Method calls and object initialization inside Quick 'describe' and 'context' blocks can be harmful

[ornithocoder]

Quick run its tests in a very specific (tree-based) order. Take the example below:

describe("MyClass") {
    // variable definition #1
    var sut: MyClass!
    var userDefaultsMock: UserDefaults!
 
    // beforeEach #1
    beforeEach { /* ... */ }
 
    // afterEach #1
    afterEach { /* ... */ }
 
    // it #1
    it("exists") { /* ... */ }
 
    // it #2
    it("conforms to the protocol ABC") { /* ... */ }
 
    describe("tableView") {
        // it #3
        it("exists") { /* ... */ }
 
        context("when user is logged out") {
            // beforeEach #2
            beforeEach { /* ... */ }
 
            // it #4
            it("has the correct background color") { /* ... */ }
        });
 
        context("when user is logged in") {
            context("and has used Facebook login") {
                // beforeEach #3
                beforeEach { /* ... */ }
 
                // it #5
                it("has the correct background color") { /* ... */ }
            });
 
            context("and has used Twitter login") {
                // beforeEach #4
                beforeEach { /* ... */ }
                 
                // it #6
                it("has the correct background color") { /* ... */ }
            }
        }
    }
}

The order of execution is:

1. Setup
   • variable definition # 1
2. Test 1
   • beforeEach # 1
   • it # 1
   • afterEach # 1
3. Test 2
   • beforeEach # 1
   • it # 2
   • afterEach # 1
4. Test 3
   • beforeEach # 1
   • it # 3
   • afterEach # 1
5. Test 4
   • beforeEach # 1
   • beforeEach # 2
   • it # 4
   • afterEach # 1
6. Test 5
   • beforeEach # 1
   • beforeEach # 3
   • it # 5
   • afterEach # 1
7. Test 6
   • beforeEach # 1
   • beforeEach # 4
   • it # 6
   • afterEach # 1

Variable definition isn't called again between tests and, for this reason, variable initialization should happen inside beforeEach blocks to avoid object state to be carried along the test file.

When the variable initialization happens outside a beforeEach and a test changes the object state, this will persist and might influence other tests. In other words, the following should be avoided:

describe("MyClass") {
    let sut = MyClass()
    let userDefaultsMock = UserDefaultsMock()
 
    beforeEach { /* ... */ }
 
    it("does something") { /* ... */ }
 
    it("does something else") { /* ... */ }
}

rule request, opt-in, no calls inside describe and context blocks.

[ornithocoder]

Merged in #1782. Thanks @marcelofabri! :D