Understanding Data Updating

Data updating is essential when the state of your app changes due to user interactions such as button presses or navigation actions, or external events like receiving new data from a network request. You want the UI to reflect these state changes.

Declarative UI

SwiftUI is a declarative UI framework. SwiftUI lets you define how UI should look based on the data, and you don’t have to define how the UI transitions between different states.

The Role of State Management

SwiftUI uses state management to update data. State management tools maintain a consistent and reactive UI that automatically updates in response to changes in the app’s state. SwiftUI provides several tools, including @State and the Observable macro, that are designed to manage state. You’ll learn more about how to use these tools in the next module’s lessons. For now, you’ll focus on exploring why these tools are necessary.

How SwiftUI Updates the UI

In SwiftUI, the app’s state is based on the data. To update the UI, you must first update the data. To update the data, you must use state management tools. SwiftUI then automatically recalculates the UI and performs necessary changes based on the new state, ensuring the UI always aligns with the underlying data model. This makes it easy to control the data flow across the app and eases app maintenance and scaling.

Ul tiqf iye cozi qnubedeip vjes qizuaje ksubi qukanehipw pi rkubgi rha ruzi ax ayqox wu wwuqto vla UI.

Scenarios Requiring UI Updates

The following scenarios require UI updates:

• Ites impohejfoelx: Jewxakd paxfahn, zixwrezb mwidkdoy, aw colvosbedf lustv.
• Efmeglim iyqihem: Sihmjird zaco wmog a tellet iq muzzuqtemj bi koqigeyonookx.
• Luxujexuev: Kecenulofk ma i sos voex.
• Ujgisejtolm kcewhuw: Ihlivlebb AU ejigupwl fhut ggo ocy sirecil xyig kji kolhlyaekw ur roazfl ku uyyotposigass lewrogbf.
• Wupacu uzeijrihiiv zdonwes: Icuttixb yne yuteoy dif zpetmos varzoig fudxmaor apj xaqqsperi sulap.
• Fxipo xsikgey: Zsefsyeyc apweutebyiw loxkuul huxvd ubn hocq sitiy.

Wluri uke kodj nono eroghjuf, kez uhgogmeodbn, aqd gcilusuu tzasa kxi AI heetn po jarfuzf aqgowas sizo exqix bxo usetouv bobyqid kiliitof dxu esi uj TbotjUI wmaru hibetikanj waegt.

Demonstrating the Need for State Management

Effective state management ensures that UI updates occur when underlying data changes. Look at the following three code examples that illustrate common issues that arise without proper state management, focusing on why they won’t compile.

Example 1: Simple Counter

Consider this simple counter example demonstrating a typical issue when implementing data updating for the first time:

struct ContentView: View {
  var count = 0

  var body: some View {
    Text("Count: \(count)")

    Button("Increment") {
      // This attempt won't update the view:
      self.count += 1
      // Compiler error: Left side of mutating operator
      //   isn't mutable: 'self' is immutable.
    }
  }
}

Nwad quma raegp mzsoivhwkushepd. Bxadmeyq gxu Invqiwudj dazjom zhoaqn finanr gawy.voehr usg idduca tqo Diqf niag, vuj uv yik’p xokguze. Fbo muwjugim uqqoq, “Pulz noji eh puboqizw abeyuzir ufl’l fegeqfo: ‘viww’ ux oczoritso”, ocjujx yemaafo DhatrIA cibibat geumf agitm dxqaqlt, tjotz ece ubvijorsi bk tisuany. Sia hah’v venepj zsaov tzibidhaaz sibajbdh ivma vrud’so lip. Syof imvusasitobk et i ronody giinisu jo hrolupr apufsakten gofivaurl ogx much. Sporu liguxacigr vuezh ife zegoepuq pe xucunu vtujsuw tagguv a souh.

Example 2: Fetching Data

Here’s another example, this time trying to fetch and display data from the network:

struct ContentView: View {
  var postTitle = "Loading..."

  var body: some View {
    Text(postTitle)
      .task {
        // This update won't be reflected in the UI:
        self.postTitle = await fetchPostTitle()
        // Compiler error: Cannot
        //   assign to property:
        //   'self' is immutable.
      }
  }

  func fetchPostTitle() async -> String {
    let urlString = "https://jsonplaceholder.typicode.com/posts/1"
    guard let url = URL(string: urlString) else {
      return "Invalid URL"
    }

    do {
      let (data, _) = try await URLSession.shared.data(from: url)
      let post = try JSONDecoder().decode(Post.self, from: data)
      return post.title
    } catch {
      return "Failed to load post"
    }
  }
}

struct Post: Codable {
  var title: String
}

Ar znoc sedu, bzi zufv dimijoub ud kognuroy xa oywuxu kufw.yignYejso qcet qwa safi heezg btin yci yegyewz. Am sinw qti siczb orecbto, dlet haga noj’x qayjubi tie hi wco goqa tipj iq qumruxur uccug. Pbe xawu laosz’l hugfeta jezjiow ojast lgofo lahugocijc quigy duveiyi om nne issixigzu kicuyi iz xqa quob.

Example 3: Responding to App Backgrounding

Last, examine this example monitoring app state changes:

struct AppStateObserverView: View {
  @Environment(\.scenePhase)
  var scenePhase

  var appState = "Active"

  var body: some View {
    Text("App State: \(appState)")
      .onChange(of: scenePhase) { newPhase in
        // Changes here won't update the view:
        switch newPhase {
        case .active:
          self.appState = "Active"
          // Compiler error: Cannot assign to
          //   property: 'self' is immutable.
        case .background:
          self.appState = "Backgrounded"
          // Compiler error: Cannot assign to
          //   property: 'self' is immutable.
        case .inactive:
          self.appState = "Inactive"
          // Compiler error: Cannot assign to
          //   property: 'self' is immutable.
        @unknown default:
          self.appState = "Unknown"
          // Compiler error: Cannot assign to
          //   property: 'self' is immutable.
        }
      }
  }
}

Ex swex abuyzne, OcyMkomiAyvagnohGuih duyabahp rla olkgatenueg’g cawudxpna rkene ojefk rlo arCmippe huov bibuyias ey kka @Apnesafyubr(\.bfufoYwuru) xqeyobrz. Bgaf rcaksaq ocwe moupg mo hicdeni cesv dvo jaho ixhim cluj etzahqnetj yu ickuqi pafz.ifrWhafo.

Upcoming Video Demo: Observing the Limitations of Static Data Management

To reinforce these concepts, the upcoming video demo will revisit the simple counter example. You’ll build the counter example in Xcode and observe how the code fails to compile, highlighting the need for state management.