rspec let vs let!

let vs let! 只有一处不同，let 是懒加载，也就是第一次调用之后，才会被创建。let! 是不管有没有被调用，强制先创建。也就是之前我记得 java 中有一个懒汉式和饿汉式的区别。ps: 不管是 let 还是 let! 每个 it 代码块之间都是独立的干净的。

下面这个 test 可以比较完整的看出区别：

RSpec.describe "LetVsLet!" do
  before do
    Timecop.return
    Timecop.travel(Time.now)
  end
  
  describe "let" do
    let(:current_time) { Time.now.to_i }

    before { puts Time.now.to_i }

    it "gets the same time over and over again" do
      sleep(1)
      puts current_time.to_i
      sleep(1)
      puts current_time.to_i
    end

    it "gets the time again" do
      sleep(1)
      puts current_time.to_i
      sleep(1)
      puts current_time.to_i
    end
  end

  describe "let!" do
    let!(:current_time) { Time.now }

    before {
      puts Time.now.to_i
    }

    it "gets the same time over and over again" do
      sleep(1)
      puts current_time.to_i
      sleep(1)
      puts current_time.to_i
    end

    it "gets the time again" do
      sleep(1)
      puts current_time.to_i
      sleep(1)
      puts current_time.to_i
    end
  end
end

执行结果：

➜  otcbtc git:(t36818) ✗ bundle exec spring rspec spec/models/test_spec.rb:1
Running via Spring preloader in process 19966
Run options: include {:locations=>{"./spec/models/test_spec.rb"=>[1]}}

LetVsLet!
  let
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    gets the same time over and over again
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    gets the time again
  let!
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    gets the same time over and over again
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    gets the time again

Finished in 8.85 seconds (files took 0.61873 seconds to load)
4 examples, 0 failures