fakeable.md

Fakeable

Instantiating models for tests is not a simple task. Sometimes, the initializer has too many attributes and the test does not rely on the full set of attributes. This inconvenience hurts our ability to efficiently unit test our app, which could discourage members from writing them at all.

To help with this, we have introduced a module called Fakes at Modules/Sources/Fakes. This module defines .fake() functions for all of our networking models. The .fake() function instantiates a type with fake values. As of now, we are defining fake values as empty values.

This, in conjunction with the copiable pattern allow us to write tests like:

func test() {
     // Given
     let initialProduct = Product.fake()
     let expectedProduct = initialProduct.copy(name: "new-name")
     let ViewModel = ViewModel(product: initialProduct)

     // When
     viewModel.updateName("new_name")

     // Then
     XCTAssertEqual(viewModel.product, expectedProduct)
}

Note: This module is meant to be used in test targets only!

Generating Fake Methods

The fake() methods are generated using Sourcery. For now, only classes, structs, and enums of the Networking, Yosemite, and Hardware targets are supported.

To generate a fake() method:

1. Make it conform to GeneratedFakeable.

   import Codegen
   
   struct ProductSettings: GeneratedFakeable {
       ...
   }

   Note: for enum types, if an enum only conforms to GeneratedFakeable its fake() method won't be generated. As a workaround, the enum can conform to basic protocols like Equatable.

2. In the terminal, navigate to the project's root folder and run rake generate.

   $ cd /path/to/root
   $ rake generate
   

   This will update the Fakes.generated file with the new fake() method.

3. Compile the project.

Modifying The Fakeable Code Generation

The rake generate command executes the Sourcery configuration files located in the CodeGeneration/Sourcery/Fakes folder.

Networking module → Networking-Fakes.yaml
Yosemite module → Yosemite-Fakes.yaml
Hardware module → Hardware-Fakes.yaml

It uses a single template, Fakes.swifttemplate, to generate the code. It's written using Swift templates.

Please refer to the Sourcery reference for more info about how to write templates.

Adding Fakeable to a new module

1. Add Codegen to the dependencies in the module target definition in Modules/Package.swift
2. Add a new file {{ModuleName}}-Fakes.yaml under CodeGeneration/Sourcery/Fakes similar to other yaml files in the same folder.
3. In Fakes.swifttemplate, add a line to import the new framework.
4. In Rakefile which includes the script for rake generate command, add the new framework to the list of frameworks for Fakeable generation similar to other frameworks.
5. Now you can try generating fake methods as instructed in an earlier section.