接受读取/分配实例变量的块

Question

考虑一下：

class Foo
  def secret
    @some_ivar
  end    
end

foo_instance = Foo.new

foo_instance.in_foos_context do
  @some_ivar = "bar"
end

foo_instance.secret
=> "bar"

除了看起来可疑之外，这可能吗？如果是，我该如何实现in_foos_context方法？

Answer 1

您可以使用instance_eval

class Foo
  def secret
    @some_ivar
  end    

  def in_foos_context(&block)
    instance_eval(&block)
  end
end

foo_instance = Foo.new

foo_instance.in_foos_context do
  @some_ivar = "bar"
end

foo_instance.secret # => "bar"

Answer 2

怎么样：

foo_instance.instance_variable_set '@some_ivar', 'bar'
foo_instance.secret
=> "bar"

组合#instance_variables，#instance_variable_get和#instance_variable_set就是为此而设的。

Answer 3

你可以这样做：

class Foo
  def secret
    @some_ivar
  end
  def in_foos_context(arg,&block)
    instance_exec(arg,&block)
  end    
end

foo_instance = Foo.new

foo_instance.in_foos_context("bar") do |v|
  @some_ivar = v
end

foo_instance.secret
# => "bar"