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