WEBVTT

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Hello. In this sequence
we'll be looking at Reflection,

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the capacity for Introspection
in Pharo and why it's useful.

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A reflective system,

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reflection is divided
into 2 major families,

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Introspection

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is a program's ability
to observe its own state,

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Intercession is a program's ability
to modify itself.

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To modify its own state
or interpretation.

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Reification is a concept that consists

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of making explicit (objects) things
that are normally implicit.

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For example, the execution stack

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in Pharo is explicit,
but can be got as a classic object.

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More classically, a class in Pharo
is a totally classic object.

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In other languages,
classes aren't objects.

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A reflective system, as I said,

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has its own representation of itself.

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It's capable of representing itself,

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and it can act on this representation
to modify itself,

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and when it modifies itself
it changes its state and representation.

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We call it a "causal connection"

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between its representation
and its state.

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If we change the state,

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the system representation changes
as well as the state of the system.

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Why is this interesting?
It's interesting because we'll use

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this introspection and intercession

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to look inside objects.

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I'm defining a collection,
which I make an "OrderedCollection"

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then I'll use the Pharo Inspector,
which is a tool,

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which can look inside this object.

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It's an instance object
of the OrderedCollection class,

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as we see here,
it has instance variables,

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"first index", "last index",

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we have the values of this
object's instance variables

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so 1, 12 and then
an array of 12 elements, OK?

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We might wonder how
this Inspector tool

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is able to look inside objects.

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How can it see
the internal state of objects?

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It uses introspection methods
to look inside the objects,

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There are lots of them,
defined as "objects".

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So "instVarAt" allows us to see

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an instance invariable
according to its number.

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I'll see instance variable
number 1 of this object.

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I can modify instance variable number 1
of this object by setting a new value,

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or I can access an instance
variable by its name,

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or change it by its name,
"instVarNamed put". OK?

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Here are some examples.
Typically we create a point,

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and then we do...
the point is 10@3

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instVarNamed: 'x'.

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I'll get the value of the instance
variable called 'x' of this point,

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which gives me 10.

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Then "instVarNamed: 'x' put:33"

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I've changed the value of
the instance variable 'x' of this point.

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It's gone from 10 to 33.

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I've been able to change
the inner state of an object,

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by using introspection and intercession,

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particularly intercession,
"instVarNamed put". OK?

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The main point is that
we've violated encapsulation.

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An external object has modified
this object, without being internal,

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so it violates encapsulation
but it's extremely useful

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when building tools
and during development.

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It's not to be used in the normal
code of an application,

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but it's extremely powerful
for building generic tools,

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typically code inspectors.

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I'll give you another example
of introspection, the class method,

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for getting the class of an object.
It's defined in "Object>>class".

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I ask this string its class,
it gives me the class string,

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this point what is its class,
the class point,

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"Smalltalk class",
I can ask the class of the class class.

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etcetera, then I add class class...

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Basically, I can query the system

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by sending the message "class"
to objects to discover their class.

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There are many methods
for querying the system.

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Here, I take "OrderedCollection"
and send it lots of query messages.

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I'll get all of its superclasses,

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all of its selectors,
the name of its instance variables,

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all of the selectors,
the name of its instance variables,

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all of its subclasses, etc...

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And all of its lines of code.

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All of this allows us to build
top level tools,

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such as the navigation system here.

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This navigation system lets us
show or browse the system.

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For example,
I can see all the classes

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that implement the method "#,".

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Here we've a window that will open.

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We'll see all the implementors of #,

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so the class "AbstractFileReference"
implements the method "#,".

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I've a list of all the classes
implemented by this method.

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Another example,
we'd like to implement

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a menu or a button.

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By clicking on it, we want to send
a message to the object behind it,

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according to the button's name,
for example.

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How do I turn a string into a message
to send to an object?

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I've an intercession method for that.
"Perform" defined under "object".

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I pass it a symbol,
the name of a message to execute,

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and it will send this message
to the object in question.

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I've a 2nd kind of message,
"perform with".

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I can give the name of the symbol,
the name of the method to execute,

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and then pass a list of arguments.

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An example, if I do "5 factorial"

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sending a classic message,
and send the reflective message,

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its "5 perform # factorial"
that's a symbol which means,

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"Object 5, please execute
or receive the message factorial".

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The normal lookup is applied
and these two forms are the same.

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Here's another example.

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Here we have a code inspector

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"Ordered collection", we can see inside
the class "Ordered Collection".

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This class contains attributes,

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instance variables,
for example "methodDict".

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In "methodDict" we see
that the class "OrderedCollection"

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comes from "Behaviour"
so it has a "methodDict' here,

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an instance of "MethodDictionary"
containing "CompiledMethod".

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So we can see
this method dictionary contains

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a compiled method here, etc...

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another compiled method...
Lots of compiled methods.

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We'll be able to ask each
of these compiled methods

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for their source code.
If I do "self getSource"

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I'll get the source code
of the compiled method.

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But we can go further.

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A compiled method can be asked
to execute itself directly

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with the message,
"valueWithReceiver:arguments".

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But watch out!
If I do that, there's no lookup.

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Seeing as I already have
a compiled method,

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it executes itself directly
without a message.

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Here I'll ask the integer class
to get its compiled method "factorial",

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which I get from the
class compiled method.

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I send it the message,
"valueWithReceiver:arguments".

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I pass it the arguments,
the receiver will be 5,

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the arguments will be (),
because it's a unary message,

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and we'll get the result.

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It triggers execution of the
compiled method without lookup.

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So to summarize,
reflection is extremely powerful.

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We've seen it's something
that allows us to introspect

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the whole system
because it represents it

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in itself as an object,

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but also to modify
the state of the system.

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It lets us build tools
in a generic way in all objects.

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So you can really talk to objects

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with a protocol that's structurally
the same for all objects.

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But watch out!
It violates encapsulation.

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It isn't used in operational code,
but for tool building.

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It facilitates the writing
of code inspectors as we saw,

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and you can see how
it's implemented in Pharo.