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Extensions Development

Overview

With the release of ExpressionEngine 1.4 comes the ability to rewrite and modify the inner workings of the program. Within ExpressionEngine are what is known as 'hooks'; little snippets of code in over 100 strategic places that allow the calling of third-party scripts. Extensions can do things like modify an entire Control Panel page, add/remove functionality, and modify the appearance of certain page elements. Extensions enable third party developers to modify aspects of ExpressionEngine without hacking the backend scripts. You can think of an Extension as a plugin. But unlike a plugin, Extensions are not used within your templates, they instead allow you to modify the core system itself.

An Extension is an add-on script that is placed in the /system/extensions/ directory and then enabled via the Extensions Manager in the Control Panel. Extensions can have their own settings and their own database tables, if necessary, but neither is required. If settings are available for an Extension, a language file is required, but unlike a module there is no control panel for Extensions.

Naming Convention

Extensions have a similar naming convention to ExpressionEngine plugins so current developers should get the hang of them quickly. There is only a single file required for an extension and inside this file should be a PHP class. The name of the class is used in the file name of the extension so that the name of the file is the lower-cased class name with the prefix 'ext.' and the standard PHP suffix of '.php'. So, if we have a class called 'Example_extension', then our file name for this extension would be 'ext.example_extension.php'.

Inside the Extension

Inside an extension file should be a class, which will be called by ExpressionEngine whenever this particular extension is required. The constructor for the class will require one parameter that will receive settings for the extension and set a class variable.

class Example_extension
{
    var $settings        = array();

    // -------------------------------
    //   Constructor - Extensions use this for settings
    // -------------------------------

    function Example_extension($settings='')
    {
        $this->settings = $settings;
    }
    // END
}
// END CLASS

Besides the $settings class variable, there are five other required class variables that your extension should have. These variables output meta information about your extension to the Extensions Manager so that it can describe your extension, provide documentation, and update settings (if any).

class Example_extension
{
    var $settings        = array();

    var $name            = 'Example Extension';
    var $version         = '1.0.0';
    var $description     = 'Our example extension';
    var $settings_exist  = 'n';
    var $docs_url        = '';//'http://expressionengine.com';

    // -------------------------------
    //   Constructor - Extensions use this for settings
    // -------------------------------

    function Example_extension($settings='')
    {
        $this->settings = $settings;
    }
    // END
}
// END CLASS

If your extension has a language file, then the $name and $description class variables can be set in the constructor by calling the language file and variables using the Language ($LANG) class. If your extension is likely to be used internationally and by non-English speakers this is a recommended course of action.

Activating and Updating

There are two required methods for your extensions class that control the activating and updating of your extension. The most important is the function used to activate the extension in ExpressionEngine. To activate an extension, you are simply inserting a query into the database with various pieces of information like the extension hook and the name of the method in your extension's class to call for this hook.

// --------------------------------
//  Activate Extension
// --------------------------------

function activate_extension()
{
    global $DB;

    $DB->query($DB->insert_string('exp_extensions',
                                  array(
                                        'extension_id' => '',
                                        'class'        => "Example_extension",
                                        'method'       => "rewrite_admin_home",
                                        'hook'         => "admin_home_page_start",
                                        'settings'     => "",
                                        'priority'     => 10,
                                        'version'      => $this->version,
                                        'enabled'      => "y"
                                      )
                                 )
              );
}
// END

Here is a quick run down of what each of these fields in the database table mean:

Updating an extension is extremely easy in ExpressionEngine. The user will simply upload the new version of the extension and ExpressionEngine will automatically update the extension the next time it is called. All that is required is an intelligent function called update_extension(). The program will automatically compare the version of the extension information in the database against the version of the extension file, and if the extension file is a newer version it calls this function.

// --------------------------------
//  Update Extension
// --------------------------------

function update_extension($current='')
{
    global $DB;

    if ($current == '' OR $current == $this->version)
    {
        return FALSE;
    }

    if ($current < '1.0.1')
    {
        // Update to next version 1.0.1
    }

    if ($current < '1.0.2')
    {
        // Update to next version 1.0.2
    }

    $DB->query("UPDATE exp_extensions
                SET version = '".$DB->escape_str($this->version)."'
                WHERE class = 'Example_extension'");
}
// END

Disabling

When an extension is enabled for the very first time, the activate_extension() function is called and all of the extension calls are inserted into the database. When an extension is disabled though, these extension calls are not removed from the database. Instead they are merely disabled, which allows settings to be preserved and not removed so that they are still there if the extension is enabled again in the future.

This causes a problem for developers who, while developing an extension, will often enable an extension to test their code but before they have added all of their extension calls to the activate_extension() function. What we have done is allowed the creation of a disable_extension() function in an extension's class. If this function exists in the class, it will be called whenever your extension is disabled. This will allow you to clear out your extension's data and basically start fresh every single time.

// --------------------------------
//  Disable Extension
// --------------------------------

function disable_extension()
{
    global $DB;

    $DB->query("DELETE FROM exp_extensions WHERE class = 'Example_extension'");
}
// END

Settings

Abstracted Settings Form and Processing

If you want to give your extension the ability to have settings, then we have written an abstracted layer to make it extremely easy. First, you have to make sure that you have your $settings_exist class variable set to 'y'. Second, you need a language file for your extension with the file name of the language file being the extension's lower-cased class name with a prefix of 'lang.' and a suffix of '.php'. Make sure the language file is put in the /system/language/ directory too. And finally, you need to have a method in your extension's class called settings(). This function will return an array in a certain form that will help the Extensions Manager automatically create a form for your settings.

// --------------------------------
//  Settings
// --------------------------------

function settings()
{
    $settings = array();

    $settings['butter']    = "Quite Tasty";
    $settings['buttery']   = array('r', array('yes' => "yes", 'no' => "no"), 'no');

    // Complex:
    // [variable_name] => array(type, values, default value)
    // variable_name => short name for setting and used as the key for language file variable
    // type:  t - textarea, r - radio buttons, s - select, ms - multiselect, f - function calls
    // values:  can be array (r, s, ms), string (t), function name (f)
    // default:  name of array member, string, nothing
    //
    // Simple:
    // [variable_name] => 'Butter'
    // Text input, with 'Butter' as the default.

    return $settings;
}
// END

The above example will create a settings form with a text input field with a variable name of 'butter' and a default value of "Not Tasty" and a radio button field with a variable name of 'buttery' with possible values of "Yes" or "No" and a default of "No".

A note about the values array for the second field, the keys will be used as the value for that item while the value will be the language text for that item. If you want, the value can be the name of a language variable from your extension's language file and the Extensions Manager will automatically retrieve it for you.

Built In Settings Form and Processing

Alternatively, if your settings require a special form that cannot created by the abstracted layer above, then ExpressionEngine permits you to create your own settings form and processing functions within your Extension. First, you need to will need have a method in your extension's class called settings_form(). This function will create the form and output it to Control Panel. Treat this function as any other page created for ExpressionEngine's Control Panel, so create it using the Display class and its methods.

function settings_form($current)
{
    global $DSP, $LANG, $IN;

    $DSP->crumbline = TRUE;

    $DSP->title  = $LANG->line('extension_settings');
    $DSP->crumb  = $DSP->anchor(BASE.AMP.'C=admin'.AMP.'area=utilities', $LANG->line('utilities')).
                               $DSP->crumb_item($DSP->anchor(BASE.AMP.'C=admin'.AMP.'M=utilities'.AMP.'P=extensions_manager', $LANG->line('extensions_manager')));
    $DSP->crumb .= $DSP->crumb_item($this->name);

    $DSP->right_crumb($LANG->line('disable_extension'), BASE.AMP.'C=admin'.AMP.'M=utilities'.AMP.'P=toggle_extension_confirm'.AMP.'which=disable'.AMP.'name='.$IN->GBL('name'));

    $DSP->body = $DSP->form_open(
                                array(
                                        'action' => 'C=admin'.AMP.'M=utilities'.AMP.'P=save_extension_settings',
                                        'name'   => 'settings_example',
                                        'id'     => 'settings_example'
                                    ),
                                array('name' => get_class($this))
                            );


    $DSP->body .=   $DSP->table('tableBorder', '0', '', '100%');
    $DSP->body .=   $DSP->tr();
    $DSP->body .=   $DSP->td('tableHeadingAlt', '', '2');
    $DSP->body .=   $this->name;
    $DSP->body .=   $DSP->td_c();
    $DSP->body .=   $DSP->tr_c();

    $DSP->body .=   $DSP->tr();
    $DSP->body .=   $DSP->td('tableCellOne', '45%');
    $DSP->body .=   $DSP->qdiv('defaultBold', $LANG->line('log_table'));
    $DSP->body .=   $DSP->td_c();

    $DSP->body .=   $DSP->td('tableCellOne');
    $DSP->body .=   $DSP->input_text('log_table', ( ! isset($current['log_table'])) ? '' : $current['log_table']);
    $DSP->body .=   $DSP->td_c();
    $DSP->body .=   $DSP->tr_c();

    $DSP->body .=   $DSP->table_c();
    $DSP->body .=   $DSP->qdiv('itemWrapperTop', $DSP->input_submit());
    $DSP->body .=   $DSP->form_c();
}

Lastly, you will need to have a method in your extension's class called save_settings(). This function will be called when your settings_form() method's form is submitted. Use it to process the data sent and put it into the exp_extensions database table. Remember that the data put into the database is a serialized array, so handle it appropriately.

Calling of the Extension

The following is an example of an ExpressionEngine Extension Hook that is available for use:

// -------------------------------------------
// 'publish_form_start' hook.
//  - Allows complete rewrite of Publish page.
//
	$edata = $EXT->call_extension('publish_form_start', $which, $submission_error, $entry_id);
	if ($EXT->end_script === TRUE) return;
//
// -------------------------------------------

The first parameter of $EXT->call_extension is the name of the hook, which lets the Extension class know what extensions to call. The other three parameters are variables taken from the function that the hook is embedded within. They provide information and data for the extensions being called for this hook, which allows those extensions to have information about the script that allow them to perform certain actions or manipulate data. When an extension is called, ExpressionEngine loads the extension file, instantiates the extension's class, and then calls the method specified for this extension hook as specified by the extension when it was activated (see above concerning activation).

When that method is called in the extension's class those other three parameters will be sent to the method automatically. Here is what the method might look like:

function replace_entry_form($which, $submission_error, $entry_id)
{
    global $DSP, $DB, $IN, $EXT;

    $EXT->end_script = TRUE;

    $DSP->body = 'New Entry Form Here';
}

The three parameters from the extension hook are mapped straight to the three parameters of the method being called, and so your extension can easily use those parameters and do what it needs to do. The ExpressionEngine.com Extension Hook library will have a record of all extension hooks and the parameters available to you, along with a suggestion or two about what can be done with the extension hook.

Multiple Extensions, Same Hook

When an extension hook is called, ExpressionEngine checks the database to see if there are any extensions available for the hook. If there are extensions, then it processes them in order based on their priority level with the lower the priority number the sooner the extension is called. Because of priority, extensions might interfere with each other, so we have provided two variables for helping with that.

$EXT->last_call

There will be rather popular hooks being used by multiple extensions and some hooks will expect you to return data to the extension hook. Because of that, there is a variable available from the Extensions class ($EXT) that will contain the returned data of any prior extensions for that hook. Say, there is a hook for formatting text and an extension before yours is called. That extension will be returning the text formatted in its own way, but then your extension is called with the original text details being sent. In such an instance of data being returned and possible prior extensions, there is a variable available to retrieve that already formatted text: $EXT->last_call. This variable will return whatever the last extension returned to this hook. If there was no prior extension, then the value of this variable is FALSE.

$EXT->end_script

Many extension hooks exist for the express purpose of totally controlling a page or script in the Control Panel. They are meant for redesigning the appearance of a form or perhaps usurping a script for processing form data. In those instances you want your extension to be the last thing called for that extension hook so that nothing else is processed after that point. The $EXT->end_script exists solely for that purpose. If you set this value to TRUE, then once your extension is done being processed the execution of the hook is finished, as is the script that the extension hook is contained within.

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