Component

Yii applications are built upon components which are objects written to a specification. A component is an instance of CComponent or its derived class. Using a component mainly involves accessing its properties and raising/handling its events. The base class CComponent specifies how to define properties and events.

1. Defining and using a Component Property ¶

A component property is like an object's public member variable. We can read its value or assign a value to it. For example,

$width=$component->textWidth;     // get the textWidth property
$component->enableCaching=true;   // set the enableCaching property

There are two different ways of defining a component property. First way is to simply declare a public member variable in the component class like the following:

class Document extends CComponent
{
    public $textWidth;
}

Another way is to use getters and setters. It is more flexible since additionally to normal properties you can declare a read only or write only property.

class Document extends CComponent
{
    private $_textWidth;
    protected $_completed=false;
 
    public function getTextWidth()
    {
        return $this->_textWidth;
    }
 
    public function setTextWidth($value)
    {
        $this->_textWidth=$value;
    }
 
    public function getTextHeight()
    {
        // calculates and returns text height
    }
 
    public function setCompleted($value)
    {
        $this->_completed=$value;
    }
}

The component above can be used like the following:

$document=new Document();
 
// we can write and read textWidth
$document->textWidth=100;
echo $document->textWidth;
 
// we can only read textHeight
echo $document->textHeight;
 
// we can only write completed
$document->completed=true;

When trying to read a component property and the property isn't defined as a public class member, Yii is trying to use a getter method i.e. for textWidth a getter method will be getTextWidth. Same thing happens when trying to write a property not defined as a public class member.

If there's a getter method but no setter method the component property becomes read only and throws an exception if we're trying to write it. If it's the other way around, the property is write-only.

Using getter and setter methods to define a property has the benefit that additional logic (e.g. performing validation, raising events) can be executed when reading and writing the property.

Note: There is a slight difference between a property defined via getter/setter methods and a class member variable. The name of the former is case-insensitive while the latter is case-sensitive.

2. Component Event ¶

Component events are special properties that take methods (called event handlers) as their values. Attaching (assigning) a method to an event will cause the method to be invoked automatically at the places where the event is raised. Therefore, the behavior of a component can be modified in a way that may not be foreseen during the development of the component.

A component event is defined by defining a method whose name starts with on. Like property names defined via getter/setter methods, event names are case-insensitive. The following code defines an onClicked event:

public function onClicked($event)
{
    $this->raiseEvent('onClicked', $event);
}

where $event is an instance of CEvent or its child class representing the event parameter.

We can attach a method to this event as follows:

$component->onClicked=$callback;

where $callback refers to a valid PHP callback. It can be a global function or a class method. If the latter, the callback must be given as an array: array($object,'methodName').

The signature of an event handler must be as follows:

function methodName($event)
{
    ......
}

where $event is the parameter describing the event (it originates from the raiseEvent() call). The $event parameter is an instance of CEvent or its derived class. At the minimum, it contains the information about who raises the event.

An event handler can also be an anonymous function which is supported by PHP 5.3 or above. For example,

$component->onClicked=function($event) {
    ......
}

If we call onClicked() now, the onClicked event will be raised (inside onClicked()), and the attached event handler will be invoked automatically.

An event can be attached with multiple handlers. When the event is raised, the handlers will be invoked in the order that they are attached to the event. If a handler decides to prevent the rest of the handlers from being invoked, it can set $event->handled to be true.

3. Component Behavior ¶

A component supports the mixin pattern and can be attached with one or several behaviors. A behavior is an object whose methods can be 'inherited' by its attached component through the means of collecting functionality instead of specialization (i.e., normal class inheritance). A component can be attached with several behaviors and thus achieve 'multiple inheritance'.

Behavior classes must implement the IBehavior interface. Most behaviors can extend from the CBehavior base class. If a behavior needs to be attached to a model, it may also extend from CModelBehavior or CActiveRecordBehavior which implements additional features specific for models.

To use a behavior, it must be attached to a component first by calling the behavior's attach() method. Then we can call a behavior method via the component:

// $name uniquely identifies the behavior in the component
$component->attachBehavior($name,$behavior);
// test() is a method of $behavior
$component->test();

An attached behavior can be accessed like a normal property of the component. For example, if a behavior named tree is attached to a component, we can obtain the reference to this behavior object using:

$behavior=$component->tree;
// equivalent to the following:
// $behavior=$component->asa('tree');

A behavior can be temporarily disabled so that its methods are not available via the component. For example,

$component->disableBehavior($name);
// the following statement will throw an exception
$component->test();
$component->enableBehavior($name);
// it works now
$component->test();

It is possible that two behaviors attached to the same component have methods of the same name. In this case, the method of the first attached behavior will take precedence.

When used together with events, behaviors are even more powerful. A behavior, when being attached to a component, can attach some of its methods to some events of the component. By doing so, the behavior gets a chance to observe or change the normal execution flow of the component.

A behavior's properties can also be accessed via the component it is attached to. The properties include both the public member variables and the properties defined via getters and/or setters of the behavior. For example, if a behavior has a property named xyz and the behavior is attached to a component $a. Then we can use the expression $a->xyz to access the behavior's property.