title: Extensions
summary: Extensions and DataExtensions let you modify and augment objects transparently.
# Extensions and DataExtensions
An [api:Extension] allows for adding additional functionality to a [api:Object] or modifying existing functionality
without the hassle of creating a subclass. Developers can add Extensions to any [api:Object] subclass within core, modules
or even their own code to make it more reusable.
Extensions are defined as subclasses of either [api:DataExtension] for extending a [api:DataObject] subclass or
the [api:Extension] class for non DataObject subclasses (such as [api:Controllers])
**mysite/code/extensions/MyMemberExtension.php**
:::php
'SS_Datetime'
);
public function SayHi() {
// $this->owner refers to the original instance. In this case a `Member`.
return "Hi " . $this->owner->Name;
}
}
Convention is for extension class names to end in `Extension`. This isn't a requirement but makes it clearer
After this class has been created, it does not yet apply it to any object. We need to tell SilverStripe what classes
we want to add the `MyMemberExtension` too. To activate this extension, add the following via the [Configuration API](../configuration).
**mysite/_config/app.yml**
:::yml
Member:
extensions:
- MyMemberExtension
Alternatively, we can add extensions through PHP code (in the `_config.php` file).
:::php
Member::add_extension('MyMemberExtension');
This class now defines a `MyMemberExtension` that applies to all `Member` instances on the website. It will have
transformed the original `Member` class in two ways:
* Added a new [api:SS_Datetime] for the users date of birth, and;
* Added a `SayHi` method to output `Hi `
From within the extension we can add more functions, database fields, relations or other properties and have them added
to the underlying `DataObject` just as if they were added to the original `Member` class but without the need to edit
that file directly.
### Adding Database Fields
Extra database fields can be added with a extension in the same manner as if they were placed on the `DataObject` class
they're applied to. These will be added to the table of the base object - the extension will actually edit the $db,
$has_one etc.
**mysite/code/extensions/MyMemberExtension.php**
:::php
'Varchar',
);
private static $has_one = array(
'Image' => 'Image',
);
public function SayHi() {
// $this->owner refers to the original instance. In this case a `Member`.
return "Hi " . $this->owner->Name;
}
}
**mysite/templates/Page.ss**
:::ss
$CurrentMember.Position
$CurrentMember.Image
## Adding Methods
Methods that have a unique name will be called as part of the `__call` method on [api:Object]. In the previous example
we added a `SayHi` method which is unique to our extension.
**mysite/templates/Page.ss**
:::ss
$CurrentMember.SayHi
// "Hi Sam"
**mysite/code/Page.php**
:::php
$member = Member::currentUser();
echo $member->SayHi;
// "Hi Sam"
## Modifying Existing Methods
If the `Extension` needs to modify an existing method it's a little trickier. It requires that the method you want to
customize has provided an *Extension Hook* in the place where you want to modify the data. An *Extension Hook* is done
through the `[api:Object->extend]` method.
**framework/security/Member.php**
:::php
public function getValidator() {
// ..
$this->extend('updateValidator', $validator);
// ..
}
Extension Hooks can be located anywhere in the method and provide a point for any `Extension` instances to modify the
variables at that given point. In this case, the core function `getValidator` on the `Member` class provides an
`updateValidator` hook for developers to modify the core method. The `MyMemberExtension` would modify the core member's
validator by defining the `updateValidator` method.
**mysite/code/extensions/MyMemberExtension.php**
:::php
addRequiredField('DateOfBirth');
}
}
The `$validator` parameter is passed by reference, as it is an object.
Another common example of when you will want to modify a method is to update the default CMS fields for an object in an
extension. The `CMS` provides a `updateCMSFields` Extension Hook to tie into.
:::php
'Varchar',
);
private static $has_one = array(
'Image' => 'Image',
);
public function updateCMSFields(FieldList $fields) {
$fields->push(new TextField('Position'));
$fields->push(new UploadField('Image', 'Profile Image'));
}
}
If you're providing a module or working on code that may need to be extended by other code, it should provide a *hook*
which allows an Extension to modify the results.
:::php
public function Foo() {
$foo = // ..
$this->extend('updateFoo', $foo);
return $foo;
}
The convention for extension hooks is to provide an `update{$Function}` hook at the end before you return the result. If
you need to provide extension hooks at the beginning of the method use `before{..}`.
## Owner
In your [api:Extension] class you can only refer to the source object through the `owner` property on the class as
`$this` will refer to your `Extension` instance.
:::php
owner);
}
}
## Checking to see if an Object has an Extension
To see what extensions are currently enabled on an object, use [api:Object->getExtensionInstances] and
[api:Object->hasExtension]
:::php
$member = Member::currentUser();
print_r($member->getExtensionInstances());
if($member->hasExtension('MyCustomMemberExtension')) {
// ..
}
## Object extension injection points
`Object` has two additional methods, `beforeExtending` and `afterExtending`, each of which takes a method name and a
callback to be executed immediately before and after `Object::extend()` is called on extensions.
This is useful in many cases where working with modules such as `Translatable` which operate on `DataObject` fields
that must exist in the `FieldList` at the time that `$this->extend('UpdateCMSFields')` is called.
Please note that each callback is only ever called once, and then cleared, so multiple extensions to the same function
require that a callback is registered each time, if necessary.
Example: A class that wants to control default values during object initialization. The code needs to assign a value
if not specified in `self::$defaults`, but before extensions have been called:
:::php
function __construct() {
$self = $this;
$this->beforeExtending('populateDefaults', function() use ($self) {
if(empty($self->MyField)) {
$self->MyField = 'Value we want as a default if not specified in $defaults, but set before extensions';
}
});
parent::__construct();
}
Example 2: User code can intervene in the process of extending cms fields.
This method is preferred to disabling, enabling, and calling field extensions manually.
:::php
public function getCMSFields() {
$this->beforeUpdateCMSFields(function($fields) {
// Include field which must be present when updateCMSFields is called on extensions
$fields->addFieldToTab("Root.Main", new TextField('Detail', 'Details', null, 255));
});
$fields = parent::getCMSFields();
// ... additional fields here
return $fields;
}
## Related Documentaion
* [Injector](injector/)
* [api:Object::useCustomClass]
## API Documentation
* [api:Extension]
* [api:DataExtension]