Performs a custom server side validation of an input value. This allows you to add your own validation logic to HTML form fields.

Start by adding a validator of the type postback to your form field in your template:

<input type="text" id="username" name="username" value="" />
{% validate id="username" type={postback event="validate_username"} %}

The event argument declares the name of the event that will be notified. Handle this event in your site or module:


%% The event name passed in your template as event="validate_username",
%% prefixed with observe_
observe_validate_username({validate_username, {postback, Id, Value, _Args}}, Context) ->
    case is_valid(Value) of
        true ->
            {{ok, Value, Context};
        false ->
            %% The validation message will be shown in the form
            {{error, Id, "Sorry, that's not valid. Try again!"}, Context}

%% Some function that returns true or false depending on the validity of the
%% input value
is_valid(Value) ->
    %% ...