<#Employee List#>
Xcode 11.5
iOS 13.0+ (for Combine framework)
No external dependencies
Focused on phone
I mostly focused on the architecture and data flow design.
I used MVVM + Combine architecture.
Data Access Layer consists of Utils and Persistence.
Utils provides asynchronous APIs for networking. Persistence provides interfaces to store and retrieve images. Both pass data as a Publisher from Combine framework. They are accessible and used only by the Services in the business layer.
Protocol-oriented networking
I defined APIRequestType protocol and APIServiceType protocol.
This approach avoids constantly changes to make room for new network requests and data types. It allows us to create as many types of requests as we need. All we need to do is add new classes that comform to these two protocols. There is no need to change existing code, respecting the Open-closed principle.
Also, this approach allows dependency injection that makes the code more testable.
Business Logic Layer includes Services and ViewModels.
For the current required features, Services consists of EmployeesAPIService that receives requests to download the employee list from the given remote endpoint, and ImageService that either receives requests to download images remotely or retrieves images from the disk locally. Note that ImageService comforms to ImageServiceType such that we can use dependency injection to inject MockImageService in ViewModel in unit tests.
ViewModels work between Views and Services, encapsulating business logic to the Views. ViewModels are downstream subscribers that receive and process whatever is passed from upstream publishers. They are marked as @ObservedObject such that SwiftUI is able to monitor ViewModels for updates and redraw the UI.
Presentation Layer is represented by Views.
Views are independent from business logic. Side effects are triggered by the view lifecycle event onAppear and are forwarded to the corresponding the ViewModel
Yet another informal data flow chart
I copied the following code snippets for making mock APIService in unit tests. Source
var stubs: [Any] = []
func stub<Request>(for type: Request.Type, response: @escaping ((Request) -> AnyPublisher<Request.ModelType, Error>)) where Request: APIRequestType {
stubs.append(response)
}
func call<Request>(from request: Request) -> AnyPublisher<Request.ModelType, Error> where Request: APIRequestType {
let response = stubs.compactMap { stub -> AnyPublisher<Request.ModelType, Error>? in
let stub = stub as? ((Request) -> AnyPublisher<Request.ModelType, Error>)
return stub?(request)
}.last
return response ?? Empty<Request.ModelType, Error>()
.eraseToAnyPublisher()
}Roughly 6.5 hours
With limited time, I implemented two unit test suites
EmployeeListViewModel
- should fetch data from the EmployeesService
- should display an error message if the request failed
CustomImageViewModel
- should fetch image from the ImageService
- should display the default photo placeholder if photo loading failed
- should dispaly an error message if any error incurred during persistence
OnDiskCache
- should store an image with a correct key
- should pass a Fail Publisher to downstreams if storing failed
- should pass a Publisher with image data with a correct key
- should pass a Fail Publisher to downstreams if fetching failed
EmployeesService
- should send a request to the given endpoint and pass a Publisher with a decoded list of Employee type
- should pass a Fail Publisher to downstreams if any error
ImageService
- should send a request to the given endpoint and pass a Publisher with the Data (or error) to downstreams
- should load the image Data from disk and pass a Publisher with the fetched image Data (or error) to downstreams
- should save an image to disk and pass a Publisher with the storing result to downstreams