带有instance_eval的Ruby块范围

Question

我对Ruby块和触发器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。 什么是魔术酱，在查看裸机时的工作量不足，与使用instance_eval相比，它改变了块在大多数常见用途下的作用范围？

以下是您可以在IRB中转储以查看我的意思的示例。 我已经包含了proc.call和block yield版本示例。 令人高兴的是，他们的行为都是一样的。

# 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

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

Answer 1

在词法范围（主要源文件）中定义@value时，您将在全局解释器中定义实例变量。 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 ，您所做的就是用您调用instance_eval的对象替换主解释器self 。