Number',
        lower_bound => 1910,
        upper_bound => 2010,
        step => 1,
        render_hints => {
            HTML => {
                field_type => 'Select',
                render_as => 'checkboxes'
            }
        }
    }
The C<render_hints> here tells the HTML render that this should be rendered
the way a Select field is rendered instead of the way a Number is normally
rendered (a text field). It also tells the renderer that when it is rendering
the Select C<field_type> it should render the select as a series of checkboxes
instead of a dropdown (the default for Select type).

The 'HTML' in the above example addresses that section to the HTML renderer, other renderers would not be affected by the render hints provided. This allows you to provide render_hints for the field that are specific for.

... More to come.

# JK - pick up here.

AUTHORS ^

Jay Kuri, jayk@cpan.org

COPYRIGHT & LICENSE ^

Copyright (c) 2010 the aforementioned authors. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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