ruby - 如何从 method_missing 获取绑定(bind)？

Question

我试图找到一种方法来在 Ruby (1.8) 的 method_missing 中从调用者那里获取绑定(bind)，但我似乎无法找到实现它的方法。

希望下面的代码能解释我想做什么:

class A
  def some_method
    x = 123
    nonexistent_method
  end

  def method_missing(method, *args, &block)
    b = caller_binding # <---- Is this possible?
    eval "puts x", b
  end
end

A.new.some_method
# expected output:
#   123

那么...有没有办法获得调用者的绑定(bind)，或者这在 Ruby (1.8) 中是不可能的？

Answer 1

这是一个(有点脆弱的)hack:

# caller_binding.rb
TRACE_STACK = []
VERSION_OFFSET = { "1.8.6" => -3, "1.9.1" => -2 }[RUBY_VERSION]
def caller_binding(skip=1)
  TRACE_STACK[ VERSION_OFFSET - skip ][:binding]
end
set_trace_func(lambda do |event, file, line, id, binding, classname|
  item = {:event=>event,:file=>file,:line=>line,:id=>id,:binding=>binding,:classname=>classname}
  #p item
  case(event)
  when 'line'
    TRACE_STACK.push(item) if TRACE_STACK.empty?
  when /\b(?:(?:c-)?call|class)\b/
    TRACE_STACK.push(item)
  when /\b(?:(?:c-)?return|end|raise)\b/
    TRACE_STACK.pop
  end
end)

这适用于您的示例，但我还没有对它进行太多测试

require 'caller_binding'
class A
  def some_method
    x = 123
    nonexistent_method
  end
  def method_missing( method, *args, &block )
    b = caller_binding
    eval "puts x", b
  end
end

x = 456
A.new.some_method #=> prints 123
A.new.nonexistent_method #=> prints 456

当然，如果绑定(bind)没有定义您要评估的变量，这将不起作用，但这是绑定(bind)的普遍问题。如果 undefined variable ，则它不知道它是什么。

require 'caller_binding'
def show_x(b)
  begin
    eval <<-SCRIPT, b
      puts "x = \#{x}"
    SCRIPT
  rescue => e
    puts e
  end
end

def y
  show_x(caller_binding)
end

def ex1
  y #=> prints "undefined local variable or method `x' for main:Object"
  show_x(binding) #=> prints "undefined local variable or method `x' for main:Object"
end

def ex2
  x = 123
  y #+> prints "x = 123"
  show_x(binding) #+> prints "x = 123"
end

ex1
ex2

要解决这个问题，您需要在评估的字符串中进行一些错误处理:

require 'caller_binding'
def show_x(b)
  begin
    eval <<-SCRIPT, b
      if defined? x
        puts "x = \#{x}"
      else
        puts "x not defined"
      end
    SCRIPT
  rescue => e
    puts e
  end
end

def y
  show_x(caller_binding)
end

def ex1
  y #=> prints "x not defined"
  show_x(binding) #=> prints "x not defined"
end

def ex2
  x = 123
  y #+> prints "x = 123"
  show_x(binding) #+> prints "x = 123"
end

ex1
ex2