RSpec：let 和 before 块有什么区别

理解 RSpec 中 let 和 before 块的区别

ruby
RSpec.describe Calculator do
  before do
    @calculator = Calculator.new
  end
  it "can add numbers" do
    result = @calculator.add(2, 3)
    expect(result).to eq(5)
  end
  it "can subtract numbers" do
    result = @calculator.subtract(5, 3)
    expect(result).to eq(2)
  end
end

ruby
RSpec.describe ShoppingCart do
  let(:item) { Item.new(name: "Widget", price: 10) }
  let(:cart) { ShoppingCart.new }
  it "can add items" do
    cart.add(item)
    expect(cart.total).to eq(10)
  end
  it "can remove items" do
    cart.add(item)
    cart.remove(item)
    expect(cart.total).to eq(0)
  end
end