mastodon-ios/Mastodon/Scene/Profile/Timeline/UserTimelineViewModel.swift

//
//  UserTimelineViewModel.swift
//  Mastodon
//
//  Created by MainasuK Cirno on 2021-3-29.
//

import os.log
import UIKit
import GameplayKit
import Combine
import CoreData
import CoreDataStack
import MastodonSDK

final class UserTimelineViewModel {
    
    var disposeBag = Set<AnyCancellable>()

    // input
    let context: AppContext
    let domain: CurrentValueSubject<String?, Never>
    let userID: CurrentValueSubject<String?, Never>
    let queryFilter: CurrentValueSubject<QueryFilter, Never>
    let statusFetchedResultsController: StatusFetchedResultsController
    var cellFrameCache = NSCache<NSNumber, NSValue>()

    let isBlocking = CurrentValueSubject<Bool, Never>(false)
    let isBlockedBy = CurrentValueSubject<Bool, Never>(false)
    let isSuspended = CurrentValueSubject<Bool, Never>(false)
    let userDisplayName = CurrentValueSubject<String?, Never>(nil)  // for suspended prompt label
    var dataSourceDidUpdate = PassthroughSubject<Void, Never>()

    // output
    var diffableDataSource: UITableViewDiffableDataSource<StatusSection, Item>?
    private(set) lazy var stateMachine: GKStateMachine = {
        let stateMachine = GKStateMachine(states: [
            State.Initial(viewModel: self),
            State.Reloading(viewModel: self),
            State.Fail(viewModel: self),
            State.Idle(viewModel: self),
            State.Loading(viewModel: self),
            State.NoMore(viewModel: self),
        ])
        stateMachine.enter(State.Initial.self)
        return stateMachine
    }()

    init(context: AppContext, domain: String?, userID: String?, queryFilter: QueryFilter) {
        self.context = context
        self.statusFetchedResultsController = StatusFetchedResultsController(
            managedObjectContext: context.managedObjectContext,
            domain: domain,
            additionalTweetPredicate: Status.notDeleted()
        )
        self.domain = CurrentValueSubject(domain)
        self.userID = CurrentValueSubject(userID)
        self.queryFilter = CurrentValueSubject(queryFilter)
        // super.init()

        self.domain
            .assign(to: \.value, on: statusFetchedResultsController.domain)
            .store(in: &disposeBag)
        
        Publishers.CombineLatest4(
            statusFetchedResultsController.objectIDs.removeDuplicates(),
            isBlocking,
            isBlockedBy,
            isSuspended
        )
        .receive(on: DispatchQueue.main)
        .debounce(for: .milliseconds(300), scheduler: DispatchQueue.main)
        .sink { [weak self] objectIDs, isBlocking, isBlockedBy, isSuspended in
            guard let self = self else { return }
            guard let diffableDataSource = self.diffableDataSource else { return }
            
            var items: [Item] = []
            var snapshot = NSDiffableDataSourceSnapshot<StatusSection, Item>()
            snapshot.appendSections([.main])

            var animatingDifferences = true
            defer {
                // not animate when empty items fix loader first appear layout issue
                diffableDataSource.apply(snapshot, animatingDifferences: animatingDifferences) { [weak self] in
                    guard let self = self else { return }
                    self.dataSourceDidUpdate.send()
                }
            }

            let name = self.userDisplayName.value
            guard !isBlocking else {
                snapshot.appendItems(
                    [Item.emptyStateHeader(attribute: Item.EmptyStateHeaderAttribute(reason: .blocking(name: name)))],
                    toSection: .main
                )
                return
            }
            
            guard !isBlockedBy else {
                snapshot.appendItems(
                    [Item.emptyStateHeader(attribute: Item.EmptyStateHeaderAttribute(reason: .blocked(name: name)))],
                    toSection: .main
                )
                return
            }
            
            guard !isSuspended else {
                snapshot.appendItems(
                    [Item.emptyStateHeader(attribute: Item.EmptyStateHeaderAttribute(reason: .suspended(name: name)))],
                    toSection: .main
                )
                return
            }

            var oldSnapshotAttributeDict: [NSManagedObjectID : Item.StatusAttribute] = [:]
            let oldSnapshot = diffableDataSource.snapshot()
            for item in oldSnapshot.itemIdentifiers {
                guard case let .status(objectID, attribute) = item else { continue }
                oldSnapshotAttributeDict[objectID] = attribute
            }

            for objectID in objectIDs {
                let attribute = oldSnapshotAttributeDict[objectID] ?? Item.StatusAttribute()
                items.append(.status(objectID: objectID, attribute: attribute))
            }
            snapshot.appendItems(items, toSection: .main)
            
            if let currentState = self.stateMachine.currentState {
                switch currentState {
                case is State.Reloading, is State.Loading, is State.Idle, is State.Fail:
                    snapshot.appendItems([.bottomLoader], toSection: .main)
                case is State.NoMore:
                    snapshot.appendItems([.emptyBottomLoader], toSection: .main)
                    animatingDifferences = false
                // TODO: handle other states
                default:
                    break
                }
            }
        }
        .store(in: &disposeBag)
    }

    deinit {
        os_log("%{public}s[%{public}ld], %{public}s", ((#file as NSString).lastPathComponent), #line, #function)
    }

}

extension UserTimelineViewModel {
    struct QueryFilter {
        let excludeReplies: Bool?
        let excludeReblogs: Bool?
        let onlyMedia: Bool?
        
        init(
            excludeReplies: Bool? = nil,
            excludeReblogs: Bool? = nil,
            onlyMedia: Bool? = nil
        ) {
            self.excludeReplies = excludeReplies
            self.excludeReblogs = excludeReblogs
            self.onlyMedia = onlyMedia
        }
    }

}