import Foundation
import CoreData

final class CoreDataManager {
    
    static let shared = CoreDataManager()
    
    func getContext() -> NSManagedObjectContext {
        return context
    }
    
    private var persistentContainer: NSPersistentContainer = {
        let container = NSPersistentContainer(name: "MyApp")
        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                debugPrint("Failed to load persistant store: \(error), \(error.userInfo)")
            }
        })
        return container
    }()
    
    private func getViewContext() -> NSManagedObjectContext {
        let context = persistentContainer.viewContext
        context.mergePolicy = NSMergePolicyType.mergeByPropertyObjectTrumpMergePolicyType
        return context
    }
    
    private func getNewBackgroundContext() -> NSManagedObjectContext {
        let context = persistentContainer.newBackgroundContext()
        context.mergePolicy = NSMergePolicyType.mergeByPropertyObjectTrumpMergePolicyType
        return context
    }
        
    private var context: NSManagedObjectContext { Thread.isMainThread ? getViewContext() : getNewBackgroundContext() }
    
    func saveContext (_ inContext: NSManagedObjectContext? = nil) {
        if let context = inContext {
            if context.hasChanges {
                do {
                    try context.save()
                } catch {
                    let nserror = error as NSError
                    debugPrint("Failed to load persistant store: \(nserror), \(nserror.userInfo)")
                }
            }
        } else {
            let context = getContext()
            context.perform {
                if context.hasChanges {
                    do {
                        try context.save()
                    } catch {
                        let nserror = error as NSError
                        debugPrint("Failed to load persistant store: \(nserror), \(nserror.userInfo)")
                    }
                }
            }
        }
    }
    
    func clear() {
        let context = getContext()
        context.perform { [self] in
            ...
            delete some data
            ...
            saveContext(context)
        }
    }
    
    private func deleteAllData(_ entity:String, _ context: NSManagedObjectContext) {
        let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: entity)
        let batchDeleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
        do {
            try context.execute(batchDeleteRequest)
        } catch let error {
            print("Delete all data in \(entity) error: ", error)
        }
    }

}