ruby - 使用 Ruby 实现类似面向 Apect 的嵌套过滤器？

Question

我正在尝试编写一个支持嵌套过滤器的类，而无需引入面向方面的库。

class Foo
  attr_accessor :around_filter

  def initialize
    #filters which wrap the following one are the ones with interesting logic
    #vanilla do-nothing filter
    @around_filter = lambda { yield } # or lambda {|&blk| blk.call}
  end

  def bar
    #would execute the around filters however deeply nested, then "meaty code"
    @around_filter.call do
      #meaty code here
      puts 'done' 
    end
  end

  #I expected to point only to the topmost filter, hoping to recurse
  def add_around_filter(&blk)
    prior_around_filter = @around_filter
    @around_filter = #??mystery code here?? refers to prior_around_filter
  end
end

目标是能够添加任意数量的环绕过滤器:

foo = Foo.new
foo.add_around_filter do
  puts 'strawberry'
  yield
  puts 'blueberry'
end
foo.add_around_filter do
  puts 'orange'
  yield
  puts 'grape'
end
foo.bar #=> orange, strawberry, done, blueberry, grape

我知道这个例子中有很多漏洞。我只写了足够的内容来传达总体方向，从更大的实际类中提炼出来。

虽然我更喜欢yield 语法，但我并不反对 block 引用:

foo.add_around_filter do |&blk|
  puts 'orange'
  blk.call
  puts 'grape'
end

我只使用一个环绕过滤器就可以做到这一点。我尝试了很多关于嵌套的事情，但从未破解过这个难题。如果您有解决方案，我将不胜感激!

Answer 1

过滤器中的 yield 将尝试屈服于在其定义点定义的 block ，这不是您想要的。它将使用显式 block 形式(在 1.9 中)，如下所示:

class Foo
  attr_accessor :around_filter

  def initialize
    #filters which wrap the following one are the ones with interesting logic
    #vanilla do-nothing filter
    @around_filter = Proc.new{|&blk| blk.call }
  end

  def bar
    #would execute the around filters however deeply nested, then "meaty code"
    @around_filter.call do
      #meaty code here
      puts 'done' 
    end
  end

  #I expected to point only to the topmost filter, hoping to recurse
  def add_around_filter(&filter)
    prior_around_filter = @around_filter
    @around_filter = Proc.new do |&blk|
      filter.call do
        prior_around_filter.call(&blk)
      end
    end
  end
end

foo = Foo.new
foo.add_around_filter do |&blk|
  puts 'strawberry'
  blk.call
  puts 'blueberry'
end
foo.add_around_filter do |&blk|
  puts 'orange'
  blk.call
  puts 'grape'
end
foo.bar #=> orange, strawberry, done, blueberry, grape