SwiftUI: การสร้างอินเทอร์เฟซที่ทันสมัยสำหรับ iOS

import SwiftUI

// Each screen is a struct that implements the View protocol
struct ContentView: View {

    // The "body" property describes what the view displays
    // "some View" means "a type of View, but Swift figures it out automatically"
    var body: some View {

        // VStack = "Vertical Stack": stacks elements vertically
        // spacing: 20 = 20 points of space between each element
        VStack(spacing: 20) {

            // A simple text with style modifiers
            Text("Welcome to SharpSkill")
                .font(.largeTitle)      // Large title font
                .fontWeight(.bold)      // Bold text

            Text("Prepare for your iOS interviews")
                .font(.subheadline)     // Smaller font
                .foregroundColor(.secondary)  // Gray secondary color

            // Button with an action (closure) and a label
            Button("Get Started") {
                print("Button tapped!")
            }
            .buttonStyle(.borderedProminent)  // Filled blue button style
        }
        .padding()  // Adds margins around the VStack
    }
}

// Example 1: padding THEN background
Text("SwiftUI")
    .padding()           // 1. Adds 16pt of space around the text
    .background(.blue)   // 2. Blue background covers text + padding
    .foregroundColor(.white)
// Result: white text on blue rectangle with margins

// Example 2: background THEN padding (reversed order!)
Text("SwiftUI")
    .background(.blue)   // 1. Tight blue background around text only
    .padding()           // 2. Transparent padding around blue background
    .foregroundColor(.white)
// Result: white text on small blue rectangle, surrounded by empty space

struct ProfileCard: View {
    var body: some View {
        // Main HStack: photo on left, info on right
        HStack(spacing: 16) {

            // ZStack to overlay the badge on the photo
            ZStack(alignment: .bottomTrailing) {
                // Profile image (circle)
                Image("avatar")
                    .resizable()
                    .frame(width: 60, height: 60)
                    .clipShape(Circle())

                // Green "verified" badge at bottom right
                Image(systemName: "checkmark.circle.fill")
                    .foregroundColor(.green)
                    .background(Circle().fill(.white))  // White circle behind
            }

            // VStack to stack name and role vertically
            VStack(alignment: .leading, spacing: 4) {
                Text("Marie Dupont")
                    .font(.headline)

                Text("iOS Developer")
                    .font(.subheadline)
                    .foregroundColor(.secondary)
            }

            // Spacer pushes everything to the left
            Spacer()

            // Chevron on right indicates it's tappable
            Image(systemName: "chevron.right")
                .foregroundColor(.gray)
        }
        .padding()
        .background(Color(.systemGray6))
        .cornerRadius(12)
    }
}

struct CounterView: View {
    // @State creates a "source of truth" for this view
    // private because state shouldn't be modified from outside
    @State private var count = 0

    var body: some View {
        VStack(spacing: 30) {
            // This Text updates automatically when count changes
            Text("\(count)")
                .font(.system(size: 72, weight: .bold))

            HStack(spacing: 40) {
                // Decrement button
                Button(action: {
                    count -= 1  // Modifies state → view refreshes
                }) {
                    Image(systemName: "minus.circle.fill")
                        .font(.largeTitle)
                }

                // Increment button
                Button(action: {
                    count += 1
                }) {
                    Image(systemName: "plus.circle.fill")
                        .font(.largeTitle)
                }
            }
        }
    }
}

// Parent view: owns the state
struct SignupForm: View {
    @State private var username = ""       // Source of truth
    @State private var isValid = false     // Validation state

    var body: some View {
        VStack(spacing: 20) {
            // We pass BINDINGS (with $) to the child view
            UsernameField(username: $username, isValid: $isValid)

            Button("Create Account") {
                // Submit the form
            }
            .disabled(!isValid)  // Disabled if invalid
            .buttonStyle(.borderedProminent)
        }
        .padding()
    }
}

// Child view: receives and modifies state via @Binding
struct UsernameField: View {
    @Binding var username: String   // Connection to parent's @State
    @Binding var isValid: Bool

    var body: some View {
        VStack(alignment: .leading, spacing: 8) {
            TextField("Username", text: $username)
                .textFieldStyle(.roundedBorder)
                .onChange(of: username) { oldValue, newValue in
                    // Validation: at least 3 characters
                    isValid = newValue.count >= 3
                }

            // Visual feedback
            HStack {
                Image(systemName: isValid ? "checkmark.circle" : "xmark.circle")
                Text("Minimum 3 characters")
            }
            .font(.caption)
            .foregroundColor(isValid ? .green : .red)
        }
    }
}

// Identifiable lets SwiftUI track each element
struct Interview: Identifiable {
    let id = UUID()           // Auto-generated unique identifier
    let technology: String
    let difficulty: String
    let questionCount: Int
}

struct InterviewListView: View {
    // Data to display (in reality, this would come from an API)
    @State private var interviews = [
        Interview(technology: "iOS", difficulty: "Intermediate", questionCount: 25),
        Interview(technology: "Android", difficulty: "Advanced", questionCount: 30),
        Interview(technology: "React", difficulty: "Beginner", questionCount: 20)
    ]

    var body: some View {
        // NavigationStack enables the navigation bar
        NavigationStack {
            List {
                // ForEach iterates over each interview
                // Thanks to Identifiable, no need to specify id:
                ForEach(interviews) { interview in
                    InterviewRow(interview: interview)
                }
            }
            .navigationTitle("My Interviews")
        }
    }
}

struct InterviewRow: View {
    let interview: Interview

    var body: some View {
        HStack {
            // Left column: title and subtitle
            VStack(alignment: .leading, spacing: 4) {
                Text(interview.technology)
                    .font(.headline)

                Text(interview.difficulty)
                    .font(.subheadline)
                    .foregroundColor(.secondary)
            }

            Spacer()

            // Badge with question count
            Text("\(interview.questionCount) Q")
                .font(.caption)
                .fontWeight(.medium)
                .padding(.horizontal, 8)
                .padding(.vertical, 4)
                .background(Color.blue.opacity(0.1))
                .cornerRadius(8)
        }
        .padding(.vertical, 4)
    }
}

struct InterviewListView: View {
    @State private var interviews = [/* ... data ... */]

    var body: some View {
        NavigationStack {
            List {
                ForEach(interviews) { interview in
                    InterviewRow(interview: interview)
                }
                // Swipe to delete
                .onDelete(perform: deleteInterview)
                // Drag and drop to reorder
                .onMove(perform: moveInterview)
            }
            .navigationTitle("My Interviews")
            .toolbar {
                // "Edit" button that activates edit mode
                EditButton()
            }
        }
    }

    // Deletes elements at specified indices
    private func deleteInterview(at offsets: IndexSet) {
        interviews.remove(atOffsets: offsets)
    }

    // Moves elements from one position to another
    private func moveInterview(from source: IndexSet, to destination: Int) {
        interviews.move(fromOffsets: source, toOffset: destination)
    }
}

struct AnimatedCard: View {
    @State private var isExpanded = false

    var body: some View {
        VStack {
            RoundedRectangle(cornerRadius: 20)
                .fill(.blue)
                // Dimensions change based on state
                .frame(
                    width: isExpanded ? 300 : 150,
                    height: isExpanded ? 200 : 100
                )

            Button(isExpanded ? "Collapse" : "Expand") {
                // withAnimation animates ALL visual changes
                // resulting from this state change
                withAnimation(.spring(response: 0.5, dampingFraction: 0.7)) {
                    isExpanded.toggle()
                }
            }
            .padding(.top)
        }
    }
}

struct TransitionDemo: View {
    @State private var showDetails = false

    var body: some View {
        VStack(spacing: 20) {
            Button("Show Details") {
                withAnimation(.easeInOut(duration: 0.3)) {
                    showDetails.toggle()
                }
            }

            // This view appears/disappears with a transition
            if showDetails {
                DetailCard()
                    // Asymmetric transition: different for entry and exit
                    .transition(
                        .asymmetric(
                            insertion: .scale.combined(with: .opacity),  // Entry: zoom + fade
                            removal: .slide                               // Exit: slide
                        )
                    )
            }
        }
    }
}

struct DetailCard: View {
    var body: some View {
        VStack(alignment: .leading, spacing: 12) {
            Text("Interview Details")
                .font(.headline)
            Text("25 questions • 45 minutes")
                .foregroundColor(.secondary)
        }
        .padding()
        .background(Color(.systemGray6))
        .cornerRadius(16)
    }
}

struct AsyncDataView: View {
    @State private var questions: [Question] = []
    @State private var isLoading = true
    @State private var errorMessage: String?

    var body: some View {
        Group {
            if isLoading {
                // State: loading in progress
                ProgressView("Loading...")

            } else if let error = errorMessage {
                // State: error
                VStack(spacing: 16) {
                    Image(systemName: "exclamationmark.triangle")
                        .font(.largeTitle)
                        .foregroundColor(.orange)
                    Text(error)
                    Button("Retry") {
                        Task { await loadQuestions() }
                    }
                }

            } else {
                // State: success, display data
                List(questions) { question in
                    Text(question.title)
                }
            }
        }
        // .task runs automatically when the view appears
        .task {
            await loadQuestions()
        }
        // Pull-to-refresh
        .refreshable {
            await loadQuestions()
        }
    }

    private func loadQuestions() async {
        isLoading = true
        errorMessage = nil

        do {
            // Async API call
            questions = try await QuestionService.shared.fetchQuestions()
        } catch {
            errorMessage = "Unable to load questions"
        }

        isLoading = false
    }
}