WEBVTT

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Hello to all.

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This sequence
will focus on class methods.

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As we saw earlier,

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class methods are normal methods,
like other methods.

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Generally, they are used
to create new instances,

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but can be used for other things.

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We'll be seeing two examples
of uses for class methods.

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Imagine we want to parse
these lines

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in a computer program.

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The lines create a structure.

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If a line starts
with an exclamation point,

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it is read as a title.

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Lines that start with dashes
are list items.

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Lines that start
with more than one dash

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are subitems on a list.

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In any other case, it is a paragraph.

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Take a program with this input:

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It creates instances
for the classes here,

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subclasses of DocumentItem.

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We also want to design
this program

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so that each subclass
of DocumentItem is responsible

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for parsing lines,

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and for deciding "canParse" (or cannot)
for a given line.

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In the "canParse" method here,
we submit a line of text.

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If the line starts
with an exclamation point,

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the canParse method must return true.

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The line belongs
to the Section Title class.

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In the ListItem class,

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the canParse method returns true
if the line starts with a dash.

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And the canParse line we see here
must always return true.

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The idea is to write an algorithm
that looks at classes one by one.

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As soon as one returns
canParse for a given line,

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we use that class to create
a new instance for that line.

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That's the goal of canParse method.

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The newFromLine method also
takes a line of text as a parameter.

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But it returns an instance in the class
in which the line is implemented.

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If I send a newFromLine message
to the SectionTitle class,

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with a chain of characters
starting with an exclamation point,

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the newFromLine method will return
an instance of the SectionTitle class.

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This is what happens:

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The ClassParser does this work.

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It searches
DocumentItem subclasses

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for the one
in charge of a given line.

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The Parser has a method
called documentClasses.

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Its goal is to return
the list of Subclasses

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in DocumentItem.

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It returns a collection
containing these three classes.

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We do this by sending the message
"allSubclasses" to DocumentItem.

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The allSubclasses method
already exists in the Pharo system.

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It returns the list of subclasses
in a given class.

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That message yields a collection

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containing SectionTitle,
ListItem, and Text.

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I want the collection
sorted by priority.

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The idea is for the text class
to come at the end

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because its canParse method
always returns true.

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The text class
can parse any line at random.

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We will test that method
after testing the others.

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We have the concept
of priority here.

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As you see, many methods
are underlined.

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If I underline the methods here,

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this indicates they are class methods.

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To execute these methods,
the message must be sent to the Class.

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I must send the canParse method
to the SectionTitle class

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to find out if the SectionTitle class
can parse a given line.

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The Parse method of the Parser class
takes a line as a parameter.

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It searches all DocumentItem subclasses
for a class that can parse the line.

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If it detects one, it creates an instance
of this class.

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Sending the command "detect:"
to a collection

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yields one among n elements
in a collection, following a predicate.

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Now we'll look at another example
of using class methods.

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We'll look at how command-line
parameters are handled.

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Here we see a program
receiving parameters,

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detecting the parameters
that will make it do things differently.

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The first parameter indicates
the action to be carried out.

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We will design the program so that
we have one class per action type.

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The "eval" action is represented
by one class:

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eval CommandLineHandler.

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What action is carried out
when the "eval" parameter appears?

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It executes...

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This chain of characters
is interpreted as Pharo code,

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and return the result
of the expression.

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3628800 is the result
of 10 factorial.

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This is implemented

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using the Command-Line Handler
class.

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EvaluateCommandLineHandler
is a subclass

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assigned to processing
the "eval" parameter.

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If "eval" is sent as the first parameter
in a command line,

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this class has to work.

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The isReponsibleFor method
must return true

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if there is one subclass

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capable of taking the command line
into account.

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Underneath, we see

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a mechanism identical to the one
we saw in the preceding example.

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Once again, we are using class methods
and sending messages to classes

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for tasks other than creating instances.

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This sequence showed us
two possible uses for class methods.

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Class methods
are mostly used to create new instances.

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But they can also be used
in other cases.