Part 1 OCL Common Expressions
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hi and welcome to the session on ocl
expressions start with a simple model
containing of fruits orange as one
property
weight type integer and gonna start a
system running this saving data to an xml file
that we save on in temp file "fruit"
I'm going to use the new debugger
so if you haven't seen this before
it's a good introduction to that as well
and the new debugger allows you to
write actions as long as ocl, so we
could do orange.create and then
we have actually created one orange object
an instance of the class, so let's
create some oranges and once we have them we
can do orange.all instances
which is a common ocl operator to show all
instances of a certain class
as we see the weight is 0. It's the default value for
integer that's not nullable
and we can index the list of all instances with
the  "at(0) operator
so at 0 there's also an at operator, but at(0) is
zero based the at operator is one based
so starting from zero to count minus 1
or length minus one
and let's do an action at first object at list
assigning the weight to 18
so this is the assign operator colon equals and
we can set the weight values
something like that and then we'd save,
now we have saved them to the xml file
that we stated initially and in ocl we can
do a lot of different operations
let's check out the "orderby", orderby(weight)
and there's also an orderby this ending
and in all these iterations, there
can be a loop variable and
it's separated with the pipe sign, so that's
what you here, the loopvar
it is just each object iterating
in the loop, so it will be of the type
that interested iteration, so orderDescending,
or orderby
results an operation for first and of
course one for last that the given a list
just returns one object and you can
combine ocl expressions
so order descending last
the ocl editor has been given an
overhaul to better support
support you as you type your expressions it's
not perfect and we will see some issues with it
but it's getting there
select is a common ocl operator
that we want to limit our results at
here we select on the weight larger than 20,
but the loop variable is optional,
so you do not always need it
all kind of comparison operators are
available larger than or equal etc.
let's add another class, apple
also has a weight
and if I do apple.all instances apple is
not a type name, because I need to
reread the model in order to have my
executor to know about it
so once I re-read and it doesn't give me
the error, but there are no apples of course
so write action: and then Apple.create
get two of those, five and the new debugger
here interprets all continuous lines
as one expression, so you need to
leave an empty roll between
expressions that are not supposed to
belong to each others
and of course it defaults to ocl but as soon as you
write action: it will do action language instead
which allows you to set values to operate on values to change state
normally ocl is totally free from side effects
but the action language that is unique for MDriven
allows you to operate objects as well
with the ocl syntax so we're
setting some weights on the apples
and we saved that state
so Apple.all instances also have the
oranges since before
these are the oranges
these are the apples and what we can do
is combine them, the operator called "union"
so there we are together, but we don't
see not very clear what they are
so let's add the default string
representation to the classes
this is one place where you actually use ocl
in the model of course you use it all
over the model but this is just one of
the places "orange with weight of
and then we grab our "selves" weight and append
grams, but since weight is an integer
we need to convert it to as #sString in
order to get use a +operator on strings
let's do the same thing for the Apple
but right Apple instead save the model
yes still don't see anything because we
haven't said that we want
to see it as string, the default string
representation on the obvious now I see them
what I can do is use to collect
operator, to build a tuple of pieces from
the result that I'm working on
so the loop variable will be one of the
objects in the iteration and that should
have weight and I can do that as string
if I can spell, but now we see that weigh it,
this is not correct weight is not a
member of eco model root
why does it say EcoModelRoot well
because that's the common denominator
between apple and orange currently,
so here we have discovered a need for an
abstraction really apple and orange are
really sharing things
we should make that available in the model
so that we can operate on it
so let's interview the abstraction
called fruit and we model this with the
generalization arrow
and we want to add the attribute
weight to fruit, but doing so we will find that there's error
reported on model,
because now orange and apple inherit weight from fruit and they
have fruit on their own so that's not allowed
but renaming it to new weight instead of weight,
so we can see that we get it work
new weight, then it's ok but of course
the new wa is 0, because we haven't set
but now we can do act on the
abstraction the fruit fruit.allinstansec
as string, that will return both
oranges and apples,
but it reports common denominator as new weight is the
only thing they share
so let's say that this is the weight and
there is actually no need for a unique
weight to apples or unique weight on oranges
it's just weight of the fruit
then we see the weight on the fruit
so this is why modeling is so good
because it works exactly as we think
about things in daily life, we use
abstractions all the time just getting
around life, just grabbing a chair on our
new office that we've never visited before
him instantly knowing that it
will work as any other chair is our ability to abstract
but having the fruits can we access
their specialization what they're
actually called what kind of fruit they are
yes, there's some ocl-type operator with
the type name that gives us this information
orange and apple, there we see, so that would
allow us to break out the hard-coded orange
to self ocl-type typename
remove the orange and of course if we do
it there we might as well
yeah let's, look at this first
orange.string is still ok, so if we can
do that we might as well move it up to fruit
going to remove it from the
subclasses just have it on the
superclass fruit, so like that, re-read
yay works like before but with less code or less ocl
the fruit is actually abstract we don't think of fruit as being
there's no, nothing existing just
being fruit is either orange or Apple in our model
so I mark it as abstract and
the class name is drawn italic instead
I can create apples, but I'm not supposed
to be able to create fruit
can't instantiate abstract class fruit, this
will help us to avoid doing illegal
things as our model evolves, orange I can
create oranges and apples but not fruits
that's ok and I can access fruit all instances or
just the oranges or just the apples
and I can act on fruits checking their
weight and I get us all the fruits
having a weight larger than 20 grams both oranges
and apples and move this or rather copy it
to a new attribute, attributes
this attribute is called presentation
not going to make it derived, derived
attribute can be backed by an derivation ocl
and ocl expression this is another
place where you commonly use ocl going
to set the same thing as in the default
string representation, but since I
defined it as a derived attribute might
as well use it in the default string representation
reduce the definitions,
re-read the model, since we changed it and
now when I execute I see I have a new
property on the object fruit called presentation
that works for apples and the first
Apple is Apple would wait 15 grams
let's see, if I can change its weight
and watch when I change the weight both
the derived presentation and the weight changes
this is an extremely large and
positive thing with derived attributes
they subscribe to every change that you
see in the model reduces the amount of
code you need to write or the amount of
ocl, so that was the first part of the ocl
I'm going to come back straight away
with another part continuing with
the same sample thank you for watching, so far

Revision as of 20:33, 15 April 2017

An introduction to OCL - the object contraints language - by example. The session also introduce the new MDriven debugger and shows how to get instant results of modelling efforts.

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Common expressions
    .create .allinstances .select .asstring .representation
at0 and at operator assign operator orderby orderbyDescending loopvar ocl editor overhaul support execution of expressions union operator ocl use in the model (string representation) collect operator member of ECOModel Root abstract classes and its inheritance specialization .OclType derived attribute

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