Proof of concept of using caches in WidgetKit.

README.md

This is a proof of concept of implementing a cache in WidgetKit. See https://codakuma.com/widgetkit-improvements for more.

MyWidget.swift

/**
Most of this file has been omitted to only focus on the caching aspect, but in reality
it would be a standard widget template.
**/

struct MyWidgetProvider: IntentTimelineProvider {
  
  // Initialise the cache.
  private let cache = MyWidgetCache()
  
  func getEntries(for configuration: ConfigurationIntent, in context: Context, completion: @escaping ([MyWidgetEntry]?) -> ()) {
    getWorkouts { error, workouts in
           
      // If something goes wrong, try to use a cached entry.
      if let error = error {
        guard let cachedEntry = cache.newEntryFromPrevious(withDate: Date(), forConfiguration: configuration) else {
          completion(nil)
          return
        }
        completion([cachedEntry])
        return
      }
                
      // If no errors, make an entry, cache it, and return it.
      let entry = MyWidgetEntry(date: Date(), configuration: configuration, workoutsCount: workouts.count)
      cache.storePreviousEntry(entry, withConfiguration: configuration)
      completion([entry])
    }
  }
}

MyWidgetCache.swift

class MyWidgetCache: WidgetCache {
  typealias TimelineEntryType = MyWidgetEntry
  typealias ConfigurationIntentType = MyWidgetConfigurationIntent
  
  var previousEntries = [MyWidgetConfigurationIntent: MyWidgetEntry]()
  
  func newEntryFromPrevious(withDate date: Date, forConfiguration configuration: MyWidgetConfigurationIntent) -> MyWidgetEntry? {
    if previousEntries.isEmpty { return nil }
    guard let indexOfPreviousEntry = previousEntries.index(forKey: configuration) else { return nil }
    
    let previousEntry = previousEntries[indexOfPreviousEntry].value
    return MyWidgetEntry(
      date: date,
      configuration: previousEntry.configuration,
      workoutsCount: previousEntry.workoutsCount
    )
  }
  
  func storeNewEntry(_ entry: MyWidgetEntry, withConfiguration configuration: MyWidgetConfigurationIntent) {
    previousEntries[configuration] = entry
  }
}

WidgetCache.swift

protocol WidgetCache  {
  associatedtype TimelineEntryType: TimelineEntry
  associatedtype ConfigurationIntentType: INIntent
  
  var previousEntries: [ConfigurationIntentType: TimelineEntryType] { get set }
  func newEntryFromPrevious(withDate date: Date, forConfiguration configuration: ConfigurationIntentType) -> TimelineEntryType?
  func storeNewEntry(_ entry: TimelineEntryType, withConfiguration configuration: ConfigurationIntentType) -> Void
}