Call method on attribute update

Hello,

I have an attribute in my class called @yn. If I change the value of @yn
by doing this:

foo.yn = 100.0

How do I cause another method in the class to automatically run? This is
what I want to do:

If @yn is updated, automatically run method “foo.calcArea”. The way I
have it now requires me to do:

foo.yn = 100.0
foo.calcArea

There must be a way to let ruby call that method for me.

Thank you!

On Oct 20, 2011, at 1:38 AM, Jason L. wrote:

If @yn is updated, automatically run method “foo.calcArea”. The way I
have it now requires me to do:

foo.yn = 100.0
foo.calcArea

There must be a way to let ruby call that method for me.

Just define the attribute writer method yourself instead of using the
helpers:

class Foo
attr_reader :yn
def yn=(value)
@yn = value
calcArea
end

def calcArea
puts “recomputing area”
end
end

f = Foo.new #=> #Foo:0x1895458
f.yn = 100 #=> 100
recomputing area

f.yn #=> 100

Gary W.

On Thu, Oct 20, 2011 at 8:01 AM, Gary W. [email protected] wrote:

puts “recomputing area”

If you find yourself doing that a lot, you could create a little
utility:

class Class
def attr_writer_with_callbacks attr, *callbacks
instance_eval do
define_method("#{attr}=") do |value|
instance_variable_set “@#{attr}”, value
callbacks.each {|callback| send callback}
end
end
end
end

ruby-1.8.7-p334 :038 > class A
ruby-1.8.7-p334 :039?> attr_writer_with_callbacks :test, :updated,
:modified
ruby-1.8.7-p334 :040?> def updated
ruby-1.8.7-p334 :041?> puts “updated”
ruby-1.8.7-p334 :042?> end
ruby-1.8.7-p334 :043?> def modified
ruby-1.8.7-p334 :044?> puts “also modified”
ruby-1.8.7-p334 :045?> end
ruby-1.8.7-p334 :046?> end
=> nil
ruby-1.8.7-p334 :047 > A.new.test = “value”
updated
also modified
=> “value”

Jesus.

Jason L. wrote in post #1027518:

Hello,

I have an attribute in my class called @yn. If I change the value of @yn
by doing this:

foo.yn = 100.0

How do I cause another method in the class to automatically run?

class Foo
attr_accessor :yn

This is the general way to wrap a method call

alias :old_yn= :yn=
def yn=(val)
self.old_yn = val
puts “Yay!”
end
end

foo = Foo.new
foo.yn = 123

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