class ObjectidColumns::DynamicMethodsModule
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 usingsuper
. You’re reduced to usingalias_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.
Public Class Methods
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!
.
# 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
Public Instance Methods
Defines a class method.
# 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
Defines a method. Works identically to Module#define_method, except that it’s public
and remove_all_methods!
will remove the method.
# 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
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.)
# 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