带有 instance_eval 的 Ruby 块作用域

Question

我对 Ruby 块和 proc 的理解是它们都是闭包。现在我已经看到它与 instance_eval 一起使用，我有点困惑。与使用 instance_eval 相比，什么是神奇的酱料，即查看裸机时的底层工作原理，它改变了块的范围在最常见的用途下的行为方式？

这是一个示例，您可以在 IRB 中转储以了解我的意思。我已经包含了 proc.call 和 block yield 版本示例。令人高兴的是，它们的行为方式相同。

# Testing block/proc and eval
class Example
  def initialize(value)
    # value defined in the instance
    @value = value
  end

  def call_a_proc(proc)
    proc.call self
  end

  def yield_to_block
    yield self
  end
end

# Value defined in the global object
@value = 1
example = Example.new 'a'

# the block/proc that prints @value
proc1 = -> instance { puts @value }

# instance calling/yielding the block/proc that prints @value
proc2 = -> instance { instance.call_a_proc proc1 }
proc3 = -> instance { instance.yield_to_block &proc1 }

# instance_eval of the block that prints @value
proc4 = -> instance { instance.instance_eval &proc1 }

# the block/proc reference @value from the global context, the context in which it was defined (standard closure)
example.call_a_proc proc1
example.yield_to_block &proc1
example.call_a_proc proc2
example.yield_to_block &proc2
example.call_a_proc proc3
example.yield_to_block &proc3

# block/proc has it's context redefined as coming from within the instance.
example.call_a_proc proc4
example.yield_to_block &proc4

我知道这就是 instance_eval 方法的重点，我只是不确定它是如何工作的。

Answer 1

当您在词法范围（您的主源文件）中定义@value 时，您正在全局解释器中定义一个实例变量。例如：

self #=> main
# "self" here refers to the main interpreter, which is of class Object
self.instance_variable_get(:@value) #=> 1
# "example" is your instance above
example.instance_variable_get(:@value) #=> "a"
# "self" has been changed to refer to "example" using instance_eval
example.instance_eval { self.instance_variable_get(:@value) } #=> "a"
# this syntax is just a shortcut for the line above
example.instance_eval { @value } #=> "a"

使用instance_eval，您只需将主解释器self 替换为您调用的对象instance_eval。