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