深入讲解Ruby中Block代码快的用法-巨人网络通讯

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

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