Tusker/Tusker/CoreData/MastodonCachePersistentStore.swift

//
//  MastodonCachePersistentStore.swift
//  Tusker
//
//  Created by Shadowfacts on 4/11/20.
//  Copyright © 2020 Shadowfacts. All rights reserved.
//

import Foundation
import CoreData
import Pachyderm
import Combine

class MastodonCachePersistentStore: NSPersistentContainer {
    
    private static let managedObjectModel: NSManagedObjectModel = {
        let url = Bundle.main.url(forResource: "Tusker", withExtension: "momd")!
        return NSManagedObjectModel(contentsOf: url)!
    }()
    
    private(set) lazy var backgroundContext: NSManagedObjectContext = {
        let context = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
        context.parent = self.viewContext
        return context
    }()
    
    private(set) lazy var prefetchBackgroundContext: NSManagedObjectContext = {
        let context = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
        context.parent = self.viewContext
        return context
    }()
    
    let statusSubject = PassthroughSubject<String, Never>()
    let accountSubject = PassthroughSubject<String, Never>()
    let relationshipSubject = PassthroughSubject<String, Never>()

    init(for accountInfo: LocalData.UserAccountInfo?, transient: Bool = false) {
        if transient {
            super.init(name: "transient_cache", managedObjectModel: MastodonCachePersistentStore.managedObjectModel)
            
            let storeDescription = NSPersistentStoreDescription()
            storeDescription.type = NSInMemoryStoreType
            persistentStoreDescriptions = [storeDescription]
        } else {
            super.init(name: "\(accountInfo!.id)_cache", managedObjectModel: MastodonCachePersistentStore.managedObjectModel)
        }
        
        loadPersistentStores { (description, error) in
            if let error = error {
                fatalError("Unable to load persistent store: \(error)")
            }
        }
        
        NotificationCenter.default.addObserver(self, selector: #selector(managedObjectsDidChange), name: .NSManagedObjectContextObjectsDidChange, object: viewContext)
    }
    
    func status(for id: String, in context: NSManagedObjectContext? = nil) -> StatusMO? {
        let context = context ?? viewContext
        let request: NSFetchRequest<StatusMO> = StatusMO.fetchRequest()
        request.predicate = NSPredicate(format: "id = %@", id)
        request.fetchLimit = 1
        if let result = try? context.fetch(request), let status = result.first {
            return status
        } else {
            return nil
        }
    }
    
    @discardableResult
    private func upsert(status: Status, context: NSManagedObjectContext) -> StatusMO {
        if let statusMO = self.status(for: status.id, in: context) {
            statusMO.updateFrom(apiStatus: status, container: self)
            return statusMO
        } else {
            return StatusMO(apiStatus: status, container: self, context: context)
        }
    }
    
    func addOrUpdate(status: Status, context: NSManagedObjectContext? = nil, completion: ((StatusMO) -> Void)? = nil) {
        let context = context ?? backgroundContext
        context.perform {
            let statusMO = self.upsert(status: status, context: context)
            if context.hasChanges {
                try! context.save()
            }
            completion?(statusMO)
            self.statusSubject.send(status.id)
        }
    }
    
    @MainActor
    func addOrUpdateOnViewContext(status: Status) -> StatusMO {
        let statusMO = self.upsert(status: status, context: viewContext)
        if viewContext.hasChanges {
            try! viewContext.save()
        }
        statusSubject.send(status.id)
        return statusMO
    }

    func addAll(statuses: [Status], completion: (() -> Void)? = nil) {
        backgroundContext.perform {
            statuses.forEach { self.upsert(status: $0, context: self.backgroundContext) }
            if self.backgroundContext.hasChanges {
                try! self.backgroundContext.save()
            }
            statuses.forEach { self.statusSubject.send($0.id) }
            completion?()
        }
    }
    
    func addAll(statuses: [Status]) async {
        return await withCheckedContinuation { continuation in
            addAll(statuses: statuses) {
                continuation.resume()
            }
        }
    }

    func account(for id: String, in context: NSManagedObjectContext? = nil) -> AccountMO? {
        let context = context ?? viewContext
        let request: NSFetchRequest<AccountMO> = AccountMO.fetchRequest()
        request.predicate = NSPredicate(format: "id = %@", id)
        request.fetchLimit = 1
        if let result = try? context.fetch(request), let account = result.first {
            return account
        } else {
            return nil
        }
    }
    
    @discardableResult
    private func upsert(account: Account) -> AccountMO {
        if let accountMO = self.account(for: account.id, in: self.backgroundContext) {
            accountMO.updateFrom(apiAccount: account, container: self)
            return accountMO
        } else {
            return AccountMO(apiAccount: account, container: self, context: self.backgroundContext)
        }
    }
    
    func addOrUpdate(account: Account, completion: ((AccountMO) -> Void)? = nil) {
        backgroundContext.perform {
            let accountMO = self.upsert(account: account)
            if self.backgroundContext.hasChanges {
                try! self.backgroundContext.save()
            }
            completion?(accountMO)
            self.accountSubject.send(account.id)
        }
    }
    
    func relationship(forAccount id: String, in context: NSManagedObjectContext? = nil) -> RelationshipMO? {
        let context = context ?? viewContext
        let request: NSFetchRequest<RelationshipMO> = RelationshipMO.fetchRequest()
        request.predicate = NSPredicate(format: "accountID = %@", id)
        request.fetchLimit = 1
        if let result = try? context.fetch(request), let relationship = result.first {
            return relationship
        } else {
            return nil
        }
    }
    
    @discardableResult
    private func upsert(relationship: Relationship) -> RelationshipMO {
        if let relationshipMO = self.relationship(forAccount: relationship.id, in: self.backgroundContext) {
            relationshipMO.updateFrom(apiRelationship: relationship, container: self)
            return relationshipMO
        } else {
            let relationshipMO = RelationshipMO(apiRelationship: relationship, container: self, context: self.backgroundContext)
            return relationshipMO
        }
    }
    
    func addOrUpdate(relationship: Relationship, completion: ((RelationshipMO) -> Void)? = nil) {
        backgroundContext.perform {
            let relationshipMO = self.upsert(relationship: relationship)
            if self.backgroundContext.hasChanges {
                try! self.backgroundContext.save()
            }
            completion?(relationshipMO)
            self.relationshipSubject.send(relationship.id)
        }
    }
    
    func addAll(accounts: [Account], completion: (() -> Void)? = nil) {
        backgroundContext.perform {
            accounts.forEach { self.upsert(account: $0) }
            if self.backgroundContext.hasChanges {
                try! self.backgroundContext.save()
            }
            completion?()
            accounts.forEach { self.accountSubject.send($0.id) }
        }
    }

    func addAll(notifications: [Pachyderm.Notification], completion: (() -> Void)? = nil) {
        backgroundContext.perform {
            let statuses = notifications.compactMap { $0.status }
            // filter out mentions, otherwise we would double increment the reference count of those accounts
            // since the status has the same account as the notification
            let accounts = notifications.filter { $0.kind != .mention }.map { $0.account }
            statuses.forEach { self.upsert(status: $0, context: self.backgroundContext) }
            accounts.forEach { self.upsert(account: $0) }
            if self.backgroundContext.hasChanges {
                try! self.backgroundContext.save()
            }
            completion?()
            statuses.forEach { self.statusSubject.send($0.id) }
            accounts.forEach { self.accountSubject.send($0.id) }
        }
    }
    
    func performBatchUpdates(_ block: @escaping (_ context: NSManagedObjectContext, _ addAccounts: ([Account]) -> Void, _ addStatuses: ([Status]) -> Void) -> Void, completion: (() -> Void)? = nil) {
        backgroundContext.perform {
            var updatedAccounts = [String]()
            var updatedStatuses = [String]()
            
            block(self.backgroundContext, { (accounts) in
                accounts.forEach { self.upsert(account: $0) }
                updatedAccounts.append(contentsOf: accounts.map { $0.id })
            }, { (statuses) in
                statuses.forEach { self.upsert(status: $0, context: self.backgroundContext) }
                updatedStatuses.append(contentsOf: statuses.map { $0.id })
            })

            updatedAccounts.forEach(self.accountSubject.send)
            updatedStatuses.forEach(self.statusSubject.send)
            
            if self.backgroundContext.hasChanges {
                try! self.backgroundContext.save()
            }
            completion?()
        }
    }
    
    @objc private func managedObjectsDidChange(_ notification: Foundation.Notification) {
        let changes = hasChangedSavedHashtagsOrInstances(notification)
        if changes.hashtags {
            NotificationCenter.default.post(name: .savedHashtagsChanged, object: nil)
        }
        if changes.instances {
            NotificationCenter.default.post(name: .savedInstancesChanged, object: nil)
        }
    }
    
    private func hasChangedSavedHashtagsOrInstances(_ notification: Foundation.Notification) -> (hashtags: Bool, instances: Bool) {
        var changes: (hashtags: Bool, instances: Bool) = (false, false)
        if let inserted = notification.userInfo?[NSInsertedObjectsKey] as? Set<NSManagedObject> {
            for object in inserted {
                if object is SavedHashtag {
                    changes.hashtags = true
                } else if object is SavedInstance {
                    changes.instances = true
                }
                if changes.hashtags && changes.instances {
                    return changes
                }
            }
        }
        if let deleted = notification.userInfo?[NSDeletedObjectsKey] as? Set<NSManagedObject> {
            for object in deleted {
                if object is SavedHashtag {
                    changes.hashtags = true
                } else if object is SavedInstance {
                    changes.instances = true
                }
                if changes.hashtags && changes.instances {
                    return changes
                }
            }
        }
        return changes
    }
    
}