深入講解Ruby中Block代碼快的用法
Block
定義
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 調(diào)用時才會起作用,如果 method 中有參數(shù),block 出現(xiàn)在最后面
Block 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(nèi)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 / sum end puts sum # => 30
如果 block 中的變量只出現(xiàn)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 += 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 的最后一個參數(shù)前面有 & 符號 (&action), 那么當此 method 被調(diào)用時,Ruby 會找一個 code block, 這個 code block 被轉(zhuǎn)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) 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 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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