WEBVTT

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Today we will carry on
with the previous session

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where I explained
that small methods were good.

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Today I'll show some examples.

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But first, let's quickly review
our previous session.

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Sending a message leads to a choice.

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A masked choice because several
possible implementations

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will be called up.

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And Pharo makes a choice
depending on the message receiver.

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Class hierarchy defines choices:
More classes = more choices.

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And you can easily add
new choices by creating subclasses.

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Subclasses can redefine or refine
the code of their superclass.

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And sending a message

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means giving subclasses
the possibility to change behavior.

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Today we'll look at
the Design Pattern template method.

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You'll find it
in the Design Pattern manual.

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What is a template method?

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It's a skeleton
that defines the overall behavior

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of an algorithm, for example,
with hooks inside.

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And these hooks can be
redefined by subclasses.

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This is an algorithm.

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It does something,
we don't exactly know what.

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Next, it does something
in hookMethod 1,

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then something else,

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and then something
in hookMethod2.

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Since hookMethods 1 and 2
are both complete methods,

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they can be redefined in the subclasses.

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There are 2 possibilities,
one each for hookMethod 1 and 2.

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These methods may or may not

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have a default behavior.

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Here, let's say that hookMethod 1
has a default behavior.

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This means that
if the subclass proposes nothing,

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hookMethod 1 has a default behavior
that functions.

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Likewise, we could say

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that hookMethod 2
does not have a default behavior.

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The subclass
must imperatively define one.

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As designer of the class,
you can choose

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whether or not
to determine a default behavior.

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We'll use printString
as a first example.

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The idea is that if I send
a printString message to an object,

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I get a string of characters
that represents this object.

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Here I have a delay.

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I've created a 10-second delay.

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If I send a printString message
to this delay,

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I get a delay and in parentheses,
a millisecond value for the delay.

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The printString method is implemented
in the object class like this.

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It sends the message
printStringLimitedTo

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This implementation

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recovers a string of characters
that represents

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the object.

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And if this string of characters
is too long,

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we can cut it off at a certain limit.

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Then at the end,
we concatenate with '...etc...'

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to say the string is not done.

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The important thing here

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is that printStringLimitedTo
sends a printOn message to self.

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This is the method

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that will or will not
be redefined in the subclasses.

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If I look at what printString returns

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for a Node and for an Apple,

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Node new returns a Node.

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This is the printString
of the Node class.

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And here in Apple, we have
the printString of the Apple class.

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We see that this behavior
and the default behavior

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are implemented in the class object.

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So the default behavior
of printString for any object whatsoever

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is one:

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Recover the class names.

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In this case, Node and Apple.

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And two, if the class name
begins with a vowel,

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we use the prefix "an".

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If it starts with a consonant,
we use the prefix "a".

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This is how we get
"a node" and "an apple".

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This is the default behavior.

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But it is possible to change
this default behavior

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by refining printOn.

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For a delay,

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we see that
the printString of a delay

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starts with
the default printOn return.

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That is "a Delay."

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But afterwards, we can add the delay
in milliseconds in parentheses.

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This is exactly what
the printOn method does.

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It starts by asking the superclass
for the default printString.

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Afterwards,
it opens a parentheses

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with a preset number of milliseconds,

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and then it closes the parentheses.

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We just looked at refinement.

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The delay class
refines the implementation

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of the printOn method
in the object class.

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But a class can also
completely redefine behavior.

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This is the case for booleans,
for example.

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If I display false,

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it will return false.

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We don't have "a false",

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simply "false".

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For this, we just send
the "false" string of characters

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in the printOn variables stream.

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That's a complete redefinition.

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Another example
of complete redefinition is intervals.

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An interval defines a set of values

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situated between
a minimum and maximum value.

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The interval 1 to 100 is displayed by:
(1 to: 100)

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The interval 1 to: 100 by: 3

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1, 4, and so on,

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is displayed in the same manner
with the additional "by".

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How is this implemented?

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The interval class
redefines the printOn method

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and sends different messages
to stream variables.

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We start by opening parentheses.

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The parentheses here and here.

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Then we display the initial value
or the first number of the interval,

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that is this "1" here,
and this "1" here.

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Then we write 'to:'.

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And we write the final value: "100",

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and "100".

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If there is a "by"
that is different from the default "1",

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we write the "a".

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And at the end,
we close the parentheses.

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We saw that printString

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uses the Design Pattern
template method so that classes

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can implement
their own textual representation.

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Now we'll look at another example,
which is object copy.

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What does copy do?

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It enables you to take an object
and create a code from it.

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Copying objects is complex.

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There can be different strategies.

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Each class can decide

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what the copy of its instances
should look like.

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There is a simple
template method solution

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that uses copy and postCopy.

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Copy is a template method
and postCopy is a hook.

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In the object class
there is a copy method.

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I'll let you read the commentary.

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In this method,
we send shallowCopy

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as a message to self,

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followed by postCopy
on the result.

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What does shallowCopy do?

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It creates a new object
that shares all instance variables

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with the base object.

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We have two objects and
their instance variables are the same.

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If I modify the instance variable
of one object, I modify the other too.

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That is the default behavior

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of shallowCopy.

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Depending on what postCopy does,

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variables will or will not be shared.

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If postCopy is empty,
all variables are shared.

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But the classes can decide
to put different things into postCopy

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in order to share certain variables
or nothing at all.

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The postCopy default setting
shares everything.

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It simply returns the current object.

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But we could also imagine
other applications for postCopy.

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For example, in the Bag class.

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Bag is a type of collection

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and its postCopy method
copies its contents.

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Thus, a Bag is
a contents instance variable,

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and the designers
of the Bag class decided that

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when you copy a Bag,

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you don't want to share
the contents variable,

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you want separate variables.

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They have the same initial value
but you can modify only one.

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Here the idea is:

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postCopy is a message sent to copy,
or the new object,

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which shares
all of the instance variables.

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If we don't want to share them,

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we create new ones
and decide upon their value.

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Here in my contents variable,

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I'll put a copy
of the initial contents.

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That way I don't share them.

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Each copy of my Bag
has its own contents.

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With the dictionary, we go even further.

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A dictionary is a collection
of key-value pairs,

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or a collection of associations.

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Not only do we want to copy
the collection...

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If we copy a dictionary,
we want to copy the collection,

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and each dictionary
has its own collection of pairs...

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But we also want each pair
to be different.

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So that if I modify one,
it won't modify the other side.

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To do this, I copy the table,
but I also copy

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each pair inside.

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This is what's going on here.

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To conclude, the Design Pattern
template method is very common.

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It's a sign of good design.

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If you do good object design,
you'll have lots of template methods.

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It's perfectly normal and a good thing.

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So don't hesitate to use them.

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This technique enables subclasses
to define behavior

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and partially modify
the behavior of the superclass.