How to Move Bootstrapping Code Out of AppDelegate

The test project I work on to develop a component of the Word Counter faster sets up a lot of service objects during launch. I shaved off 60 lines of AppDelegate by introducing bootstrapping as a thing to my code.

  • A Bootstrapping component sets up itself as it pleases.
  • A Bootstrapped collection tries to bootstrap new components. If the set-up succeeds, the collection grows.

This way you can prepare a list of Bootstrapping components and iterate over them to set up one after another.

The framework is very simple:

enum BootstrappingError: ErrorType {
    case ExpectedComponentNotFound(String)
}

protocol Bootstrapping {
    func bootstrap(bootstrapped: Bootstrapped) throws
}

struct Bootstrapped {
    private let bootstrappedComponents: [Bootstrapping]

    init() {
        self.bootstrappedComponents = []
    }

    init(components: [Bootstrapping]) {
        self.bootstrappedComponents = components
    }

    func bootstrap(component: Bootstrapping) throws -> Bootstrapped{
        try component.bootstrap(self)
    
        return Bootstrapped(components: bootstrappedComponents.add(component))
    }

    func component<T: Bootstrapping>(componentType: T.Type) throws -> T {
        guard let found = bootstrappedComponents.findFirst({ $0 is T }) as? T else {
            throw BootstrappingError.ExpectedComponentNotFound("\(T.self)")
        }
    
        return found
    }
}

bootstrap(_:) in a command which potentially grows the collection – as long as no error is thrown.

component(_:) is a query to obtain an already bootstrapped component, indicating a dependency. If the sequence is out of order, this query method will throw an .ExpectedComponentNotFound error and the bootstrapping part of the app’s launch sequence will not succeed.

Note I haven’t declared bootstrappedComponents as var. Neither have I made bootstrap(:_) mutating. That caused trouble when piping the bootstrapping with reduce like this:

// Bootstrap the sequence of components
func bootstrapped(components: [Bootstrapping]) throws -> Bootstrapped {
    return try components.reduce(Bootstrapped()) { bootstrapped, next in
        return try bootstrapped.bootstrap(next)
    }
}

bootstrapped(_:) starts with an empty collection and sequentially tries to bootstrap every component, returning the complete collection in the end. This is like a for-each loop to add up numbers, only more terse:

var result = Bootstrapped()
for nextComponent in components {
    result = result.bootstrap(nextComponent)
}
return result

To keep the following example simple, none of the components take any arguments during initialization, though in production I pass a few preset values in.

@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {

    // ...
    
    let components: [Bootstrapping] = [
        ErrorReportBootstrapping(),
        PersistenceBootstrapping(),
        DomainBootstrapping(),
        ApplicationBootstrapping()
    ]

    // Will be set during bootstrapping
    var showWindowService: ShowMainWindow!

    func applicationDidFinishLaunching(aNotification: NSNotification) {
        // Prepare everything
        bootstrap()

        // Then do something, like displaying the main window
        showWindow()
    }
    
    func bootstrap() {
        do {
            let bootstrapped = try bootstrapped(components)
            let application = try bootstrapped.component(ApplicationBootstrapping.self)
    
            showWindowService = application.showWindowService
        } catch let error as NSError {
            showLaunchError(error)
            fatalError("Application launch failed: \(error)")
        }
    }

    func bootstrapped(components: [Bootstrapping]) throws -> Bootstrapped {
        return try components.reduce(Bootstrapped()) { bootstrapped, next in
            return try bootstrapped.bootstrap(next)
        }
    }
    
    func showLaunchError(error: NSError) {
        let alert = NSAlert()
        alert.messageText = "Application launch failed. Please report to support@christiantietze.de"
        alert.informativeText = "\(error)"

        alert.runModal()
    }
    
    func showWindow() {
        showWindowService.showWindow()
    }
}

Error reporting, for example, is very simple. That’s by design since it’s so fundamental.

class ErrorReportBootstrapping: Bootstrapping {
    let errorHandler = ErrorHandler()
    let invalidDataHandler = InvalidDataHandler()

    func bootstrap(bootstrapped: Bootstrapped) throws {
        ErrorAlert.emailer = TextEmailer()
    
        // Add objects to a global registry of services used application-wide.
        ServiceLocator.sharedInstance.errorHandler = errorHandler
        ServiceLocator.sharedInstance.missingURLHandler = invalidDataHandler
        ServiceLocator.sharedInstance.fileNotFoundHandler = invalidDataHandler
    }
}

Objects can declare dependencies by querying the Bootstrapped collection:

import Foundation

class DomainBootstrapping: Bootstrapping {
    var bananaPeeler: BananaPeeler!

    func bootstrap(bootstrapped: Bootstrapped) throws {
        // may throw .ExpectedComponentNotFound("PersistenceBootstrapping")
        let persistence = try bootstrapped.component(PersistenceBootstrapping.self)
    
        bananaPeeler = BananaPeeler(bananaStore: persistence.bananaStore)
    }
}

And then ApplicationBootstrapping will be able to fetch bananaPeeler from the previously set-up DomainBootstrapping and use it to delegate from the event handler for a window controller it sets up, say.

Some of the Bootstrapping components do a similar job to VIPER’s wireframes which prepare interactor, presenter, and view. The whole Bootstrapped collection wraps the set-up of wireframes, building what some (including me, usually) call an AppDependencies object, holding the Core Data stack and other things at once, sequentially.

I like that the Bootstrapping components work in isolation and group similar concepts. That works better than // MARK‘ing groups of concepts in a single AppDependencies container.

Photo Credit: Please Clean Up Your Mess by Allen Goldblatt. License: CC-BY-2.0.