深入讲解Ruby中Block代码快的用法

block
定义

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。

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。

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

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。

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.

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

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