SwiftUI Academy · レッスン

タップカウンターの作成

タップ時に状態を更新し、UI が再描画される様子を確認します。

「タップカウンターの作成」はCoddyKit上の無料SwiftUI Academyレッスンです。これはレッスン3/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはSwiftUI Academy学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 SwiftUI Academyコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

The Goal: A Live Counter

Let's build a button that counts your taps and shows the total on screen. It is the classic first taste of reactive state. 👆

Start with State

Begin with a single @State property to hold the running total. We name it count and start it at zero.

@State private var count = 0

Show the Count

Display the value with a Text view. Using string interpolation, the label always mirrors the current count.

Text("Taps: \(count)")

Add the Button

Place a Button whose action increases the count by one. The closure runs each time you tap it.

Button("Tap me") {
    count += 1
}

Stack Them Together

Group the label and button in a VStack so they sit neatly one above the other. Now you have a complete tiny screen.

VStack {
    Text("Taps: \(count)")
    Button("Tap me") { count += 1 }
}

Watch the Loop

Tap, count changes, SwiftUI re-runs body, the Text updates. This tight cycle is the heart of every reactive SwiftUI screen.

The Full View

Here is everything wired together: state, a label, and a button living inside one view.

struct CounterView: View {
    @State private var count = 0
    var body: some View {
        VStack {
            Text("Taps: \(count)")
            Button("Tap me") { count += 1 }
        }
    }
}

Add a Reset

A second button can set count back to zero. Resetting state is just another assignment, and the UI follows instantly.

Button("Reset") { count = 0 }

Format the Label

You can pluralize or style the text based on count. The label is a function of state, so any logic you add stays in sync.

Text(count == 1 ? "1 tap" : "\(count) taps")

Decrement Too

Add a minus button to count down. Just remember to keep the value sensible, for example never letting it drop below zero.

Button("-") { if count > 0 { count -= 1 } }

No Manual Refresh

Notice you never call any refresh method. Changing count is all it takes; SwiftUI handles the redraw for you every single time.

Quick Check

In the tap counter, what makes the Text update after a tap?

Recap

You built a tap counter with one @State value, a Text that reads it, and a Button that mutates it. Change the data and the UI updates itself. 🎉

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