RSpec Let Vs Before

2018-02-08 10:20:07来源:oschina作者:Kolosek人点击

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In RSpec, there are two different ways to write DRY tests, by using before or let . Their purpose is to create variables that are common across tests. In this post, we will explore differences between before and let and explain why let is preferred by the Ruby community .


let

Let creates lazily-evaluated local variables. This means that let () is not evaluated until the method that it formed is run for the first time. It DRYs up the spec and makes it more readable.


$count = 0
describe "let" do
let(:count) { $count += 1 }
it "stores the value" do
expect(count).to eq(1)
expect(count).to eq(1)
end
it "is not cached across examples" do
expect(count).to eq(2)
end
end

The let Should not BE Used for local the Variables, Which have have to BE saved to at The Database, AS They by Will not BE saved to at The Database The unless They have have already been referenced. An In the this Case, you Should use the let! Or the before Blocks .Also, Never have a let block inside of a before the block, this is what let! is made for!


let!

Unlike let, you can use let! To force the method's invocation before each example . It means that, even if you did not invoke the helper method inside the example, it will be invoked before your example runs.


$count = 0
describe "let!" do
invocation_order = []
let!(:count) do
invocation_order << :let!
$count += 1
end
it "calls the helper method in a before hook" do
invocation_order << :example
expect(invocation_order).to eq([:let!, :example])
expect(count).to eq(1)
end
end

As with let blocks, if multiple let! blocks are defined with the same name, the most recent one will be executed. The core difference is thatlet! blockswill be executed multiple times if used like this, whereas thelet blockwill only execute the last time.


before(:each)

Before(:each) block will run before each example, even if the example doesn't use any of the instance variables defined in the block. This can noticeably slow down the setup of the instance variables.


class User
def tests
@tests ||= []
end
end
describe User do
before(:each) do
@user = User.new
end
describe "initialized in before(:each)" do
it "has 0 tests" do
@user.should have(0).tests
end
it "can accept new tests" do
@user.tests << Object.new
end
it "does not share state across examples" do
@user.should have(0).tests
end
end
end

In nearly every situation, it is better to use let over before blocks.Depending on your personal preference you could use before blocks when:

There is a reasonable amount of variables.
There are variables that don't need to be referenced directly, but are required.
There are many commands to be executed because its syntax is more clear when many commands are involved.
Creating mocks/stubs. before(:all)

This block is executed only once, before all of the examples in a group. There are certain situations this can cut down on execution and effort.


class User
def tests
@tests ||= []
end
end
describe User do
before(:all) do
@user = User.new
end
describe "initialized in before(:all)" do
it "has 0 tests" do
@user.should have(0).tests
end
it "can get accept new tests" do
@user.tests << Object.new
end
it "shares state across examples" do
@user.should have(1).tests
end
end
end

Using before(:all) in RSpec will cause you lots of trouble unless you know what you are doing!It runs outside of transactions, so the data created here will bleed into other specs.


Conclusion

Let blocks bring more to the table than before blocks.It all depends on what and how you need to make the Rspec tests. Besides being slower, one of the major problems with before blocks is that spelling errors can lead to bugs and false positives, allowing certain types of tests to pass when they shouldn't.


before(:each) do
@user = User.find(username: "kolosek")
@user.logout
end
it "should log the user out" do
expect(@usr).to be_nil
end

Since @usr was not previously defined, the test will pass, @usr is nil by default. The same test using let would raise NameError because @usr is not defined.


Hope this will help you to better understand the differences between let and before blocks.


Thank you for reading!


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