How I can get all instance method names in the baz method call, which are only present in the Bar module (without other instance methods of this class) ?
class Foo
include Bar
def a
end
def b
end
def baz
#HERE
end
end
class Foo
include Bar
def a
end
def b()
end
def baz
Bar.instance_methods(false)
end
end
puts Foo.new.baz
Related
In this example code where I have child classes (Bar, Baz) inheriting the class methods of the parent (Foo), how might I ensure #foo is only created once across all children?
class Foo
def self.foo
# only want #foo to be set once across any child classes
# that may call this inherited method.
#foo ||= expensive_operation
end
end
class Bar < Foo
def self.bar
self.foo + 'bar'
end
end
class Baz < Foo
def self.baz
self.foo + 'baz'
end
end
Instead of depending on a class-specific instance variable being present, just reference it directly:
class Baz < Foo
def self.baz
Foo.foo + 'baz'
end
end
This is exactly what class instance variable is intended for - language mechanism to have one variable shared between classes which inherit from a class. It's generally not recommended to use because it behaves like you want to - and that's confusing for people coming from other languages.
class Foo
def self.foo
# notice ##
##foo ||= expensive_operation
end
def self.expensive_operation
puts "Expensive operation"
"cached value "
end
end
class Bar < Foo
def self.bar
self.foo + 'bar'
end
end
class Baz < Foo
def self.baz
self.foo + 'baz'
end
end
Foo.foo
Bar.bar
Baz.baz
This prints Expensive operation only once.
The following prints Bar twice:
class Foo
def foo
p self.class # => prints Bar
end
end
class Bar < Foo
def foo
p self.class # => prints Bar
super
end
end
b = Bar.new
b.foo
How do I get it to print
Bar
Foo
? i.e. I want to know what class each method is defined on.
To capture the context in which a method was originally defined, you can use define_method instead of def to get the appropriate closure. A simple example:
class Foo
klass = self
define_method(:foo){p klass}
end
class Bar < Foo
def foo
p self.class
super
end
end
b = Bar.new
b.foo
You could change Foo#foo like so (provided there is just one subclass level):
class Foo
def foo
if self.class == Foo
p self.class
else
p self.class.superclass
end
end
end
class Bar < Foo
def foo
p self.class
super
end
end
Foo.new.foo
Foo
Bar.new.foo
Bar
Foo
You can use
b.class.superclass <= "Foo"
The problem you are having there is that self is the instance of Bar, b.
b.class <= always going to be Bar
self.class <= always going to be Bar if you are invoking Bar.
You say that you are defining a method at runtime, and that you don't know the class name. I don't really know what you mean ... the way I would handle this would be something like
class Bar
def initialize
puts 'In BAR class'
end
def foo
p self.class.name # => prints Bar
end
end
and then
Bar.class_eval do
def brand_new_method
# do something new
p "Still in Bar, but this is dynamically added"
end
end
Maybe you are talking about dynamically adding methods to classes higher in the inheritance chain ... to "Foo" in your example ... based on some conditional happening in an instance of "Bar". If that is the case, then why don't you use a single module to define your inherited methods:
module Foo
def foo
p self.class
end
end
and then use module_eval the same way as class_eval?
I know, I can overwrite class method from module this way
class Foo
class << self
def some_static_method
puts 'some_static_method'
end
end
end
module BAR
class << Foo
def some_static_method
puts 'another_static_method'
end
end
end
class Foo
include BAR
end
Foo.some_static_method # => 'another_static_method'
Is it possible for an instance method?
You can do the following:
class Foo
def self.some_static_method; puts "Hello from Foo" end
end
module Bar
def self.included(base)
base.instance_eval do
def some_static_method; puts "Hello from Bar" end
end
end
end
class Foo
include Bar
end
Foo.some_static_method
This should work
UPDATE
To override instance method use:
class Foo
def some_instance_method; puts "Hello from Foo" end
end
module Bar
def self.included(base)
base.class_eval do
def some_instance_method; puts "Hello from Bar" end
end
end
end
class Foo
include Bar
end
Foo.new.some_instance_method
Your question is actually not about method overriding. It is about what class is referred to within a class ... construction in a module body.
When you do
module Bar
class << Foo
p self
end
end
# => #<Class:Foo>
the << Foo points to the singleton class of the Foo in the main environment because class << Foo cannot define the singleton class directly of a class Foo that has not been defined in advance. So it looks up for Foo that is already defined, and such class is found in the main environment.
When you do
module Bar
class Foo
p self
end
end
# => Bar::Foo
a new class Bar::Foo is created; the Foo points to this Bar::Foo that is newly created, and it does not point to the Foo in the main environment. In order to point to it, you have to explicitly specify that with ::.
module Bar
class ::Foo
p self
end
end
# => Foo
If you are using Ruby > 2.0.0 then what you can use is Module#prepend. Instead of include you can prepend an module and that way all of the module's methods are overriding any existing class instance methods with the same name. You can see a quick example here.
Prior to Ruby 2, Rails had introduced a similar hack: #alias_method_chain
Here is a nice comparison of the two approaches.
Given a module with a singleton method like this:
module Foo
class << self
def bar
puts "method bar from Foo"
end
end
end
How can I override Foo.bar using another module?
This code
module Foo
class << self
def bar
puts "method bar from Foo"
end
end
end
is equal to
class << Foo
def bar
puts "method bar from Foo"
end
end
that is also equal to
def Foo.bar
puts "method bar from Foo"
end
So, you can call it to redefine this method everywhere where Foo is defined (ever withing another module, without including Foo).
Extend and alias
My problem was that I forgot to think through the inheritance chain. I was
looking for a way to override the method by modifying the inheritance chain, but
that's not possible.
The reason is that bar is defined on Foo itself, so it never looks up its inheritance chain for the method. Therefore, to change bar, I have to change it on Foo itself.
While I could just re-open Foo, like this:
module Foo
def self.bar
puts "new foo method"
end
end
... I prefer a way to be able to wrap the original bar method, as though I
were subclassing and could call super. I can achieve that by setting up an
alias for the old method.
module Foo
class << self
def bar
"method bar from Foo"
end
end
end
puts Foo.bar # => "method bar from Foo"
module FooEnhancement
# Add a hook - whenever a class or module calls `extend FooEnhancement`,
# run this code
def self.extended(base)
# In the context of the extending module or class
# (in this case, it will be Foo), do the following
base.class_eval do
# Define this new method
def self.new_bar
"#{old_bar} - now with more fiber!"
end
# Set up aliases.
# We're already in the context of the class, but there's no
# `self.alias`, so we need to call `alias` inside this block
class << self
# We can call the original `bar` method with `old_bar`
alias :old_bar :bar
# If we call `bar`, now we'll get our `new_bar` method
alias :bar :new_bar
end
end
end
end
# This will fire off the self.extended hook in FooEnhancement
Foo.extend FooEnhancement
# Calls the enhanced version of `bar`
puts Foo.bar # => 'method bar from Foo - now with more fiber!'
You can do the following:
module Foo
class << self
def bar
puts "method bar from Foo"
end
def baz
puts "method baz from Foo"
end
end
end
module Foo2
def Foo.bar
puts "new version of bar"
end
end
include Foo
Foo.baz #=> method baz from Foo
Foo.bar #=> new version of bar
Or instead of naming it Foo2 simply re-open Foo.
module Foo
class << self
def bar
puts "method bar from Foo"
end
def baz
puts "method baz from Foo"
end
end
end
module Foo
class << self
def bar
puts "new version of bar"
end
end
end
include Foo
Foo.baz #=> method baz from Foo
Foo.bar #=> new version of bar
In class Foo I'd like to include method Bar under certain conditions:
module Bar
def some_method
"orly"
end
end
class Foo
def initialize(some_condition)
if !some_condition
"bar"
else
class << self; include Bar; end
end
end
end
Is there any cleaner (and clearer) way to achieve the include in the method without having to do it inside the singleton class?
extend is the equivalent of include in a singleton class:
module Bar
def some_method
puts "orly"
end
end
class Foo
def initialize(some_condition)
extend(Bar) if some_condition
end
end
Foo.new(true).some_method # => "orly"
Foo.new(false).some_method # raises NoMethodError