Orsdxy

In the di.xml that comes with Magento2 there is a node type and a node virtualType. My questions is what is this virtualType and in what case should it be used instead of type?

In some places it looks like a symbolic link or rewrite:

<virtualType name="MagentoCoreModelSessionStorage" type="MagentoFrameworkSessionStorage">

Where one full path gets changed into another but in other places it appears to be used as a way to define a shorter alias.

<virtualType name="lessFileSourceBase" type="MagentoFrameworkViewFileCollectorBase">

Virtual types are a way to inject different dependencies into existing classes without affecting other classes.

For example, the MagentoFrameworkSessionStorage class takes a $namespace argument in its constructor, which defaults to the value 'default', and you could use the type definition to change the namespace to 'core'.

<type name="MagentoFrameworkSessionStorage">

    <arguments>

        <argument name="namespace" xsi:type="string">core</argument>

    </arguments>

</type>

The above config would make it so that all instances of MagentoFrameworkSessionStorage have a namespace of 'core'. Using a virtual type allows for the equivalent of a sub-class to be created, where only the sub-class has the altered argument values.

In the codebase we see the following two configurations:

<virtualType name="MagentoCoreModelSessionStorage" type="MagentoFrameworkSessionStorage">

    <arguments>

        <argument name="namespace" xsi:type="string">core</argument>

    </arguments>

</virtualType>



<type name="MagentoFrameworkSessionGeneric">

    <arguments>

        <argument name="storage" xsi:type="object">MagentoCoreModelSessionStorage</argument>

    </arguments>

</type>

The first snippet creates a virtual type for MagentoCoreModelSessionStorage which alters the namespace, and the second inject the virtual type into MagentoFrameworkSessionGeneric. This allows MagentoFrameworkSessionGeneric to be customized without affecting other classes that also declare a dependency on MagentoFrameworkSessionStorage

Another way to understand virtual types -

Let's say that you have a class Class1, which has the following constructor -

public function __construct(Class2 $argOfClass1){...}

And Class2 has the following constructor -

public function __construct(Class3 $argOfClass2){...}

Now, you want to change the type of $argOfClass2 from Class3 to Class4, but only when Class2 is used as $argOfClass1.

The "old" way to do that would be to add the following in di.xml -

<type name="Class1">

    <arguments>

         <argument name="argOfClass1" xsi:type="object">Class5</argument>

    </arguments>

</type>

where Class5 is the following:

class Class5 extends Class2{

    public function __construct(Class4 $argOfClass2){...}

}

Instead of using this way, you can use the virtual types to accomplish the same, by adding the following to di.xml:

<virtualType name="Class5" type="Class2">

    <arguments>

        <argument name="argOfClass2" xsi:type="string">Class4</argument>

    </arguments>

</virtualType>



<type name="Class1">

    <arguments>

         <argument name="argOfClass1" xsi:type="object">Class5</argument>

    </arguments>

</type>

As you can see, using the virtual type saved you the work of creation of Class5.

For further reference I suggest to read Alan Storm's article regarding virtual types in Magento2 - http://alanstorm.com/magento_2_object_manager_virtual_types/

In the same di.xml file I found that lessFileSourceBase is passed as an argument for lessFileSourceBaseFiltered that is passed as an argument for lessFileSourceBaseSorted that is passed as an argument for type MagentoFrameworkLessFileCollectorAggregated.

I found no other occurrence of lessFileSourceBase (or lessFileSource ) in an other file except di.xml from the core module. Only in some cache files but those are not important.

I guess if you are not going to use the virtual type in a PHP class, but only in the di xml files then you are not required to make it look like a class name and you can use an alias.

But this is just pure speculation.

It will be "fun" to try to create a class and inject in its constructor an instance of lessFileSourceBase to see how it behaves.

I wanted to throw-in my two cents to help anybody having difficulty understanding the difference between "type" and a "virtual type". A type definition tells Magento to map to a particular class when it runs into an instance of the target class. In any kind of dependency injection you need to pass a class(type) and the name of an object to the constructor or method. Keeping that in mind, a "Virtual Type" tells Magento: "No physical class exists for this injection, nor an object name given, So take my word for it and don't throw any errors. The Class and Name will be defined in this configuration." Then just like that.. Magento would create the class for you and save it in the 'generated' folder, which it would use to reference.