I understand how to implement a (validating) setter (def item=), but how do I intercept the << operation on a field?
class Bla
attr_reader :item
def initialize
#item = []
end
# only called for =, +=, -= operations (not <<)
def item=(value)
puts "Changing value to #{value}"
# pretend that there is a conditional here
#item = value
end
# This is wrong:
#def item<<(value)
# puts "adding value #{value}"
# #item << value
#end
end
b = Bla.new
b.item = ['one'] # works
b.item += ['one'] # works
b.item << 'two' # bypasses my setter
I've tried def item<<(value), that doesn't seem to work.
When you call b.item << 'two', you are calling the << method on item directly. So you have a few options here:
Implement << directly on your Bla class, then use b << 'two':
# in class Bla
def <<(value)
# do validation here
#item << value
end
Use some other, nicer-named wrapper method name like add_item:
# in class Bla
def add_item(value)
# do validation here
#item << value
end
Use a special array class for #item which has a custom definition for <<:
class MyArray < Array
def <<(item)
# run validation here
super
end
end
# in Bla class
def initialize
#item = MyArray.new
end
I would probably go with option 2, it's the most simple and readable.
Here is an alternate suggestion:
class Bla
# DO NOT DEFINE A READER:
# attr_reader :item
def initialize
#items = []
end
def set_items(new_items)
puts "Changing value to #{new_items}"
# pretend that there is a conditional here
#items = new_items
end
def remove_item(item)
# pretend that there is a conditional here
#items -= items
end
def add_item(item)
# pretend that there is a conditional here
#items += items
end
end
By not exposing the #items object directly, you remain in full control of the interface for how the variable is manipulated.
(Unless a caller does something really hacky, like bla.instance_variable_get('#items')!!)
class Pit
def <<(dirt)
puts 'shovel ' + dirt
end
end
Pit.new << 'sandy loam'
# => shovel sandy loam
Related
I have a ruby class that has an Array as one of its instance variables. I'm trying to figure out how to validate data that is being pushed onto the array.
class Something
def things
#things ||= Array.new
end
end
So I can declare an instance and add stuff to the array pretty easily this way.
#s = Something.new
#s.things << "one"
#s.things << "two"
I tried to create a class method named things<<(inString) to handle the validation but that is not valid syntax. So what approach can I take?
Try something like:
things << data if valid?(data)
where valid? is your validation method.
Example:
...
# will push only when quantity is greater than 0
def push(quantity)
things << item if valid?(quantity)
end
private
def valid?(number)
number > 0
end
If you want to have you own ThingsArray just create an Array subclass and override the push (<<) method to make the validation before pushing:
class ThingsArray < Array
def << (item)
return unless valid?(item)
super
end
private
def valid?(item)
item > 0
end
end
class Something
def things
#things ||= ThingsArray.new
end
end
How about this?
class MyThings < Array
def << (item)
# do your validation with item
self.push(item) if item.valid?
end
end
class Something
def things(item)
#things ||= MyThings.new
end
end
I wrote the following code:
class Actions
def initialize
#people = []
#commands = {
"ADD" => ->(name){#people << name },
"REMOVE" => ->(n=0){ puts "Goodbye" },
"OTHER" => ->(n=0){puts "Do Nothing" }
}
end
def run_command(cmd,*param)
#commands[cmd].call param if #commands.key?(cmd)
end
def people
#people
end
end
act = Actions.new
act.run_command('ADD','joe')
act.run_command('ADD','jack')
puts act.people
This works, however, when the #commands hash is a class variable, the code inside the hash doesn't know the #people array.
How can I make the #commands hash be a class variable and still be able to access the specific object instance variables?
You could use instance_exec to supply the appropriate context for the lambdas when you call them, look for the comments to see the changes:
class Actions
# Move the lambdas to a class variable, a COMMANDS constant
# would work just as well and might be more appropriate.
##commands = {
"ADD" => ->(name) { #people << name },
"REMOVE" => ->(n = 0) { puts "Goodbye" },
"OTHER" => ->(n = 0) { puts "Do Nothing" }
}
def initialize
#people = [ ]
end
def run_command(cmd, *param)
# Use instance_exec and blockify the lambdas with '&'
# to call them in the context of 'self'. Change the
# ##commands to COMMANDS if you prefer to use a constant
# for this stuff.
instance_exec(param, &##commands[cmd]) if ##commands.key?(cmd)
end
def people
#people
end
end
EDIT Following #VictorMoroz's and #mu's recommendations:
class Actions
def initialize
#people = []
end
def cmd_add(name)
#people << name
end
def cmd_remove
puts "Goodbye"
end
def cmd_other
puts "Do Nothing"
end
def people
p #people
end
def run_command(cmd, *param)
cmd = 'cmd_' + cmd.to_s.downcase
send(cmd, *param) if respond_to?(cmd)
end
end
act = Actions.new
act.run_command('add', 'joe')
act.run_command(:ADD, 'jill')
act.run_command('ADD', 'jack')
act.run_command('people') # does nothing
act.people
Or
class Actions
ALLOWED_METHODS = %w( add remove other )
def initialize
#people = []
end
def add(name)
#people << name
end
def remove
puts "Goodbye"
end
def other
puts "Do Nothing"
end
def people
p #people
end
def run_command(cmd, *param)
cmd = cmd.to_s.downcase
send(cmd, *param) if ALLOWED_METHODS.include?(cmd)
end
end
act = Actions.new
act.run_command('add', 'joe')
act.run_command(:add, 'jill')
act.run_command('add', 'jack')
act.run_command('people') # does nothing
act.people
Just for fun, again, but is it possible to take a block that contains method definitions and add those to an object, somehow? The following doesn't work (I never expected it to), but just so you get the idea of what I'm playing around with.
I do know that I can reopen a class with class << existing_object and add methods that way, but is there a way for code to pass that information in a block?
I guess I'm trying to borrow a little Java thinking here.
def new(cls)
obj = cls.new
class << obj
yield
end
obj
end
class Cat
def meow
puts "Meow"
end
end
cat = new(Cat) {
def purr
puts "Prrrr..."
end
}
cat.meow
# => Meow
# Not working
cat.purr
# => Prrrr...
EDIT | Here's the working version of the above, based on edgerunner's answer:
def new(cls, &block)
obj = cls.new
obj.instance_eval(&block)
obj
end
class Cat
def meow
puts "Meow"
end
end
cat = new(Cat) {
def purr
puts "Prrrr..."
end
}
cat.meow
# => Meow
cat.purr
# => Prrrr...
You can use class_eval(also aliased as module_eval) or instance_eval to evaluate a block in the context of a class/module or an object instance respectively.
class Cat
def meow
puts "Meow"
end
end
Cat.module_eval do
def purr
puts "Purr"
end
end
kitty = Cat.new
kitty.meow #=> Meow
kitty.purr #=> Purr
kitty.instance_eval do
def purr
puts "Purrrrrrrrrr!"
end
end
kitty.purr #=> Purrrrrrrrrr!
Yes
I suspect you thought of this and were looking for some other way, but just in case...
class A
def initialize
yield self
end
end
o = A.new do |o|
class << o
def purr
puts 'purr...'
end
end
end
o.purr
=> purr...
For the record, this isn't the usual way to dynamically add a method. Typically, a dynamic method starts life as a block itself, see, for example, *Module#define_method*.
(Big edit, I got part of the way there…)
I've been hacking away and I've come up with this as a way to specify things that need to be done before attributes are read:
class Class
def attr_reader(*params)
if block_given?
params.each do |sym|
define_method(sym) do
yield
self.instance_variable_get("##{sym}")
end
end
else
params.each do |sym|
attr sym
end
end
end
end
class Test
attr_reader :normal
attr_reader(:jp,:nope) { changethings if #nope.nil? }
def initialize
#normal = "Normal"
#jp = "JP"
#done = false
end
def changethings
p "doing"
#jp = "Haha!"
#nope = "poop"
end
end
j = Test.new
p j.normal
p j.jp
But changethings isn't being recognised as a method — anyone got any ideas?
You need to evaluate the block in the context of the instance. yield by default will evaluate it in its native context.
class Class
def attr_reader(*params, &blk)
if block_given?
params.each do |sym|
define_method(sym) do
self.instance_eval(&blk)
self.instance_variable_get("##{sym}")
end
end
else
params.each do |sym|
attr sym
end
end
end
end
Here's another alternative approach you can look at. It's not as elegant as what you're trying to do using define_method but it's maybe worth looking at.
Add a new method lazy_attr_reader to Class
class Class
def lazy_attr_reader(*vars)
options = vars.last.is_a?(::Hash) ? vars.pop : {}
# get the name of the method that will populate the attribute from options
# default to 'get_things'
init_method = options[:via] || 'get_things'
vars.each do |var|
class_eval("def #{var}; #{init_method} if !defined? ##{var}; ##{var}; end")
end
end
end
Then use it like this:
class Test
lazy_attr_reader :name, :via => "name_loader"
def name_loader
#name = "Bob"
end
end
In action:
irb(main):145:0> t = Test.new
=> #<Test:0x2d6291c>
irb(main):146:0> t.name
=> "Bob"
IMHO changing the context of the block is pretty counter-intuitive, from a perspective of someone who would use such attr_reader on steroids.
Perhaps you should consider plain ol' "specify method name using optional arguments" approach:
def lazy_attr_reader(*args, params)
args.each do |e|
define_method(e) do
send(params[:init]) if params[:init] && !instance_variable_get("##{e}")
instance_variable_get("##{e}")
end
end
end
class Foo
lazy_attr_reader :foo, :bar, :init => :load
def load
#foo = 'foo'
#bar = 'bar'
end
end
f = Foo.new
puts f.bar
#=> bar
Sorry for the poor title, I don't really know what to call this.
I have something like this in Ruby:
class Test
def initialize
#my_array = []
end
attr_accessor :my_array
end
test = Test.new
test.my_array << "Hello, World!"
For the #my_array instance variable, I want to override the << operator so that I can first process whatever is being inserted to it. I've tried #my_array.<<(value) as a method in the class, but it didn't work.
I think you're looking for this:
class Test
def initialize
#myarray = []
class << #myarray
def <<(val)
puts "adding #{val}" # or whatever it is you want to do first
super(val)
end
end
end
attr_accessor :myarray
end
There's a good article about this and related topics at Understanding Ruby Singleton Classes.
I'm not sure that's actually something you can do directly.
You can try creating a derived class from Array, implementing your functionality, like:
class MyCustomArray < Array
def initialize &process_append
#process_append = &process_append
end
def << value
raise MyCustomArrayError unless #process_append.call value
super.<< value
end
end
class Test
def initialize
#my_array = MyCustomArray.new
end
attr_accessor :my_array
end
Here you go...
$ cat ra1.rb
class Aa < Array
def << a
puts 'I HAVE THE CONTROL!!'
super a
end
end
class Test
def initialize
#my_array = Aa.new
end
attr_accessor :my_array
end
test = Test.new
test.my_array << "Hello, World!"
puts test.my_array.inspect
$ ruby ra1.rb
I HAVE THE CONTROL!!
["Hello, World!"]
$
a = []
a.instance_eval("alias old_add <<; def << value; puts value; old_add(value); end")
Very hackish, and off the top of my head ...
Just change 'puts value' with whatever preprocessing you want to do.
You can extend the metaclass of any individual object, without having to create a whole new class:
>> i = []
=> []
>> class << i
>> def <<(obj)
>> puts "Adding "+obj.to_s
>> super
>> end
>> end
=> nil
>> i << "foo"
Adding foo
=> ["foo"]
i extend the class, creating a method which provides access to the instance variable.
class KeywordBid
def override_ignore_price(ignore_price)
#ignorePrice = ignore_price
end
end