AlertBuilder.swift

import UIKit
import RxSwift

public typealias Alert = UIAlertController

public final class AlertBuilder {
    
    private struct Action {
        
        let title: Observable<String>
        let style: UIAlertAction.Style
        let handler: ((UIAlertAction) -> Void)?
        
        func convert() -> Observable<UIAlertAction> {
            return title.map { title in
                UIAlertAction(title: title, style: self.style, handler: self.handler)
            }
        }
    }
    
    private var _title: Observable<String> = Observable.just("")
    private var _message: Observable<String> = Observable.just("")
    private var _preferredStyle: UIAlertController.Style = .alert
    private var actions: [Action] = []
    
    public init() {}
    
    public func title(_ title: Observable<String>) -> Self {
        _title = title
        
        return self
    }
    
    public func message(_ message: Observable<String>) -> Self {
        _message = message
        
        return self
    }
    
    public func preferredStyle(_ preferredStyle: UIAlertController.Style) -> Self {
        _preferredStyle = preferredStyle
        
        return self
    }
    
    public func action(_ title: Observable<String>, style: UIAlertAction.Style = .default, handler: ((UIAlertAction) -> Void)? = nil) -> Self {
        actions.append(Action(title: title, style: style, handler: handler))
        
        return self
    }
    
    public func build() -> Observable<Alert> {
        let actions: Observable<[UIAlertAction]> = self.actions.isEmpty
            ? Observable.just([])
            : Observable.combineLatest(self.actions.map { $0.convert() })
        
        return Observable
            .combineLatest(
                actions, _title, _message, Observable.just(_preferredStyle)
            )
            .map { actions, title, message, style -> UIAlertController in
                let controller = UIAlertController(
                    title: title.isEmpty ? nil : title,
                    message: message.isEmpty ? nil : message,
                    preferredStyle: style
                )
                actions.forEach { action in
                    controller.addAction(action)
                }
                
                return controller
            }
            .take(1)
    }
    
    public func present(in controller: UIViewController?, disposeBag: DisposeBag) {
        build()
            .subscribe(onNext: { [weak controller] alert in
                controller?.present(alert: alert)
            })
            .disposed(by: disposeBag)
    }
}

extension UIViewController {
    
    public func present(alert: Alert) {
        present(alert, animated: true)
    }
}