Class: ObjectidColumns::DynamicMethodsModule

Inherits:
Module
  • Object
show all
Defined in:
lib/objectid_columns/dynamic_methods_module.rb

Overview

A DynamicMethodsModule is used to add dynamically-generated methods to an existing class.

Why do we need a module to do that? Why can’t we simply call #define_method on the class itself?

We could. However, if you do that, a few problems crop up:

  • There is no precendence that you can control. If you define a method :foo on class Bar, then that method is always run when an instance of that class is sent the message :foo. The only way to change the behavior of that class is to completely redefine that method, which brings us to the second problem…

  • Overriding and super doesn’t work. That is, you can’t override such a method and call the original method using super. You’re reduced to using alias_method_chain, which is a mess.

  • There’s no namespacing at all – at runtime, it’s not even remotely clear where these methods are coming from.

  • Finally, if you’re living in a dynamic environment – like Rails’ development mode, where classes get reloaded very frequently – once you define a method, it is likely to be forever defined. You have to write code to keep track of what you’ve defined, and remove it when it’s no longer present.

A DynamicMethodsModule fixes these problems. It’s little more than a Module that lets you define methods (and helpfully makes #define_method public to help), but it also will include itself into a target class and bind itself to a constant in that class (which magically gives the module a name, too). Further, it also keeps track of which methods you’ve defined, and can remove them all with #remove_all_methods!. This allows you to construct a much more reliable paradigm: instead of trying to figure out what methods you should remove and add when things change, you can just call #remove_all_methods! and then redefine whatever methods currently should exist.

A DynamicMethodsModule also supports class methods; if you define a method with #define_class_method, it will be added to a module that the target class has called extend on (rather than include), and hence will show up as a class method on that class. This is useful for the exact same reasons as the base DynamicMethodsModule; it allows for precedence control, use of super, namespacing, and dynamism.

Instance Method Summary collapse

Constructor Details

#initialize(target_class, name, &block) ⇒ DynamicMethodsModule

Creates a new instance. target_class is the Class into which this module should include itself; name is the name to which it should bind itself. (This will be bound as a constant inside that class, not at top-level on Object; so, for example, if target_class is User and name is Foo, then this module will end up named User::Foo, not simply Foo.)

If passed a block, the block will be evaluated in the context of this module, just like Module#new. Note that you should not use this to define methods that you want #remove_all_methods!, below, to remove; it won’t work. Any methods you add in this block using normal def will persist, even through #remove_all_methods!.

Raises:

  • (ArgumentError) 


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# File 'lib/objectid_columns/dynamic_methods_module.rb', line 38

def initialize(target_class, name, &block)
  raise ArgumentError, "Target class must be a Class, not: #{target_class.inspect}" unless target_class.kind_of?(Class)
  raise ArgumentError, "Name must be a Symbol or String, not: #{name.inspect}" unless name.kind_of?(Symbol) || name.kind_of?(String)

  @target_class = target_class
  @name = name.to_sym

  # Unfortunately, there appears to be no way to "un-include" a Module in Ruby -- so we have no way of replacing
  # an existing DynamicMethodsModule on the target class, which is what we'd really like to do in this situation.

  # Sigh. From the docs for Method#arity:
  #
  # "For Ruby methods that take a variable number of arguments, returns -n-1, where n is the number of required
  # arguments. For methods written in C, returns -1 if the call takes a variable number of arguments."
  #
  # It turns out that .const_defined? is written in C, which means it returns -1 if it takes a variable number of
  # arguments. So we can't check for arity.abs >= 2 here, but rather must look for <= -1...
  if @target_class.method(:const_defined?).arity <= -1
    if @target_class.const_defined?(@name, false)
      existing = @target_class.const_get(@name, false)

      if existing && existing != self
        raise NameError, %{You tried to define a #{self.class.name} named #{name.inspect} on class #{target_class.name},
  but that class already has a constant named #{name.inspect}: #{existing.inspect}}
      end
    end
  else
    # So...in Ruby 1.8.7, .const_defined? and .const_get don't accept the second parameter, which tells you whether
    # to search superclass constants as well. But, amusingly, we're not only not stuck, this is fine: in Ruby
    # 1.8.7, constant lookup doesn't search superclasses, either -- so we're OK.
    if @target_class.const_defined?(@name)
      existing = @target_class.const_get(@name)

      if existing && existing != self
        raise NameError, %{You tried to define a #{self.class.name} named #{name.inspect} on class #{target_class.name},
  but that class already has a constant named #{name.inspect}: #{existing.inspect}}
      end
    end
  end


  @class_methods_module = Module.new
  (class << @class_methods_module; self; end).send(:public, :private)
  @target_class.const_set("#{@name}ClassMethods", @class_methods_module)
  @target_class.send(:extend, @class_methods_module)

  @target_class.const_set(@name, self)
  @target_class.send(:include, self)

  @methods_defined = { }
  @class_methods_defined = { }

  super(&block)
end

Instance Method Details

#define_class_method(name, &block) ⇒ Object

Defines a class method.



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# File 'lib/objectid_columns/dynamic_methods_module.rb', line 117

def define_class_method(name, &block)
  @class_methods_module.send(:define_method, name, &block)
end

#define_method(name, &block) ⇒ Object

Defines a method. Works identically to Module#define_method, except that it’s public and #remove_all_methods! will remove the method.



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# File 'lib/objectid_columns/dynamic_methods_module.rb', line 110

def define_method(name, &block)
  name = name.to_sym
  super(name, &block)
  @methods_defined[name] = true
end

#remove_all_methods! ⇒ Object

Removes all methods that have been defined on this module using #define_method, below. (If you use some other mechanism to define a method on this DynamicMethodsModule, then it will not be removed when this method is called.)



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# File 'lib/objectid_columns/dynamic_methods_module.rb', line 96

def remove_all_methods!
  instance_methods.each do |method_name|
    # Important -- we use Class#remove_method, not Class#undef_method, which does something that's different in
    # some important ways.
    remove_method(method_name) if @methods_defined[method_name.to_sym]
  end

  @class_methods_module.instance_methods.each do |method_name|
    @class_methods_module.send(:remove_method, method_name) if @class_methods_defined[method_name]
  end
end