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深入讲解Ruby中Block代码块的用法

时间:2020-04-27 11:08     来源/作者:pringwq

Block
定义

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some_array.each { |value| puts value + 3 }
 
sum = 0
other_array.each do |value|
 sum += value
 puts value / sum
end
  •     A block is somewhat like the body of an anonymous method
  •     Block can take parameters
  •     Block 只有被 method 调用时才会起作用,如果 method 中有参数,block 出现在最后面

Block 中的变量
如果 block 的本地变量的名字和 block 之外但是在同样 scope 里面的 变量名字一样,那他们两个是一样的。block 内变量的值会改变 block 外变量的值。

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sum = 0
[1,2,3,4].each do |value|
 sum += value
 puts value / sum
end
puts sum # => 30

如果 block 中的变量只出现在 block 中,那么它只是 block 中本地变量,无法在 block 之外被引用。

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sum = 0
[1,2,3,4].each do |value|
 square = value * value
 sum += square
end
puts sum # => 30
puts square # undefined local variable or method 'square' for main:Object <NameError>
 
Parameters to a block are always local to a block, even if they have the same name as locals in the surrounding scope.
 
value = "some shape"
[1,2].each { |value| puts value }
puts value
 
# 1
# 2
# some shape
 
You can define a block-local variables by putting them after s semicolon in the block's parameter list
 
square = "some shape"
sum = 0
[1,2,3,4].each do |value; square|
  square = value * value
  sum += square
end
puts sum # 30
puts square # some shape
  • By making square block-local, values assigned inside the block will not affect the value of the variable with the same name in the outer scope.
  • Blocks for Transactions
  • You can use blocks to define a chunk of code that must be run under some kind of transnational control
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class File
 def self.open_and_process(*args)
  f = File.open(*args)
  yield f
  f.close
 end
end
 
File.open_and_process("testfile","r") do |file|
 while line = file.gets
  puts line
 end
end

Blocks Can Be Objects

You can convert a block into an object, store it in variables, pass it around, and then invoke its code later.

如果 method 的最后一个参数前面有 & 符号 (&action), 那么当此 method 被调用时,Ruby 会找一个 code block, 这个 code block 被转换成 class Proc 的一个对象。

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class ProcExample
 def pass_in_block(&action)
  @stored_proc = action
 end
 
 def use_proc(parameter)
  @store_proc.call(parameter)
 end
end
 
eg = ProcExample.new
eg.pass_in_block { |param| puts "The parameter is #{param}" }
eg.use_proc(99)
# => The parameter is 99
 
def create_block_object(&block)
 block
end
 
bo = create_block_object { |param| puts "You called me with #{param}" }
bo.call 99 # => You called me with 99
bo.call "cat" # => You called me with cat
 
Ruby have two built-in methods that convert a block to an object: lambda and Proc.new
 
bo = lambda { |param| puts "You called me with #{param}" }
bo.call 99 # => You called me with 99
  • Blocks Can Be Closures
  • Closure: Variables in the surrounding scope that are referenced in a block remain accessible accessible for the life of that block and the life on any Proc object created from that block.
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def n_times(thing)
 lambda {|n| thing * n}
end
 
p1 = n_times(23)
p1.call(3) #=> 69
p2.call(4) #=> 92
 
def power_proc_generator
 value = 1
 lambda { value += value }
end
 
power_proc = power_proc_generator
puts power_proc.call # 2
puts power_proc.call # 4
 
lambda 表达式的另一种简写方式
 
lambda { |params| ... }
# 与下面的写法等价
-> params { ... }
# parmas 是可选的
 
proc1 = -> arg1, arg2 {puts "#{arg1} #{arg2}"}
 
proc1.call "hello", "world"
# => hello world
 
proc2 = -> { "Hello World" }
proc2.call # => Hello World

Block Parameter List

Blocks can take default values, splat args, keyword args and a block parameter

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proc = -> a, *b, &block do
 puts "a = #{a.inspect}"
 puts "b = #{b.inspect}"
 block.call
end
 
proc.call(1,2,3,4) {puts "in block"}
# a = 1
# b = [2,3,4]
# in block

 

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