BENEDIKT TERHECHTE

This is my research notebook. I'm an OSX / iOS indie developer. After 8 years of Objective-C I really enjoy Swift nowadays. Trying to publish all my research on Development, Swift & other technologies here.

Sat, 30 Sep 2017 #

Value Types for Simple Difference Detection

Following sage advice I received from John Sundell half a year ago (I am a slow learner) I will try to write about smaller pieces instead of focusing on longform content. This should make it easier to update appventure more often. Thanks John!.

Today, I'll write about value types: Value types are a very useful addition to Swift. Objective-C did offer C struct types and (obviously) classical value types such as numbers, but Swift goes way beyond that by allowing us to also define more complex structures such as Array, Set, or Dictionary as value types. One of the best properties of value types is that they can easily be compared given that they're values:

let one = [1, 2, 3]
let two = [1, 2, 3]
print(one == two) // returns true

This way of easily comparing arrays is something we can use to implement a simple difference detection algorithm without using more heavy-weight solutions. Imagine you have a simple app that downloads a list of entries from a server and displays them in a table view. Once a minute you download a new list and reload the table view. This is not a particularly nice solution as you're reloading the table view even when there're no changes. If your data is defined as a struct and you implement the Equatable protocol, then you can simply use the equality operator to see if the table view needs to be reloaded:

struct Data: Equatable {
  let username: String
  let userid: Int
  static func ==(lhs: Data, rhs: Data) -> Bool {
    return lhs.username == rhs.username && lhs.userid == rhs.userid
  }
}
let oldData: [Data] = currentData()
let newData: [Data] = retrieveNewData()
guard oldData != newData else {
    return
}
updateData(with: newData)

However, it may be that your data is modelled using struct types, but that they're very complex and change often. So you've never implemented Equatable. Then you have three different options:

  1. Wait for Swift 4.1 which will hopefully merge a PR which will auto-generate Equatable for struct types if all properties of a struct also conform to Equatable.
  2. Use Krzysztof Zabłocki's Sourcery which is a meta programming framework that allows you to auto generate things like Equatable conformance for your types (and much more).
  3. This I will explain in more detail as it is also a pattern that you can use if your data is modelled using class types.

The idea here is to store just the absolutely necessary information in a seperate difference detection cache. Imagine the following data model:

final class Story {
  let story_id: Int
  let views: Int
  let last_updated: TimeInterval
  let description: String
}

In this example, a Story will never change its id and description. In order to create a simple cache, we can now just use the information we absolutely need to determine a change and store them in tuples. Tuples with up to 6 elements will automatically generate Equatable conformance:

let a = (1, 2, 3)
let b = (1, 2, 3)
print(a == b)

With this in mind, we can generate our tuples:

let newInformation: [(Int, Int, TimeInterval)] = 
  stories.map({ ($0.story_id, $0.views, $0.last_updated) })

guard newInformation != oldInformation else { return }

This is a simple solution that leverages value types to give us an easy solution for comparing sets of data. However, if you also need to determine insertions, deletions and moves then you can still do so with value types, but you need a proper diff algorithm such as Dwifft.

Nevertheless, for many simpler use cases it is good to remember that we can easily build a comparison cache of tuples or structs to determine general changes to data.

If you read this far, you should follow me (@terhechte)
on Twitter


    Related Articles