Today we learn how to create Inceptor or plugin in Magento 2.  At development time we need to change some function behavior so Magento provides a plugin for it.

A Plugin or Inceptor allows change behavior of any public class or method by intercepting a function call and running code. Plugins are only used for “public methods”.

Types Of Plugins In Magento 2

• Before Plugin
• After Plugin
• Around Plugin

Why we use Interceptors or Extension in Magento 2?

• we can change the behavior of the same class or method
• The plugin can be called sequentially according to sort order, so conflict with other plugin class can be avoided.
• No issue of rewriting the system
• Customize the same method in different modules.
• Ability to modify the return value of any public method.
• Ability to modify the arguments of any public method.
• Plugins do not conflict with other plugin classes.

The Magento 2 Plugins can’t be used with:

• Non-public methods (Protected & Private)
• Static methods
• Final Methods
• Final classes
• Virtual Types
• Any class that has at least one final public method
• Objects that are instantiated before Magento\Framework\Interception is bootstrapped
• __construct and __destruct

Declaring a plugin

A plugin can be declared in the di.xml file in your module.

app/code/VendoreName/ModuleName/etc/di.xml

<config>
    <type name="{ObservedType}">
      <plugin name="{pluginName}" type="{PluginClassName}" sortOrder="1" disabled="false" />
    </type>
</config>

• type name: A class or interface which the plugin observes.
• plugin name An arbitrary plugin name that identifies a plugin. Also used to merge the configurations for the plugin.
• plugin type: The name of a plugin’s class or its virtual type. Use the following naming convention when you specify this element: \Vendor\Module\Plugin\<ClassName>.
• plugin sortOrder: Plugins that call the same method run them using this order. (Optional)
• plugin disabled: To disable a plugin, set this element to true. The default value is false. (Optional)

<type name="Dolphin\PluginDemo\Controller\Index\PluginExample">
    <plugin name="demo_plugin" type="Dolphin\PluginDemo\Plugin\PluginDemo" sortOrder="1" disabled="false"  />
</type>

For example, the following code defines type name, we created PluginExample.php file at app/code/Dolphin/PluginDemo/Controller/Index/PluginExample.php

if you don’t know how to create module click here 

<?php

namespace Dolphin\PluginDemo\Controller\Index;

class PluginExample extends \Magento\Framework\App\Action\Action
{
    protected $title;

    public function execute()
    {
        echo $this->setTitle('Welcome Plugin');
        echo $this->getTitle();
    }

    public function setTitle($title)
    {
        return $this->title = $title;
    }

    public function getTitle()
    {
        return $this->title;
    }
}

Find here for more plugins: Magento 2 Extension

1) Before Plugin

• Before plugin are used whenever we need to change the arguments of the method or need to add some behavior before a method is called.
• The first method to run is an observed method.
• Before plugin is called by adding the prefix ‘before’ to the method name and setting the first letter of the original method to capital.
• It lets you modify the parameters that will be used.
• Example

<?php

    namespace Dolphin\PluginDemo\Plugin;

    class PluginDemo
    {
        public function beforeSetTitle(\Dolphin\PluginDemo\Controller\Index\PluginExample $subject, $title)
        {
            $title = $title." Before plugin method is calll</br>";
            return [$title];
        }

    }

2) After Plugin

• After Plugins are used whenever we need to change the arguments of method or need to add some behavior after a method is called.
• Starts running after the observed method is finished
• After Plugin is called by adding the prefix ‘after’ to the method name and setting the first letter of the original method to capital.
• It let you modify the output
• Responsible for editing the results of an observed method in the right way and must have a return value.
• Example

<?php

    namespace Dolphin\PluginDemo\Plugin;

    class PluginDemo
    {
        public function afterGetTitle(\Dolphin\PluginDemo\Controller\Index\PluginExample $subject, $result)
        {

            echo "After plugin method call</br>";

            return $result . 'Modify Result';

        }

    }

3) Around Plugin

• Around Plugin run before and after the observed method.
• Allows overriding a method
• Around Plugin change both the arguments and the returned values of the observed method or add behavior before and after the observed method is called.
• Around listener is called by adding the prefix ‘around’ to the method name and setting the first letter of original method to capital.
• It must have a return value
• Example

<?php

    namespace Dolphin\PluginDemo\Plugin;

    class PluginDemo
    {
        public function aroundGetTitle(\Dolphin\PluginDemo\Controller\Index\PluginExample $subject, callable $proceed)
        {
            echo "Around Method Call"
            $result = $proceed();
            echo "Around Method Call over"
            return $result;
        }

    }

Plugins are commonly used in Magento development.

I hope this helps you. For any doubts regarding this topic, please write your doubts in the comments section.