Tusker/Tusker/LazilyDecoding.swift

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

import Foundation
import CoreData

private let decoder = PropertyListDecoder()
private let encoder = PropertyListEncoder()

@propertyWrapper
public struct LazilyDecoding<Enclosing: NSManagedObject, Value: Codable> {

    private let keyPath: ReferenceWritableKeyPath<Enclosing, Data?>
    private let fallback: Value
    private var value: Value?
    private var observation: NSKeyValueObservation?
    private var skipClearingOnNextUpdate = false
    
    init(from keyPath: ReferenceWritableKeyPath<Enclosing, Data?>, fallback: Value) {
        self.keyPath = keyPath
        self.fallback = fallback
    }

    public var wrappedValue: Value {
        get { fatalError("called LazilyDecoding wrappedValue getter") }
        set { fatalError("called LazilyDecoding wrappedValue setter") }
    }
    
    public static subscript(_enclosingInstance instance: Enclosing, wrapped wrappedKeyPath: ReferenceWritableKeyPath<Enclosing, Value>, storage storageKeyPath: ReferenceWritableKeyPath<Enclosing, Self>) -> Value {
        get {
            var wrapper = instance[keyPath: storageKeyPath]
            if let value = wrapper.value {
                return value
            } else {
                guard let data = instance[keyPath: wrapper.keyPath] else { return wrapper.fallback }
                do {
                    let value = try decoder.decode(Box.self, from: data)
                    wrapper.value = value.value
                    wrapper.observation = instance.observe(wrapper.keyPath, changeHandler: { instance, _ in
                        var wrapper = instance[keyPath: storageKeyPath]
                        if wrapper.skipClearingOnNextUpdate {
                            wrapper.skipClearingOnNextUpdate = false
                        } else {
                            wrapper.removeCachedValue()
                        }
                        instance[keyPath: storageKeyPath] = wrapper
                    })
                    instance[keyPath: storageKeyPath] = wrapper
                    return value.value
                } catch {
                    return wrapper.fallback
                }
            }
        }
        set {
            var wrapper = instance[keyPath: storageKeyPath]
            wrapper.value = newValue
            wrapper.skipClearingOnNextUpdate = true
            instance[keyPath: storageKeyPath] = wrapper
            let newData = try! encoder.encode(Box(value: newValue))
            instance[keyPath: wrapper.keyPath] = newData
        }
    }
    
    mutating func removeCachedValue() {
        value = nil
        observation = nil
    }
    
}

extension LazilyDecoding {
    init<T>(arrayFrom keyPath: ReferenceWritableKeyPath<Enclosing, Data?>) where Value == [T] {
        self.init(from: keyPath, fallback: [])
    }
}

extension LazilyDecoding {
    // PropertyListEncoder only allows top-level types to be dicts or arrays, which breaks encoding nil-able values.
    // Wrapping everything in a Box ensures that it's always a dict.
    struct Box: Codable {
        let value: Value
    }
}