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Hi everyone, this video shows
some advanced points on classes.

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It has three main parts.

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Firstly, how to share a state between
instances of a class and its subclasses.

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Secondly, how to enable classes
to have their own variables.

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Thirdly, how to initialize classes.

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First up, how to share a state
between instances.

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Here we have a class
with several instances.

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To share a state, these instances
require a common object.

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In Java, we use an "instance" variable
that takes the keyword "static."

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The value of a static variable

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is the same for all instances.

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In Pharo, we use "class variables."

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This should be a familiar expression
that allows us to create classes.

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Here we create the Color class,
which is a subclass of Object.

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First, we list its instance variables,
then take a look at the line below.

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It lists the class variables
of the Color class.

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Color class has at least two variables,
ColorRegistry and ComponentMask.

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A class variable's values

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are shared between all instances
of the class and subclasses.

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These variables are accessible
from both instance and class methods.

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They start with a capital letter.

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Let's go back to our example.

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Here we have our Color class,

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which is an instance
of the metaclass Color class.

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The Color class defines
two standard variables, rgb and alpha.

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These instance variables are private,

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meaning that they're only accessible
using Color class methods.

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It also defines ColorRegistry,
which is underlined and uses capitals.

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This indicates that this variable
is shared.

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It's a class variable that is accessible

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by the methods of the Color class
and those of the class Color class.

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Here are some examples
of uses of this variable.

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In one instance method,
that of privateBlue in the Color class,

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we access the class variable
using its name.

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Likewise, if we want to
give this variable a value,

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we use a colon-equals sign,
as with any variable.

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We can do this
on the instance or class side.

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Very often,

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the values of these class variables
can be read

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within instance methods,

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and are written within class methods.

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This is what occurs most frequently,
it's not obligatory.

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Now let's look at
instance variables of classes.

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It's not the same thing.

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A class is an object,
like everything else.

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Like all objects,
a class can have instances.

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The metaclass describes the class
and lists the instance variables

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that can only be accessed
by its sole instance, its class.

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To define an instance variable
on the class side,

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we click on the "class" button
in the code browser.

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That gives us the expression here.

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So, now I'm browsing
the RPackageSet class.

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The metaclass of this class
defines a variable named cachePackages.

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These variables can only be accessed
from class methods

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and they always start
with a lowercase letter.

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Here's an example.

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The variable cachePackages is defined
in the metaclass RPackageSet class.

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This means that

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the RPackageSet class,
which is an instance of the metaclass,

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has a value
associated with this variable.

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Similarly,

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all instances of the subclasses
of the RPackageSet class

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have a specific value for that variable,
which is a different value.

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There's no sharing here.

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Each instance of the RPackageSet class
or its subclasses

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has its own value for this variable.

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Class variables and instance variables
on the class side are different things.

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We'll explain this
using the Singleton Design Pattern,

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whose purpose is to ensure
that a class has only one instance.

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One solution for executing this Pattern
is to store, within a variable,

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the instance that is freely accessible
and disable creation of a new instance.

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That's what we'll do here
with WebServer.

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Here we use an instance variable
on the class side.

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Its name takes lower case.

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This variable is defined
in the definition of the metaclass.

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Now we disable execution
of the "new" method.

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No messages can be sent to
WebServer class to create new instances.

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It's imperative
to go via uniqueInstance,

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which either returns the variable's value
if it's not nil and has content,

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or else it adds something
to the variable using "super new."

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It creates a new instance
of WebServer class,

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and stores it
in the uniqueInstance variable.

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The fact that we use
class instance variables

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leads to the following result.

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Each subclass of the WebServer class

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will have its own value

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for uniqueInstance.

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So, if WebServer has three subclasses,

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the uniqueInstance variable
will have a total of four values.

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One for WebServer and for each subclass.

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If we now use a class variable,

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we edit the class on the instance side

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and we add UniqueInstance,
using a capital U.

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Again we disallow sending "new" messages
and implement UniqueInstance.

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The only difference
is that we use a capital U.

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As a result of choosing a class variable,

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the class hierarchy
has only one singleton.

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If WebServer has three subclasses,

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these three subclasses and WebServer
share the same singleton.

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Thus we have one instance shared by all.

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Now for class initialization.

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We can have variables shared
between a class and its instances,

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but at some point
we must give them a value.

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We do this when we initialize the class.

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Objects are initialized
when the initialize message is sent.

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Likewise, we can send
the initialize message to a class.

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We decide how to initialize variables
using the initialize method code.

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If we want to initialize
the Color class,

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we send the initialize message to it.

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Typically, when a class
is loaded in the system

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by the version control tool,

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the initialize message
is sent to all classes.

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This is automatic,
there's no need to send the message

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manually to all classes loaded.

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But if we implement a class,

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if we create a new class
using the initialize method

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we must send the message manually.

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Here's an example in the Color class.

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We can see we're on the metaclass side.

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Here we initialize
several class variables,

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or instance variables on the class side.

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There are both types of variable here.

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In the initialize methods,

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we never use super initialize
on the class side.

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When we add an initialize method
on the instance side,

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we systematically call super initialize

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to initialize all instance variables
of a newly created object.

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But for classes,

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since classes and superclasses exist
when the initialize message is sent,

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we don't call super initialize
in the initialize method

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on the class side
to avoid re-initializing classes.

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Here are the takeaways.

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We use class variables to share a state.

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Classes are objects
that can have their own variables,

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so we use instance variables
on the class side.

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To initialize a class,
we send it an initialize message

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and apply the initialize method
on the class side.