loopfollow/LoopFollow/Storage/Framework/UserDefaultsValueGroups.swift

// LoopFollow
// UserDefaultsValueGroups.swift
// Created by Jon Fawcett on 2020-06-05.

import Foundation

/// UserDefaultValue groups manager class, providing change observation capabilities (keep clients informed when a change occured in a given group).
class UserDefaultsValueGroups {
    class func values(from groupName: String) -> [UserDefaultsAnyValue]? {
        return groupNameToValues[groupName]
    }

    class func add(_ value: UserDefaultsAnyValue, to groupName: String) {
        // add to "value-key to groupNames" dictionary
        var groupNames = valueKeyToGroupNames[value.key] ?? []
        guard !groupNames.contains(groupName) else {
            // already added value to this group!
            return
        }

        groupNames.append(groupName)
        valueKeyToGroupNames[value.key] = groupNames

        // add to "groupName to value" dictionary
        var values = groupNameToValues[groupName] ?? []
        values.append(value)
        groupNameToValues[groupName] = values
    }

    @discardableResult
    class func observeChanges(in groupName: String, using closure: @escaping (UserDefaultsAnyValue, String) -> Void) -> ObservationToken {
        let id = UUID()

        var observers = groupNameToObservers[groupName] ?? [:]
        observers[id] = closure
        groupNameToObservers[groupName] = observers

        return ObservationToken {
            groupNameToObservers[groupName]?.removeValue(forKey: id)
        }
    }

    // called by UserDefaultsValue instances when value changes
    class func valueChanged(_ value: UserDefaultsAnyValue) {
        valueKeyToGroupNames[value.key]?.forEach { groupName in
            notifyValueChanged(value, in: groupName)
        }
    }

    private class func notifyValueChanged(_ value: UserDefaultsAnyValue, in groupName: String) {
        groupNameToObservers[groupName]?.values.forEach { closure in
            closure(value, groupName)
        }
    }

    private static var groupNameToValues: [String: [UserDefaultsAnyValue]] = [:]
    private static var valueKeyToGroupNames: [String: [String]] = [:]
    private static var groupNameToObservers: [String: [UUID: (UserDefaultsAnyValue, String) -> Void]] = [:]
}

// user default values group definitions
extension UserDefaultsValueGroups {
    enum GroupNames {
        static let watchSync = "watchSync"
        static let alarm = "alarm"
    }
}