Question

I know this is not specific to tk, but rather a more general problem with ruby DSLs and instance_eval, but here is my question, if I want to use the DSL for tk, I can’t figure out how to make certain things work. For example, if I want to call a instance method by pressing a button, this will not work, because it thinks I am trying to call a method with the same name in the Tk parent class (i.e. Tk::Button) as in the below code:

require 'tk'
class MyApp
  def initialize
    @root = TkRoot.new
    TkFrame.new {|f|
      TkButton.new(f) {
        text "Press Me"
        command proc {do_something()}
        pack
      }
      pack
    }
  end
  def do_something
    puts "Hello!"
  end
  def run
    Tk.mainloop
  end
end

MyApp.new.run

If I rewrite it not using the DSL, I can avoid this issue, but I prefer the DSL for various reasons:

class MyApp
  def initialize
    @root = TkRoot.new
    f = TkFrame.new
    TkButton.new(f, text: "Press Me", command: proc {do_something()}).pack
    f.pack
  end
  def do_something
    puts "Hello!"
  end
  def run
    Tk.mainloop
  end
end

MyApp.new.run

The same is true for instance variables of the MyApp class. Is there any way arround this?

Answer 1

proc { do_something } is probably being evaluated in the context of the TkButton instance. That’s probably how you can call text, command and pack while inside the block.

This effectively means self is not the MyApp instance anymore; it was changed to the TkButton instance.

Try this:

def initialize
  my_app = self  # self is the MyApp instance here
  @root = TkRoot.new
  TkFrame.new do |f|
    TkButton.new f do
      text "Press Me"
      command proc { my_app.do_something }
      pack
    end
    pack
  end
end

Since blocks are closures, the my_app local variable will be available to the block.