Rspec expect( ) vs expect { }

Question

Hi I am new to rspec and was trying to figure out what is the difference between passing a block to expect{ } and just using expect( )

Here is a trivial example

require "rails_helper"
RSpec.describe "Test",type: :model do
 it "testing count" do
   arr=[1,2,3]
   expect{arr<<1}.to change{arr.count}.by(1)
 end
end

This runs fine but when I do this instead

    require "rails_helper"
    RSpec.describe "Test",type: :model do
      it "testing count" do
        arr=[1,2,3]
        expect(arr<<1).to change{arr.count}.by(1)
      end
    end

It throws an error

Failures:

  1) Test testing count
     Failure/Error: expect(arr<<1).to change{arr.count}.by(1)
       expected `arr.count` to have changed by 1, but was not given a block
     # ./spec/models/test_spec.rb:6:in block (2 levels) in <top (required)>

And even when I don't use a block with change it gives me an error

require "rails_helper"
RSpec.describe "Test",type: :model do
  it "testing count" do
    arr=[1,2,3]
    expect{arr<<1}.to change(arr.count).by(1)
  end
end

Failures:

  1) Test testing count
     Failure/Error: expect{arr<<1}.to change(arr.count).by(1)
     
     ArgumentError:
       `change` requires either an object and message (`change(obj, :msg)`) or a block (`change { }`). You passed an object but no message.

Can someone explain why is it happening??

Answer 1

该expect(...)语法用于期望括号中的语句的返回值与特定条件匹配，例如：

expect(result).to eq(3)
expect(list).not_to be_empty
expect(string).to match(/regexp/)
expect(1..10).to cover(3)

运行expect { ... }块并不真正关心块的返回值，而是关心运行块中代码的副作用。就像通过运行块或引发异常来更改另一个值一样。

expect { api_request }.to raise_error(NotFoundError)
expect { object.action }.to change(object, :value).from(old).to(new)
expect { actual }.to output("some output").to_stdout

在 RSpec 文档中查找更多示例

在你的例子中

expect { array << 1 }.to change { array.count }.by(1)

因为将值推入数组的副作用是数组中元素的数量发生变化。但

expect(array << 1).to change { arr.count }.by(1)

不起作用，因为返回值是expect(array << 1)并且[1, 2, 3, 1]此语法不支持change匹配器。