class Project {
    let name: Variable<String>
    let issues: ArrayVariable<Issue>
}

class Account {
    let name: Variable<String>
    let email: Variable<String>
}

class Issue {
    let identifier: Variable<String>
    let owner: Variable<Account>
    let isOpen: Variable<Bool>
    let created: Variable<NSDate>
}

class Document {
    let accounts: ArrayVariable<Account>
    let projects: ArrayVariable<Project>
}

// Raw Swift       ===>      // GlueKit                                    
var a = 42          ;        let b = Variable<Int>(42) 
print("a = \(a)")   ;        print("b = \(b.value\)")
a = 7               ;        b.value = 7

let cocoaKeyPath: String = "projects.issues.owner.email"

let swiftKeyPath: Document -> AnyObservableValue<[String]> = { document in 
    document.projects.flatMap{$0.issues}.flatMap{$0.owner}.map{$0.email} 
}

let document: Document = ...
let cocoaEmails: AnyObject? = document.valueForKeyPath(cocoaKeyPath)
let swiftEmails: [String] = swiftKeyPath(document).value

// Cocoa: Compiles fine, but oops, crash at runtime
document.setValue("[email protected]", forKeyPath: cocoaKeyPath)
// GlueKit/Swift: error: cannot assign to property: 'value' is a get-only property
swiftKeyPath(document).value = "[email protected]"

let issue: Issue = ...
/* Cocoa */   issue.setValue("[email protected]", forKeyPath: "owner.email") // OK
/* GlueKit */ issue.owner.map{$0.email}.value = "[email protected]"  // OK

// GlueKit
let c = document.projects.flatMap{$0.issues}.flatMap{$0.owner}.map{$0.name}.connect { emails in 
    print("Owners' email addresses are: \(emails)")
}
// Call c.disconnect() when you get bored of getting so many emails.

// Cocoa
class Foo {
    static let context: Int8 = 0
    let document: Document

    init(document: Document) {
        self.document = document
        document.addObserver(self, forKeyPath: "projects.issues.owner.email", options: .New, context:&context)
    }
    deinit {
        document.removeObserver(self, forKeyPath: "projects.issues.owner.email", context: &context)
    }
    func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, 
                                change change: [String : AnyObject]?, 
                                context context: UnsafeMutablePointer<Void>) {
        if context == &self.context {
        print("Owners' email addresses are: \(change[NSKeyValueChangeNewKey]))
        }
        else {
            super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
        }
    }
}

class ProjectSummaryViewModel {
    let currentDocument: Variable<Document> = ...
    let currentAccount: Variable<Account?> = ...

    let project: Variable<Project> = ...

    /// The name of the current project.
    var projectName: Updatable<String> { 
        return project.map { $0.name } 
    }

    /// The number of issues (open and closed) in the current project.
    var isssueCount: AnyObservableValue<Int> { 
        return project.selectCount { $0.issues }
    }

    /// The number of open issues in the current project.
    var openIssueCount: AnyObservableValue<Int> { 
        return project.selectCount({ $0.issues }, filteredBy: { $0.isOpen })
    }

    /// The ratio of open issues to all issues, in percentage points.
    var percentageOfOpenIssues: AnyObservableValue<Int> {
        // You can use the standard arithmetic operators to combine observables.
        return AnyObservableValue.constant(100) * openIssueCount / issueCount
    }

    /// The number of open issues assigned to the current account.
    var yourOpenIssues: AnyObservableValue<Int> {
        return project
            .selectCount({ $0.issues }, 
                filteredBy: { $0.isOpen && $0.owner == self.currentAccount })
    }

    /// The five most recently created issues assigned to the current account.
    var yourFiveMostRecentIssues: AnyObservableValue<[Issue]> {
        return project
            .selectFirstN(5, { $0.issues }, 
                filteredBy: { $0.isOpen && $0.owner == currentAccount }),
                orderBy: { $0.created < $1.created })
    }

    /// An observable version of NSLocale.currentLocale().
    var currentLocale: AnyObservableValue<NSLocale> {
        let center = NSNotificationCenter.defaultCenter()
        let localeSource = center
            .source(forName: NSCurrentLocaleDidChangeNotification)
            .map { _ in NSLocale.currentLocale() }
        return AnyObservableValue(getter: { NSLocale.currentLocale() }, futureValues: localeSource)
    }

    /// An observable localized string.
    var localizedIssueCountFormat: AnyObservableValue<String> {
        return currentLocale.map { _ in 
            return NSLocalizedString("%1$d of %2$d issues open (%3$d%%)",
                comment: "Summary of open issues in a project")
        }
    }

    /// An observable text for a label.
    var localizedIssueCountString: AnyObservableValue<String> {
        return AnyObservableValue
            // Create an observable of tuples containing values of four observables
            .combine(localizedIssueCountFormat, issueCount, openIssueCount, percentageOfOpenIssues)
            // Then convert each tuple into a single localized string
            .map { format, all, open, percent in 
                return String(format: format, open, all, percent)
            }
    }
}

class ProjectSummaryViewController: UIViewController {
    private let visibleConnections = Connector()
    let viewModel: ProjectSummaryViewModel

    // ...

    override func viewWillAppear() {
        super.viewWillAppear()

        viewModel.projectName.values
            .connect { name in
                self.titleLabel.text = name
            }
            .putInto(visibleConnections)

        viewModel.localizedIssueCountString.values
            .connect { text in
                self.subtitleLabel.text = text
            }
            .putInto(visibleConnections)

        // etc. for the rest of the observables in the view model
    }

    override func viewDidDisappear() {
        super.viewDidDisappear()
        visibleConnections.disconnect()
    }
}

viewModel.projectName.value = "GlueKit"   // Sets the current project's name via a key path
print(viewModel.project.name.value)       // Prints "GlueKit"