# File lib/wee/html_brushes.rb, line 546
    def initialize(canvas)
      @name = canvas.register_callback(:input, self)
      @callbacks = {} 
      @ids = Wee::SequentialIdGenerator.new 
    end

# File lib/wee/html_brushes.rb, line 552
    def add_callback(callback)
      value = @ids.next.to_s
      @callbacks[value] = callback
      return [@name, value]
    end

# File lib/wee/html_brushes.rb, line 558
    def call(value)
      if @callbacks.has_key?(value)
        cb = @callbacks[value]
        cb.call(value) if cb
      else
        raise "invalid radio button/group value"
      end
    end