Eventmachine talk - Nov 2006 - Why instance variables not visible in

When using a Connection class and setting variables via instance_eval,
they are nil in post_init, but available in receive_data.  Where is
the best place to initialize per connection variables that depend on
the variables I created via instance_eval?

Chris

On 11/6/06, snacktime <snacktime at gmail.com>
wrote:
> When using a Connection class and setting variables via instance_eval,
> they are nil in post_init, but available in receive_data.  Where is
> the best place to initialize per connection variables that depend on
> the variables I created via instance_eval?
>
> Chris
best way to achieve this..to use class_eval and defer the
evaluation...till the class is intantiated or inherited as _why has
demonstrated in Dwemthy''s Army.

Though..i am no expert in meta programming...i may be able to help..if
you post the code snippet.

--
There was only one Road; that it was like a great river: its springs
were at every doorstep, and every path was its tributary.

On 11/6/06, hemant <gethemant at gmail.com> wrote:
> On 11/6/06, snacktime <snacktime at gmail.com> wrote:
> > When using a Connection class and setting variables via instance_eval,
> > they are nil in post_init, but available in receive_data.  Where is
> > the best place to initialize per connection variables that depend on
> > the variables I created via instance_eval?
> >
> > Chris
>
> best way to achieve this..to use class_eval and defer the
> evaluation...till the class is intantiated or inherited as _why has
> demonstrated in Dwemthy''s Army.
>
> Though..i am no expert in meta programming...i may be able to help..if
> you post the code snippet.
>
Something like this:

class Foobar < EventMachine::Connection

def self.metaclass;class <<self;self;end;end;
def self.traits
  # initialize your instance variables here..with instance_eval of
preferably class_eval
end
traits :variables
end



--
There was only one Road; that it was like a great river: its springs
were at every doorstep, and every path was its tributary.

On 11/6/06, snacktime <snacktime at gmail.com>
wrote:
>  When using a Connection class and setting variables via instance_eval,
> they are nil in post_init, but available in receive_data.  Where is
> the best place to initialize per connection variables that depend on
> the variables I created via instance_eval?


Chris, I''d add this to Hemant''s good suggestion:
There is a defined protocol for how these methods get called, and I''ve
tried to be careful about not changing it. For example, I found the
following comment in the source code for EventMachine::connect:

  # Ok, added support for a user-defined block, 13Apr06.
  # This leads us to an interesting choice because of the
  # presence of the post_init call, which happens in the
  # initialize method of the new object. We call the user''s
  # block and pass the new object to it. This is a great
  # way to do protocol-specific initiation. It happens
  # AFTER post_init has been called on the object, which I
  # certainly hope is the right choice.
  # Don''t change this lightly, because accepted connections
  # are different from connected ones and we don''t want
  # to have them behave differently with respect to post_init
  # if at all possible.

On 11/6/06, hemant <gethemant at gmail.com> wrote:
> On 11/6/06, snacktime <snacktime at gmail.com> wrote:
> > When using a Connection class and setting variables via instance_eval,
> > they are nil in post_init, but available in receive_data.  Where is
> > the best place to initialize per connection variables that depend on
> > the variables I created via instance_eval?
> >
> > Chris
>
> best way to achieve this..to use class_eval and defer the
> evaluation...till the class is intantiated or inherited as _why has
> demonstrated in Dwemthy''s Army.
>
> Though..i am no expert in meta programming...i may be able to help..if
> you post the code snippet.
Basically what I want is to pass server specific variables to the
Connection class like what host/port it is running on, and there can
be multiple servers using the same Connection class.  If I use class
variables, then as far as my understanding goes they are global across
all instances of that class, which I don''t want.   I can access
instance variables in receive_data, but then I end up with something
like this:

def receive_data data
  getconfig if connected?
  # process data
end

Well the solution was to call a method in the Connection class via
instance_eval  instead of just setting the variables, that way I can
run the setup code that is needed before receive_data is called, like
so:

server1 = EmProxy::ServerConfig.new(''localhost'',8080)
server1.add_backend(''localhost'',80)
EventMachine.start_server(server1.host, server1.port, EmProxy::HttpServer) {|c|
    c.instance_eval {setconfig(server1)}
  }

On 11/7/06, snacktime <snacktime at gmail.com>
wrote:
> On 11/6/06, hemant <gethemant at gmail.com> wrote:
> > On 11/6/06, snacktime <snacktime at gmail.com> wrote:
> > > When using a Connection class and setting variables via
instance_eval,
> > > they are nil in post_init, but available in receive_data.  Where
is
> > > the best place to initialize per connection variables that depend
on
> > > the variables I created via instance_eval?
> > >
> > > Chris
> >
> > best way to achieve this..to use class_eval and defer the
> > evaluation...till the class is intantiated or inherited as _why has
> > demonstrated in Dwemthy''s Army.
> >
> > Though..i am no expert in meta programming...i may be able to help..if
> > you post the code snippet.
>
> Basically what I want is to pass server specific variables to the
> Connection class like what host/port it is running on, and there can
> be multiple servers using the same Connection class.  If I use class
> variables, then as far as my understanding goes they are global across
> all instances of that class, which I don''t want.   I can access
> instance variables in receive_data, but then I end up with something
> like this:
>
> def receive_data data
>   getconfig if connected?
>   # process data
> end
Yes true..class variables are global to class. But you didn''t get my
point. What I was saying is:

class IntelligentLife

  def IntelligentLife.home_planet
    class_eval("@@home_planet")
  end

  def IntelligentLife.home_planet=(x)
    class_eval("@@home_planet = #{ x} ")
  end

   #...
end

class Terran < IntelligentLife
  @@home_planet = "Earth"
  #...
end

class Martian < IntelligentLife
  @@home_planet = "Mars"
  #...
end

puts Terran.home_planet            # Earth
puts Martian.home_planet           # Mars

You remove class_eval and both your puts will return the same
thing.(example is from Hal fulton''s Ruby way)

Another approach could be:

class Class

  def initialize(klass, &block)
    block = Proc.new
    class_eval(&block)
  end

end


class IntelligentLife

  def IntelligentLife.home_planet
    class_eval("@@home_planet")
  end

  def IntelligentLife.home_planet=(x)
    class_eval("@@home_planet = #{ x} ")
  end

  #...

end

Terran = Class.new(IntelligentLife) do
   @@home_planet = "Earth"
end

Martian = Class.new(IntelligentLife) do
   @@home_planet = "Mars"
end

I am already using it for writting connection classes. Where server
and client both classes will derive from this ConnectionClass and
override some of the defaults. I will dynamically hookup couple of
parsers and ready to go.

Anyway..in the end, whatever works for you.

--
There was only one Road; that it was like a great river: its springs
were at every doorstep, and every path was its tributary.

On 11/7/06, hemant <gethemant at gmail.com> wrote:
> On 11/7/06, snacktime <snacktime at gmail.com> wrote:
> > On 11/6/06, hemant <gethemant at gmail.com> wrote:
> > > On 11/6/06, snacktime <snacktime at gmail.com> wrote:
> > > > When using a Connection class and setting variables via
instance_eval,
> > > > they are nil in post_init, but available in receive_data. 
Where is
> > > > the best place to initialize per connection variables that
depend on
> > > > the variables I created via instance_eval?
> > > >
> > > > Chris
> > >
> > > best way to achieve this..to use class_eval and defer the
> > > evaluation...till the class is intantiated or inherited as _why
has
> > > demonstrated in Dwemthy''s Army.
> > >
> > > Though..i am no expert in meta programming...i may be able to
help..if
> > > you post the code snippet.
> >
> > Basically what I want is to pass server specific variables to the
> > Connection class like what host/port it is running on, and there can
> > be multiple servers using the same Connection class.  If I use class
> > variables, then as far as my understanding goes they are global across
> > all instances of that class, which I don''t want.   I can
access
> > instance variables in receive_data, but then I end up with something
> > like this:
> >
> > def receive_data data
> >   getconfig if connected?
> >   # process data
> > end
>
> Yes true..class variables are global to class. But you didn''t get
my
> point. What I was saying is:
>
> class IntelligentLife
>
>   def IntelligentLife.home_planet
>     class_eval("@@home_planet")
>   end
>
>   def IntelligentLife.home_planet=(x)
>     class_eval("@@home_planet = #{ x} ")
>   end
>
>    #...
> end
>
> class Terran < IntelligentLife
>   @@home_planet = "Earth"
>   #...
> end
>
> class Martian < IntelligentLife
>   @@home_planet = "Mars"
>   #...
> end
>
> puts Terran.home_planet            # Earth
> puts Martian.home_planet           # Mars
>
> You remove class_eval and both your puts will return the same
> thing.(example is from Hal fulton''s Ruby way)
>
> Another approach could be:
>
> class Class
>
>   def initialize(klass, &block)
>     block = Proc.new
>     class_eval(&block)
>   end
>
> end
>
>
> class IntelligentLife
>
>   def IntelligentLife.home_planet
>     class_eval("@@home_planet")
>   end
>
>   def IntelligentLife.home_planet=(x)
>     class_eval("@@home_planet = #{ x} ")
>   end
>
>   #...
>
> end
>
> Terran = Class.new(IntelligentLife) do
>    @@home_planet = "Earth"
> end
>
> Martian = Class.new(IntelligentLife) do
>    @@home_planet = "Mars"
> end
>
> I am already using it for writting connection classes. Where server
> and client both classes will derive from this ConnectionClass and
> override some of the defaults. I will dynamically hookup couple of
> parsers and ready to go.
>
> Anyway..in the end, whatever works for you.
>
> --
> There was only one Road; that it was like a great river: its springs
> were at every doorstep, and every path was its tributary.
>
If you update your local EM code...you may find that Francis has added
a reconnect method in EventMachine::Connection class. I find it pretty
useful.

So..i have reconnect hook defined in ConnectionClass itself. I define
an attribute like:

class ClientClass < ActiveTcp::ConnectionClass
  acts_as_client true
end

note that..though acts_as_client is a class method, but it sets
corresponding instance variable in derived class. So, whenever
ConnectionClass finds that acts_as_client is true for the current
object..and if connection is lost it will automatically try to
reconnect.

ClientClass, won''t be even aware of this.


--
There was only one Road; that it was like a great river: its springs
were at every doorstep, and every path was its tributary.

Thanks for the explanation, I didn''t understand it until your example.
 But what happens if you are using the same class for multiple
servers, like so:

class TcpConnection < EventMachine::Connection

    def TcpConnection.config
      class_eval("@@config")
    end

    def TcpConnection.config=(config)
      class_eval("@@config = #{config}")
    end
  end

class HttpServer < TcpConnection
end

And then you have multiple http servers:

EventMachine.start_server(host, port, EmProxy::HttpServer) {|c|
    c.class_eval {@@config=config1}
  }

EventMachine.start_server(host, port, EmProxy::HttpServer) {|c|
    c.class_eval {@@config=config2}
  }

Will that work, or will @@config = config2 for both instances of HttpServer?

Chris

Ya nevermind that won''t work, but you can see what I''m trying
to do.