WEBVTT

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Hello, this video will explain
an important feature of Pharo,

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the doesNotUnderstand hook.

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You've no doubt already seen
a debugger window with this message.

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Where does it come from
and what is it for?

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Let's look at an example.

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We have the object node1.

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We'll send this object
the message coucou: stef.

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From node1,
we follow the lookup method

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back to the object's class.

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We don't find the coucou method
in this class.

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So, we go to the superclass,
but it's not there either.

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The virtual system informs us

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that it can't find a matching method,
so it reifies the message.

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We discussed reification in the video
on introspection and reflection.

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Reify means to represent
an implicit concept as an object.

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Here, it's a message.

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We create an object
to represent the message.

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It's an instance of Message class.

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The virtual system will resend a message

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to the object node1.

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It sends the doesNotUnderstand message,
passing the object as a parameter.

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This is a new execution
of the lookup algorithm.

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The system will not find the
doesNotUnderstand method in Node class.

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So, it goes back
to search in the superclass.

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It finds the method, abbreviated to DNU.

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Now it can now execute the method.

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DoesNotUnderstand is a message

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that the virtual machine
sends for you to objects

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when a message has failed.

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All classes can redefine this method

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to give it a specific behavior
when a message is not understood.

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This method is an important tool

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that allows us to build many features.

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It's used in automatic delegation,

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distributed programming, and so on.

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Here we'll look at
some uses of doesNotUnderstand.

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Suppose we want to redirect
all our messages to a different object.

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In a simple delegation, I create
an object that stores the target,

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where messages will be sent.

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I redefine the doesNotUnderstand method.

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It takes the object aMessage
as a parameter,

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containing the failed message selector.

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Then I can ask this message
to resend itself.

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I use sendTo: self target,
which is the instance variable.

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I resend the message to another object.

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Be careful, this is a powerful function

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that can interfere with
the legibility of the code.

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As it's explained here.

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The code will show
who ultimately receives the message.

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It's very useful for constructing tools

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and building advanced mechanisms.

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Another example we'll look at
is LoggingProxy.

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The basic idea here

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is to create a minimal object
that contains few methods,

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and to customize
its doesNotUnderstand: method.

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Then we'll swap a domain object

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with this proxy, or minimal object,
using "become."

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First, we create a proxy object.

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We give the proxy object a subject,

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which is the object
we're going to replace,

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or the target.

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We add InvocationCount to increment
the counter with each message.

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We put the counter in initialize at 0,
since it has received no messages.

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Then we add the subject,
which is what will be replaced.

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Now we redefine
the DNU method on this proxy.

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Every time a new message
is not understood,

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its reception appears on the Transcript,
and the counter is incremented.

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Then we forward the message
to the subject.

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Like before,
we redirect the message elsewhere.

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For sendTo, as you see
in the Message class implementation,

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we simply use the Perform method
with Arguments.

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We already explained this
in an earlier course.

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Let's look at an example.

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How do we use this LoggingProxy?

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We'll create an instance,
the Object point.

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Then we'll use "become"

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so that everything
that points to this point

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will inadvertently
point to a LoggingProxy,

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which we instantiate.

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Now if we send messages
to the Object point,

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noting that "become" is now
an instance of LoggingProxy,

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every time we send it a message,

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it will display in Transcript

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and the counter will be incremented.

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This is what we did
in doesNotUnderstand.

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At the end,
the counter shows a value of 3.

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There are limits
to the proxy's framework,

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as seen in the last example.

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For example, we can't capture
messages an object sends to itself.

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That gets tricky.

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We can't use "become" with classes,
as reflective models have limitations.

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It's also risky when the proxy
and the object to be replaced

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both understand the same message.

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When I send the message to the proxy,

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it will reply,
rather than capturing it with DNU

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and transferring it
to the target object.

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Pharo has other more powerful
proxy frameworks that trap everything,

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but they're more complex
than this simple example.

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Another example
of how to apply this technique

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is to dynamically generate accessors,
or even methods.

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Here I've redefined
the doesNotUnderstand method.

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I see a message has been received,

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so I test if I have the instance
variables that include the message.

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If so, I generate a new method
using "compile,"

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to define a new method for the class.

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This will return
the value of the variable.

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Here I'm generating
the variable's read-accessor.

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If the message sent
does not match the name

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of an instance variable,
I send a super doesNotUnderstand.

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This enables us to load
read-accessors automatically,

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depending on
if they've been called or not.

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In conclusion,

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we've seen how to use minimal objects.

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They're not direct instances
of the Object class,

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but of the proto Object class.

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They can be the basis for proxies.

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By redefining the doesNotUnderstand
method, we can capture

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the existence of a failed message
and then redirect the message.

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It's a powerful hook that provides
the basis of many tools in Pharo.

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But you must be very careful
when you use it.

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It's only to be used when necessary.

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Don't put it in the domain code yet,
it's a highly advanced technique.