Block
定義
some_array.each { |value| puts value + 3 }sum = 0other_array.each do |value| sum += value puts value / sumendBlock 中的變量
如果 block 的本地變量的名字和 block 之外但是在同樣 scope 里面的 變量名字一樣，那他們兩個是一樣的。block 內變量的值會改變 block 外變量的值。
sum = 0[1,2,3,4].each do |value| sum += value puts value / sumendputs sum # => 30
如果 block 中的變量只出現在 block 中，那么它只是 block 中本地變量，無法在 block 之外被引用。
sum = 0[1,2,3,4].each do |value| square = value * value sum += squareendputs sum # => 30puts 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 shapeYou can define a block-local variables by putting them after s semicolon in the block's parameter listsquare = "some shape"sum = 0[1,2,3,4].each do |value; square| square = value * value sum += squareendputs sum # 30puts square # some shapeclass File def self.open_and_process(*args) f = File.open(*args) yield f f.close endendFile.open_and_process("testfile","r") do |file| while line = file.gets puts line endendBlocks 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 的最后一個參數前面有 & 符號 （&action）, 那么當此 method 被調用時，Ruby 會找一個 code block, 這個 code block 被轉換成 class Proc 的一個對象。
class ProcExample def pass_in_block(&action) @stored_proc = action end def use_proc(parameter) @store_proc.call(parameter) endendeg = ProcExample.neweg.pass_in_block { |param| puts "The parameter is #{param}" }eg.use_proc(99)# => The parameter is 99def create_block_object(&block) blockendbo = create_block_object { |param| puts "You called me with #{param}" }bo.call 99 # => You called me with 99bo.call "cat" # => You called me with catRuby have two built-in methods that convert a block to an object: lambda and Proc.newbo = lambda { |param| puts "You called me with #{param}" }bo.call 99 # => You called me with 99def n_times(thing) lambda {|n| thing * n}endp1 = n_times(23)p1.call(3) #=> 69p2.call(4) #=> 92def power_proc_generator value = 1 lambda { value += value }endpower_proc = power_proc_generatorputs power_proc.call # 2puts power_proc.call # 4lambda 表達式的另一種簡寫方式lambda { |params| ... }# 與下面的寫法等價-> params { ... }# parmas 是可選的proc1 = -> arg1, arg2 {puts "#{arg1} #{arg2}"}proc1.call "hello", "world"# => hello worldproc2 = -> { "Hello World" }proc2.call # => Hello WorldBlock 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.callendproc.call(1,2,3,4) {puts "in block"}# a = 1# b = [2,3,4]# in block新聞熱點
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